Power conversion device

Abstract

Provided is a power conversion device that is capable of performing zero-voltage soft switching and is capable of reducing switching loss. In a power conversion device (100), a control device (50) performs shortening control if it has been determined that, when a control signal for a high-level period set in advance is applied to each of a plurality of switches (8), resonance currents passing through two or more switches (8) of the plurality of switches (8) will flow simultaneously to a resonance inductor (L1). In the shortening control, the control device (50): applies a control signal in which the high-level period set in advance has been shortened to a first switch that is one of the two or more switches (8); and applies a control signal that has the high-level period set in advance to a second switch that is one of the two or more switches (8) other than the first switch.

Description

Power Conversion Device

[0001] The present disclosure relates to a power conversion device, and more particularly to a power conversion device capable of converting DC power into AC power.

[0002] Patent Document 1 discloses a power conversion device that converts direct current into multi-phase alternating current.

[0003] The power conversion device disclosed in Patent Document 1 includes a main switching means (power conversion circuit), two capacitors, one coil (resonant inductor), multiple auxiliary switching elements, and a control means. The main switching means is composed of a pair of main switching elements connected in series between both terminals of a DC power supply, and a main switching circuit is provided for each phase of the multi-phase AC, with the interconnection point of the pair of main switching elements serving as the output point for each phase. The two capacitors divide the voltage of the DC power supply. One end of the coil is connected to the voltage division point between the two capacitors. The multiple auxiliary switching elements connect the other end of the coil to the output points for each phase. When the control means determines that multiple phase currents are flowing through the coil, it controls the multiple auxiliary switching elements so that the current flowing in at least one phase is smaller than a preset magnitude.

[0004] JP 2010-233306 A

[0005] The power conversion device described in Patent Document 1 is configured to perform zero-current soft switching of the main switch, and is not capable of performing zero-voltage soft switching.

[0006] An object of the present disclosure is to provide a power conversion device that is capable of performing zero-voltage soft switching and reducing switching loss.

[0007] A power conversion device according to one aspect of the present disclosure includes a first DC terminal and a second DC terminal, a power conversion circuit, multiple AC terminals, multiple switches, multiple resonant capacitors, a resonant inductor, a regenerative capacitor, and a control device. The power conversion circuit includes multiple first switching elements and multiple second switching elements. In the power conversion circuit, multiple switching circuits are connected in parallel, with the multiple first switching elements and the multiple second switching elements connected in series in a one-to-one relationship. In the power conversion circuit, the multiple first switching elements are connected to the first DC terminals, and the multiple second switching elements are connected to the second DC terminals. The multiple AC terminals correspond one-to-one to the multiple switching circuits. Each of the multiple AC terminals is connected to a connection point between the first switching element and the second switching element in the corresponding switching circuit. The multiple switches correspond one-to-one to the multiple switching circuits. Each of the multiple switches has a first end connected to the connection point between the first switching element and the second switching element in the corresponding switching circuit, and a second end connected in common to a common connection point. The resonant capacitors correspond one-to-one to the multiple switches. Each of the multiple resonant capacitors is connected between the first end and the second DC terminal of the corresponding switch. The resonant inductor has a first end and a second end. The first end of the resonant inductor is connected to the common connection point. The regenerative capacitor has a third end and a fourth end. The third end of the regenerative capacitor is connected to the first DC terminal or the second DC terminal. The control device provides a control signal whose potential changes between a high level and a low level to each of the multiple first switching elements, the multiple second switching elements, and the multiple switches.The control device sets a dead time period between a high-level period of a control signal to the first switching element and a high-level period of a control signal to the second switching element for each of the multiple switching circuits, and sets the high-level period of the control signal to each of the multiple switches based on the dead time period for a corresponding switching circuit among the multiple switching circuits. A load current flows through each of the multiple AC terminals, passing through the first switching element or the second switching element of the corresponding switching circuit. The control device performs reduction control when it determines that resonant currents passing through two or more of the multiple switches simultaneously flow through the resonant inductor when a control signal with a predetermined high-level period is provided to each of the multiple switches. In the reduction control, the control device provides a control signal with the predetermined high-level period reduced to a first switch that is one of the two or more switches, and provides a control signal with the predetermined high-level period to a second switch of the two or more switches that is different from the first switch.

[0008] The power conversion device of the present disclosure has the advantage of being capable of performing zero-voltage soft switching and reducing switching loss.

[0009] FIG. 1 is a circuit diagram of a system including a power conversion device according to a first embodiment. FIG. 2 is an explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being charged. FIG. 3 is another explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being charged. FIG. 4 is another explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being charged. FIG. 5 is a diagram illustrating a time change in duty and a time change in load current corresponding to voltage command values ​​for each of three phases in an AC load connected to multiple AC terminals of the power conversion device according to the first embodiment. FIG. 6 is an explanatory diagram illustrating a first current threshold and a second current threshold used by the control device in the power conversion device according to the first embodiment. FIG. 7 is an explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being discharged. FIG. 8 is an explanatory diagram of the operation of the control device in the power conversion device of the first embodiment when the control device performs a basic operation in the case where the load current is less than 0 and the resonant capacitor is discharging. FIG. 9 is an explanatory diagram of the operation of the control device in the power conversion device of the first embodiment when the load current is less than 0 and the resonant capacitor is charging. FIG. 10 is a timing chart for explaining an example of a boundary condition between a case where currents flowing through the resonant inductors do not overlap (do not flow simultaneously) and a case where they overlap (flow simultaneously) in the power conversion device of the first embodiment. FIG. 11 is a timing chart for explaining an example of a boundary condition between a case where the U-phase resonant current and the V-phase resonant current do not overlap (do not flow simultaneously) and a case where they overlap (flow simultaneously) in the power conversion device of the first embodiment. FIG. 12 is an explanatory diagram of the conditions under which the control device performs shortening control in the power conversion device of the first embodiment. FIG. 13 is a timing chart for explaining the operation of the control device in the power conversion device of the first embodiment when the control device determines that the resonant currents overlap when the load current is greater than 0 and the resonant capacitor is charging. FIG. 14 is a timing chart for explaining the operation when the control device in the power conversion device of the first embodiment performs the reduction control.FIG. 15 is an explanatory diagram of conditions under which the control device performs reduction control in the power conversion device of embodiment 1. FIG. 16 is a timing chart for explaining the operation when the control device in the power conversion device of embodiment 1 determines that the resonant currents overlap when the load current is less than 0 and the resonant capacitor is discharging. FIG. 17 is a timing chart for explaining the operation when the control device in the power conversion device of embodiment 1 performs reduction control. FIG. 18 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 2 performs reduction control. FIG. 19 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 2 performs reduction control. FIG. 20 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 3 determines that the three-phase resonant currents overlap when the load current is greater than 0 and the resonant capacitor is charging. FIG. 21 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 3 performs reduction control. FIG. 22 is a timing chart for explaining the operation when the control device in the power conversion device of embodiment 3 determines that three-phase resonant currents overlap when the load current<0 and the resonant capacitor is discharging. FIG. 23 is a timing chart for explaining the operation when the control device in the power conversion device of embodiment 3 performs shortening control. FIG. 24 is an explanatory diagram of conditions under which the control device performs shortening control in the power conversion device according to Modification 2 of embodiment 3. FIG. 25 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 4 performs shortening control. FIG. 26 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 4 performs shortening control. FIG. 27 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 5 performs shortening control. FIG. 28 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 5 performs shortening control. FIG. 29 is a timing chart for explaining the operation when the control device in the power conversion device according to embodiment 6 performs shortening control.FIG. 30 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 6 performs the reduction control. FIG. 31 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 7 performs the reduction control. FIG. 32 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 7 performs the reduction control. FIG. 33 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 8 performs the reduction control. FIG. 34 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 8 performs the reduction control. FIG. 35 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 9 performs the reduction control. FIG. 36 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 9 performs the reduction control. FIG. 37 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 10 performs the reduction control. FIG. 38 is a timing chart for explaining the operation when the control device in the power conversion apparatus of embodiment 10 performs the reduction control. FIG. 39 is a circuit diagram of a system including the power conversion apparatus of embodiment 11. Fig. 40 is a circuit diagram of a system including a power conversion device according to embodiment 12. Fig. 41 is a circuit diagram of a system including a power conversion device according to embodiment 13.

[0010] First Embodiment A power conversion device 100 according to a first embodiment will be described below with reference to FIGS. 1 to 17. FIG.

[0011] (1) Overall Configuration of the Power Conversion Device FIG. 1 is a circuit diagram of a system including a power conversion device 100 according to the first embodiment. As shown in FIG. 1 , the power conversion device 100 includes, for example, a first DC terminal 31, a second DC terminal 32, and multiple (e.g., three) AC terminals 41. A DC power source E1 is connected between the first DC terminal 31 and the second DC terminal 32, and an AC load RA1 is connected to the multiple AC terminals 41. The AC load RA1 is, for example, a three-phase servo motor. The power conversion device 100 converts DC output from the DC power source E1 into AC power and outputs it to the AC load RA1. The DC power source E1 includes, for example, a solar cell or a fuel cell. The DC power source E1 may also include a DC-DC converter. In the power conversion device 100, when the multiple AC terminals 41 are three AC terminals 41, the AC power is, for example, three-phase AC power having a U phase, a V phase, and a W phase.

[0012] The power conversion device 100 includes a power conversion circuit 11, a plurality of (e.g., three) switches 8, a plurality of (e.g., three) resonant capacitors 9, a resonant inductor L1, a regenerative capacitor 15, and a control device 50. The power conversion device 100 also includes a protection circuit 17 and a capacitor C10. Each of the plurality of switches 8 is, for example, a bidirectional switch.

[0013] The power conversion circuit 11 has a plurality of (e.g., three) first switching elements 1 and a plurality of (e.g., three) second switching elements 2. In the power conversion circuit 11, a plurality of (e.g., three) switching circuits 10, each having a plurality of first switching elements 1 and a plurality of second switching elements 2 connected in series in a one-to-one relationship, are connected in parallel to one another. In the power conversion circuit 11, the plurality of first switching elements 1 are connected to first DC terminals 31, and the plurality of second switching elements 2 are connected to second DC terminals 32. The plurality of AC terminals 41 correspond one-to-one to the plurality of switching circuits 10. Each of the plurality of AC terminals 41 is connected to a connection point 3 between the first switching elements 1 and the second switching elements 2 in the corresponding switching circuit 10. The plurality of switches 8 correspond one-to-one to the plurality of switching circuits 10. Each of the plurality of switches 8 has a first terminal 81 connected to the connection point 3 between the first switching elements 1 and the second switching elements 2 in the corresponding switching circuit 10, and a second terminal 82 commonly connected to a common connection point 25. The multiple resonant capacitors 9 correspond one-to-one to the multiple switches 8. Each of the multiple resonant capacitors 9 is connected between the first end 81 of the corresponding switch 8 and the second DC terminal 32. The resonant inductor L1 has a first end and a second end. The first end of the resonant inductor L1 is connected to the common connection point 25. The regenerative capacitor 15 has a third end 153 and a fourth end 154. The regenerative capacitor 15 has a third end connected to the second DC terminal 32 and a fourth end connected to the common connection point 25 via the resonant inductor L1. The control device 50 controls the multiple first switching elements 1, the multiple second switching elements 2, and the multiple switches 8.

[0014] (2) Details of the Power Conversion Device Hereinafter, for convenience of explanation, the switching circuits 10 corresponding to the U phase, V phase, and W phase of the multiple switching circuits 10 may be referred to as switching circuit 10U, switching circuit 10V, and switching circuit 10W, respectively. Hereinafter, the first switching element 1 and the second switching element 2 of switching circuit 10U may be referred to as first switching element 1U and second switching element 2U. Hereinafter, the first switching element 1 and the second switching element 2 of switching circuit 10V may be referred to as first switching element 1V and second switching element 2V. Hereinafter, the first switching element 1 and the second switching element 2 of switching circuit 10W may be referred to as first switching element 1W and second switching element 2W. In the following, the connection point 3 between the first switching element 1U and the second switching element 2U will be referred to as connection point 3U, the connection point 3 between the first switching element 1V and the second switching element 2V will be referred to as connection point 3V, and the connection point 3 between the first switching element 1W and the second switching element 2W will be referred to as connection point 3W. In the following, the AC terminal 41 connected to connection point 3U will be referred to as AC terminal 41U, the AC terminal 41 connected to connection point 3V will be referred to as AC terminal 41V, and the AC terminal 41 connected to connection point 3W will be referred to as AC terminal 41W. In the following, the resonant capacitor 9 connected in parallel to the second switching element 2U will be referred to as resonant capacitor 9U, the resonant capacitor 9 connected in parallel to the second switching element 2V will be referred to as resonant capacitor 9V, and the resonant capacitor 9 connected in parallel to the second switching element 2W will be referred to as resonant capacitor 9W. In the following, the switch 8 connected to connection point 3U may be referred to as switch 8U, the switch 8 connected to connection point 3V may be referred to as switch 8V, and the switch 8 connected to connection point 3W may be referred to as switch 8W.

[0015] In the power conversion device 100, a high-potential output terminal (positive electrode) of the DC power supply E1 is connected to the first DC terminal 31, and a low-potential output terminal (negative electrode) of the DC power supply E1 is connected to the second DC terminal 32. In addition, in the power conversion device 100, for example, a U-phase terminal, a V-phase terminal, and a W-phase terminal of an AC load RA1 are connected to three AC terminals 41U, 41V, and 41W, respectively.

[0016] In the power conversion circuit 11, each of a plurality of (e.g., three) first switching elements 1 and a plurality of (e.g., three) second switching elements 2 has a control terminal, a first main terminal, and a second main terminal. The control terminals of the plurality of first switching elements 1 and the plurality of second switching elements 2 are connected to a control device 50. In each of the plurality of switching circuits 10 of the power conversion device 100, a first main terminal of the first switching element 1 is connected to a first DC terminal 31, a second main terminal of the first switching element 1 is connected to a first main terminal of the second switching element 2, and a second main terminal of the second switching element 2 is connected to a second DC terminal 32. In each of the plurality of switching circuits 10, the first switching element 1 is a high-side switching element (P-side switching element), and the second switching element 2 is a low-side switching element (N-side switching element). Each of the plurality of first switching elements 1 and the plurality of second switching elements 2 is, for example, an IGBT (Insulated Gate Bipolar Transistor). Therefore, the control terminal, the first main terminal and the second main terminal of each of the plurality of first switching elements 1 and the plurality of second switching elements 2 are the gate terminal, the collector terminal and the emitter terminal, respectively.

[0017] The power conversion circuit 11 further includes a plurality of (three) first diodes 4 connected in anti-parallel to the plurality of (three) first switching elements 1 in a one-to-one relationship, and a plurality of (three) second diodes 5 connected in anti-parallel to the plurality of (three) second switching elements 2 in a one-to-one relationship. In each of the plurality of first diodes 4, the anode of the first diode 4 is connected to the second main terminal (emitter terminal) of the first switching element 1 corresponding to the first diode 4, and the cathode of the first diode 4 is connected to the first main terminal (collector terminal) of the first switching element 1 corresponding to the first diode 4. In each of the plurality of second diodes 5, the anode of the second diode 5 is connected to the second main terminal (emitter terminal) of the second switching element 2 corresponding to the second diode 5, and the cathode of the second diode 5 is connected to the first main terminal (collector terminal) of the second switching element 2 corresponding to the second diode 5.

[0018] A connection point 3U between the first switching element 1U and the second switching element 2U is connected to, for example, the U-phase terminal of the AC load RA1 via an AC terminal 41U. A connection point 3V between the first switching element 1V and the second switching element 2V is connected to, for example, the V-phase of the AC load RA1 via an AC terminal 41V. A connection point 3W between the first switching element 1W and the second switching element 2W is connected to, for example, the W-phase of the AC load RA1 via an AC terminal 41W.

[0019] The multiple resonant capacitors 9 correspond one-to-one to the multiple switches 8. Each of the multiple resonant capacitors 9 is connected between a first end of the corresponding switch 8 and the second DC terminal 32. The power conversion device 100 has multiple resonant circuits. The multiple resonant circuits include a resonant circuit having a resonant capacitor 9U and a resonant inductor L1, a resonant circuit having a resonant capacitor 9V and a resonant inductor L1, and a resonant circuit having a resonant capacitor 9W and a resonant inductor L1. The multiple resonant circuits share the resonant inductor L1 in common.

[0020] Each of the multiple switches 8 has, for example, two IGBTs, a first IGBT 6 and a second IGBT 7, connected in anti-parallel. In each of the switches 8, the collector terminal of the first IGBT 6 is connected to the emitter terminal of the second IGBT 7, and the emitter terminal of the first IGBT 6 is connected to the collector terminal of the second IGBT 7. In each of the multiple switches 8, the emitter terminal of the first IGBT 6 is connected to the connection point 3 of the switching circuit 10 corresponding to the switch 8 having the first IGBT 6. In each of the multiple switches 8, the collector terminal of the second IGBT 7 is connected to the connection point 3 of the switching circuit 10 corresponding to the switch 8 having the second IGBT 7. The switch 8U is connected to the connection point 3U between the first switching element 1U and the second switching element 2U. The switch 8V is connected to the connection point 3V between the first switching element 1V and the second switching element 2V. The switch 8W is connected to a connection point 3W between the first switching element 1W and the second switching element 2W. Hereinafter, for convenience of explanation, the first IGBT 6 and the second IGBT 7 of the switch 8U may be referred to as the first IGBT 6U and the second IGBT 7U, the first IGBT 6 and the second IGBT 7 of the switch 8V may be referred to as the first IGBT 6V and the second IGBT 7V, and the first IGBT 6 and the second IGBT 7 of the switch 8W may be referred to as the first IGBT 6W and the second IGBT 7W, respectively.

[0021] The plurality of switches 8 are controlled by the control device 50. In other words, the first IGBT 6U, the second IGBT 7U, the first IGBT 6V, the second IGBT 7V, the first IGBT 6W, and the second IGBT 7W are controlled by the control device 50.

[0022] The resonant inductor L1 has a first end and a second end. The first end of the resonant inductor L1 is connected to the common connection point 25. The second end of the resonant inductor L1 is connected to the fourth end 154 of the regenerative capacitor 15.

[0023] The regenerative capacitor 15 is connected between the second end of the resonant inductor L1 and the second DC terminal 32. The regenerative capacitor 15 is, for example, a film capacitor.

[0024] The protection circuit 17 includes a third diode 13 and a fourth diode 14. The third diode 13 is connected between the common connection point 25 and the first DC terminal 31. In the third diode 13, the anode of the third diode 13 is connected to the common connection point 25. In addition, in the third diode 13, the cathode of the third diode 13 is connected to the first DC terminal 31. The fourth diode 14 is connected between the common connection point 25 and the second DC terminal 32. In the fourth diode 14, the anode of the fourth diode 14 is connected to the second DC terminal 32. In the fourth diode 14, the cathode of the fourth diode 14 is connected to the common connection point 25. Therefore, the fourth diode 14 is connected in series with the third diode 13.

[0025] The capacitor C10 is connected between the first DC terminal 31 and the second DC terminal 32, and is connected in parallel to the power conversion circuit 11. The capacitor C10 is, for example, an electrolytic capacitor.

[0026] The control device 50 controls a plurality of first switching elements 1, a plurality of second switching elements 2, and a plurality of switches 8. The execution entity of the control device 50 includes a computer system. The computer system has one or more computers. The computer system is primarily composed of a processor and memory as hardware. The processor executes a program stored in the memory of the computer system, thereby realizing the function of the control device 50 as the execution entity in the present disclosure. The program may be pre-stored in the memory of the computer system, provided via a telecommunications line, or provided by being recorded on a non-transitory recording medium such as a memory card, optical disk, or hard disk drive (magnetic disk) readable by the computer system. The processor of the computer system is composed of one or more electronic circuits including a semiconductor integrated circuit (IC) or a large-scale integrated circuit (LSI). The multiple electronic circuits may be integrated into a single chip or distributed across multiple chips. The multiple chips may be integrated into a single device or distributed across multiple devices.

[0027] The control device 50 outputs control signals SU1, SV1, and SW1 that control the on / off of the multiple first switching elements 1U, 1V, and 1W, respectively. Each of the control signals SU1, SV1, and SW1 is, for example, a PWM (Pulse Width Modulation) signal whose potential level changes between a first potential level (hereinafter also referred to as a low level) and a second potential level (hereinafter also referred to as a high level) that is higher than the first potential level. The first switching elements 1U, 1V, and 1W are turned on when the control signals SU1, SV1, and SW1 are at a high level, and turned off when the control signals SU1, SV1, and SW1 are at a low level. The control device 50 also outputs control signals SU2, SV2, and SW2 that control the on / off of the multiple second switching elements 2U, 2V, and 2W, respectively. Each of the control signals SU2, SV2, and SW2 is, for example, a PWM signal whose potential level changes between a first potential level (hereinafter also referred to as a low level) and a second potential level (hereinafter also referred to as a high level) that is higher than the first potential level. The second switching elements 2U, 2V, and 2W are turned on when the control signals SU2, SV2, and SW2 are at a high level, and turned off when they are at a low level.

[0028] The control device 50 uses a sawtooth-wave carrier signal (see FIGS. 2 and 11 ) to generate control signals SU1, SV1, and SW1 corresponding to the plurality of first switching elements 1U, 1V, and 1W, respectively, and control signals SU2, SV2, and SW2 corresponding to the plurality of second switching elements 2U, 2V, and 2W, respectively. More specifically, the control device 50 generates the control signals SU1 and SU2 to be provided to the first switching element 1U and the second switching element 2U, respectively, based on at least the carrier signal and a U-phase voltage command value. The control device 50 also generates the control signals SV1 and SV2 to be provided to the first switching element 1V and the second switching element 2V, respectively, based on at least the carrier signal and a V-phase voltage command value. The control device 50 also generates the control signals SW1 and SW2 to be provided to the first switching element 1W and the second switching element 2W, respectively, based on at least the carrier signal and a W-phase voltage command value. The U-phase voltage command value, V-phase voltage command value, and W-phase voltage command value are, for example, sinusoidal signals whose phases are different from each other by 120°, and whose values ​​change over time. The waveform of the carrier signal is not limited to a sawtooth waveform and may be, for example, a triangular wave or a sawtooth wave obtained by inverting a sawtooth wave as shown in FIG. 11 . The U-phase voltage command value, V-phase voltage command value, and W-phase voltage command value have the same cycle length. The U-phase voltage command value, V-phase voltage command value, and W-phase voltage command value have a longer cycle length than the carrier signal.

[0029] The duty of the control signals SU1 and SU2 provided by the control device 50 to the first switching element 1U and the second switching element 2U, respectively, varies based on the U-phase voltage command value. In FIG. 5, the duty of the control signal SU1 is shown as the U-phase duty. The control device 50 (see FIG. 1) compares the U-phase voltage command value with a carrier signal to generate the control signal SU1 provided to the first switching element 1U. The control device 50 also inverts the control signal SU1 provided to the first switching element 1U to generate the control signal SU2 provided to the second switching element 2U. The control device 50 also sets a dead time Td (see FIG. 2) between the high-level period of the control signal SU1 and the high-level period of the control signal SU2 so that the on-periods of the first switching element 1U and the second switching element 2U do not overlap.

[0030] The duties of the control signals SV1 and SV2 provided from the control device 50 to the first switching element 1V and the second switching element 2V, respectively, vary based on the V-phase voltage command value. In FIG. 5, the duty of the control signal SV1 is shown as the V-phase duty. The control device 50 (see FIG. 1) compares the V-phase voltage command value with a carrier signal to generate the control signal SV1 provided to the first switching element 1V. The control device 50 also inverts the control signal SV1 provided to the first switching element 1V to generate the control signal SV2 provided to the second switching element 2V. The control device 50 also sets a dead time Td (see FIG. 3) between the high-level period of the control signal SV1 and the high-level period of the control signal SV2 so that the on-periods of the first switching element 1V and the second switching element 2V do not overlap.

[0031] The duty of the control signals SW1 and SW2 provided by the control device 50 to the first switching element 1W and the second switching element 2W, respectively, varies based on the W-phase voltage command value. In FIG. 5, the duty of the control signal SW1 is shown as the W-phase duty. The control device 50 (see FIG. 1) compares the W-phase voltage command value with a carrier signal to generate the control signal SW1 provided to the first switching element 1W. The control device 50 also inverts the control signal SW1 provided to the first switching element 1W to generate the control signal SW2 provided to the second switching element 2W. The control device 50 also sets a dead time period Td (see FIG. 4) between the high-level period of the control signal SW1 and the high-level period of the control signal SW2 so that the on periods of the first switching element 1W and the second switching element 2W do not overlap.

[0032] The U-phase voltage command value, V-phase voltage command value, and W-phase voltage command value are, for example, sinusoidal signals that are out of phase with each other by 120°, and their values ​​change over time. Therefore, the duty of control signal SU1 (U-phase duty), the duty of control signal SV1 (V-phase duty), and the duty of control signal SW1 (W-phase duty) change into sinusoidal waves that are out of phase with each other by 120°, for example, as shown in Fig. 5. Similarly, the duty of control signal SU2, the duty of control signal SV2, and the duty of control signal SW2 change into sinusoidal waves that are out of phase with each other by 120°.

[0033] The control device 50 generates each of the control signals SU1, SU2, SV1, SV2, SW1, and SW2 based on the carrier signal, each voltage command value, and information about the state of the AC load RA1. For example, if the AC load RA1 is a three-phase servo motor, the information about the state of the AC load RA1 includes, for example, detection values ​​from a plurality of current sensors that detect output currents (hereinafter also referred to as load currents) iU, iV, and iW flowing through the U-phase, V-phase, and W-phase of the AC load RA1, respectively.

[0034] The plurality of switches 8, the resonant inductor L1, the plurality of resonant capacitors 9, and the regenerative capacitor 15 are provided to perform zero-voltage soft switching of the plurality of first switching elements 1 and the plurality of second switching elements 2.

[0035] In the power conversion device 100 , the control device 50 controls the plurality of switches 8 in addition to the plurality of first switching elements 1 and second switching elements 2 of the power conversion circuit 11 .

[0036] The control device 50 generates control signals SU6, SU7, SV6, SV7, SW6, and SW7 that control the on / off of the first IGBT6U, the second IGBT7U, the first IGBT6V, the second IGBT7V, the first IGBT6W, and the second IGBT7W, respectively, and outputs them to the gate terminals of the first IGBT6U, the second IGBT7U, the first IGBT6V, the second IGBT7V, the first IGBT6W, and the second IGBT7W, respectively.

[0037] When the first IGBT 6U is in the ON state and the second IGBT 7U is in the OFF state, the switch 8U allows a charging current to pass through the path of the regenerative capacitor 15, resonant inductor L1, switch 8U, and resonant capacitor 9U. The charging current is a current that charges the resonant capacitor 9U. When the first IGBT 6U is in the OFF state and the second IGBT 7U is in the ON state, the switch 8U allows a discharging current to pass through the path of the resonant capacitor 9U, switch 8U, resonant inductor L1, and regenerative capacitor 15. The discharging current is a current that discharges the charge of the resonant capacitor 9U.

[0038] When the first IGBT 6V is in the ON state and the second IGBT 7V is in the OFF state, the switch 8V allows a charging current to pass through the path of the regenerative capacitor 15-resonant inductor L1-switch 8V-resonant capacitor 9V. The charging current is a current that charges the resonant capacitor 9V. When the first IGBT 6V is in the OFF state and the second IGBT 7V is in the ON state, the switch 8V allows a discharging current to pass through the path of the resonant capacitor 9V-switch 8V-resonant inductor L1-regenerative capacitor 15. The discharging current is a current that discharges the charge of the resonant capacitor 9V.

[0039] When the first IGBT 6W is in the ON state and the second IGBT 7W is in the OFF state, the switch 8W allows a charging current to pass through the path of the regenerative capacitor 15, resonant inductor L1, switch 8W, and resonant capacitor 9W. The charging current is a current that charges the resonant capacitor 9W. When the first IGBT 6W is in the OFF state and the second IGBT 7W is in the ON state, the switch 8W allows a discharging current to pass through the path of the resonant capacitor 9W, switch 8W, resonant inductor L1, and regenerative capacitor 15. The discharging current is a current that discharges the charge of the resonant capacitor 9W.

[0040] (3) Operation of the Power Conversion Device In the following description, the polarity of current iL1 flowing through resonant inductor L1 will be defined as positive when flowing in the direction of the arrow in Fig. 1, and as negative when flowing in the direction opposite to the direction of the arrow in Fig. 1. Also, in the following description, the polarity of load currents iU, iV, and iW flowing through the U, V, and W phases of AC load RA1 will be defined as positive when flowing in the direction of the arrow in Fig. 1, and as negative when flowing in the direction opposite to the direction of the arrow in Fig. 1. Also, in the following description, the polarity of currents i9U, i9V, and i9W flowing through resonant capacitors 9U, 9V, and 9W will be defined as positive when flowing in the direction of the arrow in Fig. 1, and as negative when flowing in the direction opposite to the direction of the arrow in Fig. 1. Therefore, in the case of a discharge operation in which the resonant capacitors 9U, 9V, and 9W are discharged, the polarity of the currents i9U, i9V, and i9W is positive, and in the case of a charge operation in which the resonant capacitors 9U, 9V, and 9W are charged, the polarity of the currents i9U, i9V, and i9W is negative.

[0041] In the power conversion device 100, for example, the first IGBT 6U of the switch 8U may change from an ON state in which the first IGBT 6U of the switch 8U is in an ON state and current iL1 is flowing through the resonant inductor L1 with positive polarity to an OFF state. In this case, the current iL1 flowing through the resonant inductor L1 is regenerated to the power conversion circuit 11 via the third diode 13 until the energy of the resonant inductor L1 is consumed and the current iL1 becomes zero. In addition, in the power conversion device 100, for example, the second IGBT 7U of the switch 8U may change from an ON state in which the second IGBT 7U of the switch 8U is in an ON state and current iL1 is flowing through the resonant inductor L1 with negative polarity to an OFF state. In this case, the current iL1 flowing through the resonant inductor L1 flows through the path of the fourth diode 14, the resonant inductor L1, and the regenerative capacitor 15 until the energy of the resonant inductor L1 is consumed and the current iL1 becomes zero.

[0042] Furthermore, in the power conversion device 100, for example, a state in which the first IGBT 6V of the switch 8V is in the on state and current iL1 is flowing through the resonant inductor L1 with positive polarity may change to an off state, where the first IGBT 6V of the switch 8V is in the on state. In this case, the current iL1 flowing through the resonant inductor L1 is regenerated to the power conversion circuit 11 via the third diode 13 until the energy of the resonant inductor L1 is consumed and the current iL1 becomes zero. Also, in the power conversion device 100, for example, a state in which the second IGBT 7V of the switch 8V is in the on state and current iL1 is flowing through the resonant inductor L1 with negative polarity may change to an off state, where the current iL1 flowing through the resonant inductor L1 flows through the path of the fourth diode 14, resonant inductor L1, and regenerative capacitor 15 until the energy of the resonant inductor L1 is consumed and the current iL1 becomes zero.

[0043] Furthermore, in the power conversion device 100, for example, the first IGBT 6W of the switch 8W may be switched from an ON state in which the current iL1 is flowing through the resonant inductor L1 with positive polarity to an OFF state in which the first IGBT 6W of the switch 8W is switched. In this case, the current iL1 flowing through the resonant inductor L1 is regenerated to the power conversion circuit 11 via the third diode 13 until the energy of the resonant inductor L1 is consumed and the current iL1 becomes zero. Furthermore, in the power conversion device 100, for example, the second IGBT 7W of the switch 8W may be switched from an ON state in which the current iL1 is flowing through the resonant inductor L1 with negative polarity to an OFF state in which the second IGBT 7W of the switch 8W is switched. In this case, the current iL1 flowing through the resonant inductor L1 flows through the path of the fourth diode 14, the resonant inductor L1, and the regenerative capacitor 15 until the energy of the resonant inductor L1 is consumed and the current iL1 becomes zero.

[0044] The control device 50 sets a dead time period Td between the high-level period of the control signals SU1, SV1, and SW1 to the first switching elements 1U, 1V, and 1W and the high-level period of the control signals SU2, SV2, and SW2 to the second switching elements 2U, 2V, and 2W for each of the multiple switching circuits 10. The control device 50 also sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Here, the control device 50 sets the length of the high-level period of the control signal to each of the multiple switches 8 as, for example, the sum of the length of the first period and the length of the second period. The length of the first period is the length of a resonance half cycle determined by the capacitance of the resonance capacitor 9 corresponding to the switch 8 and the inductance of the resonance inductor L1. If the resonance cycle is Tres, the length of the first period is Tres / 2. The end point of the first period, i.e., the end point of an integral multiple of the resonant half cycle, preferably coincides with the end point of the dead time period Td for the switching circuit 10 corresponding to the switch 8. For example, in the example of FIG. 2 , the length between time t2 and time t3 of the control signal SU6 is the length of the first period. The length of the second period is, for example, an additional time Tau determined by the voltage of the regenerative capacitor 15, the inductance of the resonant inductor L1, and the load current value. The length of the first period described above is an ideal design example and may be 90% to 110% of the length of Tres / 2. Furthermore, the length of the second period described above is an ideal design example and may be 90% to 110% of the additional time (additional time Tau in the example of FIG. 2 ) determined by the voltage of the regenerative capacitor 15, the inductance of the resonant inductor L1, and the load current value.

[0045] 1 to 9, the basic operation of the zero voltage soft switching of each of the plurality of first switching elements 1 and the plurality of second switching elements 2 is described below. The basic operation is an operation performed when a resonant current passing through two or more of the plurality of switches 8 does not flow simultaneously through the resonant inductor L1. After describing the basic operation, the operation performed when the control device 50 determines that a resonant current passing through two or more of the plurality of switches 8 will flow simultaneously will be described.

[0046] (3.1) Basic Operation In zero voltage soft switching of the first switching element 1, the voltage across the first switching element 1 must be set to zero immediately before the first switching element 1, which is the target of zero voltage soft switching, is turned on. In addition, in zero voltage soft switching of the second switching element 2, the voltage across the second switching element 2 must be set to zero immediately before the second switching element 2, which is the target of zero voltage soft switching, is turned on. Hereinafter, the switching element (first switching element 1 or second switching element 2) that is the target of zero voltage soft switching is also referred to as the target switching element.

[0047] The basic operation of the control device 50 differs depending on the polarity (positive / negative) of the load current flowing through the AC terminal 41 connected to the target switching element and the operation (charging operation / discharging operation) of the resonant capacitor 9 connected in series or parallel to the target switching element. The load current is positive when flowing from the AC terminal 41 to the AC load RA1, and negative when flowing from the AC load RA1 to the AC terminal 41. When the resonant capacitor 9 is charging, the voltage across the resonant capacitor 9 increases. When the resonant capacitor 9 is discharging, the voltage across the resonant capacitor 9 decreases. The voltage across each of the multiple second switching elements 2 is the same as the voltage across the resonant capacitor 9 connected in parallel to the second switching element 2.

[0048] (3.1.1) Operation for Soft-Switching the First Switching Element When Load Current > 0 When the target of soft switching is the first switching element 1 (hereinafter also referred to as the target first switching element 1), and the polarity of the load current flowing through the AC terminal 41 connected to the target first switching element 1 is positive, the control device 50 turns on the first IGBT 6 corresponding to the target first switching element 1. As a result, the control device 50 causes resonance between the resonant inductor L1 and the resonant capacitor 9 connected to the target first switching element 1, charging the resonant capacitor 9 from the regenerative capacitor 15 and setting the voltage across the target first switching element 1 to zero. In this way, the power conversion device 100 can achieve zero-voltage soft switching of the target first switching element 1.

[0049] 2 is an explanatory diagram illustrating the operation of the control device 50 in the power conversion device 100 of the first embodiment when performing basic operation in the case where the load current is greater than 0 and the resonant capacitor 9 is being charged. In the case where the target first switching element is the first switching element 1U of the switching circuit 10U, control signals SU1 and SU2 are illustrated which are given from the control device 50 to the first switching element 1U and the second switching element 2U of the switching circuit 10U, respectively. Also illustrated in FIG. 2 are the control signal SU6 given from the control device 50 to the first IGBT 6U of the switch 8U, the load current iU flowing in the U-phase of the AC load RA1, the current iL1 flowing in the resonant inductor L1, the voltage V1u across the first switching element 1U, and the voltage V2u across the second switching element 2U.

[0050] 2 also shows a dead time period Td set in the control device 50 to prevent the first switching element 1 and the second switching element 2, which are in phase, from being turned on simultaneously. Also shown in Fig. 2 is an additional time Tau set in the control device 50 for the control signal SU6 of the first IGBT 6U of the switch 8U. The additional time Tau will be described later.

[0051] FIG. 3 is another operational diagram illustrating the basic operation of the control device 50 in the power conversion device 100 of the first embodiment when the load current is greater than 0 and the resonant capacitor 9 is being charged. FIG. 3 illustrates control signals SV1 and SV2 provided by the control device 50 to the first switching element 1V and the second switching element 2V of the switching circuit 10V, respectively, for the case where the target first switching element is the first switching element 1V of the switching circuit 10V. FIG. 3 also illustrates the control signal SV6 provided by the control device 50 to the first IGBT 6V of the switch 8V, the load current iV flowing through the V-phase of the AC load RA1, the current iL1 flowing through the resonant inductor L1, the voltage V1v across the first switching element 1V, and the voltage V2v across the second switching element 2V. In FIG. 3, the voltage value of the DC power source E1 is indicated as Vd.

[0052] 3 also shows a dead time period Td set in the control device 50 to prevent the first switching element 1 and the second switching element 2, which are in phase, from being turned on simultaneously. Also, FIG. 3 shows an additional time Tav set in the control device 50 for the control signal SV6 of the first IGBT 6V of the switch 8V. The additional time Tav will be described later.

[0053] FIG. 4 is another explanatory diagram of the basic operation of the control device 50 in the power conversion device 100 of the first embodiment when the load current is greater than 0 and the resonant capacitor 9 is being charged. FIG. 4 illustrates the control signals SW1 and SW2 provided by the control device 50 to the first switching element 1W and the second switching element 2W of the switching circuit 10W, respectively, when the target first switching element is the first switching element 1W of the switching circuit 10W. FIG. 4 also illustrates the control signal SW6 provided by the control device 50 to the first IGBT 6W of the switch 8W and the load current iW flowing through the W-phase of the AC load RA1. FIG. 4 also illustrates the current iL1 flowing through the resonant inductor L1. FIG. 4 also illustrates the voltage V1w across the first switching element 1W and the voltage V2w across the second switching element 2W. In FIG. 4, the voltage value of the DC power source E1 is indicated as Vd.

[0054] 4 also shows a dead time period Td set in the control device 50 to prevent the first switching element 1W and the second switching element 2W from being turned on simultaneously. Also, Fig. 4 shows an additional time Taw set in the control device 50 for the control signal SW6 of the first IGBT 6W of the switch 8W. The additional time Taw will be described later.

[0055] As shown in FIG. 2, the additional time Tau is set to advance the start time t1 of the high-level period of the control signal SU6 relative to the start time t2 of the first period, thereby making the high-level period of the control signal SU6 longer than the first period. The length of the additional time Tau is set based on the value of the load current iU. To start LC resonance at the start time t2 of the first period, it is desirable that the value of the current iL1 match the value of the load current iU at the start time t2 of the first period. This is because, while iL1 < iU, all of the current iL1 flows through the AC load RA1, preventing the resonant capacitor 9U from being charged. The end time of the high-level period of the control signal SU6 may be the same as or later than the end time t3 of the dead time period Td. FIG. 2 shows an example in which the end time of the high-level period of the control signal SU6 is set to the same as the end time t3 of the dead time period Td. For example, the control device 50 sets the high-level period of the control signal SU6 to Tau + (Tres / 2). That is, the control device 50 sets the length of the first period to Tres / 2. In the switching circuit 10U, the voltage V2u across the second switching element 2U becomes Vd at time t3 when the dead time period Td ends, and the voltage V1u across the first switching element 1U becomes zero at time t3 when the dead time period Td ends. In the example of FIG. 2 , the current iL1 flowing through the resonance inductor L1 begins at time t1 when the high-level period of the control signal SU6 starts, after time t0 when the dead time period Td starts, and becomes zero at time t4 when additional time Tau has elapsed since time t3 when the dead time period Td ends. As for the current iL1, as a result of iL1≧iU being satisfied from time t2 when the first period starts, the current iL1 in the shaded region of the current waveform in the fifth row from the top in Fig. 2 flows into the resonant capacitor 9U, causing LC resonance. After time t3 when the dead time period Td ends, the current iL1 is regenerated in the power conversion circuit 11 via the third diode 13 directly connected to the resonant inductor L1.

[0056] As described above, the control device 50 determines the additional time Tau based on the load current iU so that iL1 = iU at time t2 when the first period starts, in order to start LC resonance at time t2 when the first period starts and end the resonance half cycle at time t3 when the dead time period Td ends. More specifically, the control device 50 determines the additional time Tau by calculating Tau = iU × (L / V15) using, for example, the detection result of the load current iU by a current sensor or its signal processed value, or an estimated value of the load current iU, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 of the regenerative capacitor 15 (the potential V15 at the fourth end of the regenerative capacitor 15). In this case, the detection result of the load current iU or its signal processed value is the detection value at the carrier period to which the additional time Tau is added, or at the timing closest to that carrier period. The estimated value of the load current iU at this time is, for example, a value obtained by estimating the load current iU in the carrier period to which the additional time Tau is added. The resonance half period in the case of basic operation is half the resonance period, which is the reciprocal of the resonance frequency of the resonance circuit including the resonance inductor L1 and one resonance capacitor 9. Therefore, if the inductance of the resonance inductor L1 is L and the capacitance of the resonance capacitor 9 is C, then the resonance half period is π × (L C) １／２ In the control device 50, the length of the dead time period Td is set to be longer than the length of the resonance half cycle, but the length of the dead time period Td may be set to be shorter than the length of the resonance half cycle. However, it is desirable to design the length of the resonance half cycle to be shorter than the length of the dead time period Td.

[0057] As shown in FIG. 3 , the additional time Tav is set to advance the start time t11 of the high-level period of the control signal SV6 relative to the start time t12 of the first period, thereby making the high-level period of the control signal SV6 longer than the first period. The length of the additional time Tav is set based on the value of the load current iV. To start LC resonance at the start time t12 of the first period, it is desirable that the value of the current iL1 match the value of the load current iV at the start time t12 of the first period. This is because, while iL1 < iV, all of the current iL1 flows through the AC load RA1, preventing the resonant capacitor 9V from being charged. The end time of the high-level period of the control signal SV6 may be the same as or later than the end time t13 of the dead time period Td. FIG. 3 shows an example in which the end time of the high-level period of the control signal SV6 is set to the same as the end time t13 of the dead time period Td. The control device 50 sets the high-level period of the control signal SV6 to Tav + (Tres / 2). The voltage V1v across the first switching element 1V becomes zero at time t13, when the dead time period Td ends. In the example of FIG. 3 , the current iL1 flowing through the resonant inductor L1 begins at time t11, when the high-level period of the control signal SV6 begins, after time t10, when the dead time period Td begins, and becomes zero at time t14, when the additional time Tav has elapsed since time t13, when the dead time period Td ends. Since iL1≧iV holds true from time t12, when the first period begins, the current iL1 in the shaded region of the current waveform in the fourth row from the top in FIG. 3 flows into the resonant capacitor 9V, generating LC resonance. After time t13, when the dead time period Td ends, the current iL1 is regenerated in the power conversion circuit 11 via the third diode 13, which is directly connected to the resonant inductor L1.

[0058] As described above, the control device 50 determines the additional time Tav based on the load current iV so that iL1 = iV at time t12, when the first period starts. More specifically, the control device 50 determines the additional time Tav by calculating Tav = iV × (L / V15) using, for example, the detection result of the load current iV by a current sensor, its signal-processed value, or an estimated value of the load current iV, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 of the regenerative capacitor 15. The detection result of the load current iV or its signal-processed value at this time uses a detection value at the carrier cycle to which the additional time Tav is added, or at a timing closest to that carrier cycle. Furthermore, the estimated value of the load current iV at this time uses, for example, a value estimated from the load current iV at the carrier cycle to which the additional time Tav is added.

[0059] As shown in FIG. 4 , the additional time Taw is set to advance the start time t9 of the high-level period of the control signal SW6 to earlier than the start time t22 of the first period, thereby making the high-level period of the control signal SW6 longer than the first period. The length of the additional time Taw is set based on the value of the load current iW. To start LC resonance at the start time t22 of the first period, it is desirable that the value of the current iL1 match the value of the load current iW at the start time t22 of the first period. This is because, while iL1 < iW, all of the current iL1 flows through the AC load RA1, preventing the resonant capacitor 9W from being charged. The end time of the high-level period of the control signal SW6 may be the same as or later than the end time t23 of the dead time period Td. FIG. 4 illustrates an example in which the end time of the high-level period of the control signal SW6 is set to the same as the end time t23 of the dead time period Td. The control device 50 sets the high-level period of the control signal SW6 to Taw + (Tres / 2). The voltage V1w across the first switching element 1W becomes zero at time t23, when the dead time period Td ends. In the example of FIG. 4 , the current iL1 flowing through the resonant inductor L1 begins at time t21, when the high-level period of the control signal SW6 begins, after time t20, when the dead time period Td begins, and becomes zero at time t24, when the additional time Taw has elapsed since time t23, when the dead time period Td ends. Since iL1≧iW holds true from time t22, when the first period begins, the current iL1 in the shaded region of the current waveform in the fourth row from the top in FIG. 4 flows into the resonant capacitor 9W, generating LC resonance. After time t23, when the dead time period Td ends, the current iL1 is regenerated in the power conversion circuit 11 via the third diode 13, which is directly connected to the resonant inductor L1.

[0060] The control device 50 determines the additional time Taw based on the load current iW. More specifically, the control device 50 determines the additional time Taw by calculating Taw = iW × (L / V15) using the detection result of the load current iW by the current sensor, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 of the regenerative capacitor 15. The detection result of the load current iW or its signal processed value at this time uses a detection value at the carrier cycle to which the additional time Taw is added or at a timing closest to that carrier cycle. Furthermore, the estimated value of the load current iW at this time uses, for example, a value estimated from the load current iW at the carrier cycle to which the additional time Taw is added.

[0061] (3.1.2) Operation for Soft-Switching the Second Switching Element When Load Current > 0 FIG. 6 is an explanatory diagram of the first current threshold I1 and the second current threshold I2 used by the control device 50 in the power conversion device 100 of embodiment 1. When the soft-switching target is the second switching element 2 (hereinafter also referred to as the target second switching element 2), and the polarity of the load current (load current iU, load current iV, or load current iW) flowing through the AC terminal 41 connected to the target second switching element 2 is positive, the control device 50 compares the current value of the load current with the first current threshold I1 (=Ith, see FIG. 6 ). As an example of designing the operation of the control device 50, the control device 50 does not turn on the switch 8 when the current value of the load current is greater than the first current threshold I1, and turns on the switch 8 during the dead time period Td when the current value of the load current is smaller than the first current threshold I1. Here, as in section (3.1.1), it is assumed that the length of the dead time period Td is set to be longer than the resonant half cycle. In the power conversion device 100, when the current value of the load current is greater than the first current threshold I1, the control device 50 can discharge the resonant capacitor 9U connected in parallel to the target second switching element 2 using the load current iU without turning on the switch 8 corresponding to the target second switching element 2. This allows the power conversion device 100 to achieve zero-voltage soft switching of the target second switching element 2.

[0062] 7 is an explanatory diagram illustrating the operation of the control device 50 in the power conversion device 100 of the first embodiment when the control device 50 performs basic operation when the load current is greater than 0 and the resonant capacitor 9 is discharging. Fig. 7 illustrates the control signals SU1, SU2, and SU7, the load current iU, the current i9U flowing from the resonant capacitor 9U, and the voltage V2u across the second switching element 2U when the target second switching element 2 is the second switching element 2U of the switching circuit 10U and the current value of the load current is greater than the first current threshold I1. Fig. 7 also illustrates the dead time period Td and the additional time Tau set by the control device 50 for the control signal SU7 of the second IGBT 7U of the switch 8U.

[0063] When the current value of the load current iU is greater than the first current threshold I1, the control device 50 does not provide a high-level period for the control signal SU7. In this case, in the power conversion device 100, the current i9U begins to flow from the resonant capacitor 9U at time t32, when the first period starts, and the current i9U decreases to zero before time t33, when the dead time period Td ends, and the voltage V2u across the second switching element 2U becomes zero before time t33, when the dead time period Td ends. As a result, in the power conversion device 100, when the control signal SU2 changes from low to high at time t33, when the dead time period Td ends, the second switching element 2U is subjected to zero-voltage soft switching.

[0064] When the current value of the load current iU is smaller than the first current threshold I1, the control device 50 provides a high-level period for the control signal SU7, as shown by the two-dot chain line in FIG. 7 . In this case, the start time t32 of the high-level period of the control signal SU7 is, for example, after the start time t31 of the dead time period Td. The end time of the high-level period of the control signal SU7 is the same as the end time t33 of the dead time period Td. As a result, in the power conversion device 100, the voltage V2u across the second switching element 2U becomes zero before the end time t33 of the dead time period Td. Therefore, in the power conversion device 100, when the control signal SU2 changes from low to high at the end time t33 of the dead time period Td, the second switching element 2U is subjected to zero-voltage soft switching. The start time of the high-level period of the control signal SU7 may be time t30, which is earlier than the start time of the first period by an additional time Tau. The end point of the high-level period of the control signal SU7 may be time t34, which is later than time t33, at which the dead time period Td ends, by the additional time Tau. The time before and after the first period in the high-level period is not limited to the additional time Tau, and may be other set times. The relationship between the load current threshold and the method for determining the high-level period of the control signal to the switch 8 described above is an ideal design example and is not limited to the above design example. For example, the control device 50 may set the high-level period of the control signal to the switch 8 so that the switch 8 is turned on, for example, during the first period, even if the current value of the load current is greater than the first current threshold I1. The control device 50 may not turn on the switch 8 during the first period, even if the current value of the load current is smaller than the first current threshold I1. The control device 50 may set the high-level period of the control signal to the switch 8 so that the switch 8 is always turned on, for example, during the first period, regardless of the first current threshold I1. The control device 50 may also keep the switch 8 always in the off state, regardless of the first current threshold I1. The control device 50 may also combine the operations described in (3.1.2) as appropriate. The control device 50 may not necessarily match the high-level period of the control signal to the switch 8 with the first period as described above.For example, the high level period of the control signal to the switch 8 may be designed to have a length other than the first period depending on the design time of the resonance half cycle.

[0065] (3.1.3) Operation for Soft-Switching the Second Switching Element When Load Current<0 When the polarity of the load current (load current iU, load current iV, or load current iW) flowing through the AC terminal 41 connected to the target second switching element 2 is negative, the control device 50 turns on the second IGBT 7 corresponding to the target second switching element 2. As a result, the control device 50 causes the resonant capacitor 9 and the resonant inductor L1 connected to the target second switching element 2 to resonate, discharging the resonant capacitor 9 and setting the voltage across the target second switching element 2 to zero. This allows the power conversion device 100 to achieve zero-voltage soft switching of the target second switching element 2.

[0066] 8 is an explanatory diagram of the operation when the control device 50 in the power conversion device 100 of embodiment 1 performs basic operation when the load current is less than 0 and the resonant capacitor 9 is discharging. Fig. 8 illustrates the control signals SU1, SU2, and SU7, the load current iU, the current iL1 flowing through the resonant inductor L1, and the voltage V2u across the second switching element 2U in the case where the target second switching element 2 is the second switching element 2U of the switching circuit 10U.

[0067] FIG. 8 also illustrates the dead time period Td set by the control device 50 to prevent the first switching element 1 and the second switching element 2, which are in phase, from being simultaneously turned on. FIG. 8 also illustrates the additional time Tau set by the control device 50 for the control signal SU7 of the second IGBT 7U of the switch 8U. The end point of the high-level period of the control signal SU7 may be the same as or later than the end point t43 of the dead time period Td. In FIG. 8, the end point of the high-level period of the control signal SU7 is set to the same as the end point t43 of the dead time period Td. For example, the control device 50 sets the high-level period of the control signal SU7 to Tau + (Tres / 2). In the switching circuit 10U, the voltage V2u across the second switching element 2U becomes zero at the end point t43 of the dead time period Td. 8 , the current iL1 flowing through the resonant inductor L1 begins at time t41, when the high-level period of the control signal SU7 starts, after time t40, when the dead time period Td starts, and becomes zero at time t44, when the additional time Tau has elapsed since time t43, when the dead time period Td ends. With regard to the current iL1, since time t42, when the first period starts, iL1≦iU, LC resonance occurs, and a resonant current (discharge current of the resonant capacitor 9U) flows from the resonant capacitor 9U toward the resonant inductor L1. After time t43, when the dead time period Td ends, the current iL1 is regenerated in the power conversion circuit 11 via the fourth diode 14, which is directly connected to the resonant inductor L1.

[0068] The control device 50 determines the additional time Tau based on the load current iU so that iL1 = iU at time t42, when the first period starts, to start LC resonance and end the resonant half cycle at time t43, when the dead time period Td ends. More specifically, the control device 50 determines the additional time Tau by calculating Tau = |iU| × (L / V15) using, for example, the detection result of the output current iU by a current sensor or its signal-processed value, or an estimated value of the load current iU, the pre-stored inductance L of the resonance inductor L1, and the detection result of the voltage V15 of the regenerative capacitor 15. The load current value (the detection result of the load current iU or its signal-processed value) used here is the detection value at the carrier cycle to which the additional time Tau is added, or at the timing closest to that carrier cycle. The estimated value of the load current iU here is, for example, the load current iU estimated for the carrier cycle to which the additional time Tau is added. The resonance half period in the case of basic operation is half the resonance period, which is the reciprocal of the resonance frequency of the resonance circuit including the resonance inductor L1 and one resonance capacitor 9. Therefore, if the inductance of the resonance inductor L1 is L and the capacitance of the resonance capacitor 9 is C, the resonance half period is π × (L C) １／２ In the control device 50, the length of the dead time period Td during basic operation is set to be, for example, longer than the half resonance period during basic operation, but may also be set to be the same as the half resonance period during basic operation.

[0069] (3.1.4) Operation for Soft-Switching the First Switching Element When Load Current < 0 When the polarity of the load current (load current iU, load current iV, or load current iW) flowing through the AC terminal 41 connected to the target first switching element 1 is negative, the control device 50 compares the current value of the load current with the second current threshold I2 (= −Ith, see FIG. 6 ). As an example of designing the operation of the control device 50, the control device 50 does not turn on the switch 8 when the current value of the load current is smaller than the second current threshold I2, and turns on the switch 8 during the first period when the current value of the load current is greater than the second current threshold I2. Here, as in section (3.1.1), it is assumed that the length of the dead time period Td is set to be longer than the length of the resonant half cycle, for example. When the current value of the load current is smaller than the second current threshold I2, the power conversion device 100 can charge the resonant capacitor 9U connected in series to the target first switching element 1 with the load current without the control device 50 turning on the switch 8 corresponding to the target first switching element 1. This allows the power conversion device 100 to achieve zero-voltage soft switching of the target first switching element 1.

[0070] 9 is an explanatory diagram illustrating the operation of the control device 50 in the power conversion device 100 of the first embodiment when the control device 50 performs basic operation when the load current is less than 0 and the resonant capacitor 9 is charging. In FIG. 9, the control signals SU1, SU2, and SU6, the load current iU, the current i9U flowing from the resonant capacitor 9U, and the voltage V2u across the second switching element 2U are shown for the case where the target first switching element 1 is the first switching element 1U of the switching circuit 10U and the current value of the load current is greater than the second current threshold I2 (in other words, the absolute value of the current value of the load current is less than the absolute value of the second current threshold I2). Also shown in FIG. 9 is the dead time period Td.

[0071] When the load current value is smaller than the second current threshold I2 (in other words, when the absolute value of the load current is greater than the absolute value of the second current threshold I2), the control device 50 does not provide a high-level period for the control signal SU6. In this case, in the power conversion device 100, the current i9U begins to flow through the resonant capacitor 9U at time t51, when the first period begins. As a result, in the power conversion device 100, the resonant capacitor 9U is charged, and the voltage V2u across the second switching element 2U increases. The current i9U becomes zero before time t52, when the dead time period Td ends, and the voltage V1u across the first switching element 1U becomes zero before time t52, when the dead time period Td ends. As a result, in the power conversion device 100, the first switching element 1U is zero-voltage soft-switched when the control signal SU1 changes from low to high at time t52, when the dead time period Td ends.

[0072] When the load current value is greater than the second current threshold I2 (in other words, when the absolute value of the load current is less than the absolute value of the second current threshold), the control device 50 sets a high-level period for the control signal SU6, as shown by the two-dot chain line in FIG. 9 . In this case, the high-level period of the control signal SU6 starts at time t51, which is later than time t50, when the dead time period Td starts. The high-level period of the control signal SU6 ends at time t52, when the dead time period Td ends. As a result, in the power conversion device 100, the voltage V1u across the first switching element 1U becomes zero before time t52, when the dead time period Td ends. Therefore, in the power conversion device 100, when the control signal SU1 changes from low to high at time t52, when the dead time period Td ends, the first switching element 1U is subjected to zero-voltage soft switching. Furthermore, the relationship between the load current threshold and the method for determining the high-level period of the control signal to the switch 8 described above is an ideal design example and is not limited to the above design example. For example, the control device 50 may set the high-level period of the control signal to the switch 8 so that the switch 8 is turned on, for example, during the first period, even when the current value of the load current is smaller than the second current threshold I2. The control device 50 may not turn on the switch 8 during the first period even when the current value of the load current is larger than the second current threshold I2. The control device 50 may set the high-level period of the control signal to the switch 8 so that the switch 8 is always turned on, for example, during the first period, regardless of the second current threshold I2. The control device 50 may also keep the switch 8 always in the off state regardless of the second current threshold I2. The control device 50 may also appropriately combine the operations described in (3.1.3). The control device 50 may not necessarily set the high-level period of the control signal to the switch 8 to coincide with the first period as described above. For example, depending on the design time of the resonance half cycle, the control device 50 may set the high level period of the control signal to the switch 8 to a length other than the first period.

[0073] (3.2) Operation of the Shortening Control When the control device 50 determines that a resonant current passing through each of two or more of the multiple switches 8 will flow simultaneously through the resonant inductor L1 when a control signal with a preset high-level period is given to each of the multiple switches 8, the control device 50 performs the shorting control. In the shorting control, a control signal with a preset high-level period shortened is given to a first switch that is one of the two or more switches 8, and a control signal with a preset high-level period is given to a second switch of the two or more switches 8 that is different from the first switch. When the control device 50 "determines that a resonant current passing through each of two or more of the multiple switches 8 will flow simultaneously" means that the control device 50 has previously estimated that a resonant current passing through each of the two or more switches 8 will flow simultaneously through the resonant inductor L1.

[0074] (3.2.1) Determination of whether two-phase resonant currents flow simultaneously Figure 5 is a diagram showing time changes in duties corresponding to voltage command values ​​for each of three phases and time changes in load current in an AC load connected to multiple AC terminals of the power conversion device 100 of embodiment 1. In the power conversion device 100, the phases of the voltage command values ​​for three phases (U phase, V phase, and W phase) are different from each other by 120°, but the voltage command values ​​for two phases approach each other by an electrical angle of 60°, and the duties of the control signals for the two phases approach each other (see the first region A1 and the second region A2 in Figure 5). In the first region A1 in Figure 5, the duties of the U-phase control signal and the V-phase control signal are approximately 0.75. In the second region A2 in Figure 5, the duties of the U-phase control signal and the V-phase control signal are approximately 0.25. In the power conversion device 100, the polarity of the resonant current is the same as the polarity of the current iL1, and in the first region A1, the polarity of the resonant current is positive, and in the second region A2, the polarity of the resonant current is negative. In the case of the first region A1 of the power conversion device 100, for example, during one cycle of the carrier signal, the time difference between time t1 (see FIG. 2 ) at which the high-level period of the control signal SU6 applied to the first IGBT 6U starts and time t11 (see FIG. 3 ) at which the high-level period of the control signal SV6 applied to the first IGBT 6V starts is short, and there is a possibility that the U-phase resonant current and the V-phase resonant current will simultaneously flow through the resonant inductor L1. In the power conversion device 100, in the second region A2, the direction of the resonant current is opposite to that in the first region A1, but there is a possibility that the U-phase resonant current and the V-phase resonant current will simultaneously flow through the resonant inductor L1.

[0075] Assuming that the capacitance of each of the multiple resonant capacitors 9U, 9V, and 9W is C, if a U-phase current and a V-phase current flow simultaneously through the resonant inductor L1, then in terms of an equivalent circuit, a capacitor having a combined capacitance (=2×C) of the resonant capacitors 9U and 9V is connected in series to the resonant inductor L1. Therefore, in the power conversion device 100, if two-phase currents flow simultaneously through the resonant inductor L1, the resonant frequency of the resonant circuit including the resonant inductor L1 will change compared to when a single-phase current flows through the resonant inductor L1, and zero-voltage soft switching may not be achieved.

[0076] (3.2.1.1) In the Case of Charging Operation of Resonant Capacitor FIG. 10 is a diagram showing an example of boundary conditions between a case where the U-phase current (current passing through switch 8U) and a V-phase current (current passing through switch 8V) do not overlap (flow simultaneously) and a case where they overlap (flow simultaneously) in resonant inductor L1. These boundary conditions will be described with reference to FIG. 10. FIG. 11 is a diagram showing an example of boundary conditions between a case where the U-phase resonant current and the V-phase resonant current do not overlap (flow simultaneously) and a case where they overlap (flow simultaneously) in resonant inductor L1. These boundary conditions will be described with reference to FIG. 11.

[0077] 10, in the power conversion device 100, if the time difference ΔTuv is equal to or greater than Tau+Tav+(Tres / 2), the U-phase current and the V-phase current do not overlap in the resonant inductor L1, and if the time difference ΔTuv is less than Tau+Tav+(Tres / 2), the U-phase current and the V-phase current overlap. The time difference ΔTuv is the time difference between time t3 when the high-level period of the control signal SU1 starts and time t13 when the high-level period of the control signal SV1 starts.

[0078] In the power conversion device 100, as shown in FIG. 11, if the time difference ΔTuv is Tres / 2 or more, the U-phase resonant current and the V-phase resonant current do not overlap in the resonant inductor L1, and if the time difference ΔTuv is less than Tres / 2, the U-phase resonant current and the V-phase resonant current overlap in the resonant inductor L1.

[0079] The control device 50 sets the threshold for the time difference ΔTuv to Tres / 2. If the time difference ΔTuv is less than Tres / 2, the control device 50 estimates that the U-phase resonant current and the V-phase resonant current flow simultaneously in the resonant inductor L1. The above threshold setting is an example, and other values ​​may be set. For example, the threshold may be set to a value greater than Tres / 2, taking into account errors in the value of Tres / 2, etc. For example, the threshold may be set to Tau + Tav + (Tres / 2). In this case, the control device 50 estimates that the U-phase current (current passing through the switch 8U) and the V-phase current (current passing through the switch 8V) overlap in the resonant inductor L1 if the time difference ΔTuv is less than Tau + Tav + (Tres / 2). Furthermore, taking into consideration an error in the value of Tau + Tav + (Tres / 2), the threshold value may be set to a value even greater than Tau + Tav + (Tres / 2). Furthermore, the method for calculating the time difference ΔTuv used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example, and any other calculation method may be used as long as it can calculate a time difference equivalent to the time difference ΔTuv. For example, the time difference ΔTuv used to determine whether two-phase resonant currents flow simultaneously may be the time difference between time t0 (see FIG. 2 ) at which the high-level period of the control signal SU2 ends and time t10 (see FIG. 3 ) at which the high-level period of the control signal SV2 ends.

[0080] In the following description, the time difference between time t3 (see FIG. 2) when the high-level period of control signal SU1 starts and time t23 (see FIG. 4) when the high-level period of control signal SW1 starts is referred to as ΔTuw. In power conversion device 100, if the time difference ΔTuw is Tres / 2 or more, the U-phase resonant current and the W-phase resonant current do not overlap in resonant inductor L1, and if the time difference ΔTuw is less than Tres / 2, the U-phase resonant current and the W-phase resonant current overlap in resonant inductor L1.

[0081] The control device 50 sets a threshold for the time difference ΔTuw to Tres / 2. If the time difference ΔTuw is less than the threshold, the control device 50 estimates that a U-phase resonant current and a W-phase resonant current flow simultaneously through the resonant inductor L1. The above threshold setting is an example, and other values ​​may be set. For example, taking into account errors in the value of Tres / 2, the threshold may be set to a value greater than Tres / 2. For example, the threshold may be set to Tau + Taw + (Tres / 2). In this case, the control device 50 estimates that a U-phase current (current passing through the switch 8U) and a W-phase current (current passing through the switch 8W) overlap in the resonant inductor L1 if the time difference ΔTuw is less than Tau + Taw + (Tres / 2). Furthermore, taking into consideration an error in the value of Tau + Taw + (Tres / 2), the threshold value may be set to a value even greater than Tau + Taw + (Tres / 2). Furthermore, the method for calculating the time difference ΔTuw used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example, and any other calculation method may be used as long as it can calculate a time difference equivalent to the time difference ΔTuw. For example, the time difference ΔTuw used to determine whether two-phase resonant currents flow simultaneously may be the time difference between time t0 (see FIG. 2 ) at which the high-level period of the control signal SU2 ends and time t20 (see FIG. 4 ) at which the high-level period of the control signal SW2 ends.

[0082] In the following description, the time difference between time t13 (see FIG. 3 ) at which the high-level period of the control signal SV1 given to the first switching element 1V of the switching circuit 10V starts and time t23 (see FIG. 4 ) at which the high-level period of the control signal SW1 given to the first switching element 1W of the switching circuit 10W starts will be referred to as ΔTvw. In the power conversion device 100, if the time difference ΔTvw is Tres / 2 or more, the V-phase resonant current and the W-phase resonant current do not overlap in the resonant inductor L1, and if the time difference ΔTvw is less than Tres / 2, the V-phase resonant current and the W-phase resonant current overlap in the resonant inductor L1.

[0083] The control device 50 sets a threshold value for the time difference ΔTvw to Tres / 2. If the time difference ΔTvw is less than the threshold value, the control device 50 estimates that a V-phase resonant current and a W-phase resonant current flow simultaneously through the resonant inductor L1. The above threshold value is an example, and may be set to another value. For example, taking into account an error in the value of Tres / 2, the threshold value may be set to a value greater than Tres / 2. For example, the threshold value may be set to Tav + Taw + (Tres / 2). In this case, the control device 50 estimates that a V-phase current (current passing through the switch 8V) and a W-phase current (current passing through the switch 8W) overlap in the resonant inductor L1 if the time difference ΔTvw is less than Tav + Taw + (Tres / 2). Furthermore, taking into consideration an error in the value of Tav + Taw + (Tres / 2), the threshold value may be set to a value even greater than Tav + Taw + (Tres / 2). Furthermore, the method for calculating the time difference ΔTvw used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example, and any other calculation method may be used as long as it can calculate a time difference equivalent to the time difference ΔTvw. For example, the time difference ΔTvw used to determine whether two-phase resonant currents flow simultaneously may be the time difference between time t10 (see FIG. 3 ) at which the high-level period of the control signal SV2 ends and time t20 (see FIG. 4 ) at which the high-level period of the control signal SW2 ends.

[0084] (3.2.1.2) In the case of discharging operation of the resonant capacitor In the case of discharging operation of the resonant capacitor 9, the control device 50 can determine whether two-phase resonant currents flow simultaneously using the same time difference and threshold value as in the case of charging operation of the resonant capacitor 9.

[0085] For example, if the time difference between the start of the high-level period of the control signal SU2 and the start of the high-level period of the control signal SV2 is less than a threshold value (e.g., Tres / 2), the control device 50 estimates that the U-phase resonant current and the V-phase resonant current overlap. The above threshold value is an example, and other values ​​may be used. For example, taking into account errors in the value of Tres / 2, the threshold value may be set to a value greater than Tres / 2. For example, the threshold value may be set to Tau + Tav + (Tres / 2). In this case, if the time difference ΔTuv is less than Tau + Tav + (Tres / 2), the control device 50 estimates that the U-phase current (current passing through the switch 8U) and the V-phase current (current passing through the switch 8V) overlap in the resonant inductor L1. Furthermore, taking into consideration an error in the value of Tau+Tav+(Tres / 2), the threshold value may be set to a value even greater than Tau+Tav+(Tres / 2).

[0086] Furthermore, if the time difference between the start of the high-level period of the control signal SU2 and the start of the high-level period of the control signal SW2 is less than a threshold value (e.g., Tres / 2), the control device 50 estimates that the U-phase resonant current and the W-phase resonant current overlap. The above threshold value is an example, and may be set to another value. For example, the threshold value may be set to a value greater than Tres / 2, taking into account errors in the value of Tres / 2, etc. For example, the threshold value may be set to Tau + Taw + (Tres / 2). In this case, the control device 50 estimates that the U-phase current (current passing through the switch 8U) and the W-phase current (current passing through the switch 8W) overlap in the resonant inductor L1 if the time difference ΔTuw is less than Tau + Taw + (Tres / 2). Furthermore, taking into consideration an error in the value of Tau+Taw+(Tres / 2), the threshold value may be set to a value even greater than Tau+Taw+(Tres / 2).

[0087] Furthermore, if the time difference between the start of the high-level period of the control signal SV2 and the start of the high-level period of the control signal SW2 is less than a threshold value (e.g., Tres / 2), the control device 50 estimates that the V-phase resonant current and the W-phase resonant current overlap. The above threshold value is an example, and may be set to another value. For example, the threshold value may be set to a value greater than Tres / 2, taking into account errors in the value of Tres / 2, etc. For example, the threshold value may be set to Tav + Taw + (Tres / 2). In this case, if the time difference ΔTvw is less than Tav + Taw + (Tres / 2), the control device 50 estimates that the V-phase current (current passing through the switch 8V) and the W-phase current (current passing through the switch 8W) overlap in the resonant inductor L1. Furthermore, taking into consideration an error in the value of Tav+Taw+(Tres / 2), the threshold value may be set to a value even greater than Tav+Taw+(Tres / 2).

[0088] (3.2.2) Shortening Control When It is Determined that Two-Phase Resonant Currents Flow Simultaneously In shortening control, the control device 50 provides a control signal with a preset high-level period shortened to a first switch that is one of the two or more switches 8, and provides a control signal with a preset high-level period to a second switch that is different from the first switch among the two or more switches 8. When performing shorting control, the control device 50 defines the first switch as the switch 8 that corresponds to the switching circuit 10 that has the largest absolute value of the load current flowing through each of the two or more switching circuits 10 that correspond to the two or more switches 8 in the plurality of switching circuits 10.

[0089] (3.2.2.1) In the case of charging operation of the resonant capacitor Below, we will explain the operation of the shortening control when the control device 50 has determined in advance that two-phase resonant currents, U phase and V phase, will flow simultaneously during the period corresponding to the third region A3 in Figure 12, with reference to Figures 13 and 14.

[0090] FIG. 12 is an explanatory diagram of conditions under which the control device 50 performs shortening control in the power conversion device 100 of the first embodiment. The upper part of FIG. 12 illustrates a voltage command value Vu for the U-phase switching circuit 10U, a voltage command value Vv for the V-phase switching circuit 10V, and a voltage command value Vw for the W-phase switching circuit 10W, which are given to the control device 50 from an external device. The lower part of FIG. 12 illustrates a U-phase load current iU, a V-phase load current iV, and a W-phase load current iW. In a third region A3 of FIG. 12, the absolute value of the voltage command value Vv is greater than the absolute value of the voltage command value Vu. In the third region A3 of FIG. 12, the absolute value of the load current iV is greater than the absolute value of the load current iU.

[0091] Fig. 13 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 of the first embodiment when the control device 50 determines that the resonant currents overlap when the load current is greater than 0 and the resonant capacitor 9 is being charged. Fig. 13 shows a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, the current iL1, and the voltages V2u, V2v across the second switching elements 2U, 2V, respectively, before performing the shortening control when the control device 50 determines that the U-phase resonant current and the V-phase resonant current flow simultaneously. Fig. 14 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 of the first embodiment when performing the shortening control. 14 shows timing charts of the control signals SU1, SU2, SV1, SV2, control signals SU6, SV6, load currents iU, iV, current iL1, and voltages V2u, V2v across the second switching elements 2U, 2V, respectively, when the control device 50 performs shortening control. Note that FIGS. 13 and 14 show timing charts for a portion of one period within one cycle of the carrier signal.

[0092] In the power conversion device 100, when the control device 50 does not perform the speed reduction control, the voltages V2u and V2v across the second switching elements 2U and 2V do not rise to Vd when the control signals SU1 and SV1 change from a low level to a high level (the end of the dead time periods corresponding to the U phase and the V phase, respectively), as shown in FIG. 13 . In other words, when the control device 50 does not perform the speed reduction control, the charging of the resonant capacitors 9U and 9V does not finish when the dead time periods corresponding to the U phase and the V phase, respectively, end. Therefore, when the speed reduction control is not performed, the voltages across the first switching elements 1U and 1V do not decrease to zero when the dead time periods corresponding to the U phase and the V phase, respectively, end. As a result, in the power conversion device 100, the switching of the first switching elements 1U and 1V becomes hard switching.

[0093] When the control device 50 determines that the U-phase resonant current and the V-phase resonant current flow simultaneously during the charging operation of the resonant capacitor, it shortens one of the high-level periods of the control signal SU6 and the control signal SV6. FIG. 14 shows an example in which the high-level period of the control signal SV6 is shortened by a shortening period Tredv. The control device 50 may shorten the high-level period of the control signal SU6 instead of the high-level period of the control signal SV6. Specifically, the control device 50 shortens the high-level period of the control signal SV6 by the shortening period Tredv so that the control signal SU6 and the control signal SV6 do not overlap. In the example of FIG. 14, the control device 50 sets the length of the shortening period Tredv to be equal to the length of the high-level period of the control signal SU6 before the shortening control and the overlap period Tov (see FIG. 13) between the control signal SV6 before the shortening control. In the example of Fig. 13, the length of the overlap period Tov is the time difference between the start point of the high-level period of the control signal SU6 and the end point of the high-level period of the control signal SV6. Therefore, in the example of Fig. 14, the length of the shortening period Tredv is the same as the length of the overlap period Tov. In the example of Fig. 14, the control device 50 shortens the high-level period of the control signal SV6 in a direction that advances the end point of the high-level period of the control signal SV6 before the shortening control by the shortening period Tredv.

[0094] As can be seen from the waveform of current iL1 in Figure 13 and the waveform of current iL1 in Figure 14, in the power conversion device 100, the control device 50 performs shortening control when it determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby shortening the overlap period of the U-phase resonant current and the V-phase resonant current. Furthermore, from the voltages V2u and V2v across the second switching elements 2U and 2V in Figure 13 and the voltages V2u and V2v across the second switching elements 2U and 2V in Figure 14, it can be seen that hard switching of the first switching element 1U is mitigated, and hard switching of the first switching element 1V is also mitigated. Since the value of the voltage across the first switching circuit 10U is Vd, the sum of the voltage V1u across the first switching element 1U and the voltage V2u across the second switching element 2U is Vd. "Hard switching of the first switching element 1U is alleviated" means that zero-voltage soft switching of the first switching element 1U is realized or the value of the voltage V1u across the first switching element 1U when the first switching element 1U is turned on is reduced. With regard to each of the other first switching elements 1V and 1W, "hard switching of the switching element is alleviated" has the same meaning as when hard switching of the first switching element 1U is alleviated. In other words, "hard switching of the switching element is alleviated" means that zero-voltage soft switching of the switching element is realized or the value of the voltage across the switching element when the switching element is turned on is reduced.

[0095] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period between the U-phase resonant current and the W-phase resonant current, and also reduces the hard switching of each of the switching elements 1U and 1W.

[0096] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period of the V-phase resonant current and the W-phase resonant current, and also alleviate the hard switching of each of the switching elements 1V and 1W.

[0097] (3.2.2.2) In the case of discharge operation of the resonant capacitor Below, we will explain the operation of the shortening control when the control device 50 has determined in advance that two-phase resonant currents, U phase and V phase, will flow simultaneously during the period corresponding to the fourth region A4 in Figure 15, with reference to Figures 16 and 17.

[0098] Fig. 15 is an explanatory diagram of the conditions under which the control device 50 performs shortening control in the power conversion device 100 of the first embodiment. How to interpret Fig. 15 is the same as how to interpret Fig. 12 , so the explanation will be omitted. In the fourth region A4 of Fig. 15 , the absolute value of the voltage command value Vv is greater than the absolute value of the voltage command value Vu. Also, in the fourth region A4 of Fig. 15 , the absolute value of the load current iV is greater than the absolute value of the load current iU.

[0099] FIG. 16 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 of the first embodiment when the control device 50 determines that the resonant currents overlap when the load current is less than 0 and the resonant capacitor 9 is discharging. FIG. 16 illustrates a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and iL1, and the voltages V2u and V2v across the second switching elements 2U and 2V, respectively, before performing the shortening control when the control device 50 determines that the U-phase resonant current and the V-phase resonant current flow simultaneously. FIG. 17 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 of the first embodiment when performing the shortening control. FIG. 17 illustrates a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and iL1, respectively, when performing the shortening control. 16 and 17 show timing charts for a part of one period of the carrier signal.

[0100] In the power conversion device 100, when the control device 50 does not perform the speed reduction control, the voltages V2u and V2v across the second switching elements 2U and 2V do not decrease to zero at the time when the control signals SU1 and SV1 change from a low level period to a high level period (the time when the dead time periods corresponding to the U phase and the V phase, respectively, end), as shown in Fig. 16. In other words, when the control device 50 does not perform the speed reduction control, the discharge of the resonant capacitors 9U and 9V does not finish at the time when the dead time periods corresponding to the U phase and the V phase, respectively, end. For this reason, in the power conversion device 100, when the control device 50 does not perform the speed reduction control, the switching of the second switching elements 2U and 2V becomes hard switching.

[0101] The control device 50 compares the absolute value of the load current iU with the absolute value of the load current iV and shortens the high-level period of the control signal SV7 to the switch 8V having the larger absolute value of the load current iV by the shortening period Tredv. Specifically, the control device 50 shortens the high-level period of the control signal SV7 by the shortening period Tredv so that the control signal SU7 and the control signal SV7 do not overlap. In the example of FIG. 17 , the control device 50 sets the length of the shortening period Tredv to be equal to the length of the high-level period of the control signal SU7 before the shortening control and the overlap period Tov (see FIG. 16 ) of the control signal SV7 before the shortening control. In the example of FIG. 16 , the length of the overlap period Tov is the time difference between the start point of the high-level period of the control signal SU7 and the end point of the high-level period of the control signal SV7. Therefore, in the example of FIG. 17 , the length of the shortening period Tredv is the same as the length of the overlap period Tov. In the example of FIG. 17, the control device 50 shortens the high-level period of the control signal SV7 in a direction that advances the end point of the high-level period of the control signal SV7 before the shortening control by the shortening period Tredv.

[0102] As can be seen from the waveform of current iL1 in Figure 16 and the waveform of current iL1 in Figure 17, in the power conversion device 100, the control device 50 performs shortening control when it determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby shortening the overlap period of the U-phase resonant current and the V-phase resonant current. Furthermore, from the voltages V2u and V2v across the second switching elements 2U and 2V in Figure 16 and the voltages V2u and V2v across the second switching elements 2U and 2V in Figure 17, it can be seen that hard switching of the second switching element 2U is alleviated, and hard switching of the second switching element 2V is also alleviated. "Alleviating hard switching of the second switching element 2U" means that zero-voltage soft switching of the second switching element 2U is realized, or the value of the voltage V2u across the second switching element 2U when the second switching element 2U is turned on is reduced. For each of the other switching elements 2V and 2W, "hard switching of the switching element is alleviated" has the same meaning as when hard switching of the second switching element 2U is alleviated. In other words, alleviating hard switching of a switching element means that zero-voltage soft switching of the switching element is realized or the value of the voltage across the switching element when the switching element is turned on is reduced.

[0103] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period between the U-phase resonant current and the W-phase resonant current, and also reduces the hard switching of each of the switching elements 1U and 1W.

[0104] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period of the V-phase resonant current and the W-phase resonant current, and also alleviate the hard switching of each of the switching elements 1V and 1W.

[0105] (4) Advantages In the power conversion device 100 according to the first embodiment, the control device 50 sets a dead time period Td between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2 for each of the multiple switching circuits 10, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through the resonant inductor L1 and passes through two or more of the multiple switches 8 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs shortening control. In the shortening control, the control device 50 provides a control signal with a predetermined shortened high-level period to a first switch that is one of the two or more switches 8, and provides a control signal with a predetermined high-level period to a second switch that is different from the first switch among the two or more switches 8.

[0106] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0107] Furthermore, in the power conversion device 100, when the control device 50 performs shortening control, the first switch is the switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the largest absolute value of the load current flowing through each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the multiple switching circuits 10.

[0108] According to the above configuration, the switches 8 that are subjected to less hard switching among the plurality of switches 8 are equalized.

[0109] (Modification of First Embodiment) The circuit configuration of the power conversion device 100 according to a modification of the first embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0110] The power conversion device 100 according to the modification of the first embodiment differs only in the way the first switch and the second switch are determined when the control device 50 performs reduction control.

[0111] The first switch is a switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the largest absolute value of the voltage command value for each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the plurality of switching circuits 10.

[0112] Therefore, for example, in the third region A3 of Fig. 12 , the absolute value of the voltage command value Vv for the V-phase switching circuit 10V is greater than the absolute value of the voltage command value Vu for the U-phase switching circuit 10U. Diagrams illustrating the operation of the compaction control of the control device 50 in this case are similar to Figs. 13 and 14 of the first embodiment. Furthermore, in the fourth region of Fig. 15 , the absolute value of the voltage command value Vv for the V-phase switching circuit 10 is greater than the absolute value of the voltage command value Vu for the U-phase switching circuit 10. Diagrams illustrating the operation of the compaction control of the control device 50 in this case are similar to Figs. 16 and 17 of the first embodiment.

[0113] Second Embodiment The circuit configuration of a power conversion device 100 according to a second embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0114] In the power conversion device 100 according to the second embodiment, the method of determining the first switch and the second switch when the control device 50 performs the reduction control is different.

[0115] In the power conversion device 100 according to the second embodiment, when the control device 50 performs the shortening control, the first switch is the switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the smallest absolute value of the load current flowing through each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the plurality of switching circuits 10.

[0116] (1.1) In the case of charging operation of the resonant capacitor Hereinafter, the operation of the speed reduction control when the control device 50 determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously in the period corresponding to the third region A3 in Fig. 12 will be described with reference to Figs. 13 and 18. In the third region A3 in Fig. 12, the absolute value of the load current iU is smaller than the absolute value of the load current iV.

[0117] 13 shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, and iL1, and the voltages V2u and V2v across the second switching elements 2U and 2V, respectively, before performing shortening control when the control device 50 determines that the U-phase resonant current and the V-phase resonant current flow simultaneously. Also, FIG. 18 shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, and iL1, respectively, when the control device 50 performs shortening control. Note that FIGS. 13 and 18 show timing charts for a portion of one period of the carrier signal.

[0118] In the power conversion device 100, when the control device 50 does not perform the shortening control, the voltages V2u and V2v across the second switching elements 2U and 2V do not rise to Vd at the time when the control signals SU1 and SV1 change from a low level period to a high level period (the time when the dead time periods corresponding to the U phase and the V phase, respectively, end), as shown in Fig. 13. As a result, in the power conversion device 100, the switching of the first switching elements 1U and 1V becomes hard switching.

[0119] The control device 50 compares the absolute value of the load current iU with the absolute value of the load current iV and shortens the high-level period of the control signal SU6 to the switch 8U having the smaller absolute value of the load current by the shortening period Tredu. Specifically, the control device 50 shortens the high-level period of the control signal SU6 by the shortening period Tredu so that the control signal SU6 and the control signal SV6 do not overlap. FIG. 18 is a timing chart illustrating the operation of the power conversion device 100 according to the second embodiment when the control device 50 performs shortening control. In the example of FIG. 18 , the control device 50 sets the length of the shortening period Tredu to be equal to the length of the overlap period Tov (see FIG. 13 ) between the high-level period of the control signal SU6 before the shortening control and the control signal SV6 before the shortening control. In the example of FIG. 18 , the length of the shortening period Tredu is the same as the length of the overlap period Tov. In the example of FIG. 18, the control device 50 shortens the high-level period of the control signal SU6 in such a way that the start point of the high-level period of the control signal SVU before the shortening control is delayed by the shortening period Tredu.

[0120] As can be seen from the waveform of current iL1 in Fig. 13 and the waveform of current iL1 in Fig. 18, in power conversion device 100, control device 50 performs shortening control when it has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby shortening the overlap period of the U-phase resonant current and the V-phase resonant current. Furthermore, from the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 13 and the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 18, it can be seen that hard switching of first switching element 1U is alleviated and zero-voltage soft switching of first switching element 1V is performed.

[0121] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period between the U-phase resonant current and the W-phase resonant current, and also reduces the hard switching of each of the switching elements 1U and 1W.

[0122] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period of the V-phase resonant current and the W-phase resonant current, and also alleviate the hard switching of each of the switching elements 1V and 1W.

[0123] (1.2) In the case of the discharge operation of the resonant capacitor Below, we will explain the operation of the shortening control when the control device 50 has determined in advance that two-phase resonant currents, U phase and V phase, will flow simultaneously during the period corresponding to the fourth region A4 in Figure 15, with reference to Figures 16 and 19.

[0124] The way to interpret Fig. 15 is the same as that of Fig. 12, and therefore the explanation will be omitted. In the fourth region A4 of Fig. 15, the absolute value of the voltage command value Vv is greater than the absolute value of the voltage command value Vu. Also, in the fourth region A4 of Fig. 15, the absolute value of the load current iU is greater than the absolute value of the load current iV.

[0125] 16 shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and iL1, and the voltages V2u and V2v across the second switching elements 2U and 2V, respectively, before performing the shortening control when the control device 50 determines that the U-phase resonant current and the V-phase resonant current flow simultaneously. FIG. 19 shows timing charts for explaining the operation of the power conversion device 100 of the second embodiment when the control device 50 performs the shortening control. FIG. 19 also shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and iL1 when the control device 50 performs the shortening control. Note that FIGS. 16 and 20 show timing charts for a portion of one cycle of the carrier signal.

[0126] In the power conversion device 100, when the control device 50 does not perform the shortening control, the voltages V2u and V2v across the second switching elements 2U and 2V do not decrease to zero when the control signals SU1 and SV1 change from a low-level period to a high-level period, as shown in Fig. 16. For this reason, in the power conversion device 100, when the control device 50 does not perform the shortening control, the switching of the second switching elements 2U and 2V becomes hard switching.

[0127] The control device 50 compares the absolute value of the load current iU with the absolute value of the load current iV and shortens the high-level period of the control signal SU7 sent to the switch 8U having the smaller absolute value of the load current by the shortening period Tredu. Specifically, the control device 50 shortens the high-level period of the control signal SU7 by the shortening period Tredu so that the control signal SU7 and the control signal SV7 do not overlap. In the example of FIG. 19 , the control device 50 sets the length of the shortening period Tredu to be equal to the length of the high-level period of the control signal SU7 before the shortening control and the overlap period Tov (see FIG. 16 ) between the control signal SV7 before the shortening control. In the example of FIG. 16 , the length of the overlap period Tov is the time difference between the start point of the high-level period of the control signal SU7 and the end point of the high-level period of the control signal SV7. Therefore, in the example of FIG. 19 , the length of the shortening period Tredu is the same as the length of the overlap period Tov. In the example of FIG. 19, the control device 50 shortens the high-level period of the control signal SU7 in such a way that the start point of the high-level period of the control signal SU7 before the shortening control is delayed by the shortening period Tredu.

[0128] As can be seen from the waveform of current iL1 in Fig. 16 and the waveform of current iL1 in Fig. 19, in power conversion device 100, control device 50 performs shortening control when it has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby shortening the overlap period of the U-phase resonant current and the V-phase resonant current. Furthermore, from the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 16 and the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 19, it can be seen that hard switching of second switching element 2U is alleviated and zero-voltage soft switching of second switching element 2V is achieved.

[0129] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period of the U-phase resonant current and the W-phase resonant current, and also alleviates hard switching of one of the switching elements 2U and 2W, while realizing zero-voltage soft switching of the other.

[0130] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shortening control to shorten the overlap period of the V-phase resonant current and the W-phase resonant current, and also alleviates hard switching of one of the switching elements 2U and 2W, while realizing zero-voltage soft switching of the other.

[0131] (2) Advantages In the power conversion device 100 according to the second embodiment, the control device 50 sets a dead time period Td between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2 for each of the multiple switching circuits 10, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through the resonant inductor L1 and through two or more of the multiple switches 8 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs shortening control. In the shortening control, the control device 50 provides a control signal with a predetermined shortened high-level period to a first switch that is one of the two or more switches 8, and provides a control signal with a predetermined high-level period to a second switch that is different from the first switch among the two or more switches 8.

[0132] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0133] Furthermore, in the power conversion device 100, when the control device 50 performs shortening control, the first switch is the switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the smallest absolute value of the load current flowing through each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the multiple switching circuits 10.

[0134] The above configuration equalizes the switches 8 that are alleviated from hard switching among the multiple switches 8. Furthermore, the above configuration shortens the high-level period of the control signal to the switch 8 corresponding to the switching circuit 10 with the smallest absolute value of the load current, thereby making it possible to further reduce switching loss.

[0135] (Modification of Second Embodiment) The circuit configuration of the power conversion device 100 according to a modification of the second embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0136] The power conversion device 100 according to the modification of the second embodiment differs only in the way the first switch and the second switch are determined when the control device 50 performs reduction control.

[0137] The first switch is a switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the smallest absolute value of the voltage command value for each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the plurality of switching circuits 10.

[0138] Therefore, for example, in the third region A3 of Fig. 12 , the absolute value of the voltage command value Vu for the U-phase switching circuit 10U is smaller than the absolute value of the voltage command value Vv for the V-phase switching circuit 10V. Diagrams illustrating the operation of the reduction control of the control device 50 in this case are similar to Figs. 13 and 18 of the first embodiment. Furthermore, in the fourth region of Fig. 15 , the absolute value of the voltage command value Vu for the U-phase switching circuit 10 is smaller than the absolute value of the voltage command value Vv for the V-phase switching circuit 10. Diagrams illustrating the operation of the reduction control of the control device 50 in this case are similar to Figs. 16 and 19 of the second embodiment.

[0139] Third Embodiment The circuit configuration of the power conversion device 100 according to a third embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0140] The power conversion device 100 according to the third embodiment differs from the power conversion device 100 according to the first embodiment in that the control device 50 also performs time-saving control when the control device 50 determines that resonant currents flowing through three of the multiple switches 8 simultaneously (i.e., when it is determined in advance that three-phase resonant currents will flow simultaneously). The phrase "when it is determined that resonant currents flowing through three of the multiple switches 8 simultaneously" refers to a case where it is estimated in advance that resonant currents flowing through the three switches 8 simultaneously through the resonant inductor L1. The control device 50 determines that three-phase resonant currents will flow simultaneously when, for example, the time difference between the start of a high-level period of the control signal SU6 corresponding to the U phase and the start of a high-level period of the control signal SV6 corresponding to the V phase, the time difference between the start of a high-level period of the control signal SV6 corresponding to the V phase and the start of a high-level period of the control signal SW6 corresponding to the W phase, and the time difference between the start of a high-level period of the control signal SW6 corresponding to the W phase and the start of a high-level period of the control signal SU6 corresponding to the U phase are all less than a threshold.

[0141] Assuming that the capacitance of each of the multiple resonant capacitors 9U, 9V, and 9W is C, if U-phase, V-phase, and W-phase currents simultaneously flow through the resonant inductor L1, then in terms of an equivalent circuit, a capacitor having a combined capacitance (=3×C) of the resonant capacitors 9U, 9V, and 9W is connected in series to the resonant inductor L1. Therefore, in the power conversion device 100, if three-phase currents simultaneously flow through the resonant inductor L1, the resonant frequency of the resonant circuit including the resonant inductor L1 changes compared to when one-phase current flows through the resonant inductor L1, potentially making it impossible to achieve zero-voltage soft switching. Furthermore, if the control device 50 does not perform short-circuiting control, three-phase resonant currents are likely to flow simultaneously, for example, when the servo motor, which is the AC load RA1, is rotating at low speed.

[0142] In the reduction control, the control device 50 provides a control signal with a preset high-level period shortened to a first switch that is one of the three switches 8, and provides a control signal having a preset high-level period to a second switch that is different from the first switch. The control device 50 also provides a control signal having a high-level period shortened from the preset high-level period to a third switch that is different from the first and second switches. When the control device 50 performs the reduction control, the first switch is the switch 8 of the three switches 8 corresponding to the switching circuit 10 that has the smallest absolute value of the load current flowing through each of the three switching circuits 10 corresponding to the three switches 8 in the multiple switching circuits 10. The third switch is the switch 8 of the three switches 8 corresponding to the switching circuit 10 that has the second smallest absolute value of the load current flowing through each of the three switching circuits 10 corresponding to the three switches 8 in the multiple switching circuits 10.

[0143] (1.1) In the case of charging operation of the resonant capacitor Below, we will explain the operation of the shortening control when the control device 50 has determined in advance that three-phase resonant currents, namely U phase, V phase, and W phase, will flow simultaneously, with reference to Figures 20 and 21.

[0144] Fig. 20 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 according to the third embodiment when the control device 50 determines that three-phase resonant currents overlap when the load current is greater than 0 and the resonant capacitor 9 is being charged. Fig. 20 illustrates a timing chart of the control signals SU1, SU2, SV1, SV2, SW1, SW2, the control signals SU6, SV6, SW6, the load currents iU, iV, iW, the current iL1, and the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W, respectively, before performing the shortening control when the control device 50 determines that three-phase resonant currents flow simultaneously. Fig. 21 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 according to the third embodiment when performing the shortening control. 21 shows timing charts of the control signals SU1, SU2, SV1, SV2, SW1, SW2, control signals SU6, SV6, SW6, load currents iU, iV, iW, current iL1, and voltages V2u, V2v, V2w across the second switching elements 2U, 2V, 2W, respectively, when the control device 50 performs shortening control. Note that FIGS. 20 and 21 show timing charts for a portion of one period within one cycle of the carrier signal.

[0145] (1.2) Time-Shortening Control When It Is Determined That Three-Phase Resonant Currents Flow Simultaneously In the power conversion device 100, unless the control device 50 performs time-shortening control, as shown in FIG. 20 , the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W do not rise to Vd when the control signals SU1, SV1, and SW1 change from low to high (the end of the dead time periods corresponding to the U, V, and W phases, respectively). In other words, unless the control device 50 performs time-shortening control, the charging of the resonant capacitors 9U, 9V, and 9W does not finish when the dead time periods corresponding to the U, V, and W phases, respectively, end. Therefore, unless time-shortening control is performed, the voltages across the first switching elements 1U, 1V, and 1W do not decrease to zero when the dead time periods corresponding to the U, V, and W phases, respectively, end. As a result, in the power conversion device 100, the switching of the first switching elements 1U, 1V, and 1W becomes hard switching.

[0146] The control device 50 compares the absolute values ​​of the load current iU, the load current iV, and the load current iW, and shortens the high-level period of the control signal SW6 sent to the switch 8W having the largest absolute value of the load current by a shortening period Tredw, and shortens the high-level period of the control signal SV6 sent to the switch 8V having the second largest absolute value of the load current by a shortening period Tredv. Specifically, as shown in FIG. 21 , the control device 50 shortens the high-level period of the control signal SW6 by a shortening period Tredw and shortens the high-level period of the control signal SV6 by a shortening period Tredv so that the control signals SU6, SV6, and SW6 do not overlap. In the example of FIG. 21 , the control device 50 sets the length of the shortening period Tredw to be the length of the overlapping period between the high-level period of the control signal SW6 and the high-level period of the control signal SU6 before the shortening control. The control device 50 also sets the length of the reduction period Tredv to be equal to the length of the high-level period of the control signal SV6 before the reduction control and the overlap period of the control signal SW6 before the reduction control. In the example of FIG. 21 , the control device 50 reduces the high-level period of the control signal SW6 in a direction that advances the end point of the high-level period of the control signal SW6 before the reduction control by the reduction period Tredw. The control device 50 also reduces the high-level period of the control signal SV6 in a direction that advances the end point of the high-level period of the control signal SV6 before the reduction control by the reduction period Tredv. It can be seen from the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W in FIG. 20 and the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W in FIG. 21 that the hard switching of the first switching element 1U is mitigated.

[0147] (1.3) In the Case of Discharge Operation of Resonant Capacitor FIG. 22 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 of the third embodiment when the control device 50 determines that the three-phase resonant currents overlap when the load current is less than 0 and the resonant capacitor 9 is discharging. In the power conversion device 100, if the control device 50 does not perform the speed-saving control, as shown in FIG. 22 , the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W do not decrease to zero when the control signals SU2, SV2, and SW2 change from low to high (the end of the dead time periods corresponding to the U, V, and W phases, respectively). In other words, if the control device 50 does not perform the speed-saving control, the discharge of the resonant capacitors 9U, 9V, and 9W does not end when the dead time periods corresponding to the U, V, and W phases, respectively, end. As a result, in the power conversion device 100, the switching of the second switching elements 2U, 2V, and 2W becomes hard switching.

[0148] FIG. 23 is a timing chart illustrating the operation of the control device 50 in the power conversion device 100 of the third embodiment when the control device 50 performs the shortening control. The control device 50 compares the absolute values ​​of the load current iU, the load current iV, and the load current iW, and shortens the high-level period of the control signal SW7 sent to the switch 8W having the largest absolute value of the load current by a shortening period Tredw, and shortens the high-level period of the control signal SV7 sent to the switch 8V having the second largest absolute value of the load current by a shortening period Tredv. Specifically, as shown in FIG. 23 , the control device 50 shortens the high-level period of the control signal SW7 by a shortening period Tredw so that the control signal SU7, the control signal SV7, and the control signal SW7 do not overlap, and shortens the high-level period of the control signal SV7 by a shortening period Tredv. In the example of FIG. 23 , the control device 50 sets the length of the shortening period Tredw to be the length of the overlapping period between the high-level period of the control signal SW7 and the high-level period of the control signal SU7 before the shortening control. Furthermore, the control device 50 sets the length of the shortening period Tredv to be equal to the length of the high-level period of the control signal SV7 before the shortening control and the overlapping period of the control signal SW7 before the shortening control. In the example of Fig. 23, the control device 50 shortens the high-level period of the control signal SW7 in a direction that advances the end point of the high-level period of the control signal SW7 before the shortening control by the shortening period Tredw. Furthermore, the control device 50 shortens the high-level period of the control signal SV7 in a direction that advances the end point of the high-level period of the control signal SV7 before the shortening control by the shortening period Tredv.

[0149] As can be seen from the waveform of current iL1 in Fig. 22 and the waveform of current iL1 in Fig. 23, in power conversion device 100, control device 50 performs shortening control when it has determined in advance that three-phase resonant currents of U-phase, V-phase, and W-phase will flow simultaneously, thereby shortening the overlap period of the U-phase resonant current, the V-phase resonant current, and the W-phase resonant current. Furthermore, from the voltages V2u, V2v, and V2w across second switching elements 2U, 2V, and 2W in Fig. 22 and the voltages V2u, V2v, and V2w across second switching elements 2U, 2V, and 2W in Fig. 23, it can be seen that hard switching of second switching element 2U is mitigated.

[0150] (First Modification of Third Embodiment) In the power conversion device 100 according to a first modification of the third embodiment, the method of determining the first switch and the third switch when the control device 50 performs shortening control differs from that of the third embodiment. In the first modification of the third embodiment, the third switch is the switch 8 corresponding to the switching circuit 10, among the three switches 8, having the smallest absolute value of the load current flowing through each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The first switch is the switch 8 corresponding to the switching circuit 10, among the three switches 8, having the second smallest absolute value of the load current flowing through each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10.

[0151] (Second Modification of Third Embodiment) The circuit configuration of the power conversion device 100 according to a second modification of the third embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0152] The power conversion device 100 according to the second modification of the third embodiment differs only in the way the first switch, the second switch, and the third switch are determined when the control device 50 performs reduction control.

[0153] The first switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the smallest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The second switch is the switch 8 of the three switches that corresponds to the switching circuit 10 having the largest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The third switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the second smallest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10.

[0154] 24 is an explanatory diagram of conditions under which the control device 50 performs shortening control in the power conversion device 100 according to the second modification of the third embodiment. For example, in a fifth region A5 in Fig. 24 , the absolute value of the voltage command value Vv for the V-phase switching circuit 10V is greater than the absolute value of the voltage command value Vw for the W-phase switching circuit 10W, and the absolute value of the voltage command value Vw is greater than the absolute value of the voltage command value Vu for the U-phase switching circuit 10U. In this case, the first switch, the second switch, and the third switch are the switch 8W, the switch 8V, and the switch 8U, respectively.

[0155] (Third Modification of Third Embodiment) The circuit configuration of the power conversion device 100 according to a third modification of the third embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0156] The power conversion device 100 according to the third modification of the third embodiment differs only in the way the first switch, the second switch, and the third switch are determined when the control device 50 performs reduction control.

[0157] The first switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the largest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The second switch is the switch 8 of the three switches that corresponds to the switching circuit 10 having the smallest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The third switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the second largest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10.

[0158] 24 , the absolute value of the voltage command value Vv for the V-phase switching circuit 10V is greater than the absolute value of the voltage command value Vu for the U-phase switching circuit 10W, and the absolute value of the voltage command value Vu is greater than the absolute value of the voltage command value Vw for the W-phase switching circuit 10W. In this case, the first switch, the second switch, and the third switch are the switch 8V, the switch 8W, and the switch 8U, respectively.

[0159] Fourth Embodiment The circuit configuration of the power conversion device 100 according to a fourth embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0160] The operation of the power conversion device 100 according to the fourth embodiment is substantially the same as the operation of the power conversion device 100 according to the third embodiment, except that when performing the shortening control, the direction in which the high-level period of the control signal to the first switch is shortened is opposite to the direction in which the high-level period of the control signal to the third switch is shortened.

[0161] Fig. 25 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to embodiment 4 performs shortening control. When the load current is greater than 0 and the resonant capacitor 9 is being charged, the control device 50 shortens the high-level period of the control signal SU6 to the switch 8U in a direction that delays the start of the high-level period, and shortens the high-level period of the control signal SV6 to the switch 8V by Tredv in a direction that advances the end of the high-level period, as shown in Fig. 25 .

[0162] Fig. 26 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of embodiment 4 performs shortening control. Furthermore, when the load current is less than 0 and the resonant capacitor 9 is discharging, the control device 50 shortens the high-level period of the control signal SU6 to the switch 8U in a direction that delays the start point of the high-level period, and shortens the high-level period of the control signal SV6 to the switch 8V by Tredv in a direction that advances the end point of the high-level period, as shown in Fig. 26 .

[0163] In the power conversion device 100 according to the fourth embodiment, the control device 50 sets a dead time period Td for each of the multiple switching circuits 10 between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current passing through two or more of the multiple switches 8 simultaneously flows through the resonant inductor L1 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs shortening control. In the shortening control, the control device 50 provides a control signal with a predetermined shortened high-level period to a first switch that is one of the two or more switches 8, and provides a control signal with a predetermined high-level period to a second switch of the two or more switches 8 that is different from the first switch.

[0164] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0165] Furthermore, in the power conversion device 100 according to the fourth embodiment, when the control device 50 performs shortening control, the first switch is the switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the smallest absolute value of the load current flowing through each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the plurality of switching circuits 10.

[0166] Furthermore, with the above configuration, the high-level periods of the control signals to the switch 8 corresponding to the switching circuit 10 with the smallest absolute value of the load current and the switch 8 corresponding to the switching circuit 10 with the second smallest absolute value of the load current are shortened, thereby making it possible to further reduce switching loss.

[0167] Fifth Embodiment The circuit configuration of the power conversion device 100 according to a fifth embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0168] The control device 50 differs from embodiment 1 in that when it determines that resonant currents passing through two of the multiple switches 8 simultaneously flow through the resonant inductor L1, it shortens the high-level period of the control signal of the first switch so that the high-level period becomes zero.

[0169] In the reduction control, the control device 50 provides a control signal (i.e., a low-level control signal) to a first switch that is one of the two switches 8, the control signal having a preset high-level period reduced so as to set the preset high-level period to zero, and provides a control signal having a preset high-level period to a second switch that is different from the first switch 8. When the control device 50 performs the reduction control, the first switch is the switch 8 of the two switches 8 that corresponds to the switching circuit 10 that has the largest absolute value of the voltage command value for each of the two switching circuits 10 that correspond to the two switches 8 in the plurality of switching circuits 10.

[0170] (1.1) In the Case of Charging Operation of Resonant Capacitor Hereinafter, the operation of the reduction control when the control device 50 has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously in the period corresponding to the third region A3 in Fig. 12 will be described with reference to Fig. 13 and Fig. 27. Fig. 27 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to the fifth embodiment performs reduction control.

[0171] 12 shows a voltage command value Vu for the U-phase switching circuit 10U, a voltage command value Vv for the V-phase switching circuit 10V, and a voltage command value Vw for the W-phase switching circuit 10W, which are given to the control device 50 from an external device. In a third region A3 of FIG. 12, the voltage command value Vv is greater than the voltage command value Vu.

[0172] 13 shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, the current iL1, and the voltages V2u, V2v across the second switching elements 2U, 2V before the control device 50 performs shortening control when it determines that the U-phase resonant current and the V-phase resonant current flow simultaneously. Also, FIG. 27 shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, the current iL1, and the voltages V2u, V2v across the second switching elements 2U, 2V when the control device 50 performs shortening control. Note that FIGS. 13 and 27 show timing charts for a portion of one cycle of the carrier signal.

[0173] In the power conversion device 100, when the control device 50 does not perform the speed reduction control, the voltages V2u and V2v across the second switching elements 2U and 2V do not rise to Vd at the time when the control signals SU1 and SV1 change from a low level period to a high level period (the time when the dead time periods corresponding to the U phase and the V phase, respectively, end), as shown in FIG. 13 . Therefore, when the speed reduction control is not performed, the voltages across the first switching elements 1U and 1V do not decrease to zero at the time when the dead time periods corresponding to the U phase and the V phase, respectively, end. As a result, in the power conversion device 100, the switching of the first switching elements 1U and 1V becomes hard switching.

[0174] The control device 50 compares the absolute value of the voltage command value Vu with the absolute value of the voltage command value Vv, and shortens the high level period of the control signal SV6 sent to the switch 8V corresponding to the switching circuit 10V having the larger absolute value of the voltage command value to zero.

[0175] As can be seen from the waveform of current iL1 in Fig. 13 and the waveform of current iL1 in Fig. 27, in power conversion device 100, control device 50 performs shortening control when it determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby making it possible to make the V-phase resonant current of the U-phase resonant current and the V-phase resonant current zero. Furthermore, from the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 13 and the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 27, it can be seen that zero-voltage soft switching of first switching element 1U connected to second switching element 2U is achieved.

[0176] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, the U phase and the W phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the U phase resonant current and the W phase resonant current.

[0177] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, the V-phase and the W-phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the V-phase resonant current and the W-phase resonant current (the overlap period can be reduced to zero).

[0178] (1.2) In the Case of Discharge Operation of Resonant Capacitor Hereinafter, the operation of the reduction control when the control device 50 has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously in the period corresponding to the fourth region A4 in Fig. 15 will be described with reference to Fig. 16 and Fig. 28. Fig. 28 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of embodiment 5 performs reduction control.

[0179] In the fourth region A4 in FIG. 15, the absolute value of the voltage command value Vv is greater than the absolute value of the voltage command value Vu.

[0180] 16 shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and iL1, and the voltages V2u and V2v across the second switching elements 2U and 2V, respectively, before performing shortening control when the control device 50 determines that the U-phase resonant current and the V-phase resonant current flow simultaneously. Also, FIG. 28 shows timing charts of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and iL1, respectively, when the control device 50 performs shortening control. Note that FIGS. 16 and 28 show timing charts for a portion of one period of the carrier signal.

[0181] In the power conversion device 100, when the control device 50 does not perform the shortening control, the voltages V2u and V2v across the second switching elements 2U and 2V do not decrease to zero when the control signals SU1 and SV1 change from a low-level period to a high-level period, as shown in Fig. 16. Therefore, in the power conversion device 100, when the control device 50 does not perform the shortening control, the switching of the second switching elements 2U and 2V becomes hard switching.

[0182] The control device 50 compares the absolute value of the voltage command value Vu with the absolute value of the voltage command value Vv, and shortens the high-level period of the control signal SV7 sent to the switch 8V corresponding to the switching circuit 10V having the larger absolute value of the voltage command value so that the high-level period is zero.

[0183] As can be seen from the waveform of current iL1 in Fig. 16 and the waveform of current iL1 in Fig. 28, in power conversion device 100, control device 50 performs shortening control when it has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby making it possible to make the V-phase resonant current of the U-phase resonant current and the V-phase resonant current zero. Furthermore, from the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 16 and the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 28, it can be seen that zero-voltage soft switching of second switching element 2U is achieved.

[0184] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the U-phase resonant current and the W-phase resonant current (the overlap period can be reduced to zero).

[0185] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, the V phase and the W phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the V phase resonant current and the W phase resonant current (the overlap period can be reduced to zero).

[0186] (2) Advantages In the power conversion device 100 according to the fifth embodiment, the control device 50 sets a dead time period Td for each of the multiple switching circuits 10 between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through the resonant inductor L1 and passes through two or more of the multiple switches 8 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs shortening control. In the shortening control, the control device 50 provides a control signal with a predetermined shortened high-level period to a first switch that is one of the two or more switches 8, and provides a control signal with a predetermined high-level period to a second switch that is different from the first switch among the two or more switches 8.

[0187] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0188] Furthermore, in the power conversion device 100 according to the fifth embodiment, when the control device 50 performs the reduction control, the first switch is supplied with a control signal in which the preset high level period is reduced to zero.

[0189] According to the above configuration, it is possible to realize zero-voltage soft switching of the first switching element 1 and the second switching element 2 of the switching circuit 10 corresponding to the second switch, and it is possible to reduce switching loss.

[0190] Sixth Embodiment The circuit configuration of the power conversion device 100 according to a sixth embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0191] The power conversion device 100 according to the sixth embodiment differs from the power conversion device 100 according to the fifth embodiment in the way in which the first switch and the second switch are determined when the control device 50 performs shortening control.

[0192] The first switch is the switch 8 corresponding to the switching circuit 10 among the two switches 8, in which the absolute value of the load current flowing through each of the two switching circuits 10 corresponding to the two switches 8 in the plurality of switching circuits 10 is smallest.

[0193] (1.1) In the Case of Charging Operation of Resonant Capacitor Hereinafter, the operation of the speed reduction control when the control device 50 has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously in the period corresponding to the third region A3 in Fig. 12 will be described with reference to Fig. 13 and Fig. 29. Fig. 29 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to the sixth embodiment performs speed reduction control.

[0194] In the power conversion device 100, when the control device 50 does not perform the speed reduction control, the voltages V2u and V2v across the second switching elements 2U and 2V do not rise to Vd at the time when the control signals SU1 and SV1 change from a low level period to a high level period (the time when the dead time periods corresponding to the U phase and the V phase, respectively, end), as shown in FIG. 13 . Therefore, when the speed reduction control is not performed, the voltages across the first switching elements 1U and 1V do not decrease to zero at the time when the dead time periods corresponding to the U phase and the V phase, respectively, end. As a result, in the power conversion device 100, the switching of the first switching elements 1U and 1V becomes hard switching.

[0195] The control device 50 compares the absolute value of the load current iU with the absolute value of the load current iV, and shortens the high level period of the control signal SU6 sent to the switch 8U corresponding to the switching circuit 10U having the smaller absolute value of the load current to zero.

[0196] As can be seen from the waveform of current iL1 in Fig. 13 and the waveform of current iL1 in Fig. 29, in power conversion device 100, control device 50 performs shortening control when it determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby making it possible to make the V-phase resonant current of the U-phase resonant current and the V-phase resonant current zero. Furthermore, from the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 13 and the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 29, it can be seen that zero-voltage soft switching of first switching element 1U connected to second switching element 2U is achieved.

[0197] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, the U phase and the W phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the U phase resonant current and the W phase resonant current (reducing the overlap period to zero).

[0198] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, the V-phase and the W-phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the V-phase resonant current and the W-phase resonant current (the overlap period can be reduced to zero).

[0199] (1.2) In the Case of Discharge Operation of Resonant Capacitor Hereinafter, the operation of the reduction control when the control device 50 has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously in the period corresponding to the fourth region A4 in Fig. 15 will be described with reference to Fig. 16 and Fig. 30. Fig. 30 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of embodiment 6 performs reduction control.

[0200] 16 shows a timing chart of the control signals SU1, SU2, SV1, SV2, control signals SU7, SV7, load currents iU, iV, current iL1, and voltages V2u, V2v across the second switching elements 2U, 2V, respectively, before performing shortening control when the control device 50 determines that the U-phase resonant current and the V-phase resonant current flow simultaneously. Also, FIG. 30 shows a timing chart of the control signals SU1, SU2, SV1, SV2, control signals SU7, SV7, load currents iU, iV, and current iL1 when the control device 50 performs shortening control.

[0201] In the power conversion device 100, when the control device 50 does not perform the shortening control, the voltages V2u and V2v across the second switching elements 2U and 2V do not decrease to zero when the control signals SU1 and SV1 change from a low-level period to a high-level period, as shown in Fig. 16. Therefore, in the power conversion device 100, when the control device 50 does not perform the shortening control, the switching of the second switching elements 2U and 2V becomes hard switching.

[0202] The control device 50 compares the absolute value of the load current iU with the absolute value of the load current iV, and shortens the high-level period of the control signal SU7 sent to the switch 8U corresponding to the switching circuit 10U having the smaller absolute value of the load current so that the high-level period is zero.

[0203] As can be seen from the waveform of current iL1 in Fig. 16 and the waveform of current iL1 in Fig. 30, in power conversion device 100, control device 50 performs shortening control when it has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, thereby making it possible to make the U-phase resonant current zero out of the U-phase resonant current and the V-phase resonant current. Furthermore, from the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 16 and the voltages V2u and V2v across second switching elements 2U and 2V in Fig. 30, it can be seen that zero-voltage soft switching of second switching element 2V is achieved.

[0204] Similarly, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the U-phase resonant current and the W-phase resonant current (the overlap period can be reduced to zero).

[0205] Furthermore, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, the V phase and the W phase, will flow simultaneously, it can perform shortening control to shorten the overlap period between the V phase resonant current and the W phase resonant current (the overlap period can be reduced to zero).

[0206] (2) Advantages In the power conversion device 100 according to the sixth embodiment, the control device 50 sets a dead time period Td for each of the multiple switching circuits 10 between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through the resonant inductor L1 and passes through two or more of the multiple switches 8 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs shortening control. In the shortening control, the control device 50 provides a control signal with a predetermined shortened high-level period to a first switch that is one of the two or more switches 8, and provides a control signal with a predetermined high-level period to a second switch that is different from the first switch among the two or more switches 8.

[0207] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0208] Furthermore, in the power conversion device 100 according to the sixth embodiment, when the control device 50 performs the reduction control, the first switch is supplied with a control signal in which the preset high level period is reduced to zero.

[0209] According to the above configuration, it is possible to realize zero-voltage soft switching of the first switching element 1 and the second switching element 2 of the switching circuit 10 corresponding to the second switch, and it is possible to reduce switching loss.

[0210] Furthermore, in the power conversion device 100 according to the sixth embodiment, when the control device 50 performs shortening control, the first switch is the switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the smallest absolute value of the load current flowing through each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the plurality of switching circuits 10.

[0211] Furthermore, with the above configuration, the high level period of the control signal to the switch 8 corresponding to the switching circuit 10 with the smallest absolute value of the load current is shortened, so that switching loss can be further reduced.

[0212] Seventh Embodiment The circuit configuration of the power conversion device 100 according to a seventh embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0213] The operation of the power conversion device 100 according to the seventh embodiment is substantially the same as the operation of the power conversion device 100 according to the third embodiment. The basic operation of the control device 50 in the power conversion device 100 according to the seventh embodiment is the same as the basic operation of the control device 50 in the power conversion device 100 according to the first embodiment, as in the third embodiment, and therefore will not be described again.

[0214] The power conversion device 100 according to the seventh embodiment differs from the third embodiment in the way in which the first switch, the second switch, and the third switch are determined when the control device 50 performs the reduction control.

[0215] The first switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the smallest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The second switch is the switch 8 of the three switches that corresponds to the switching circuit 10 having the smallest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The third switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the second smallest absolute value of the voltage command value for each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10.

[0216] In the power conversion device 100 according to the seventh embodiment, the control device 50 shortens the high-level period of the control signal to the first switch to zero, and also shortens the high-level period of the control signal to the third switch to zero.

[0217] (1.1) Case of Charging Operation of Resonant Capacitor As an example, a case where the condition of absolute value of voltage command value Vv > absolute value of voltage command value Vw > absolute value of voltage command value Vu is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8V, and switch 8W, respectively, will be described with reference to Fig. 20 and Fig. 31. Fig. 31 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to the seventh embodiment performs shortening control.

[0218] 20 shows a timing chart before the control device 50 performs the shortening control, and FIG. 31 shows a timing chart when the control device 50 performs the shortening control. In the shortening control, the control device 50 shortens the high-level period of the control signal SU6 sent to the switch 8U (first switch) so that the high-level period of the control signal SU6 is zero, does not shorten the high-level period of the control signal SV6 sent to the switch 8V (second switch), and shortens the high-level period of the control signal SW6 sent to the switch 8W (third switch) so that the high-level period of the control signal SW6 is zero. Only the resonant current passing through the switch 8V (second switch) of the three switches 8 flows through the resonant inductor L1, and the voltage V2v across the resonant capacitor 9V is charged to Vd, thereby achieving zero-voltage soft switching of the first switching element 1V.

[0219] When the condition of absolute value of voltage command value Vw > absolute value of voltage command value Vu > absolute value of voltage command value Vv is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SV6 for switch 8V, control signal SW6 for switch 8W, and control signal SU6 for switch 8U, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SV6 so that it becomes zero, and shortens the high-level period of control signal SU6 so that it becomes zero. This achieves zero-voltage soft switching of first switching element 1W.

[0220] Furthermore, when the condition of absolute value of voltage command value Vu > absolute value of voltage command value Vv > absolute value of voltage command value Vw is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SW6 for switch 8W, control signal SU6 for switch 8U, and control signal SV6 for switch 8V, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SW6 so that it becomes zero, and shortens the high-level period of control signal SV6 so that it becomes zero. This achieves zero-voltage soft switching of first switching element 1U.

[0221] (1.2) Case of Discharge Operation of Resonant Capacitor As an example, a case where the condition of absolute value of voltage command value Vv > absolute value of voltage command value Vw > absolute value of voltage command value Vu is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8V, and switch 8W, respectively, will be described with reference to Fig. 22 and Fig. 32. Fig. 32 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of embodiment 7 performs shortening control.

[0222] 22 shows a timing chart before the control device 50 performs the speed reduction control, and FIG. 32 shows a timing chart when the control device 50 performs the speed reduction control. In the speed reduction control, the control device 50 shortens the high-level period of the control signal SU7 sent to the switch 8U (first switch) so that the high-level period of the control signal SU7 is zero, does not shorten the high-level period of the control signal SV7 sent to the switch 8V (second switch), and shortens the high-level period of the control signal SW7 sent to the switch 8W (third switch) so that the high-level period of the control signal SW7 is zero. Only the resonant current passing through the switch 8V (second switch) of the three switches 8 flows through the resonant inductor L1, and the resonant capacitor 9V is discharged until the voltage V2v across the second switching element 2V reaches zero volts, thereby achieving zero-voltage soft switching of the second switching element 2V.

[0223] When the condition of absolute value of voltage command value Vw > absolute value of voltage command value Vu > absolute value of voltage command value Vv is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SV7 for switch 8V, control signal SW7 for switch 8W, and control signal SU7 for switch 8U, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SV7 to zero, and shortens the high-level period of control signal SU7 to zero. This achieves zero-voltage soft switching of second switching element 2W.

[0224] Furthermore, when the condition of absolute value of voltage command value Vu > absolute value of voltage command value Vv > absolute value of voltage command value Vw is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SW7 for switch 8W, control signal SU7 for switch 8U, and control signal SV7 for switch 8V, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SW7 to zero, and shortens the high-level period of control signal SV7 to zero. This achieves zero-voltage soft switching of second switching element 2U.

[0225] (2) Advantages In the power conversion device 100 according to the seventh embodiment, the control device 50 sets a dead time period Td between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2 for each of the multiple switching circuits 10, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through two or more of the multiple switches 8 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs reduction control. In the reduction control, the control device 50 provides a control signal with a predetermined reduced high-level period to a first switch that is one of the three switches 8, provides a control signal with a predetermined high-level period to a second switch that is different from the first switch, and provides a control signal with a predetermined reduced high-level period to a third switch that is different from the first and second switches.

[0226] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0227] Furthermore, in the power conversion device 100 according to the seventh embodiment, when the control device 50 performs the shortening control, it provides the first switch with a control signal in which the preset high level period is shortened to zero, and it provides the third switch with a control signal in which the preset high level period is shortened to zero.

[0228] According to the above configuration, it is possible to realize zero-voltage soft switching of the first switching element 1 and the second switching element 2 of the switching circuit 10 corresponding to the second switch, and it is possible to reduce switching loss.

[0229] Eighth Embodiment The circuit configuration of the power conversion device 100 according to an eighth embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0230] The operation of the power conversion device 100 according to the eighth embodiment is substantially the same as the operation of the power conversion device 100 according to the third embodiment. The basic operation of the control device 50 in the power conversion device 100 according to the eighth embodiment is the same as the basic operation of the control device 50 in the power conversion device 100 according to the first embodiment, as in the third embodiment, and therefore will not be described again.

[0231] The power conversion device 100 according to the eighth embodiment differs from the third embodiment in the way in which the first switch, the second switch, and the third switch are determined when the control device 50 performs the reduction control.

[0232] The first switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the smallest absolute value of the load current flowing through each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The second switch is the switch 8 of the three switches that corresponds to the switching circuit 10 having the smallest absolute value of the load current flowing through each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10. The third switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the second smallest absolute value of the load current flowing through each of the three switching circuits 10 corresponding to the three switches 8 in the plurality of switching circuits 10.

[0233] In the power conversion device 100 according to the eighth embodiment, the control device 50 shortens the high-level period of the control signal to the first switch to zero, and also shortens the high-level period of the control signal to the third switch to zero.

[0234] (1.1) Case of Charging Operation of Resonant Capacitor As an example, a case where the condition of absolute value of load current iW > absolute value of load current iV > absolute value of load current iU is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8V, and switch 8W, respectively, will be described with reference to Fig. 20 and Fig. 33. Fig. 33 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to embodiment 8 performs shortening control. In Fig. 20 and Fig. 33, the polarity of the load current iW is negative, and the polarities of the load current iV and the load current iU are positive.

[0235] 20 shows a timing chart before the control device 50 performs the shortening control, and FIG. 33 shows a timing chart when the control device 50 performs the shortening control. In the shortening control, the control device 50 shortens the high-level period of the control signal SU6 sent to the switch 8U (first switch) so that the high-level period of the control signal SU6 is zero, does not shorten the high-level period of the control signal SW6 sent to the switch 8W (second switch), and shortens the high-level period of the control signal SV6 sent to the switch 8V (third switch) so that the high-level period of the control signal SV6 is zero. Only the resonant current passing through the switch 8W (second switch) of the three switches 8 flows through the resonant inductor L1, and the voltage V2w across the resonant capacitor 9W is charged to Vd, thereby achieving zero-voltage soft switching of the first switching element 1W.

[0236] When the condition of absolute value of load current iU > absolute value of load current iW > absolute value of load current iV is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SV6 for switch 8V, control signal SU6 for switch 8U, and control signal SW6 for switch 8W, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SV6 to zero, and shortens the high-level period of control signal SW6 to zero. This achieves zero-voltage soft switching of first switching element 1U.

[0237] Furthermore, when the condition of absolute value of load current iV > absolute value of load current iU > absolute value of load current iW is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SW6 for switch 8W, control signal SV6 for switch 8V, and control signal SU6 for switch 8U, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SW6 to zero, and shortens the high-level period of control signal SU6 to zero. This achieves zero-voltage soft switching of first switching element 1V.

[0238] (1.2) Case of Discharge Operation of Resonant Capacitor As an example, a case where the condition of absolute value of load current iW > absolute value of load current iV > absolute value of load current iU is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8V, and switch 8W, respectively, will be described with reference to Figs. 22 and 34. Fig. 34 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of embodiment 8 performs shortening control. In Figs. 22 and 34, the polarity of the load current iW is positive, and the polarities of the load current iV and the load current iU are negative.

[0239] 22 shows a timing chart before the control device 50 performs the shortening control, and FIG. 34 shows a timing chart when the control device 50 performs the shortening control. In the shortening control, the control device 50 shortens the high-level period of the control signal SU7 sent to the switch 8U (first switch) so that the high-level period of the control signal SU7 is zero, does not shorten the high-level period of the control signal SW7 sent to the switch 8W (second switch), and shortens the high-level period of the control signal SV7 sent to the switch 8V (third switch) so that the high-level period of the control signal SV7 is zero. Only the resonant current passing through the switch 8W (second switch) of the three switches 8 flows through the resonant inductor L1, and the resonant capacitor 9W is discharged until the voltage V2v across the second switching element 2W reaches zero volts, thereby achieving zero-voltage soft switching of the second switching element 2W.

[0240] When the condition of absolute value of load current iU > absolute value of load current iW > absolute value of load current iV is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SV7 for switch 8V, control signal SU7 for switch 8U, and control signal SW7 for switch 8W, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SV7 to zero, and shortens the high-level period of control signal SW7 to zero. This achieves zero-voltage soft switching of second switching element 2U.

[0241] Furthermore, when the condition of absolute value of load current iV > absolute value of load current iU > absolute value of load current iW is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SW7 for switch 8W, control signal SV7 for switch 8V, and control signal SU7 for switch 8U, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SW6 to zero, and shortens the high-level period of control signal SU7 to zero. This achieves zero-voltage soft switching of second switching element 2V.

[0242] (2) Advantages In the power conversion device 100 according to the eighth embodiment, the control device 50 sets a dead time period Td between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2 for each of the multiple switching circuits 10, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through two or more of the multiple switches 8 in the resonant inductor L1 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs shortening control. In the shortening control, the control device 50 provides a control signal with a predetermined shortened high-level period to a first switch that is one of the three switches 8, provides a control signal with a predetermined high-level period to a second switch that is different from the first switch, and provides a control signal with a predetermined shortened high-level period to a third switch that is different from the first and second switches.

[0243] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0244] Furthermore, in the power conversion device 100 according to the eighth embodiment, when the control device 50 performs the shortening control, it provides the first switch with a control signal in which the preset high level period is shortened to zero, and it provides the third switch with a control signal in which the preset high level period is shortened to zero.

[0245] According to the above configuration, it is possible to realize zero-voltage soft switching of the first switching element 1 and the second switching element 2 of the switching circuit 10 corresponding to the second switch, and it is possible to reduce switching loss.

[0246] Furthermore, in the power conversion device 100 according to the eighth embodiment, when the control device 50 performs shortening control, the first switch is the switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the smallest absolute value of the load current flowing through each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the plurality of switching circuits 10.

[0247] Furthermore, with the above configuration, the high level period of the control signal to the switch 8 corresponding to the switching circuit 10 with the smallest absolute value of the load current is shortened, so that switching loss can be further reduced.

[0248] Ninth Embodiment The circuit configuration of the power conversion device 100 according to a ninth embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0249] The operation of the power conversion device 100 according to the ninth embodiment is substantially the same as the operation of the power conversion device 100 according to the seventh embodiment. The power conversion device 100 according to the ninth embodiment differs from the seventh embodiment in that, when the control device 50 shortens the high-level period of the control signal of the first switch, the control device 50 shortens the high-level period so that there is zero overlap with the high-level period of the control signal of the second switch.

[0250] (1.1) Case of Charging Operation of Resonant Capacitor As an example, a case where the condition of absolute value of voltage command value Vv > absolute value of voltage command value Vw > absolute value of voltage command value Vu is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8V, and switch 8W, respectively, will be described with reference to Fig. 20 and Fig. 35. Fig. 35 is a timing chart for explaining the operation when the control device 100 in the power conversion device according to the ninth embodiment performs shortening control 50.

[0251] 20 shows a timing chart before the control device 50 performs the reduction control, and FIG. 35 shows a timing chart when the control device 50 performs the reduction control. In the reduction control, the control device 50 reduces the high-level period of the control signal SU6 to the switch 8U (first switch) by the reduction period Tredu, does not reduce the high-level period of the control signal SV6 to the switch 8V (second switch), and reduces the high-level period of the control signal SW6 to the switch 8W (third switch) so that the high-level period of the control signal SW6 is zero. As a result, the voltage V2v across the resonant capacitor 9V is charged to Vd, thereby achieving zero-voltage soft switching of the first switching element 1V.

[0252] When the condition of absolute value of voltage command value Vw > absolute value of voltage command value Vu > absolute value of voltage command value Vv is satisfied, the control signal for the first switch, the control signal for the second switch, and the control signal for the third switch are control signal SV6 for switch 8V, control signal SW6 for switch 8W, and control signal SU6 for switch 8U, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SV6 by shortening period Tredv (see FIG. 21 ), thereby shortening the high-level period of control signal SU6 to zero. This achieves zero-voltage soft switching of first switching element 1W.

[0253] Furthermore, when the condition of absolute value of voltage command value Vu > absolute value of voltage command value Vv > absolute value of voltage command value Vw is satisfied, the control signal for the first switch, the control signal for the second switch, and the control signal for the third switch are control signal SW6 for switch 8W, control signal SU6 for switch 8U, and control signal SV6 for switch 8V, respectively. In this case, the reduction control of control device 50 reduces the high-level period of control signal SW6 by a reduction period Tredw (see FIG. 21 ), thereby reducing the high-level period of control signal SV6 to zero. This achieves zero-voltage soft switching of first switching element 1U.

[0254] (1.2) Case of Discharge Operation of Resonant Capacitor As an example, a case where the condition of absolute value of voltage command value Vv > absolute value of voltage command value Vw > absolute value of voltage command value Vu is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8V, and switch 8W, respectively, will be described with reference to Fig. 22 and Fig. 36. Fig. 36 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of embodiment 9 performs shortening control.

[0255] 22 shows a timing chart before the control device 50 performs the reduction control, and FIG. 36 shows a timing chart when the control device 50 performs the reduction control. In the reduction control, the control device 50 reduces the high-level period of the control signal SU7 to the switch 8U (first switch) by the reduction period Tredu, does not reduce the high-level period of the control signal SV7 to the switch 8V (second switch), and reduces the high-level period of the control signal SW7 to the switch 8W (third switch) so that the high-level period of the control signal SW7 is zero. As a result, the resonant capacitor 9V is discharged until the voltage V2v across the second switching element 2V reaches zero volts, thereby achieving zero-voltage soft switching of the second switching element 2V.

[0256] When the condition of absolute value of voltage command value Vw > absolute value of voltage command value Vu > absolute value of voltage command value Vv is satisfied, the control signal for the first switch, the control signal for the second switch, and the control signal for the third switch are control signal SV7 for switch 8V, control signal SW7 for switch 8W, and control signal SU7 for switch 8U, respectively. In this case, the shortening control of control device 50 shortens the high-level period of control signal SV7 by shortening period Tredv, thereby shortening the high-level period of control signal SU7 to zero. This achieves zero-voltage soft switching of second switching element 2W.

[0257] Furthermore, when the condition of absolute value of voltage command value Vu > absolute value of voltage command value Vv > absolute value of voltage command value Vw is satisfied, the control signal for the first switch, the control signal for the second switch, and the control signal for the third switch are control signal SW7 for switch 8W, control signal SU7 for switch 8U, and control signal SV7 for switch 8V, respectively. In this case, the reduction control of control device 50 reduces the high-level period of control signal SW7 by reduction period Tredw, thereby reducing the high-level period of control signal SV7 to zero. This achieves zero-voltage soft switching of second switching element 2U.

[0258] (2) Advantages In the power conversion device 100 according to the ninth embodiment, the control device 50 sets a dead time period Td between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2 for each of the multiple switching circuits 10, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through two or more of the multiple switches 8 in the resonant inductor L1 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs shortening control. In the shortening control, the control device 50 provides a control signal with a predetermined shortened high-level period to a first switch that is one of the three switches 8, provides a control signal with a predetermined high-level period to a second switch that is different from the first switch, and provides a control signal with a predetermined shortened high-level period to a third switch that is different from the first and second switches.

[0259] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0260] Furthermore, in the power conversion device 100 according to the ninth embodiment, when the control device 50 performs the shortening control, it provides the first switch with a control signal in which the preset high-level period is shortened by the shortening period, and it provides the third switch with a control signal in which the preset high-level period is shortened to zero.

[0261] According to the above configuration, it is possible to realize zero-voltage soft switching of the first switching element 1 and the second switching element 2 of the switching circuit 10 corresponding to the second switch, and it is possible to reduce switching loss.

[0262] (First Modification of the Ninth Embodiment) In the power conversion device 100 according to a first modification of the ninth embodiment, the method of determining the first switch and the third switch when the control device 50 performs shortening control differs from that of the ninth embodiment. In the first modification of the ninth embodiment, the first switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the smallest absolute value of the voltage command value for each of the three switching circuits 10. The third switch is the switch 8 of the three switches 8 that corresponds to the switching circuit 10 having the second smallest absolute value of the voltage command value for each of the three switching circuits 10.

[0263] (Second Modification of Ninth Embodiment) In a power conversion device 100 according to a second modification of the ninth embodiment, the method of determining the first switch and the third switch when the control device 50 performs shortening control differs from that of the ninth embodiment. In the second modification of the ninth embodiment, the third switch is the switch 8 corresponding to the switching circuit 10 having the smallest absolute value of the voltage command value for each of the three switching circuits 10, among the three switches 8. The first switch is the switch 8 corresponding to the switching circuit 10 having the second smallest absolute value of the voltage command value for each of the three switching circuits 10, among the three switches 8.

[0264] Tenth Embodiment The circuit configuration of the power conversion device 100 according to a tenth embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0265] The operation of the power conversion device 100 according to the tenth embodiment is substantially the same as the operation of the power conversion device 100 according to the eighth embodiment. The power conversion device 100 according to the tenth embodiment differs from the seventh embodiment in that, when the control device 50 shortens the high-level period of the control signal of the third switch, it shortens the high-level period so that there is zero overlap with the high-level period of the control signal of the second switch.

[0266] (1.1) Case of Charging Operation of Resonant Capacitor As an example, a case where the condition of absolute value of load current iW > absolute value of load current iV > absolute value of load current iU is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8V, and switch 8W, respectively, will be described with reference to Fig. 20 and Fig. 37. Fig. 37 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to embodiment 10 performs shortening control. In Fig. 20 and Fig. 37, the polarity of the load current iW is negative, and the polarities of the load current iV and the load current iU are positive.

[0267] FIG. 20 shows a timing chart before the control device 50 performs the shortening control, and FIG. 37 shows a timing chart after the control device 50 performs the shortening control. In the shortening control, the control device 50 shortens the high-level period of the control signal SU6 sent to the switch 8U (first switch) so that the high-level period of the control signal SU6 is zero. The control device 50 does not shorten the high-level period of the control signal SW6 sent to the switch 8W (second switch), but shortens the high-level period of the control signal SV6 sent to the switch 8V (third switch) by the shortening period Tredv. This causes the voltage V2w across the resonant capacitor 9W to be charged to Vd, thereby achieving zero-voltage soft switching of the first switching element 1W. Furthermore, the control device 50 prioritizes shortening the high-level period of the switch 8 corresponding to the switching circuit 10 with a relatively small absolute value of the load current, thereby further reducing switching loss in the switching circuit 10.

[0268] When the condition of absolute value of load current iU > absolute value of load current iW > absolute value of load current iV is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SV6 for switch 8V, control signal SU6 for switch 8U, and control signal SW6 for switch 8W, respectively. In this case, the shortening control of the control device 50 shortens the high-level period of the control signal SV6 so that the high-level period of the control signal SV6 becomes zero, and shortens the high-level period of the control signal SW6 by the shortened period Tredw (see FIG. 21 ). This achieves zero-voltage soft switching of the first switching element 1U. Furthermore, because the control device 50 prioritizes shortening the high-level period of the switch 8 corresponding to the switching circuit 10 with a relatively small absolute value of the load current, switching loss in the switching circuit 10 can be further reduced.

[0269] Furthermore, when the condition of absolute value of load current iV > absolute value of load current iU > absolute value of load current iW is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SW6 for switch 8W, control signal SV6 for switch 8V, and control signal SU6 for switch 8U, respectively. In this case, the shortening control of the control device 50 shortens the high-level period of the control signal SW6 so that the high-level period of the control signal SW6 becomes zero, and shortens the high-level period of the control signal SU6 by the shortening period Tredu. This achieves zero-voltage soft switching of the first switching element 1V. Furthermore, because the control device 50 prioritizes shortening the high-level period of the switch 8 corresponding to the switching circuit 10 with a relatively small absolute value of the load current, switching loss in the switching circuit 10 can be further reduced.

[0270] (1.2) Case of Discharge Operation of Resonant Capacitor As an example, a case where the condition of absolute value of load current iW > absolute value of load current iV > absolute value of load current iU is satisfied and the first switch, second switch, and third switch are switch 8U, switch 8W, and switch 8V, respectively, will be described with reference to Figs. 22 and 38. Fig. 38 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of embodiment 10 performs shortening control. In Figs. 22 and 38, the polarity of the load current iW is positive, and the polarities of the load current iV and the load current iU are negative.

[0271] FIG. 22 shows a timing chart before the control device 50 performs the shortening control, and FIG. 38 shows a timing chart after the control device 50 performs the shortening control. In the shortening control, the control device 50 shortens the high-level period of the control signal SU7 sent to the switch 8U (first switch) so that the high-level period of the control signal SU7 is zero. The control device 50 does not shorten the high-level period of the control signal SW7 sent to the switch 8W (second switch), but shortens the high-level period of the control signal SV7 sent to the switch 8V (third switch) by the shortening period Tredv. This allows the resonant capacitor 9W to discharge until the voltage V2v across the second switching element 2W reaches zero volts, thereby achieving zero-voltage soft switching of the second switching element 2W. Furthermore, the control device 50 prioritizes shortening the high-level period of the switch 8 corresponding to the switching circuit 10 with a relatively small absolute value of the load current, thereby further reducing switching loss in the switching circuit 10.

[0272] When the condition of absolute value of load current iU > absolute value of load current iW > absolute value of load current iV is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SV7 for switch 8V, control signal SU7 for switch 8U, and control signal SW7 for switch 8W, respectively. In this case, the shortening control of the control device 50 shortens the high-level period of the control signal SV7 so that the high-level period of the control signal SV7 becomes zero, and shortens the high-level period of the control signal SW7 by the shortening period Tredw. This achieves zero-voltage soft switching of the second switching element 2U. Furthermore, because the control device 50 prioritizes shortening the high-level period of the switch 8 corresponding to the switching circuit 10 with a relatively small absolute value of the load current, switching loss in the switching circuit 10 can be further reduced.

[0273] Furthermore, when the condition of absolute value of load current iV > absolute value of load current iU > absolute value of load current iW is satisfied, the control signals for the first switch, the second switch, and the third switch are control signal SW7 for switch 8W, control signal SV7 for switch 8V, and control signal SU7 for switch 8U, respectively. In this case, the shortening control of the control device 50 shortens the high-level period of the control signal SW6 so that the high-level period of the control signal SW6 becomes zero, and shortens the high-level period of the control signal SU7 by the shortening period Tredu. This achieves zero-voltage soft switching of the second switching element 2V. Furthermore, because the control device 50 prioritizes shortening the high-level period of the switch 8 corresponding to the switching circuit 10 with a relatively small absolute value of the load current, switching loss in the switching circuit 10 can be further reduced.

[0274] (2) Advantages In the power conversion device 100 according to the tenth embodiment, the control device 50 sets a dead time period Td between the high-level period of the control signal to the first switching element 1 and the high-level period of the control signal to the second switching element 2 for each of the multiple switching circuits 10, and sets the high-level period of the control signal to each of the multiple switches 8 based on the dead time period Td for the corresponding switching circuit 10 among the multiple switching circuits 10. Furthermore, when the control device 50 determines that a resonant current flows simultaneously through two or more of the multiple switches 8 when a control signal with a predetermined high-level period is provided to each of the multiple switches 8, the control device 50 performs reduction control. In the reduction control, the control device 50 provides a control signal with a predetermined reduced high-level period to a first switch that is one of the three switches 8, provides a control signal with a predetermined high-level period to a second switch that is different from the first switch, and provides a control signal with a predetermined reduced high-level period to a third switch that is different from the first and second switches.

[0275] According to the above configuration, it is possible to perform zero voltage soft switching and reduce switching loss.

[0276] Furthermore, in the power conversion device 100 according to the tenth embodiment, when the control device 50 performs the shortening control, it provides the first switch with a control signal in which the preset high level period is shortened to zero, and it provides the third switch with a control signal in which the preset high level period is shortened by the shortening period.

[0277] According to the above configuration, it is possible to realize zero-voltage soft switching of the first switching element 1 and the second switching element 2 of the switching circuit 10 corresponding to the second switch, and it is possible to reduce switching loss.

[0278] Furthermore, in the power conversion device 100 according to the tenth embodiment, when the control device 50 performs shortening control, the first switch is the switch 8 corresponding to the switching circuit 10 among the two or more switches 8, which has the smallest absolute value of the load current flowing through each of the two or more switching circuits 10 corresponding to the two or more switches 8 in the plurality of switching circuits 10.

[0279] Furthermore, with the above configuration, the high level period of the control signal to the switch 8 corresponding to the switching circuit 10 with the smallest absolute value of the load current is shortened, so that switching loss can be further reduced.

[0280] (First Modification of Tenth Embodiment) In a power conversion device 100 according to a first modification of the tenth embodiment, the method of determining the first switch and the third switch when the control device 50 performs shortening control differs from that of the tenth embodiment. In the first modification of the tenth embodiment, the first switch is the switch 8 corresponding to the switching circuit 10 having the largest absolute value of the voltage command value for each of the three switching circuits 10, among the three switches 8. The third switch is the switch 8 corresponding to the switching circuit 10 having the second largest absolute value of the voltage command value for each of the three switching circuits 10, among the three switches 8.

[0281] (Second Modification of Tenth Embodiment) In a power conversion device 100 according to a second modification of the tenth embodiment, the method of determining the first switch and the third switch when the control device 50 performs shortening control differs from that of the tenth embodiment. In the second modification of the tenth embodiment, the third switch is the switch 8 corresponding to the switching circuit 10 having the largest absolute value of the voltage command value for each of the three switching circuits 10, among the three switches 8. The first switch is the switch 8 corresponding to the switching circuit 10 having the second largest absolute value of the voltage command value for each of the three switching circuits 10, among the three switches 8.

[0282] (Embodiment 11) A power conversion device 100A according to embodiment 11 will be described with reference to Fig. 39. Fig. 39 is a circuit diagram of a system including the power conversion device 100A according to embodiment 11. Constituent elements of the power conversion device 100A according to embodiment 11 that are the same as those of the power conversion device 100 according to embodiment 1 will be assigned the same reference numerals and descriptions thereof will be omitted.

[0283] In a power conversion device 100A according to the eleventh embodiment, a first IGBT 6 and a second IGBT 7 are connected in anti-series in each of the plurality of switches 8. In the power conversion device 100A according to the eleventh embodiment, in each of the plurality of switches 8, an emitter terminal of the first IGBT 6 and an emitter terminal of the second IGBT 7 are connected, a collector terminal of the first IGBT 6 is connected to a connection point 3 of a corresponding one of the plurality of switching circuits 10, and a collector terminal of the second IGBT 7 is connected to a common connection point 25. Each of the plurality of switches 8 further includes a diode 61 connected in anti-parallel to the first IGBT 6 and a diode 71 connected in anti-parallel to the second IGBT 7.

[0284] The operation of the control device 50 is, for example, the same as the operation of the control device 50 of embodiment 1, but is not limited to this and may be the same as the operation of the control device 50 of any of embodiments 2 to 10, or these operations may be performed in an appropriate combination.

[0285] In the power conversion device 100A according to the eleventh embodiment, each of the first IGBT 6 and the second IGBT 7 may be replaced with a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or a bipolar transistor. In this case, the diode 61 and the diode 71 in FIG. 21 may be substituted with a parasitic diode of the replaced element, or an element built into the chip of the replaced element. Furthermore, in the power conversion device 100A according to the eleventh embodiment, the diode 61 and the diode 71 are not limited to being externally attached to the first IGBT 6 and the second IGBT 7, but may also be an element built into the chip.

[0286] In addition, in each of the multiple switches 8, the collector terminal of the first IGBT 6 and the collector terminal of the second IGBT 7 may be connected, the emitter terminal of the first IGBT 6 may be connected to the connection point 3 of a corresponding one of the multiple switching circuits 10, and the emitter terminal of the second IGBT 7 may be connected to the common connection point 25.

[0287] (Twelfth Embodiment) Hereinafter, a power conversion device 100B according to a twelfth embodiment will be described with reference to Fig. 40. Fig. 40 is a circuit diagram of a system including the power conversion device 100B according to the twelfth embodiment. The power conversion device 100B according to the twelfth embodiment differs from the power conversion device 100 according to the first embodiment in that it further includes a capacitor 16 connected between the second end of the resonance inductor L1 and the first DC terminal 31. With regard to the power conversion device 100B according to the twelfth embodiment, components similar to those of the power conversion device 100 according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

[0288] The power conversion device 100B does not include the capacitor C10 in the power conversion device 100 according to the first embodiment. The capacitor 16 is connected in series with the regenerative capacitor 15. Therefore, in the power conversion device 100B, the series circuit of the capacitor 16 and the regenerative capacitor 15 is connected between the first DC terminal 31 and the second DC terminal 32. The capacitance of the capacitor 16 is the same as the capacitance of the regenerative capacitor 15. The phrase "the capacitance of the capacitor 16 is the same as the capacitance of the regenerative capacitor 15" does not necessarily mean that the capacitance of the capacitor 16 exactly matches the capacitance of the regenerative capacitor 15, but may mean that the capacitance of the capacitor 16 is within a range of 95% to 105% of the capacitance of the regenerative capacitor 15.

[0289] In the power conversion device 100B according to the twelfth embodiment, the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is equal to the voltage value Vd of the DC power supply E1 divided by the capacitor 16 and the regenerative capacitor 15. Therefore, the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is approximately Vd / 2. In the power conversion device 100B according to the twelfth embodiment, the control device 50 may store the value of the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 in advance.

[0290] The control device 50 of the power conversion device 100B according to the twelfth embodiment performs shortening control, similar to the control device 50 of the power conversion device 100 according to the first embodiment. Therefore, the power conversion device 100B according to the twelfth embodiment can reduce switching loss, similar to the power conversion device 100 according to the first embodiment.

[0291] The operation of the reduction control of the control device 50 is, for example, similar to the operation of the reduction control of the control device 50 of embodiment 1, but is not limited to this and may be similar to the operation of the reduction control of the control device 50 of any of embodiments 2 to 10, or these operations may be executed in an appropriate combination.

[0292] (Embodiment 13) A power conversion device 100C according to embodiment 13 will be described below with reference to Fig. 41. Fig. 41 is a circuit diagram of a system including the power conversion device 100C according to embodiment 13. The power conversion device 100C according to embodiment 13 differs from the power conversion device 100 according to embodiment 1 in that a regenerative capacitor 15 is connected between the second end of the resonance inductor L1 and the first DC terminal 31. With regard to the power conversion device 100C according to embodiment 13, components similar to those of the power conversion device 100 according to embodiment 1 are denoted by the same reference numerals, and description thereof will be omitted.

[0293] The control device 50 of the power conversion device 100C according to the thirteenth embodiment performs shortening control, similar to the control device 50 of the power conversion device 100 according to the first embodiment. Therefore, the power conversion device 100C according to the thirteenth embodiment can reduce switching loss, similar to the power conversion device 100 according to the first embodiment.

[0294] (Other Modifications) The above-described first to thirteenth embodiments are merely examples of various embodiments of the present disclosure. The above-described first to thirteenth embodiments can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved.

[0295] In the control device 50, the operation of "determining that multiple resonant currents are flowing simultaneously" is not limited to the operation of "determining that multiple resonant currents are flowing simultaneously" when the time difference is less than the threshold value described in embodiment 1.

[0296] For example, the control device 50 may determine that two-phase resonant currents are flowing simultaneously when any one of the current difference between the U-phase load current iU and the V-phase load current iV, the current difference between the V-phase load current iV and the W-phase load current iW, and the current difference between the W-phase load current iW and the U-phase load current iU is less than a current difference threshold.

[0297] In addition, the control device 50 may determine that three-phase resonant currents are flowing simultaneously when any of the current differences between the U-phase load current iU and the V-phase load current iV, the V-phase load current iV and the W-phase load current iW, and the W-phase load current iW and the U-phase load current iU are less than the current difference threshold.

[0298] In addition, the control device 50 may determine that "two-phase resonant currents flow simultaneously" when the electrical angle calculated from sensor information output from a sensor device (e.g., an encoder or resolver) for detecting the rotation speed of the motor, or the estimated electrical angle, is within a first rotation angle range (e.g., 55 degrees or more and 65 degrees or less), a second rotation angle range (e.g., 115 degrees or more and 125 degrees or less), a third rotation angle range (e.g., 175 degrees or more and 185 degrees or less), a fourth rotation angle range (e.g., 235 degrees or more and 245 degrees or less), a fifth rotation angle range (295 degrees or more and 305 degrees or less), or a sixth rotation angle range (e.g., 355 degrees or more and 365 degrees or less).

[0299] In the modified example of the first embodiment, the modified example of the second embodiment, and the fifth embodiment, the first switch and the second switch among the plurality of switches 8 are determined based on the magnitude relationship of the absolute values ​​of the voltage command values, but the first switch and the second switch among the plurality of switches 8 may be determined based on the magnitude relationship of the duties. In the modified example 2 of the third embodiment, the modified example 3 of the third embodiment, the seventh embodiment, the ninth embodiment, the modified example 1 of the ninth embodiment, the modified example 2 of the ninth embodiment, the modified example 1 of the tenth embodiment, and the modified example 2 of the tenth embodiment, the first switch, the second switch, and the third switch among the plurality of switches 8 are determined based on the magnitude relationship of the absolute values ​​of the voltage command values, but the first switch, the second switch, and the third switch among the plurality of switches 8 may be determined based on the magnitude relationship of the duties.

[0300] Furthermore, each of the plurality of first switching elements 1 and the plurality of second switching elements 2 is not limited to an IGBT, but may be a MOSFET. In this case, each of the plurality of first diodes 4 may be substituted with a parasitic diode of the MOSFET constituting the corresponding first switching element 1. Also, each of the plurality of second diodes 5 may be substituted with a parasitic diode of the MOSFET constituting the corresponding second switching element 2. The MOSFET is, for example, a Si-based MOSFET or a SiC-based MOSFET. Each of the plurality of first switching elements 1 and the plurality of second switching elements 2 may be, for example, a bipolar transistor or a GaN-based GIT.

[0301] Furthermore, in the power conversion devices 100, 100A, 100B, and 100C, if the capacitance of each of the multiple resonant capacitors 9 is relatively small, instead of attaching the multiple resonant capacitors 9 externally, the parasitic capacitance between both ends of the multiple second switching elements 2 may also serve as the multiple resonant capacitors 9.

[0302] The dead time period may be set by a dead time generation circuit included in a gate driver IC (Integrated Circuit) or the like that is provided separately from the control device 50. Alternatively, the control device 50 may include a gate driver IC, and the dead time period may be set by a dead time generation circuit included in the gate driver IC.

[0303] Furthermore, the power conversion devices 100, 100A, 100B, and 100C are not limited to being configured to output three-phase AC, but may be configured to output three or more phases of polyphase AC. Regarding the method for determining the additional times Tau, Tav, and Taw described in the section "(3.1) Basic Operation" of the first embodiment, the formulas used are ideal design examples, and the invention is not limited to always calculating using such formulas. In some cases, the additional times Tau, Tav, and Taw may be set to 0 or another fixed time. Furthermore, values ​​calculated using another formula may be used as long as the purpose of the additional times Tau, Tav, and Taw can be achieved. For example, in the basic example, Tau = iU × (L / V15) is calculated, but this is not limiting. Tau may be set to 0, set between 0 and iU × (L / V15), or always set to a constant additional time. It may also be calculated using another formula, or a combination of these.

[0304] Furthermore, the length of each of the shortened periods Tredu, Tredv, and Tredw is not limited to the same value as the time difference, and may be any value.

[0305] (Aspects) The following aspects are disclosed in this specification.

[0306] A power conversion device (100; 100A; 100B; 100C) according to a first aspect includes a first DC terminal (31), a second DC terminal (32), a power conversion circuit (11), a plurality of AC terminals (41), a plurality of switches (8), a plurality of resonant capacitors (9), a resonant inductor (L1), a regenerative capacitor (15), and a control device (50). The power conversion circuit (11) has a plurality of first switching elements (1) and a plurality of second switching elements (2). In the power conversion circuit (11), a plurality of switching circuits (10) are connected in parallel to each other, each of which has a plurality of first switching elements (1) and a plurality of second switching elements (2) connected in series in a one-to-one relationship. In the power conversion circuit (11), the plurality of first switching elements (1) are connected to the first DC terminal (31), and the plurality of second switching elements (2) are connected to the second DC terminal (32). The plurality of AC terminals (41) correspond one-to-one to the plurality of switching circuits (10). Each of the plurality of AC terminals (41) is connected to a connection point (3) between a first switching element (1) and a second switching element (2) in the corresponding switching circuit (10). The plurality of switches (8) correspond one-to-one to the plurality of switching circuits (10). Each of the plurality of switches (8) has a first end (81) connected to a connection point (3) between the first switching element (1) and the second switching element (2) in the corresponding switching circuit (10), and a second end (82) commonly connected to a common connection point (25). The resonant capacitors (9) correspond one-to-one to the plurality of switches (8). Each of the plurality of resonant capacitors (9) is connected between a first end (81) of the corresponding switch (8) and a second DC terminal (32). The resonant inductor (L1) has a first end and a second end. In the resonant inductor (L1), a first end of the resonant inductor (L1) is connected to the common connection point (25). The regenerative capacitor (15) has a third end (153) and a fourth end (154). In the regenerative capacitor (15), the third end (153) is connected to the first DC terminal (31) or the second DC terminal (32).A control device (50) provides a control signal whose potential changes between a high level and a low level to each of a plurality of first switching elements (1), a plurality of second switching elements (2), and a plurality of switches (8). The control device (50) sets a dead time period (Td) between a high-level period of the control signal to the first switching element (1) and a high-level period of the control signal to the second switching element (2) for each of a plurality of switching circuits (10), and sets the high-level period of the control signal to each of the plurality of switches (8) based on the dead time period (Td) for a corresponding switching circuit (10) among the plurality of switching circuits (10). A load current flows through each of a plurality of AC terminals (41) through the first switching element (1) or the second switching element (2) of the corresponding switching circuit (10). The control device (50) performs shortening control when it determines that, when a control signal with a preset high-level period is provided to each of the plurality of switches (8), a resonant current passing through two or more of the plurality of switches (8) simultaneously flows through a resonant inductor (L1). In the shortening control, the control device (50) provides a control signal with a shortened high level period set in advance to a first switch that is one of the two or more switches (8), and provides a control signal with a high level period set in advance to a second switch that is different from the first switch among the two or more switches (8).

[0307] According to this aspect, it is possible to perform zero voltage soft switching and reduce switching loss.

[0308] In the power conversion device (100; 100A; 100B; 100C) according to the second aspect, in the first aspect, when performing the shortening control, the control device (50) provides the first switch with a control signal in which the preset high level period is shortened to zero.

[0309] According to this aspect, it is possible to realize zero-voltage soft switching of the first switching element (1) and the second switching element (2) of the switching circuit (10) corresponding to the second switch, thereby making it possible to reduce switching loss.

[0310] In the power conversion device (100; 100A; 100B; 100C) according to the third aspect, in the first or second aspect, the first switch is a switch (8) corresponding to a switching circuit (10) among the two or more switches (8) in which the absolute value of the load current flowing through each of the two or more switching circuits (10) corresponding to the two or more switches (8) in the plurality of switching circuits (10) is smallest.

[0311] According to this aspect, it is possible to further reduce switching loss.

[0312] In the power conversion device (100; 100A; 100B; 100C) according to the fourth aspect, in the first or second aspect, the first switch is the switch (8) corresponding to the switching circuit (10) among the two or more switches (8) that has the smallest absolute value of the voltage command value for each of the two or more switching circuits (10) corresponding to the two or more switches (8) in the plurality of switching circuits (10).

[0313] According to this aspect, the switches (8) in which hard switching is alleviated among the plurality of switches (8) are equalized.

[0314] In the power conversion device (100; 100A; 100B; 100C) according to the fifth aspect, in the first or second aspect, the first switch is the switch (8) corresponding to the switching circuit (10) having the largest absolute value of the voltage command value for each of the two or more switching circuits (10) corresponding to the two or more switches (8) among the plurality of switching circuits (10).

[0315] According to this aspect, the switches (8) in which hard switching is alleviated among the plurality of switches (8) are equalized.

[0316] In a power conversion device (100; 100A; 100B; 100C) according to a sixth aspect, in the first aspect, the two or more switches (8) are three switches (8). When performing the shortening control, the control device (50) provides, to the first switch, a control signal in which a preset high-level period is shortened to zero, and provides, to a third switch of the three switches (8), which is different from the first switch and the second switch, a control signal having a high-level period in which the preset high-level period is shortened.

[0317] According to this aspect, by performing the shortening control, it is possible to realize zero-voltage soft switching of each of the first switching element (1) and the second switching element (2) of the switching circuit (10) corresponding to the second switch.

[0318] In the power conversion device (100; 100A; 100B; 100C) according to the seventh aspect, in the sixth aspect, the first switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the smallest absolute value of the load current flowing through each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10). The third switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the second smallest absolute value of the load current flowing through each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10).

[0319] According to this aspect, it is possible to further reduce switching loss.

[0320] In the power conversion device (100; 100A; 100B; 100C) according to the eighth aspect, in the sixth aspect, the third switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the smallest absolute value of the load current flowing through each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10). The first switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the second smallest absolute value of the load current flowing through each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10).

[0321] According to this aspect, it is possible to further reduce switching loss.

[0322] In the power conversion device (100; 100A; 100B; 100C) according to the ninth aspect, in the sixth aspect, the first switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the smallest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10). The third switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the second smallest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10).

[0323] According to this aspect, when the shortening control is performed, the switches (8) that realize zero-voltage soft switching among the plurality of switches (8) are equalized.

[0324] In the power conversion device (100; 100A; 100B; 100C) according to the tenth aspect, in the sixth aspect, the third switch is the switch (8) among the three switches (8) corresponding to the switching circuit (10) having the smallest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10). The first switch is the switch (8) among the three switches (8) corresponding to the switching circuit (10) having the second smallest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10).

[0325] According to this aspect, when the shortening control is performed, the switches (8) that realize zero-voltage soft switching among the plurality of switches (8) are equalized.

[0326] In the power conversion device (100; 100A; 100B; 100C) according to an eleventh aspect, in the sixth aspect, the first switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the largest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10). The third switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the second largest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10).

[0327] According to this aspect, it is possible to further reduce switching loss.

[0328] In the power conversion device (100; 100A; 100B; 100C) according to the twelfth aspect, in the sixth aspect, the third switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the largest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10). The first switch is the switch (8) of the three switches (8) corresponding to the switching circuit (10) having the second largest voltage command value for each of the three switching circuits (10) corresponding to the three switches (8) in the plurality of switching circuits (10).

[0329] The power conversion device of the present disclosure is capable of performing zero-voltage soft switching and reducing switching loss, making it industrially useful.

[0330] 1 First switching element 2 Second switching element 3 Connection point 8 Switch 9 Resonant capacitor 10 Switching circuit 11 Power conversion circuit 15 Regenerative capacitor 31 First DC terminal 32 Second DC terminal 41 AC terminal 50 Control device 100, 100A, 100B, 100C Power conversion device iU, iV, iW Output current (load current) L1 Resonant inductor RA1 AC load SU1, SU2, SU6, SU7 Control signal SV1, SV2, SV6, SV7 Control signal SW1, SW2, SW6, SW7 Control signal Tad Additional time Tred Reduction period Tres Resonant half cycle V15 Voltage Vu Voltage command value Vv Voltage command value Vw Voltage command value

Claims

1. A power conversion circuit having a first DC terminal and a second DC terminal, the power conversion circuit having a plurality of first switching elements and a plurality of second switching elements, the plurality of first switching elements and the plurality of second switching elements being connected in series in a one-to-one relationship, the plurality of first switching elements being connected to the first DC terminal, and the plurality of second switching elements being connected to the second DC terminal; a plurality of AC terminals corresponding one-to-one to the plurality of switching circuits, each of which is connected to a connection point of the first switching elements and the second switching elements in the corresponding switching circuit; a plurality of switches corresponding one-to-one to the plurality of switching circuits, each of which has a first end connected to the connection point of the first switching elements and the second switching elements in the corresponding switching circuit and a second end commonly connected to a common connection point; a plurality of resonant capacitors corresponding one-to-one to the plurality of switches, each of which is connected between the first end and the second DC terminal of the corresponding switch; and a resonant inductor having a first end and a second end, the first end being connected to the common connection point. a regenerative capacitor having a third end and a fourth end, the third end being connected to the first DC terminal or the second DC terminal; and a control device that provides a control signal, the potential of which changes between a high level and a low level, to each of the plurality of first switching elements, the plurality of second switching elements and the plurality of switches, wherein the control device sets a dead time period for each of the plurality of switching circuits between a high level period of the control signal to the first switching element and a high level period of the control signal to the second switching element, and sets the high level period of the control signal to each of the plurality of switches based on the dead time period for a corresponding one of the plurality of switching circuits; a load current flows through each of the plurality of AC terminals and passes through the first switching element or the second switching element of the corresponding switching circuit,When the control device determines that a resonant current passing through each of two or more of the multiple switches simultaneously flows through the resonant inductor when a control signal with a preset high level period is given to each of the multiple switches, the control device performs shortening control, in which a control signal with the preset high level period is given to a first switch that is one of the two or more switches, and a control signal having the preset high level period is given to a second switch among the two or more switches that is different from the first switch.

2. The power conversion device according to claim 1, wherein, when performing the shortening control, the control device applies to the first switch a control signal that shortens the preset high level period to zero.

3. A power conversion device as described in claim 1 or 2, wherein the first switch is a switch corresponding to a switching circuit having the smallest absolute value of a load current flowing through each of two or more switching circuits corresponding to the two or more switches in the plurality of switching circuits, among the two or more switches.

4. A power conversion device as described in claim 1 or 2, wherein the first switch is a switch corresponding to a switching circuit among the two or more switches, the switching circuit having the smallest absolute value of a voltage command value for each of two or more switching circuits corresponding to the two or more switches in the plurality of switching circuits.

5. A power conversion device as described in claim 1 or 2, wherein the first switch is a switch corresponding to a switching circuit among the two or more switches, the switching circuit having the largest absolute value of a voltage command value for each of two or more switching circuits corresponding to the two or more switches in the plurality of switching circuits.

6. The power conversion device according to claim 1, wherein the two or more switches are three switches, and when performing the shortening control, the control device provides, to the first switch, a control signal in which the preset high level period is shortened to zero, and provides, to a third switch of the three switches, which is different from the first switch and the second switch, a control signal having a high level period in which the preset high level period is shortened.

7. A power conversion device as described in claim 6, wherein the first switch is a switch corresponding to a switching circuit having the smallest absolute value of a load current flowing through each of the three switching circuits corresponding to the three switches in the plurality of switching circuits, among the three switches, and the third switch is a switch corresponding to a switching circuit having the second smallest absolute value of a load current flowing through each of the three switching circuits corresponding to the three switches in the plurality of switching circuits, among the three switches.

8. The power conversion device according to claim 6, wherein the third switch is a switch corresponding to a switching circuit having the smallest absolute value of a load current flowing through each of the three switching circuits corresponding to the three switches in the plurality of switching circuits, among the three switches, and the first switch is a switch corresponding to a switching circuit having the second smallest absolute value of a load current flowing through each of the three switching circuits corresponding to the three switches in the plurality of switching circuits, among the three switches.

9. The power conversion device according to claim 6, wherein the first switch is a switch corresponding to a switching circuit, among the three switches, having the smallest absolute value of a voltage command value for each of three switching circuits corresponding to the three switches in the plurality of switching circuits, and the third switch is a switch corresponding to a switching circuit, among the three switches, having the second smallest absolute value of a voltage command value for each of three switching circuits corresponding to the three switches in the plurality of switching circuits.

10. The power conversion device according to claim 6, wherein the third switch is a switch corresponding to a switching circuit having the smallest absolute value of a voltage command value for each of three switching circuits corresponding to the three switches in the plurality of switching circuits, among the three switches, and the first switch is a switch corresponding to a switching circuit having the second smallest absolute value of a voltage command value for each of three switching circuits corresponding to the three switches in the plurality of switching circuits, among the three switches.

11. The power conversion device according to claim 6, wherein the first switch is a switch corresponding to a switching circuit, among the three switches, having the largest absolute value of a voltage command value for each of three switching circuits corresponding to the three switches in the plurality of switching circuits, and the third switch is a switch corresponding to a switching circuit, among the three switches, having the second largest absolute value of a voltage command value for each of three switching circuits corresponding to the three switches in the plurality of switching circuits.

12. The power conversion device according to claim 6, wherein the third switch is a switch corresponding to a switching circuit, among the three switches, having the largest absolute value of a voltage command value for each of the three switching circuits corresponding to the three switches in the plurality of switching circuits, and the first switch is a switch corresponding to a switching circuit, among the three switches, having the second largest absolute value of a voltage command value for each of the three switching circuits corresponding to the three switches in the plurality of switching circuits.

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