Power conversion device

Abstract

The present invention more reliably performs soft switching for a power conversion device. A power conversion device (100) comprises a power conversion circuit (11), a plurality of switches (8), a plurality of resonant capacitors (9), a resonant inductor (L1), a regeneration capacitor (15), and a control device (50). The power conversion circuit (11) comprises a plurality of first switching elements (1), and a plurality of second switching elements (2). In the power conversion device (100), when it is determined that resonant currents passing through two or more switches (8), respectively, among the plurality of switches (8), will simultaneously flow in the resonant inductor (L1), the control device (50) performs shift control for shifting a high level period of a control signal to at least one switch (8) among the two or more switches (8), so as to prevent the resonant currents passing through the two or more switches (8), respectively, from simultaneously flowing in the resonant inductor (L1).

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] In the power conversion device described in Patent Document 1, when the control means determines that multiple phase currents are flowing through the coil, it controls multiple auxiliary switch elements so that the current flowing through at least one phase is smaller than a preset magnitude, and therefore soft switching of the main switch corresponding to the at least one phase is not performed.

[0006] An object of the present disclosure is to provide a power conversion device that can perform soft switching more reliably.

[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.When the control device determines that resonant currents passing through two or more switches of the plurality of switches simultaneously flow through the resonant inductor, it performs shift control to shift the high-level period of the control signal to at least one of the two or more switches so that the resonant currents passing through the two or more switches simultaneously do not flow through the resonant inductor.

[0008] The power conversion device of the present disclosure has the effect of enabling soft switching to be performed more reliably.

[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 of the control device of the power conversion device when the control device performs a basic operation in the case where the load current is greater than 0 and the resonant capacitor is charging. FIG. 3 is another explanatory diagram illustrating an operation of the control device of the power conversion device when the control device performs a basic operation in the case where the load current is greater than 0 and the resonant capacitor is charging. FIG. 4 is a diagram illustrating a time change in duty corresponding to a voltage command for each of three phases in an AC load connected to multiple AC terminals of the power conversion device, and a time change in load current. FIG. 5 is an explanatory diagram illustrating a first current threshold and a second current threshold used by the control device of the power conversion device. FIG. 6 is an explanatory diagram illustrating an operation of the control device of the power conversion device when the control device performs a basic operation in the case where the load current is greater than 0 and the resonant capacitor is discharging. FIG. 7 is an explanatory diagram illustrating an operation of the control device of the power conversion device when the load current is less than 0 and the resonant capacitor is discharging. FIG. 8 is an explanatory diagram illustrating an operation of the control device of the power conversion device when the control device performs a basic operation in the case where the load current is less than 0 and the resonant capacitor is charging. FIG. 9 is a timing chart illustrating the operation of a control device in the power conversion apparatus of the same. FIG. 10 is an explanatory diagram of the operation of a control device in the power conversion apparatus of the same. FIG. 11 is a timing chart illustrating the operation of a control device in the power conversion apparatus of the same. FIG. 12 is a timing chart illustrating the operation of a control device in a power conversion apparatus according to a second embodiment. FIG. 13 is a timing chart illustrating the operation of a control device in the power conversion apparatus of the same. FIG. 14 is a timing chart illustrating the operation of a control device in the power conversion apparatus of the same. FIG. 15 is a timing chart illustrating a case where a control device executes shift control in a power conversion apparatus according to a third embodiment. FIG. 16 is a timing chart illustrating a case where a control device does not execute shift control in the power conversion apparatus of the same. FIG. 17 is a timing chart illustrating a case where a control device executes shift control in the power conversion apparatus of the same. FIG. 18 is a timing chart illustrating a case where a control device does not execute shift control in the power conversion apparatus of the same.FIG. 19 is a timing chart when the control device executes shift control in the power conversion device according to embodiment 4. FIG. 20 is a timing chart when the control device executes shift control in the power conversion device according to embodiment 5. FIG. 21 is a timing chart when the control device does not execute shift control in the power conversion device according to embodiment 6. FIG. 22 is a circuit diagram of a system including a power conversion device according to embodiment 6. FIG. 23 is a circuit diagram of a system including a power conversion device according to embodiment 7. FIG. 24 is a circuit diagram of a system including a power conversion device according to embodiment 8. FIG. 25 is a circuit diagram of a system including a power conversion device according to embodiment 9. FIG. 26 is a circuit diagram of a system including a power conversion device according to embodiment 10. FIG. 27 is a circuit diagram of a system including a power conversion device according to embodiment 11. FIG. 28 is a circuit diagram of a system including a power conversion device according to embodiment 12. FIG. 29 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.

[0011] (1) Overall Configuration of the Power Conversion Device As shown in FIG. 1 , the power conversion device 100 includes 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 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 regenerative capacitor 15, a resonant inductor L1, 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. A first end 81 of each of the plurality of switches 8 is connected to the connection point 3 between the first switching elements 1 and the second switching elements 2 in the corresponding switching circuit 10. The plurality of resonant capacitors 9 correspond one-to-one to the plurality of switches 8. Each of the multiple resonant capacitors 9 is connected between the first end 81 and the second DC terminal 32 of the corresponding switch 8. The resonant inductor L1 has a first end and a second end, and the first end 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 the third end 153 connected to the second DC terminal 32, and the fourth end 154 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 mainly composed of a processor and memory as hardware. The processor executes a program recorded 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-recorded in the memory of the computer system, provided via a telecommunications line, or recorded and provided 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 carrier signal (see FIG. 2 ) to generate control signals SU1, SV1, and SW1 corresponding to the multiple first switching elements 1U, 1V, and 1W, respectively, and control signals SU2, SV2, and SW2 corresponding to the multiple 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. 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. 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. The U-phase voltage command, V-phase voltage command, and W-phase voltage command are, for example, sinusoidal signals whose phases differ by 120° from each other, and whose amplitudes (voltage command 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 the sawtooth wave shown in FIG. 2 . The U-phase voltage command, V-phase voltage command, and W-phase voltage command each have the same cycle length. The U-phase voltage command, V-phase voltage command, and W-phase voltage command each 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. In FIG. 4, 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 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 by 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. In FIG. 2, 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 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 period Td (see FIG. 2) 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 duties 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, vary based on the W-phase voltage command. In FIG. 4, 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 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. 3) 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, V-phase voltage command, and W-phase voltage command are, for example, sinusoidal signals whose phases are different from each other by 120°, and whose amplitudes change over time. Therefore, the duty of the control signal SU1 (U-phase duty), the duty of the control signal SV1 (V-phase duty), and the duty of the control signal SW1 (W-phase duty) change like sinusoidal waves whose phases are different from each other by 120°, as shown in FIG. 4 . Similarly, the duty of the control signal SU2, the duty of the control signal SV2, and the duty of the control signal SW2 change like sinusoidal waves whose phases are different from each other by 120°.

[0033] The control device 50 generates the control signals SU1, SU2, SV1, SV2, SW1, and SW2 based on the carrier signal, the voltage commands, and information about the state of the AC load RA1. For example, if the AC load RA1 is a three-phase 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, SW1 to the first switching elements 1U, 1V, 1W and the high level period of the control signals SU2, SV2, SW2 to the second switching elements 2U, 2V, 2W for each of the multiple switching circuits 10.

[0045] 1 to 8, 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 illustrates control signals SU1 and SU2 provided from the control device 50 to the first switching element 1U and the second switching element 2U of the switching circuit 10U when the target first switching element is the first switching element 1U of the switching circuit 10U. Also illustrated in FIG. 2 are a control signal SU6 provided from the control device 50 to the first IGBT 6U of the switch 8U, a load current iU flowing through the U-phase of the AC load RA1, a current iL1 flowing through the resonant inductor L1, a voltage V1u across the first switching element 1U, and a voltage V2u across the second switching element 2U. Also illustrated in FIG. 2 are control signals SV1 and SV2 provided from the control device 50 to the first switching element 1V and the second switching element 2V of the switching circuit 10V when the target first switching element is the first switching element 1V of the switching circuit 10V. FIG. 2 also illustrates the control signal SV6 given from 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.

[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, and 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 Tau and the additional time Tav will be described later.

[0051] FIG. 3 illustrates control signals SW1 and SW2 provided from 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. 3 also illustrates a control signal SW6 provided from the control device 50 to the first IGBT 6W of the switch 8W and a load current iW flowing through the W-phase of the AC load RA1. FIG. 3 also illustrates a current iL1 flowing through the resonant inductor L1. FIG. 3 also illustrates a voltage V1w across the first switching element 1W and a voltage V2w across the second switching element 2W. In FIG. 3, the voltage value of the DC power supply 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 1W and the second switching element 2W from being turned on simultaneously. Also, Fig. 3 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.

[0053] As shown in FIG. 2, the additional time Tau is set to advance the start point t1 of the high-level period of the control signal SU6 relative to the start point t2 of the dead time period Td, thereby making the high-level period of the control signal SU6 longer than the dead time period Td. 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 point t2 of the dead time period Td, it is desirable that the value of the current iL1 match the value of the load current iU at the start point t2 of the dead time period Td. 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 point of the high-level period of the control signal SU6 may be the same as or later than the end point t3 of the dead time period Td. FIG. 2 shows an example in which the end point of the high-level period of the control signal SU6 is set to the same as the end point t3 of the dead time period Td. The control device 50 sets the high-level period of the control signal SU6 to Tau + Td. In the switching circuit 10U, the voltage V2u across the second switching element 2U becomes Vd at the end t3 of the dead time period Td, and the voltage V1u across the first switching element 1U becomes zero at the end t3 of the dead time period Td. In the example of FIG. 2 , the current iL1 flowing through the resonant inductor L1 begins at the start t1 of the high-level period of the control signal SU6 and becomes zero at the time t4, when the additional time Tau has elapsed since the end t3 of the dead time period Td. Since iL1≧iU holds true from the start t2 of the dead time period Td, 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, generating LC resonance. After the end t3 of the dead time period Td, the current iL1 is regenerated in the power conversion circuit 11 via the third diode 13 directly connected to the resonant inductor L1.

[0054] As described above, the control device 50 determines the additional time Tau based on the load current iU so that iL1 = iU at the start time t2 of the dead time period Td, in order to start LC resonance at the start time t2 of the dead time period Td and end the resonance half cycle at the end time of the dead time period Td. 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, 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 potential V15 of the regenerative capacitor 15. The detection result of the load current iU or its signal-processed value is a detection value at the carrier cycle to which the additional time Tau is added, or at a timing closest to that carrier cycle. The estimated value of the load current iU is a value, for example, an estimate of the load current iU at 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 resonance half cycle during basic operation is set to be equal to the length of the dead time period Td, for example.

[0055] As shown in FIG. 2, the additional time Tav is set to advance the start point t5 of the high-level period of the control signal SV6 relative to the start point t6 of the dead time period Td, thereby making the high-level period of the control signal SV6 longer than the dead time period Td. 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 point t6 of the dead time period Td, it is desirable that the value of the current iL1 match the value of the load current iV at the start point t6 of the dead time period Td. 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 point of the high-level period of the control signal SV6 may be the same as or later than the end point t7 of the dead time period Td. FIG. 2 shows an example in which the end point of the high-level period of the control signal SV6 is set to the same as the end point t7 of the dead time period Td. The control device 50 sets the high-level period of the control signal SV6 to Tav + Td. The voltage V1V across the first switching element 1V becomes zero at the end t7 of the dead time period Td. In the example of FIG. 2 , the current iL1 flowing through the resonant inductor L1 begins to flow at the start t5 of the high-level period of the control signal SV6 and becomes zero at the time t8, when the additional time Tav has elapsed from the end t7 of the dead time period Td. With regard to the current iL1, since iL1≧iV holds true from the start t6 of the dead time period Td, the current iL1 in the shaded region of the current waveform in the tenth row from the top in FIG. 2 flows into the resonant capacitor 9V, causing LC resonance. After the end t7 of the dead time period Td, 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, in order to start LC resonance at the start time t6 of the dead time period Td, the control device 50 determines the additional time Tav based on the load current iV so that iL1 = iV at the start time t6 of the dead time period Td. 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 potential V15 of the regenerative capacitor 15. The detection result of the load current iV or its signal-processed value at this time is a detection value at the carrier cycle to which the additional time Tav is added, or at a timing closest to that carrier cycle. The estimated value of the load current iV at this time is, for example, a value estimated from the load current iV at the carrier cycle to which the additional time Tav is added.

[0057] As shown in FIG. 3 , the additional time Taw is set to advance the start point t9 of the high-level period of the control signal SW6 relative to the start point t10 of the dead time period Td, thereby making the high-level period of the control signal SW6 longer than the dead time period Td. 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 point t10 of the dead time period Td, it is desirable that the value of the current iL1 match the value of the load current iW at the start point t10 of the dead time period Td. 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 point of the high-level period of the control signal SW6 may be the same as or later than the end point t11 ​​of the dead time period Td. FIG. 3 illustrates an example in which the end point of the high-level period of the control signal SW6 is set to the same as the end point t11 ​​of the dead time period Td. The control device 50 sets the high-level period of the control signal SW6 to Taw+Td. The voltage V1W across the first switching element 1W becomes zero at the end t11 of the dead time period Td. In the example of FIG. 3 , the current iL1 flowing through the resonant inductor L1 begins at the start t9 of the high-level period of the control signal SW6 and becomes zero at the time t12, when the additional time Taw has elapsed since the end t11 of the dead time period Td. Since iL1≧iW holds true from the start t10 of the dead time period Td, 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 9W, generating LC resonance. After the end t11 of the dead time period Td, the current iL1 is regenerated in the power conversion circuit 11 via the third diode 13 directly connected to the resonant inductor L1.

[0058] 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 potential 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.

[0059] (3.1.2) Operation for Soft-Switching the Second Switching Element When Load Current > 0 When the soft-switching target is the second switching element 2 (hereinafter also referred to as the target second switching element 2), and 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 positive, the control device 50 compares the current value of the load current with the first current threshold I1 (=Ith, see FIG. 5 ). When the current value of the load current is greater than the first current threshold I1, the control device 50 does not turn on the switch 8, and when the current value of the load current is less than the first current threshold I1, the control device 50 turns on the switch 8 during the dead time period Td. 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.

[0060] 6 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. Also illustrated in FIG. 6 is the dead time period Td and the additional time Tau that the control device 50 sets for the control signal SU7 of the second IGBT 7U of the switch 8U.

[0061] 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 the start time t22 of the dead time period Td, and the current i9U decreases to zero before the end time t23 of the dead time period Td, causing the voltage V2u across the second switching element 2U to become zero before the end time t23 of the dead time period Td. As a result, in the power conversion device 100, when the control signal SU2 changes from low level to high level at the end time t23 of the dead time period Td, the second switching element 2U is subjected to zero-voltage soft switching.

[0062] 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. 6 . The start point of the high-level period of the control signal SU7 in this case is, for example, the same as the start point t22 of the dead time period Td. The end point of the high-level period of the control signal SU7 is the same as the end point t23 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 point t23 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 point t23 of the dead time period Td, the second switching element 2U is subjected to zero-voltage soft switching. The start point of the high-level period of the control signal SU7 may be time t21, which is earlier than the start point of the dead time period Td by an additional time Tau. The end point of the high level period of the control signal SU7 may be time t24, which is later than the end point t23 of the dead time period Td by the additional time Tau. Note that the time before and after the period in the high level period that overlaps with the dead time period Td is not limited to the additional time Tau and may be another set time.

[0063] (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.

[0064] Figure 7 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 when the target second switching element 2 is the second switching element 2U of the switching circuit 10U.

[0065] FIG. 7 also illustrates the 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. FIG. 7 also illustrates the additional time Tau set in 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 t33 of the dead time period Td. FIG. 7 illustrates an example in which the end point of the high-level period of the control signal SU7 is set to the same as the end point t33 of the dead time period Td. The control device 50 sets the high-level period of the control signal SU7 to Tau + Td. In the switching circuit 10U, the voltage V2u across the second switching element 2U becomes zero at the end point t33 of the dead time period Td. 7 , the current iL1 flowing through the resonant inductor L1 starts flowing at time t31, the start time of the high-level period of the control signal SU7, and becomes zero at time t34, when the additional time Tau has elapsed since time t33, the end time Td. With regard to the current iL1, when iL1≦iU from time t32, the start time Td, the 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 t33, the end time Td, 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.

[0066] The control device 50 determines the additional time Tau based on the load current iU so that iL1 = iU at the start time t32 of the dead time period Td, in order to start LC resonance at the start time t32 of the dead time period Td and end the resonance half cycle at the end time t33 of the dead time period Td. 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 inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the potential V15 of the regenerative capacitor 15. The detection result of the load current iU or its signal-processed value at this time uses a detection value at the carrier cycle to which the additional time Tau is added, or at a timing closest to that carrier cycle. The estimated value of the load current iU at this time uses, for example, an estimated value of the load current iU at 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 resonance half cycle during basic operation is set to be equal to the length of the dead time period Td, for example.

[0067] (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. 5 ). When the current value of the load current is smaller than the second current threshold I2, the control device 50 does not turn on the switch 8. When the current value of the load current is larger than the second current threshold I2, the control device 50 turns on the switch 8 during the dead time period Td. 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 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.

[0068] 8 illustrates 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 when 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, when the absolute value of the current value of the load current is smaller than the absolute value of the second current threshold I2). Also illustrated in FIG. 8 is the dead time period Td.

[0069] 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 the start time t41 of the dead time period Td. 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 the end time t23 of the dead time period Td, and the voltage V1u across the first switching element 1U becomes zero before the end time t42 of the dead time period Td. As a result, in the power conversion device 100, the first switching element 1U is subjected to zero-voltage soft switching when the control signal SU1 changes from low to high at the end time t42 of the dead time period Td.

[0070] When the current value of the load current is greater than the second current threshold I2 (in other words, when the absolute value of the load current is smaller than the absolute value of the second current threshold), the control device 50 sets a high-level period of the control signal SU6, as shown by the two-dot chain line in FIG. 8 . The start point of the high-level period of the control signal SU6 at this time is the same as the start point t41 of the dead time period Td. The end point of the high-level period of the control signal SU6 is the same as the end point t42 of the dead time period Td. As a result, in the power conversion device 100, the voltage V1u across the first switching element 1U becomes zero before the end point t42 of the dead time period Td. Therefore, in the power conversion device 100, when the control signal SU1 changes from low to high at the end point t42 of the dead time period Td, the first switching element 1U is subjected to zero-voltage soft switching.

[0071] (3.2) Operation of Shift Control When the control device 50 determines that resonant currents passing through two or more switches 8 out of the multiple switches 8 flow simultaneously through the resonant inductor L1, the control device 50 performs shift control to shift the high-level period of the control signal to at least one switch 8 out of the two or more switches 8 so that the resonant currents passing through the two or more switches 8 do not flow simultaneously through the resonant inductor L1. "When it is determined that resonant currents passing through two or more switches 8 out of the multiple switches 8 will flow simultaneously" means that it has been estimated in advance that resonant currents passing through two or more switches 8 will flow simultaneously through the resonant inductor L1.

[0072] (3.2.1) Determination of whether two-phase resonant currents flow simultaneously In the power conversion device 100, the phases of the voltage commands for three phases (U phase, V phase, and W phase) differ from each other by 120°, but the command values ​​of the voltage commands 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 areas A1 and A2 in FIG. 4 ). In area A1 in FIG. 4 , the duties of the control signals for the U phase and the V phase are near 0.75. In area A2 in FIG. 4 , the duties of the control signals for the U phase and the V phase are near 0.25. The polarity of the resonant current is the same as the polarity of the current iL1; in area A1, the polarity of the resonant current is positive, and in area A2, the polarity of the resonant current is negative. In the case of region A1, for example, during one cycle of the carrier signal, the time difference between start time t1 (see FIG. 2) of the high-level period of the control signal SU6 supplied to the first IGBT 6U and start time t5 (see FIG. 2) of the high-level period of the control signal SV6 supplied to the first IGBT 6V becomes short, and there is a possibility that the U-phase resonant current and the V-phase resonant current will flow simultaneously through the resonant inductor L1. In the power conversion device 100, in the case of region A2, the direction of the resonant current is opposite to that in region A1, but there is a possibility that the U-phase resonant current and the V-phase resonant current will flow simultaneously through the resonant inductor L1.

[0073] 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.

[0074] (3.2.1.1) In the case of charging the resonant capacitor Figure 2 is a diagram showing an example of the boundary conditions between the case where the U-phase resonant current and the V-phase resonant current do not overlap (flow simultaneously) and the case where they overlap (flow simultaneously). The boundary conditions will be described with reference to Figure 2.

[0075] In the power conversion device 100, if the time difference ΔTuv between the start point t3 of the high-level period of the control signal SU1 and the start point t7 of the high-level period of the control signal SV1 is equal to or greater than (Tau + Tav + Td), the U-phase resonant current and the V-phase resonant current do not overlap, and if the time difference ΔTuv is less than (Tau + Tav + Td), the U-phase resonant current and the V-phase resonant current overlap. The control device 50 sets a threshold for the time difference ΔTuv to, for example, (Tau + Tav + Td), and if the time difference ΔTuv is less than the threshold, it estimates that resonant currents corresponding to two phases, switching circuit 10U and switching circuit 10V, of the multiple switching circuits 10, will flow simultaneously through the resonant inductor L1. The above threshold setting is an example, and other values ​​can also be considered. For example, it is conceivable to set the threshold value to a value even greater than (Tau + Tav + Td) in consideration of errors in the additional times Tau and Tav. 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. For example, the time difference ΔTuv used to determine whether two-phase resonant currents flow simultaneously may be the time difference between the end point t2 of the high-level period of the control signal SU2 and the end point t6 of the high-level period of the control signal SV2.

[0076] Furthermore, in the power conversion device 100, if the time difference between the start time t3 of the high-level period of the control signal SU1 and the start time t11 of the high-level period of the control signal SW1 is equal to or greater than (Tau + Taw + Td), the U-phase resonant current and the W-phase resonant current do not overlap. If the time difference is less than (Tau + Taw + Td), the U-phase resonant current and the W-phase resonant current overlap. The control device 50 sets a threshold for the time difference to, for example, (Tau + Taw + Td). If the time difference is less than the threshold, the control device 50 estimates that resonant currents corresponding to two phases, the switching circuit 10U and the switching circuit 10W, of the multiple switching circuits 10 simultaneously flow through the resonant inductor L1. The above threshold setting is merely an example, and other values ​​may also be used. For example, the threshold may be set to a value greater than (Tau + Taw + Td) in consideration of errors in the additional time Tau and the additional time Taw. Furthermore, the method of calculating the time difference 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. For example, the time difference used to determine whether two-phase resonant currents flow simultaneously may be the time difference between the end point t2 of the high-level period of the control signal SU2 and the end point t10 of the high-level period of the control signal SW2.

[0077] Furthermore, in the power conversion device 100, if the time difference between the start point t7 of the high-level period of the control signal SV1 provided to the first switching element 1V of the switching circuit 10V and the start point t11 ​​of the high-level period of the control signal SW1 provided to the first switching element 1W of the switching circuit 10W is (Tav + Taw + Td) or more, the V-phase resonant current and the W-phase resonant current do not overlap, and if the time difference is less than (Tav + Taw + Td), the V-phase resonant current and the W-phase resonant current overlap. The control device 50 sets a threshold for the time difference to, for example, (Tav + Taw + Td), and if the time difference is less than the threshold, it estimates that resonant currents corresponding to two phases, the switching circuit 10V and the switching circuit 10W, of the multiple switching circuits 10 will flow simultaneously through the resonant inductor L1. The above threshold setting is an example, and other values ​​can also be considered. For example, it is possible to set the threshold value to a value greater than (Tav + Taw + Td) in consideration of errors in the additional times Tav and Taw. Furthermore, the method for calculating the time difference 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. For example, the time difference used to determine whether two-phase resonant currents flow simultaneously may be the time difference between the end point t6 of the high-level period of the control signal SV2 and the end point t10 of the high-level period of the control signal SW2.

[0078] (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.

[0079] For example, the control device 50 estimates that the U-phase resonant current and the V-phase resonant current overlap if the time difference between the start point of the high-level period of the control signal SU2 and the start point of the high-level period of the control signal SV2 is less than a threshold value (e.g., Tau + Tav + Td).

[0080] In addition, the control device 50 estimates that the U-phase resonant current and the W-phase resonant current overlap if the time difference between the start point of the high-level period of the control signal SU2 and the start point of the high-level period of the control signal SW2 is less than a threshold value (e.g., Tau + Taw + Td).

[0081] In addition, the control device 50 estimates that the V-phase resonant current and the W-phase resonant current overlap 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., Tav + Taw + Td).

[0082] (3.2.2) Shift Control When It is Determined that Two-Phase Resonant Currents Flow Simultaneously The control device 50 performs shift control to shift the high-level periods of the control signals to the two switches 8, for example, so that the resonant currents passing through the two switches 8 respectively do not flow simultaneously through the resonant inductor L1.

[0083] When performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 so as not to change the lengths of the high-level periods of the control signals supplied to the first switching element 1 and the second switching element 2 of the two switching circuits 10 corresponding to the two switches 8. For example, when shifting the high-level period of the control signal SU6 or SU7 supplied to the switch 8U, the control device 50 shifts the high-level periods of the control signal SU1 and the control signal SU2, but does not change the duties of the control signal SU1 and the control signal SU2 in one period of the carrier signal. Furthermore, when shifting the high-level period of the control signal SV6 or SV7 supplied to the switch 8V, the control device 50 shifts the high-level periods of the control signal SV1 and the control signal SV2, but does not change the duties of the control signal SV1 and the control signal SV2 in one period of the carrier signal. Furthermore, when shifting the high-level period of the control signal SW6 or SW7 provided to the switch 8W, the control device 50 shifts the high-level periods of the control signal SW1 and the control signal SW2, but does not change the duty cycle of each of the control signals SW1 and SW2 in one cycle of the carrier signal. Hereinafter, for convenience of explanation, the shift time of the high-level period of the control signal SU6 or SU7 provided to the switch 8U when shifting the high-level period of the control signal SU6 or SU7 is referred to as Tus. Furthermore, the shift time of the high-level period of the control signal SV6 or SV7 provided to the switch 8V when shifting the high-level period of the control signal SW6 or SW7 is referred to as Tvs. Furthermore, the shift time of the high-level period of the control signal SW6 or SW7 provided to the switch 8W when shifting the high-level period of the control signal SW6 or SW7 is referred to as Tws.

[0084] (3.2.2.1) Operation for Soft-Switching First Switching Element When performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 in different directions. When performing shift control, the control device 50 first compares the duties of the control signals for two first switching elements 1 corresponding to the two switches 8 among the multiple first switching elements 1 when the polarity of the load current flowing through each of the two AC terminals 41 connected to the two switches 8 is positive. The control device 50 then advances the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively large duty, and delays the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively small duty.

[0085] Fig. 9 shows an example of operation when shift control is performed in a period corresponding to region A1 in Fig. 4. The upper part of Fig. 9 shows a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, and the current iL1 before a shift (when shift control is not performed) when the control device 50 determines that two-phase resonant currents, U-phase and V-phase, flow simultaneously. The lower part of Fig. 9 shows a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, and the current iL1 when the control signals SU6 and SV6 are shifted so that the total shift time of the high-level period of the control signal SU6 and the high-level period of the control signal SV6 is a predetermined period. In the example of FIG. 9, the control device 50 shifts the start point of the high-level period of the control signal SU6 to the switch 8U by a shift time Tus, and shifts the high-level period of the control signal SV6 to the switch 8V by a shift time Tvs.

[0086] The control device 50 sets the predetermined period to a length equal to or longer than the overlap time Tov of the two-phase resonant currents when it is determined that the two-phase resonant currents flow simultaneously. The overlap time Tov will be described with reference to FIG. 10. FIG. 10 is an explanatory diagram illustrating the waveform of the resonant current when the resonant inductor L1 is not shared (i.e., when three resonant inductors L1 are provided in one-to-one correspondence with the three resonant capacitors). The overlap time Tov between the U-phase resonant current (fourth row from the top in FIG. 10) and the V-phase resonant current (eighth row from the top in FIG. 10) is calculated by the formula Tov=(Tau+Tav+Td)-ΔTuv. If Tres is the resonance period of the resonant circuit formed by the inductance L of the resonant inductor L1 and the capacitance C of one resonant capacitor 9, Tres=1 / {2π(L·C) １／２}, where Td=Tres / 2. The resonant current in the twelfth row from the top of Figure 10 shows the waveform of the V-phase resonant current when the high-level periods of the control signal SV1 in the fifth row from the top, the control signal SV2 in the sixth row, and the control signal SV6 in the seventh row from the top of Figure 10 are shifted in a direction to delay the overlap time Tov. It can be seen from Figure 10 that the resonant current in the twelfth row from the top of Figure 10 avoids overlapping with the resonant current in the fourth row from the top of Figure 10. Figure 10 shows a case where the U-phase resonant current and the V-phase resonant current overlap, but the methods for determining the overlap time Tov and the shift direction of the control signals are similar in both cases where the U-phase resonant current and the W-phase resonant current overlap and where the V-phase resonant current and the W-phase resonant current overlap.

[0087] 9 shows an example in which the control device 50 sets the predetermined period to Tov+ΔT. That is, in the example in FIG. 9, the control device 50 sets the predetermined period to Tov+ΔT=Tus+Tvs.

[0088] 9, the polarities of the load currents iU and iV flowing through the two AC terminals 41U and 41V connected to the two switches 8U and 8V are positive. In this case, the control device 50 compares the duties of the control signals SU1 and SV1 for the two first switching elements 1U and 1V that correspond one-to-one to the two switches 8U and 8V. The control device 50 then advances the high-level period of the control signal SV6 provided to the switch 8V corresponding to the first switching element 1V that provides the control signal SV1 with a relatively large duty by the shift time Tvs, and delays the high-level period of the control signal SU6 provided to the switch 8U corresponding to the first switching element 1U that provides the control signal SU1 with a relatively small duty by the shift time Tus.

[0089] As can be seen from the waveform of current iL1 in Figure 9, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the V-phase resonant current from overlapping (see current iL1 in the lower part of Figure 9). Similarly, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the W-phase resonant current from overlapping. Furthermore, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shift control to prevent the V-phase resonant current and the W-phase resonant current from overlapping. The upper limit (maximum value) of the shift time when the high-level period of the control signal to switch 8 is shifted earlier is the shift time when the time between the start of one cycle of the carrier signal and the start of the high-level period of the shifted control signal (the control signal to switch 8 after the shift) is minimized (for example, 0) without changing the length of the high-level period. Also, the upper limit (maximum value) of the shift time when the high-level period of the control signal to switch 8 is shifted later is the shift time when the time between the end of one cycle of the carrier signal and the end of the high-level period of the shifted control signal (the control signal to switch 8 after the shift) is minimized (for example, 0) without changing the length of the high-level period.

[0090] In the power conversion device 100, when the control device 50 does not execute shift 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 end of the dead time period Td corresponding to the U phase and the V phase, respectively). In other words, when the control device 50 does not execute shift control, charging of the resonant capacitors 9U and 9V does not finish at the end of the dead time period Td corresponding to the U phase and the V phase, respectively. Therefore, when the shift control is not executed, the voltages across the first switching elements 1U and 1V do not decrease to zero at the end of the dead time period Td corresponding to the U phase and the V phase, respectively. As a result, in the power conversion device 100, the switching of the first switching elements 1U and 1V becomes hard switching.

[0091] In contrast, when the control device 50 executes shift control, as shown in Fig. 9, the voltages V2u and V2v across the second switching elements 2U and 2V 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 end of the dead time period Td corresponding to the U phase and the V phase, respectively). In other words, when the control device 50 executes shift control, charging of the resonant capacitors 9U and 9V ends at the end of the dead time period Td corresponding to the U phase and the V phase, respectively. For this reason, in the power conversion device 100, when the control device 50 executes shift control, the switching of the first switching elements 1U and 1V becomes zero-voltage soft switching.

[0092] 9 above shows an example of shift control when the control device 50 determines in advance that a U-phase resonant current and a V-phase resonant current will simultaneously flow through the resonant inductor L1, but this is not limiting. For example, when the control device 50 determines in advance that a V-phase resonant current and a W-phase resonant current will simultaneously flow through the resonant inductor L1, the control device 50 also performs shift control in cases where it has determined in advance that a W-phase resonant current and a U-phase resonant current will simultaneously flow through the resonant inductor L1, thereby enabling zero-voltage soft switching.

[0093] (3.2.2.2) Operation for Soft-Switching the Second Switching Element When performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 in different directions. When performing shift control, the control device 50 first compares the duties of the control signals for two first switching elements 1 corresponding to the two switches 8 among the multiple first switching elements 1 when the polarity of the load current flowing through each of the two AC terminals 41 connected to the two switches 8 is negative. Then, the control device 50 shifts later the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively large duty, and shifts earlier the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively small duty.

[0094] Fig. 11 shows an example of operation when shift control is performed in a period corresponding to region A2 in Fig. 4. The upper part of Fig. 11 shows a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and the current iL1 before the shift (when shift control is not performed) when the control device 50 determines that two-phase resonant currents, U-phase and V-phase, flow simultaneously. The lower part of Fig. 11 shows a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU7, SV7, the load currents iU, iV, and the current iL1 when the control signals SU7 and SV7 are shifted so that the total shift time of the high-level period of the control signal SU7 and the high-level period of the control signal SV7 is a predetermined period. In the example of FIG. 11, the control device 50 shifts the start point of the high level period of the control signal SU7 to the switch 8U by a shift time Tus, and shifts the high level period of the control signal SV7 to the switch 8V by a shift time Tvs.

[0095] The control device 50 sets the predetermined period to a length equal to or longer than the overlap time Tov of the two-phase resonant currents when it is determined that the two-phase resonant currents flow simultaneously.

[0096] 11 shows an example in which the control device 50 sets the predetermined period to Tov+ΔT. That is, in the example in FIG. 11, the control device 50 sets the predetermined period to Tov+ΔT=Tus+Tvs.

[0097] 11 , the polarity of the load currents iU, iV flowing through the two AC terminals 41U, 41V connected to the two switches 8U, 8V is negative. In this case, the control device 50 compares the duties of the control signals SU1, SV1 for the two first switching elements 1U, 1V that correspond one-to-one to the two switches 8U, 8V. The control device 50 then delays, by a shift time Tvs, the high-level period of the control signal SV7 provided to the switch 8V corresponding to the first switching element 1V that provides the control signal SV1 with a relatively large duty, and shifts, by a shift time Tus, the high-level period of the control signal SU7 provided to the switch 8U corresponding to the first switching element 1U that provides the control signal SU1 with a relatively small duty.

[0098] As can be seen from the waveform of current iL1 in Figure 11 , in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the V-phase resonant current from overlapping (see current iL1 in the lower part of Figure 11 ). Similarly, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the W-phase resonant current from overlapping. Furthermore, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shift control to prevent the V-phase resonant current and the W-phase resonant current from overlapping.

[0099] In the power conversion device 100, when the control device 50 does not execute shift control, the voltages V1u and V1v across the first switching elements 1U and 1V do not rise to Vd at the time when the control signals SU2 and SV2 change from a low-level period to a high-level period (the end of the dead time period Td corresponding to the U phase and the V phase, respectively). In other words, when the control device 50 does not execute shift control, the discharge of the resonant capacitors 9U and 9V does not finish at the end of the dead time period Td corresponding to the U phase and the V phase, respectively. Therefore, when the shift control is not executed, the voltages across the second switching elements 2U and 2V do not decrease to zero at the end of the dead time period Td corresponding to the U phase and the V phase, respectively. As a result, in the power conversion device 100, the switching of the second switching elements 2U and 2V becomes hard switching.

[0100] In contrast, when the control device 50 executes shift control, as shown in Fig. 11, the voltages V1u and V1v across the first switching elements 1U and 1V rise to Vd at the point in time when the control signals SU2 and SV2 change from a low-level period to a high-level period (the end point of the dead time period Td corresponding to the U phase and the V phase, respectively). In other words, when the control device 50 executes shift control, the discharge of the resonant capacitors 9U and 9V ends at the end point of the dead time period Td corresponding to the U phase and the V phase, respectively. For this reason, in the power conversion device 100, when the control device 50 executes shift control, the switching of the second switching elements 2U and 2V becomes zero-voltage soft switching.

[0101] 11 shows an example of shift control when the control device 50 determines in advance that a U-phase resonant current and a V-phase resonant current will simultaneously flow through the resonant inductor L1, but this is not limiting. For example, when the control device 50 determines in advance that a V-phase resonant current and a W-phase resonant current will simultaneously flow through the resonant inductor L1, the control device 50 also performs shift control in a case where it has determined in advance that a W-phase resonant current and a U-phase resonant current will simultaneously flow through the resonant inductor L1, thereby enabling zero-voltage soft switching.

[0102] (4) Summary In the power conversion device 100 according to the first embodiment, when the control device 50 determines that resonant currents passing through two of the multiple switches 8 respectively flow through the resonant inductor L1 simultaneously, the control device 50 performs control to shift the high-level periods of the control signals to the two switches 8 respectively so that the resonant currents passing through the two switches 8 respectively do not flow through the resonant inductor L1 simultaneously. This enables the power conversion device 100 to more reliably achieve soft switching.

[0103] Furthermore, in the power conversion device 100 according to the first embodiment, when performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 so as to maintain the lengths of the high-level periods of the control signals to be applied to the first switching element 1 and the second switching element 2 of the switching circuit 10 connected to the two switches 8 among the multiple switching circuits 10. This enables the power conversion device 100 according to the first embodiment to suppress changes in the line voltage.

[0104] Furthermore, in the power conversion device 100 according to the first embodiment, when performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 in different directions. This allows the power conversion device 100 according to the first embodiment to achieve a higher frequency.

[0105] Furthermore, in the power conversion device 100 according to the first embodiment, when performing shift control, the control device 50 compares the duties of the control signals for the two first switching elements 1 corresponding to the two switches 8 when the polarity of the load current flowing through each of the two AC terminals 41 connected to the two switches 8 is positive. The control device 50 then advances the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively large duty, and shifts the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively small duty in a later direction. Furthermore, the control device 50 compares the duties of the control signals for the two first switching elements 1 corresponding to the two switches 8 when the polarity of the load current flowing through each of the two AC terminals 41 connected to the two switches 8 is negative. The control device 50 then delays the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides the control signal with a relatively large duty, and shifts the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides the control signal with a relatively small duty, thereby enabling the power conversion device 100 according to the first embodiment to achieve higher frequencies.

[0106] 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.

[0107] In the power conversion device 100 according to the second embodiment, the operation of the shift control of the control device 50 when it is determined that the two-phase resonant currents overlap is different from the operation of the control device 50 according to the first embodiment.

[0108] In the following, when the control device 50 determines that two-phase resonant currents are flowing simultaneously through the resonant inductor L1, the operation of the control device 50 for soft-switching the first switching element 1 will be explained based on Figures 12 and 13, and the operation of the control device 50 for soft-switching the second switching element 2 will be explained based on Figure 14.

[0109] (1.1) Operation for Soft-Switching First Switching Element When performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 in different directions. When performing shift control, the control device 50 first compares the duties of the control signals for two first switching elements 1 corresponding to the two switches 8 among the multiple first switching elements 1 when the polarity of the load current flowing through each of the two AC terminals 41 connected to the two switches 8 is positive. The control device 50 then delays the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively large duty, and advances the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively small duty.

[0110] Fig. 12 shows an example of operation when shift control is performed in a period corresponding to region A1 in Fig. 4. The upper part of Fig. 12 shows a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, and the current iL1 before a shift (when shift control is not performed) when the control device 50 determines that two-phase resonant currents, U-phase and V-phase, flow simultaneously. The lower part of Fig. 12 shows a timing chart of the control signals SU1, SU2, SV1, SV2, the control signals SU6, SV6, the load currents iU, iV, and the current iL1 when the control signals SU6 and SV6 are shifted so that the total shift time of the high-level period of the control signal SU6 and the high-level period of the control signal SV6 is a predetermined period. In the example of FIG. 12, the control device 50 shifts the start point of the high-level period of the control signal SU6 to the switch 8U by a shift time Tus, and shifts the high-level period of the control signal SV6 to the switch 8V by a shift time Tvs.

[0111] The control device 50 sets the predetermined period as ΔTuv+Ti+ΔT, where Ti is the resonant current time and ΔT is the time margin. The additional time during the high-level period of the switch 8 connected to the AC terminal 41 with the larger absolute value of the load current of the two switches 8 is set as Ta, and the resonant current time Ti is calculated using the formula 2×Ta+Tres / 2. Figure 12 illustrates the resonant current time Tiu of the U-phase and the resonant current time Tiv of the V-phase. In the example of Figure 12, the absolute value of the V-phase load current iV is larger than the absolute value of the U-phase load current iU, so the resonant current time Ti is set as the resonant current time Tiv of the V-phase, and the additional time Ta is set as the additional time Tav during the high-level period of the switch 8V. FIG. 12 shows a case where the U-phase resonant current and the V-phase resonant current overlap, but the method for determining the predetermined period and the method for determining the shift direction of the control signal are the same in both cases where the U-phase resonant current and the W-phase resonant current overlap, and where the V-phase resonant current and the W-phase resonant current overlap.

[0112] 12 shows an example in which the control device 50 sets the predetermined period to ΔTuv+(2×Tav+Tres / 2)+ΔT. That is, in the example in FIG. 12, the control device 50 sets the predetermined period to ΔTuv+(2×Tav+Tres / 2)+ΔT.

[0113] 12, the polarities of the load currents iU and iV flowing through the two AC terminals 41U and 41V connected to the two switches 8U and 8V are positive. In this case, the control device 50 compares the duties of the control signals SU1 and SV1 for the two first switching elements 1U and 1V that correspond one-to-one to the two switches 8U and 8V. The control device 50 then delays, by a shift time Tvs, the high-level period of the control signal SV6 provided to the switch 8V corresponding to the first switching element 1V that provides the control signal SV1 with a relatively large duty, and shifts, by a shift time Tus, the high-level period of the control signal SU6 provided to the switch 8U corresponding to the first switching element 1U that provides the control signal SU1 with a relatively small duty. Note that Figure 13 shows that when the total time of the shift time for shifting the high level period of the control signal SU6 in the direction of advancing it and the shift time for shifting the high level period of the control signal SV6 in the direction of delaying it is set to ΔTuv and the end 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 are made to coincide with each other, the resonant current of the U phase and the resonant current of the V phase overlap.

[0114] As can be seen from the waveform of current iL1 in Figure 12, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the V-phase resonant current from overlapping (see current iL1 in the lower part of Figure 12). Similarly, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the W-phase resonant current from overlapping. Furthermore, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shift control to prevent the V-phase resonant current and the W-phase resonant current from overlapping. The upper limit (maximum value) of the shift time when the high-level period of the control signal to switch 8 is shifted in a later direction is the shift time when the time between the end of one cycle of the carrier signal and the end of the high-level period of the shifted control signal (the control signal to switch 8 after the shift) is zero. Also, the upper limit (maximum value) of the shift time when the high-level period of the control signal to switch 8 is shifted in an earlier direction is the shift time when the time between the start of one cycle of the carrier signal and the start of the high-level period of the shifted control signal (the control signal to switch 8 after the shift) is zero.

[0115] In the power conversion device 100 according to the second embodiment, when the control device 50 determines that two-phase resonant currents flow simultaneously through the resonant inductor L1 and executes shift control, overlapping of the resonant currents is avoided as shown in Fig. 12, and therefore charging of the resonant capacitors 9U and 9V ends at the end of the dead time periods Td corresponding to the U phase and the V phase, respectively. Therefore, in the power conversion device 100, when the control device 50 executes shift control, the switching of the first switching elements 1U and 1V becomes zero-voltage soft switching.

[0116] 12 shows an example of shift control when the control device 50 determines in advance that a U-phase resonant current and a V-phase resonant current will simultaneously flow through the resonant inductor L1, but this is not limiting. For example, when the control device 50 determines in advance that a V-phase resonant current and a W-phase resonant current will simultaneously flow through the resonant inductor L1, the control device 50 also performs shift control in a case where it has determined in advance that a W-phase resonant current and a U-phase resonant current will simultaneously flow through the resonant inductor L1, thereby enabling zero-voltage soft switching.

[0117] (1.2) Operation for Soft-Switching the Second Switching Element When performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 in different directions. When performing shift control, the control device 50 first compares the duties of the control signals for two first switching elements 1 corresponding to the two switches 8 among the multiple first switching elements 1 when the polarity of the load current flowing through each of the two AC terminals 41 connected to the two switches 8 is negative. The control device 50 then advances the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively large duty, and delays the high-level period of the control signal provided to the switch 8 corresponding to the first switching element 1 that provides a control signal with a relatively small duty.

[0118] Fig. 14 shows an example of operation when shift control is performed in a period corresponding to region A2 in Fig. 4. The upper part of Fig. 14 shows a timing chart of the control signals SU1, SU2, SV1, SV2, control signals SU7, SV7, load currents iU, iV, and current iL1 before shifting (when shift control is not performed) when the control device 50 determines that two-phase resonant currents, U-phase and V-phase, flow simultaneously. The lower part of Fig. 14 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 signals SU7 and SV7 are shifted so that the total shift time of the high-level period of the control signal SU7 and the high-level period of the control signal SV7 is a predetermined period. In the example of FIG. 14, the control device 50 shifts the start point of the high-level period of the control signal SU7 to the switch 8U by a shift time Tus, and shifts the high-level period of the control signal SV7 to the switch 8V by a shift time Tvs.

[0119] In the example of FIG. 14, the control device 50 sets the predetermined period to ΔTuv+(2×Tau+Tres / 2)+ΔT.

[0120] 14, the polarity of the load currents iU, iV flowing through the two AC terminals 41U, 41V connected to the two switches 8U, 8V is negative. In this case, the control device 50 compares the duties of the control signals SU1, SV1 for the two first switching elements 1U, 1V that correspond one-to-one to the two switches 8U, 8V. The control device 50 then shifts the high-level period of the control signal SV7 provided to the switch 8V corresponding to the first switching element 1V that provides the control signal SV1 with a relatively large duty, in an earlier direction by the shift time Tvs, and shifts the high-level period of the control signal SU7 provided to the switch 8U corresponding to the first switching element 1U that provides the control signal SU1 with a relatively small duty, in a later direction by the shift time Tus.

[0121] As can be seen from the waveform of current iL1 in Figure 14, in the power conversion device 100, if the control device 50 determines in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the V-phase resonant current from overlapping (see current iL1 in the lower part of Figure 14). Similarly, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, U-phase and W-phase, will flow simultaneously, it performs shift control to prevent the U-phase resonant current and the W-phase resonant current from overlapping. Furthermore, in the power conversion device 100, if the control device 50 determines that two-phase resonant currents, V-phase and W-phase, will flow simultaneously, it performs shift control to prevent the V-phase resonant current and the W-phase resonant current from overlapping.

[0122] In the power conversion device 100, when the control device 50 executes shift control, the discharge of the resonant capacitors 9U, 9V ends at the time when the control signals SU2, SV2 change from a low level period to a high level period (the end time of the dead time period Td corresponding to the U phase and the V phase, respectively) as shown in Fig. 14. Therefore, in the power conversion device 100, when the control device 50 executes shift control, the switching of the second switching elements 2U, 2V becomes zero-voltage soft switching.

[0123] 14 described above shows an example of shift control when the control device 50 determines in advance that a U-phase resonant current and a V-phase resonant current will simultaneously flow through the resonant inductor L1, but this is not limiting. For example, when the control device 50 determines in advance that a V-phase resonant current and a W-phase resonant current will simultaneously flow through the resonant inductor L1, the control device 50 also performs shift control in a case where it has determined in advance that a W-phase resonant current and a U-phase resonant current will simultaneously flow through the resonant inductor L1, thereby enabling zero-voltage soft switching.

[0124] (Modification of Embodiment 2) In a power conversion device 100 according to a modification of Embodiment 2, the control device 50 is configured to combine the shift control of the control device 50 of Embodiment 2 with the shift control of the control device 50 of Embodiment 1, and to execute the shift control of the control device 50 of Embodiment 2 and the shift control of the control device 50 of Embodiment 1 alternately or at an arbitrary ratio. As a result, the power conversion device 100 according to the modification can reduce the bias of the fluctuation of the ripple of the line voltage compared to the power conversion device 100 according to Embodiment 1 and the power conversion device 100 according to Embodiment 2. Furthermore, the power conversion device 100 according to the modification can distribute the period during which a resonant current flows through the resonant inductor L1 compared to the power conversion device 100 according to Embodiment 1 and the power conversion device 100 according to Embodiment 2, and can reduce the thermal burden on the resonant inductor L1.

[0125] 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.

[0126] In the power conversion device 100 according to the third embodiment, the shift control operation that the control device 50 performs when it determines that the three-phase resonant currents overlap is different from the shift control operation of the control device 50 according to the first embodiment.

[0127] 15 and 17 , the operation of the control device 50 will be described when it is determined that resonant currents flowing through three of the switches 8 simultaneously through the resonant inductor L1 are passing through three of the switches 8. The control device 50 executes shift control when it is determined that resonant currents flowing through three of the switches 8 simultaneously through the resonant inductor L1. The phrase "when it is determined that resonant currents flowing through three of the switches 8 simultaneously" refers to a case where it has been estimated in advance that resonant currents flowing through the three switches 8 simultaneously through the resonant inductor L1. An example of a case where three-phase resonant currents overlap is when the AC load RA1 is in a light load state, where iU = 0, iV = 0, and iW = 0. This situation occurs, for example, when the AC load RA1 is a motor, particularly when the motor is rotating at a low speed or when the motor's rotational speed is zero (e.g., when the motor is locked). Therefore, the control device 50 determines that "three-phase resonant currents flow simultaneously" when, for example, the rotational speed of the motor (e.g., the number of rotations [rpm]) falls below a rotational speed threshold. Here, the control device 50 determines that "three-phase resonant currents flow simultaneously" when, for example, the rotational speed calculated from sensor information output from a sensor device (e.g., an encoder or resolver) for detecting the rotational speed of the motor, or the estimated rotational speed, falls below the rotational speed threshold.

[0128] Assuming that the capacitance of each of the multiple resonant capacitors 9U, 9V, and 9W is C, if a U-phase current, a V-phase current, and a W-phase current 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 will change compared to when a single-phase current flows through the resonant inductor L1, and zero-voltage soft switching may not be achieved.

[0129] When the control device 50 determines that the resonant currents passing through the three switches 8 respectively flow simultaneously through the resonant inductor L1, it performs shift control to shift the high level periods of the control signals to two of the three switches 8 so that the resonant currents passing through the three switches 8 respectively do not flow simultaneously through the resonant inductor L1.

[0130] When performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 so that the lengths of the high-level periods of the control signals to the first switching element 1 and the second switching element 2 of the switching circuit 10 connected to the two switches 8 among the multiple switching circuits 10 do not change.

[0131] When performing shift control, the control device 50 targets the control signals to any two of the three switches 8 as the shift targets, and shifts the high-level periods of the two switches 8 targeted for shifting in different directions so that the resonant currents passing through the three switches 8 do not simultaneously flow through the resonant inductor L1.

[0132] (1) Operation of the Power Conversion Device (1.1) Basic Operation The basic operation of the control device 50 is the same as that of the first embodiment, and therefore a description thereof will be omitted.

[0133] (1.2) Shift Control (1.2.1) Operation for Soft-Switching the First Switching Element Fig. 15 shows a timing chart of the power conversion device 100 when the control device 50 executes shift control in a case where the control device 50 determines that three-phase resonant currents of the U phase, the V phase, and the W phase flow simultaneously when the polarity of the current iL1 is positive, and Fig. 16 shows a timing chart of the power conversion device 100 when the control device 50 does not execute shift control. Note that Figs. 15 and 16 show timing charts for a part of one period within one cycle of the carrier signal.

[0134] 15 and 16 each show timing charts of the control signals SU1, SU2, SV1, SV2, SW1, SW2, SU6, SV6, and SW6, the load currents iU and iV, the current iL1, and the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W. The control device 50 sets the length of the high-level period of each of the control signals SU6, SV6, and SW6 to a half resonance period (Tre / 2), and sets the dead time period Td to a length equal to the half resonance period. Note that, as shown in FIGS. 15 and 16, when the load currents iU, iV, and iW are 0, the control device 50 sets the additional times Tau, Tav, and Taw described in the first embodiment to 0.

[0135] 15 , the control device 50 targets the control signals SU6 and SW6 for the two switches 8U and 8W, respectively, for shifting. However, this is not limited to this. In the example of FIG. 15 , the control device 50 shifts the high-level period of the control signal SU6 for the switch 8U by a first shift time T1 in a direction that advances the start point of the high-level period of the control signal SU6. This prevents overlapping of resonant currents between a combination of one of the two switches 8 targeted for shifting and another switch 8 that is not targeted for shifting (here, so that the U-phase resonant current and the V-phase resonant current do not flow simultaneously). Furthermore, the control device 50 shifts the high-level period of the control signal SW6 for the switch 8W by a second shift time T2 in a direction that delays the start point of the high-level period of the control signal SW6. This prevents overlapping of resonant currents between a combination of the remaining one of the two switches 8 targeted for shifting and the other two switches 8 (here, so that the W-phase resonant current does not flow simultaneously with both the U-phase resonant current and the V-phase resonant current). In FIG. 15, the state before the high level period of the control signals SU6 and SW6 is shifted (that is, the state in FIG. 16) is shown by a two-dot chain line, and the state after the shift is shown by a solid line.

[0136] The example of FIG. 15 shows a case where the control device 50 sets the first shift time T1 to T1 = Tres / 2 + Δ. The example of FIG. 15 also shows a case where the control device 50 sets T2 = Tres / 2 + Δ. Δ is a time margin for more reliably avoiding overlap of the two-phase resonant currents. Note that in the example of FIG. 15, the high-level period of the control signal SU6 to the switch 8U is shifted by the first shift time T1 in a direction to advance the start point of the high-level period of the control signal SU6, and the high-level period of the control signal SW6 to the switch 8W is shifted by the second shift time T2 in a direction to delay the start point of the high-level period of the control signal SW6 to the switch 8W. However, this is not limiting. For example, the high-level period of the control signal SW6 to the switch 8W may be shifted by the first shift time T1 in a direction to advance the start point of the high-level period of the control signal SW6, and the high-level period of the control signal SU6 to the switch 8U may be shifted by the second shift time T2 in a direction to delay the start point of the high-level period of the control signal SU6 to the switch 8U. Furthermore, the combination of two switches 8 to be shifted is not limited to the combination of switches 8U and 8W, but may be the combination of switches 8U and 8V, or the combination of switches 8V and 8W.

[0137] In the power conversion device 100 according to the third embodiment, the first shift time T1 and the second shift time T2 are each a predetermined period. The upper limit (maximum) of the first shift time T1 when the high-level period of the control signal to the switch 8 is shifted earlier is the shift time when the time between the start of one cycle of the carrier signal and the start of the high-level period of the shifted control signal (the control signal to the switch 8 after the shift) is minimized (for example, 0) without changing the length of the high-level period. Furthermore, the upper limit (maximum) of the second shift time T2 when the high-level period of the control signal to the switch 8 is shifted later is the shift time when the time between the end of one cycle of the carrier signal and the end of the high-level period of the shifted control signal (the control signal to the switch 8 after the shift) is minimized (for example, 0) without changing the length of the high-level period.

[0138] In the power conversion device 100, when the control device 50 does not execute shift control, the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W do not rise to Vd at the time when the control signals SU1, SV1, and SW1 change from a low-level period to a high-level period (the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively), as shown in FIG. 16 . In other words, in the power conversion device 100, when the control device 50 does not execute shift control, the charging of the resonant capacitors 9U, 9V, and 9W does not finish at the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively. Therefore, in the power conversion device 100, when the control device 50 does not execute shift operation, the voltages across the first switching elements 1U, 1V, and 1W do not decrease to zero at the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively, and the switching of the first switching elements 1U, 1V, and 1W becomes hard switching.

[0139] In contrast, in the power conversion device 100, when the control device 50 executes shift control, the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W rise to Vd at the time when the control signals SU1, SV1, and SW1 change from a low-level period to a high-level period (the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively), as shown in Fig. 15. In other words, in the power conversion device 100, when the control device 50 executes shift control, the charging of the resonant capacitors 9U, 9V, and 9W ends at the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively. Therefore, in the power conversion device 100, when the control device 50 executes shift control, the switching of the first switching elements 1U, 1V, and 1W becomes zero-voltage soft switching.

[0140] Incidentally, when it is determined that the three-phase resonant currents overlap, the relationship between the polarities and magnitudes of the three-phase load currents is not limited to the relationship iU = 0, iV = 0, iW = 0, but may be, for example, a relationship iU > 0 > iV or a relationship iW > iV > 0 > iU.

[0141] Furthermore, as long as the control signals to the three switches 8 at least partially overlap, there are no limitations on the temporal relationship between the control signals to the first switching element 1 and the temporal relationship between the control signals to the second switching element 2. For example, the start point of the high-level period of the control signal SV6 may be earlier than the start point of the high-level period of the control signal SU6, and the start point of the control signal SU6 may be earlier than the start point of the control signal SW6. Furthermore, for example, in the case of an operation for soft-switching the first switching element (in the case of an operation for charging the resonant capacitor 9), when three-phase resonant currents flow simultaneously, when the load current is positive, it is desirable to add additional times Tau, Tav, and Taw to the high-level periods of the switches 8 corresponding to the phases through which the positive load current flows, as in the basic operation.

[0142] (1.2.2) Operation for soft-switching the second switching element Fig. 17 shows a timing chart of the power conversion device 100 when the control device 50 executes shift control in a case where the control device 50 determines that three-phase resonant currents of the U phase, the V phase, and the W phase flow simultaneously when the polarity of the current iL1 is negative, and Fig. 18 shows a timing chart of the power conversion device 100 when the control device 50 does not execute shift control. Note that Figs. 17 and 18 show timing charts for a part of one period within one cycle of the carrier signal.

[0143] 17 and 18 each show timing charts of the control signals SU1, SU2, SV1, SV2, SW1, SW2, SU7, SV7, SW7, load currents iU, iV, current iL1, and voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W. The control device 50 sets the length of the high-level period of each of the control signals SU7, SV7, and SW7 to a half resonance period (Tre / 2), and sets the dead time period Td to a length equal to the half resonance period. Note that, as shown in FIGS. 17 and 18, when the load currents iU, iV, and iW are 0, the control device 50 sets the additional times Tau, Tav, and Taw described in the first embodiment to 0.

[0144] 17 , the control device 50 targets the control signals SU7 and SW7 for the two switches 8U and 8W, respectively, for shifting, but this is not limited to this. In the example of FIG. 17 , the control device 50 shifts the start point of the high-level period of the control signal SU7 for switch 8U by a first shift time T1 in a direction toward an earlier start time so that overlapping of resonant currents does not occur when one of the two switches 8 targeted for shifting is combined with one switch 8 that is not targeted for shifting (here, so that the U-phase resonant current and the V-phase resonant current do not flow simultaneously). Furthermore, the control device 50 shifts the start point of the high-level period of the control signal SW7 for switch 8W by a second shift time T2 in a direction toward a later start time so that overlapping of resonant currents does not occur when the remaining one of the two switches 8 targeted for shifting is combined with the other two switches 8 (here, so that the W-phase resonant current does not flow simultaneously with both the U-phase resonant current and the V-phase resonant current). In FIG. 17, the state before the high level period of the control signals SU7 and SW7 is shifted (that is, the state in FIG. 18) is shown by a two-dot chain line, and the state after the shift is shown by a solid line.

[0145] The example of FIG. 17 shows a case where the control device 50 sets the first shift time T1 to T1 = Tres / 2 + Δ. The example of FIG. 17 also shows a case where the control device 50 sets T2 = Tres / 2 + Δ. Δ is a time margin for more reliably avoiding overlap of the two-phase resonant currents. Note that in the example of FIG. 17, the high-level period of the control signal SU7 to the switch 8U is shifted by the first shift time T1 in a direction to advance the start point of the high-level period of the control signal SU7, and the high-level period of the control signal SW7 to the switch 8W is shifted by the second shift time T2 in a direction to delay the start point of the high-level period of the control signal SW7 to the switch 8W. However, this is not limiting. For example, the high-level period of the control signal SW7 to the switch 8W may be shifted by the first shift time T1 in a direction to advance the start point of the high-level period of the control signal SW7 to the switch 8W, and the high-level period of the control signal SU7 to the switch 8U may be shifted by the second shift time T2 in a direction to delay the start point of the high-level period of the control signal SU7 to the switch 8U. Furthermore, the combination of two switches 8 to be shifted is not limited to the combination of switches 8U and 8W, but may be the combination of switches 8U and 8V, or the combination of switches 8V and 8W.

[0146] In the power conversion device 100, when the control device 50 does not execute shift control, the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W do not drop to zero at the time when the control signals SU2, SV2, and SW2 change from a low-level period to a high-level period (the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively), as shown in FIG. 18 . In other words, in the power conversion device 100, when the control device 50 does not execute shift control, the discharge of the resonant capacitors 9U, 9V, and 9W does not finish at the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively. Therefore, in the power conversion device 100, when the control device 50 does not execute a shift operation, the voltages across the second switching elements 2U, 2V, and 2W do not drop to zero at the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively, and the switching of the second switching elements 2U, 2V, and 2W becomes hard switching.

[0147] In contrast, in the power conversion device 100, when the control device 50 executes shift control, the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W decrease to zero at the time when the control signals SU2, SV2, and SW2 change from a low-level period to a high-level period (the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively), as shown in Fig. 17. In other words, in the power conversion device 100, when the control device 50 executes shift control, the discharge of the resonant capacitors 9U, 9V, and 9W ends at the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively. Therefore, in the power conversion device 100, when the control device 50 executes shift control, the switching of the second switching elements 2U, 2V, and 2W becomes zero-voltage soft switching.

[0148] (2) Summary In the power conversion device 100 according to the third embodiment, when the control device 50 determines that the resonant currents passing through the three switches 8 respectively flow simultaneously through the resonant inductor L1, the control device 50 performs control to shift the high-level periods of the control signals to two of the three switches 8 so that the resonant currents passing through the three switches 8 respectively do not flow simultaneously through the resonant inductor L1. This enables the power conversion device 100 to more reliably achieve soft switching.

[0149] 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.

[0150] In the power conversion device 100 according to the fourth embodiment, the shift control operation that the control device 50 executes when it determines that the three-phase resonant currents overlap is different from the shift control operation of the control device 50 according to the third embodiment.

[0151] 19, the operation of the control device 50 when it is determined that resonant currents passing through three of the multiple switches 8 simultaneously flow through the resonant inductor L1 will be described. When it is determined that resonant currents passing through three of the multiple switches 8 simultaneously flow through the resonant inductor L1, the control device 50 executes shift control.

[0152] As in the third embodiment, when performing shift control, the control device 50 shifts the high-level periods of the control signals to the two switches 8 among the multiple switching circuits 10 so that the lengths of the high-level periods of the control signals to be applied to the first switching element 1 and the second switching element 2 of the switching circuit 10 connected to the two switches 8 do not change.

[0153] In the fourth embodiment, when performing shift control, the control device 50 targets the control signals to any two of the three switches 8 as the shift targets, and shifts the high-level periods of the two switches 8 as the shift targets in the same direction so that the resonant currents passing through the three switches 8 do not simultaneously flow through the resonant inductor L1.

[0154] Fig. 19 shows a timing chart of the power conversion device 100 when the control device 50 executes shift control in a case where the control device 50 determines that three-phase resonant currents of the U phase, the V phase, and the W phase flow simultaneously when the polarity of the current iL1 is positive, while Fig. 16 described in the third embodiment shows a timing chart of the power conversion device 100 when the control device 50 does not execute shift control. Note that Fig. 19 illustrates a timing chart for a part of one period within one cycle of the carrier signal.

[0155] 19, like FIG. 16, illustrates a timing chart of the control signals SU1, SU2, SV1, SV2, SW1, SW2, SU6, SV6, SW6, load currents iU, iV, current iL1, and voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W. The control device 50 sets the length of the high-level period of each of the control signals SU6, SV6, and SW6 to a half resonance period (Tre / 2), and sets the dead time period Td to a length equal to the half resonance period. Note that, as shown in FIG. 19, the control device 50 sets the additional times Tau, Tav, and Taw described in the first embodiment to 0 when the load currents iU=0, iV=0, and iW=0.

[0156] In the example of Fig. 19, the control device 50 targets the control signals SV6 and SW6 to the two switches 8V and 8W, respectively, for shifting, but this is not limited thereto. In the example of Fig. 19, the control device 50 shifts the high-level period of the control signal SV6 to the switch 8V by a first shift time T1 in a direction delaying the start point of the high-level period of the control signal SV6 to the switch 8V so that overlapping of resonant currents does not occur when one of the two switches 8 to be shifted is combined with one switch 8 that is not to be shifted (here, so that the U-phase resonant current and the V-phase resonant current do not flow simultaneously). Furthermore, the control device 50 shifts the high-level period of the control signal SW6 to the switch 8W by a second shift time T2 in a direction delaying the start point of the high-level period of the control signal SW6 to the switch 8W so that overlapping of resonant currents does not occur when the remaining one of the two switches 8 to be shifted is combined with the other two switches 8 (here, so that the W-phase resonant current does not flow simultaneously with both the U-phase resonant current and the V-phase resonant current). In FIG. 19, the state before the high level period of the control signals SV6 and SW6 is shifted (that is, the state in FIG. 16) is shown by a two-dot chain line, and the state after the shift is shown by a solid line.

[0157] 19 shows an example in which the control device 50 sets the first shift time T1 to T1=Tres / 2+Δ. Also, in the example in FIG. 19, an example in which the control device 50 sets T2 to T2=2×(Tres / 2)+Δ. Δ is a time margin for more reliably avoiding overlap of the resonant currents of the two phases.

[0158] In the power conversion device 100 according to the fourth embodiment, when the control device 50 executes shift control, as shown in FIG. 19 , the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W increase to Vd at the time when the control signals SU1, SV1, and SW1 change from a low-level period to a high-level period (the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively). That is, in the power conversion device 100, when the control device 50 executes shift control, the charging of the resonant capacitors 9U, 9V, and 9W ends at the end of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively. Therefore, in the power conversion device 100, when the control device 50 executes shift control, the switching of the first switching elements 1U, 1V, and 1W becomes zero-voltage soft switching.

[0159] In the power conversion device 100 according to the fourth embodiment, when the control device 50 determines that the resonant currents passing through the three switches 8 respectively flow simultaneously through the resonant inductor L1, the control device 50 performs control to shift the high-level periods of the control signals to two of the three switches 8 so that the resonant currents passing through the three switches 8 respectively do not flow simultaneously through the resonant inductor L1. This enables the power conversion device 100 to more reliably achieve soft switching.

[0160] 19 , the high-level period of the control signal SV6 to the switch 8V is shifted by a first shift time T1 to delay the start of the high-level period of the control signal SV6, and the high-level period of the control signal SW6 to the switch 8W is shifted by a second shift time T2 to delay the start of the high-level period of the control signal SW6. However, this is not limiting. For example, the high-level period of the control signal SW6 to the switch 8W may be shifted by a first shift time T1 to delay the start of the high-level period of the control signal SW6, and the control signal SV6 to the switch 8V may be shifted by a second shift time T2 to delay the start of the high-level period of the control signal SV6. Furthermore, the combination of two switches 8 to be shifted is not limited to the combination of switches 8V and 8W, but may also be the combination of switches 8U and 8V or the combination of switches 8U and 8W. Furthermore, the shift direction is not limited to the direction of delaying the high-level periods of the control signals of the two switches 8, but may also be the direction of advancing the high-level periods of the control signals of the two switches 8. Furthermore, in the example of FIG. 19, the second shift time T2 is longer than the first shift time T1, but this is not limiting, and the first shift time T1 may be longer than the second shift time T1.

[0161] Furthermore, while FIG. 19 explains the operation for soft-switching the first switching elements 1U, 1V, and 1W, the shift control operation is similar in the case of the operation for soft-switching the second switching elements 2U, 2V, and 2W.

[0162] In the power conversion device 100 according to the modification of the fourth embodiment, the control device 50 can reduce bias in the fluctuation of the ripple of the line voltage by, for example, appropriately changing the combination of two switches 8 that are the targets of shift control.

[0163] 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.

[0164] In the power conversion device 100 according to the fifth embodiment, the shift control operation that the control device 50 executes when it determines that the three-phase resonant currents overlap is different from the shift control operation of the control device 50 according to the third embodiment. In the fifth embodiment, the control device 50 executes the shift control so that the resonant currents do not overlap in the combinations of one switch 8 and the remaining two switches 8 out of the three switches 8.

[0165] In the fifth embodiment, when the control device 50 determines that resonant currents passing through the three switches 8 simultaneously flow through the resonant inductor L1, in the case of a charging operation for charging a plurality of resonant capacitors 9, the control device 50 performs a first shift control as a shift control when a first condition is satisfied, and in the case of a discharging operation for discharging a plurality of resonant capacitors 9, the control device 50 performs a second shift control as a shift control when a second condition is satisfied.

[0166] The first condition is that the time difference between the start points of the high-level periods of the control signals SU1, SV1, and SW1 having the longest high-level period and the control signal having the shortest high-level period, among the control signals SU1, SV1, and SW1 supplied to the three first switching elements 1, respectively, is longer than the resonant half period Tres. The resonant half period Tres is half the resonant period determined by the reciprocal of the resonant frequency of a resonant circuit including the resonant inductor L1 and one of the multiple resonant capacitors 9. The first shift control includes control to shift the high-level period of the control signal supplied to the switch 8 corresponding to the first switching element 1 that supplies the control signal having the second-longest high-level period, among the control signals SU1, SV1, and SW1 supplied to the three first switching elements 1, respectively. The second condition is that the time difference between the start points of the high-level periods of the control signals SU2, SV2, and SW2 having the longest high-level period and the control signal having the shortest high-level period, among the control signals SU2, SV2, and SW2 supplied to the three second switching elements 2, respectively, is longer than the resonant half period Tres. The second shift control includes control for shifting the high-level period of the control signal to the switch 8 corresponding to the second switching element 2 that supplies the control signal having the second longest high-level period among the control signals SU2, SV2, SW2 supplied to the three second switching elements 2. When performing shift control (first shift control or second shift control), the control device 50 shifts the high-level period of the control signal to the switch 8 so that the length of the high-level period of the control signal does not change.

[0167] Fig. 20 shows a timing chart of the power conversion device 100 when the control device 50 executes the first shift control in the charging operation for charging the plurality of resonance capacitors 9 when the control device 50 determines that resonance currents flow simultaneously through the three switches 8, and Fig. 21 shows a timing chart of the power conversion device 100 when the control device 50 does not execute the first shift control. Note that Figs. 20 and 21 show timing charts for a part of one period of the carrier signal.

[0168] 20 and 21 each show timing charts of the control signals SU1, SU2, SV1, SV2, SW1, SW2, SU6, SV6, SW6, the load currents iU, iV, iW, and the current iL1. The control device 50 sets the length of the high-level period of each of the control signals SU6, SV6, and SW6 to a half resonance period (Tre / 2), and sets the dead time period Td to a length equal to the half resonance period. Note that, as shown in FIGS. 20 and 21, when the load currents iU=0, iV=0, and iW=0, the control device 50 sets the additional times Tau, Tav, and Taw described in the first embodiment to 0.

[0169] In the example of Figure 20, since the relationship is such that [length of the high-level period of the control signal SU1] > [length of the high-level period of the control signal SV1] > [length of the high-level period of the control signal SW1], the control device 50 shifts the high-level period of the control signal SV6 to the switch 8V corresponding to the first switching element 1V that supplies the control signal SV1 by the shift time Ts in the direction of accelerating the period. In the fifth embodiment, the shift time Ts is a predetermined period. In Figure 20, the state of the control signal SV6 before the high-level period is shifted (i.e., the state in Figure 21) is shown by a two-dot chain line, and the state after the shift is shown by a solid line. In the example of FIG. 20 , the control device 50 shifts the high-level period of the control signal SV6 to the switch 8V by a shift time Ts in a direction to advance the start point of the high-level period of the control signal SV6 so that the resonant current flowing through the switch 8V and the resonant current flowing through the switch 8U do not flow through the resonant inductor L1 simultaneously, and the resonant current flowing through the switch 8V and the resonant current flowing through the switch 8W do not flow through the resonant inductor L1 simultaneously. The length of the predetermined period (shift time Ts) is, for example, 2×Tres / 2. The length of the predetermined period is not limited to this, and may be any length that prevents overlapping of the resonant currents. Furthermore, the direction in which the high-level period of the control signal SV6 is shifted is not limited to advancing the high-level period, and may also be delaying the high-level period.

[0170] In the power conversion device 100 according to the fifth embodiment, when the control device 50 executes shift control, the voltages V2u, V2v, and V2w across the second switching elements 2U, 2V, and 2W increase to Vd at the time when the control signals SU1, SV1, and SW1 change from a low-level period to a high-level period (the end time of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively). That is, in the power conversion device 100, when the control device 50 executes shift control, the charging of the resonant capacitors 9U, 9V, and 9W ends at the end time of the dead time period Td corresponding to the U phase, V phase, and W phase, respectively. Therefore, in the power conversion device 100, when the control device 50 executes shift control, the switching of the first switching elements 1U, 1V, and 1W becomes zero-voltage soft switching.

[0171] Furthermore, in the power conversion device 100 according to the fifth embodiment, when the second condition is satisfied, the control device 50 performs the second shift control, thereby enabling the second switching elements 2U, 2V, and 2W to be subjected to zero voltage soft switching.

[0172] As described above, the power conversion device 100 according to the fifth embodiment can perform soft switching more reliably.

[0173] Sixth Embodiment A power conversion device 100A according to a sixth embodiment will be described with reference to Fig. 22. Regarding the power conversion device 100A according to the sixth embodiment, components similar to those of the power conversion device 100 according to the first embodiment will be denoted by the same reference numerals and descriptions thereof will be omitted.

[0174] In the power conversion device 100A according to the sixth embodiment, the first IGBT 6 and the second IGBT 7 are connected in anti-series in each of the multiple switches 8. In the power conversion device 100A according to the sixth embodiment, the collector terminal of the first IGBT 6 and the collector terminal of the second IGBT 7 are connected in each of the multiple switches 8, the emitter terminal of the first IGBT 6 is 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 is connected to the common connection point 25. Each of the multiple 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.

[0175] In the power conversion device 100A according to the sixth embodiment, each of the first IGBT 6 and the second IGBT 7 may be replaced with a MOSFET or a bipolar transistor. In this case, the diode 61 and the diode 71 in FIG. 22 may be substituted with a parasitic diode of the replaced element, or an element built into the chip of the replaced element. In addition, in the power conversion device 100A according to the sixth embodiment, the diode 61 and the diode 71 are not limited to being externally connected to the first IGBT 6 and the second IGBT 7, but may be built into the chip.

[0176] 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 5, or may perform the shift control of the control device 50 of embodiments 1 to 5.

[0177] Seventh Embodiment A power conversion device 100A according to a seventh embodiment will be described with reference to Fig. 23. Regarding the power conversion device 100A according to the seventh embodiment, components similar to those of the power conversion device 100 according to the first embodiment will be denoted by the same reference numerals and descriptions thereof will be omitted.

[0178] In a power conversion device 100A according to the seventh embodiment, the first IGBT 6 and the second IGBT 7 are connected in anti-series in each of the multiple switches 8. In the power conversion device 100A according to the seventh embodiment, the emitter terminal of the first IGBT 6 and the emitter terminal of the second IGBT 7 are connected in each of the multiple switches 8, the collector terminal of the second IGBT 7 is connected to the connection point 3 of a corresponding one of the multiple switching circuits 10, and the collector terminal of the first IGBT 6 is connected to the common connection point 25. Each of the multiple 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.

[0179] In the power conversion device 100A according to the seventh embodiment, each of the first IGBT 6 and the second IGBT 7 may be replaced with a MOSFET or a bipolar transistor. In this case, the diode 61 and the diode 71 in FIG. 23 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 seventh embodiment, the diode 61 and the diode 71 are not limited to being externally connected to the first IGBT 6 and the second IGBT 7, but may be built into the chip.

[0180] 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 5, or may perform the shift control of the control device 50 of embodiments 1 to 5.

[0181] Eighth Embodiment A power conversion device 100A according to an eighth embodiment will be described with reference to Fig. 24. Regarding the power conversion device 100A according to the eighth embodiment, components similar to those of the power conversion device 100 according to the first embodiment will be denoted by the same reference numerals and descriptions thereof will be omitted.

[0182] In a power conversion device 100A according to the eighth embodiment, a first MOSFET 6A and a second MOSFET 7A are connected in anti-series in each of the multiple switches 8. In the power conversion device 100A according to the eighth embodiment, the drain terminal of the first MOSFET 6A and the drain terminal of the second MOSFET 7A are connected in anti-parallel in each of the multiple switches 8. Each of the multiple switches 8 further includes a diode 61 connected in anti-parallel to the first MOSFET 6A and a diode 71 connected in anti-parallel to the second MOSFET 7A. In each of the multiple switches 8, the source terminal of the second MOSFET 7A is connected to a common connection point 25. In each of the multiple switches 8, the source terminal of the first MOSFET 6A is connected to a connection point 3 of a switching circuit 10 corresponding to the switch 8 having the first MOSFET 6A. Control signals SU6 and SU7 are provided from a control device 50 to the first MOSFET 6A and the second MOSFET 7A of a switch 8U. The first MOSFET 6A and the second MOSFET 7A of the switch 8V are supplied with control signals SV6 and SV7 from the control device 50. The first MOSFET 6A and the second MOSFET 7A of the switch 8W are supplied with control signals SW6 and SW7 from the control device 50.

[0183] The operation of the control device 50 is, for example, similar to the operation of the control device 50 of embodiment 1, but is not limited to this and may be similar to the operation of any of the control devices 50 of embodiments 2 to 5, or may perform the shift control of the control device 50 of embodiments 1 to 5.

[0184] Ninth Embodiment A power conversion device 100A according to a ninth embodiment will be described with reference to Fig. 25. Regarding the power conversion device 100A according to the ninth embodiment, components similar to those of the power conversion device 100 according to the first embodiment will be denoted by the same reference numerals and descriptions thereof will be omitted.

[0185] In the power conversion device 100A according to the ninth embodiment, a diode 63 is connected in series to the first MOSFET 6A, and a diode 73 is connected in series to the second MOSFET 7A in each of the multiple switches 8. In the power conversion device 100A according to the ninth embodiment, a series circuit of the first MOSFET 6A and the diode 63 and a series circuit of the second MOSFET 7A and the diode 73 are connected in anti-parallel.

[0186] The operation of the control device 50 is, for example, similar to the operation of the control device 50 of embodiment 1, but is not limited to this and may be similar to the operation of any of the control devices 50 of embodiments 2 to 5, or may perform the shift control of the control device 50 of embodiments 1 to 5.

[0187] Tenth Embodiment A power conversion device 100A according to a tenth embodiment will be described with reference to Fig. 26. Regarding the power conversion device 100A according to the tenth embodiment, components similar to those of the power conversion device 100 according to the first embodiment will be denoted by the same reference numerals and descriptions thereof will be omitted.

[0188] In the power conversion device 100A according to the tenth embodiment, each of the multiple switches 8 includes one MOSFET 80, a diode 83 connected in anti-parallel to the MOSFET 80, a series circuit of two diodes 84 and 85 connected in anti-parallel to the MOSFET 80, and a series circuit of two diodes 86 and 87 connected in anti-parallel to the MOSFET 80. In each of the multiple switches 8, a connection point between the diode 84 and the diode 85 in the switch 8 (a first end of the switch 8) is connected to a connection point 3 of a corresponding switching circuit 10 among the multiple switching circuits 10, and a connection point between the diode 86 and the diode 87 (a second end of the switch 8) is connected to the common connection point 25. In each of the switches 8, the switch 8 is in an on state when the MOSFET 80 is in an on state, and is in an off state when the MOSFET 80 is in an off state.

[0189] The MOSFETs 80 of the multiple switches 8 are controlled by a control device 50. The control device 50 outputs a control signal SU8 that controls the on / off state of the MOSFET 80 of the switch 8U, a control signal SV8 that controls the on / off state of the MOSFET 80 of the switch 8V, and a control signal SW8 that controls the on / off state of the MOSFET 80 of the switch 8W.

[0190] In the switch 8, when the MOSFET 80 is in the on state, a resonant current flows due to a resonant circuit including the resonant inductor L1 and the resonant capacitor 9. In the power conversion device 100A, when one of the multiple switches 8 is in the on state, a charging current including the resonant current flows through the path of the regenerative capacitor 15, the resonant inductor L1, the diode 86, the MOSFET 80, the diode 85, and the resonant capacitor 9. In addition, in the power conversion device 100A, when one of the multiple switches 8 is in the on state, a discharging current including the resonant current flows through the path of the resonant capacitor 9, the diode 84, the MOSFET 80, the diode 87, the resonant inductor L1, and the regenerative capacitor 15.

[0191] In the power conversion device 100A according to the tenth embodiment, each of the plurality of MOSFETs 80 may be replaced with an IGBT. Also, in the power conversion device 100A according to the tenth embodiment, each of the plurality of switches 8 may have, instead of the MOSFET 80, a bipolar transistor or a GaN-based GIT (Gate Injection Transistor), for example.

[0192] The operation of the control device 50 is, for example, similar to the operation of the control device 50 of embodiment 1, but is not limited to this and may be similar to the operation of any of the control devices 50 of embodiments 2 to 5, or may perform the shift control of the control device 50 of embodiments 1 to 5.

[0193] (Embodiment 11) A power conversion device 100A according to embodiment 11 will be described with reference to Fig. 27. Regarding the power conversion device 100A according to embodiment 11, components similar to those of the power conversion device 100 according to embodiment 1 are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0194] In the power conversion device 100A according to the eleventh embodiment, each of the multiple switches 8 is a dual-gate GaN-based GIT having a first source terminal, a first gate terminal, a second gate terminal, and a second source terminal. In the power conversion device 100A according to the eleventh embodiment, a control signal SU6 is applied between the first gate terminal and the first source terminal of the dual-gate GaN-based GIT constituting the switch 8U, and a control signal SU7 is applied between the second gate terminal and the second source terminal. Furthermore, a control signal SV6 is applied between the first gate terminal and the first source terminal of the dual-gate GaN-based GIT constituting the switch 8V, and a control signal SV7 is applied between the second gate terminal and the second source terminal. Furthermore, a control signal SW6 is applied between the first gate terminal and the first source terminal of the dual-gate GaN-based GIT constituting the switch 8W, and a control signal SW7 is applied between the second gate terminal and the second source terminal.

[0195] The operation of the control device 50 is, for example, similar to the operation of the control device 50 of embodiment 1, but is not limited to this and may be similar to the operation of any of the control devices 50 of embodiments 2 to 5, or may perform the shift control of the control device 50 of embodiments 1 to 5.

[0196] (Twelfth Embodiment) A power conversion device 100B according to a twelfth embodiment will be described below with reference to Fig. 28. 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. In 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 descriptions thereof will be omitted.

[0197] 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.

[0198] 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 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.

[0199] The control device 50 of the power conversion device 100B according to the twelfth embodiment performs shift control in the same manner as 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 realize zero voltage soft switching of each of the plurality of first switching elements 1 and the plurality of second switching elements 2, similar to the power conversion device 100 according to the first embodiment.

[0200] The operation of the control device 50 is, for example, similar to the operation of the control device 50 of embodiment 1, but is not limited to this and may be similar to the operation of any of the control devices 50 of embodiments 2 to 5, or may perform the shift control of the control device 50 of embodiments 1 to 5.

[0201] (Embodiment 13) A power conversion device 100C according to embodiment 13 will be described below with reference to Fig. 29. 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 L0 and the first DC terminal 31. In 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 descriptions thereof will be omitted.

[0202] The control device 50 of the power conversion device 100C according to the thirteenth embodiment performs shift control in the same manner as 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 perform soft switching more reliably, similar to the power conversion device 100 according to the first embodiment.

[0203] (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.

[0204] 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, or the operation of "determining that three-phase resonant currents are flowing simultaneously" when the rotation speed of the above-mentioned motor is less than the rotation speed threshold value.

[0205] For example, the control device 50 may determine that three-phase resonant currents are flowing simultaneously when all of the following are less than a threshold value: the time difference between the start of the high-level period of the control signal corresponding to the U phase and the start of the high-level period of the control signal corresponding to the V phase; the time difference between the start of the high-level period of the control signal corresponding to the V phase and the start of the high-level period of the control signal corresponding to the W phase; and the time difference between the start of the high-level period of the control signal corresponding to the W phase and the start of the high-level period of the control signal corresponding to the U phase.

[0206] In addition, 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.

[0207] 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.

[0208] 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).

[0209] 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.

[0210] 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.

[0211] Furthermore, the length of the dead time period Td is not limited to being set to be the same as the resonance half cycle, and may be set to a length different from the resonance half cycle.

[0212] The dead time period Td 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 generation circuit included in the gate driver IC may set the dead time period Td.

[0213] 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 polyphase AC with three or more phases.

[0214] (Aspects) The following aspects are disclosed in this specification.

[0215] 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).The control device (50) supplies a control signal whose potential changes between a high level and a low level to each of the plurality of first switching elements (1), the plurality of second switching elements (2), and the plurality of switches (8). When the control device (50) determines that a resonant current passing through each of two or more switches (8) of the plurality of switches (8) simultaneously flows through the resonant inductor (L1), the control device (50) performs shift control to shift a high level period of the control signal to at least one switch (8) of the two or more switches (8) so that the resonant current passing through each of the two or more switches (8) does not simultaneously flow through the resonant inductor (L1).

[0216] According to this aspect, soft switching can be performed more reliably.

[0217] In the power conversion device (100; 100A; 100B; 100C) according to the second aspect, in the first aspect, when performing shift control, the control device (50) shifts the high-level period of the control signal to the at least one switch (8) so that the length of the high-level period of the control signal to be applied to each of the first switching element (1) and the second switching element (2) of the switching circuit (10) connected to the at least one switch (8) among the plurality of switching circuits (10) does not change.

[0218] According to this aspect, it is possible to suppress changes in the line voltage.

[0219] In the power conversion device (100; 100A; 100B; 100C) according to the third aspect, in the first or second aspect, the control device (50) shifts the high-level periods of the control signals to two of the two or more switches (8) in directions different from each other when performing shift control.

[0220] According to this aspect, it is possible to achieve a higher frequency.

[0221] A power conversion device (100; 100A; 100B; 100C) according to a fourth aspect is based on any one of the first to third aspects. When a control device (50) determines that a resonant current passing through each of two switches (8) among a plurality of switches (8) simultaneously flows through a resonant inductor (L1), and when the polarity of a load current flowing through each of two AC terminals (41) connected to two switches (8) among a plurality of AC terminals (41) is positive, the control device (50) compares the duties of control signals for two first switching elements (1) corresponding to two switches (8) among a plurality of first switching elements (1). The control device (50) then advances the high-level period of the control signal provided to the switch (8) corresponding to the first switching element (1) that provides a control signal with a relatively large duty, and delays the high-level period of the control signal provided to the switch (8) corresponding to the first switching element (1) that provides a control signal with a relatively small duty. When the polarity of the load current flowing through each of the two AC terminals (41) connected to two switches (8) among the plurality of AC terminals (41) is negative, the control device (50) compares the duties of the control signals for the two first switching elements (1) corresponding to the two switches (8) among the plurality of first switching elements (1).The control device (50) then shifts in a later direction the high-level period of the control signal to be given to the switch (8) corresponding to the first switching element (1) that gives the control signal with a relatively large duty, and shifts in an earlier direction the high-level period of the control signal to be given to the switch (8) corresponding to the first switching element (1) that gives the control signal with a relatively small duty.

[0222] According to this aspect, when it is determined that resonant currents passing through two of the multiple switches (8) simultaneously flow through the resonant inductor (L1), soft switching can be performed more reliably.

[0223] A power conversion device (100; 100A; 100B; 100C) according to a fifth aspect is based on any one of the first to third aspects. When a control device (50) determines that a resonant current passing through each of two switches (8) among a plurality of switches (8) simultaneously flows through a resonant inductor (L1), and when the polarity of a load current flowing through each of two AC terminals (41) connected to the two switches (8) among a plurality of AC terminals (41) is positive, the control device (50) compares the duties of control signals for two first switching elements (1) among a plurality of first switching elements (1) corresponding to the two switches (8). The control device (50) then delays the high-level period of the control signal provided to the switch (8) corresponding to the first switching element (1) that provides a control signal with a relatively large duty, and advances the high-level period of the control signal provided to the switch (8) corresponding to the first switching element (1) that provides a control signal with a relatively small duty. When the polarity of the load current flowing through each of the two AC terminals (41) connected to two switches (8) among the plurality of AC terminals (41) is negative, the control device (50) compares the duties of the control signals for the two first switching elements (1) corresponding to the two switches (8) among the plurality of first switching elements (1).The control device (50) then shifts the high-level period of the control signal given to the switch (8) corresponding to the first switching element (1) that gives the control signal with a relatively large duty in an earlier direction, and shifts the high-level period of the control signal given to the switch (8) corresponding to the first switching element (1) that gives the control signal with a relatively small duty in a later direction.

[0224] According to this aspect, when it is determined that resonant currents passing through two of the multiple switches (8) simultaneously flow through the resonant inductor (L1), soft switching can be performed more reliably.

[0225] A power conversion device (100; 100A; 100B; 100C) according to a sixth aspect is based on any one of the first to fifth aspects. When the control device (50) determines that resonant currents passing through three switches (8) of the plurality of switches (8) simultaneously flow through the resonant inductor (L1), the control device (50) shifts high-level periods of control signals given to two switches (8) of the three switches (8) in the same direction in shift control.

[0226] According to this aspect, when it is determined that the resonant currents flowing through the three switches (8) simultaneously, it is possible to perform soft switching more reliably.

[0227] A power conversion device (100; 100A; 100B; 100C) according to a seventh aspect is based on any one of the first to sixth aspects. The plurality of first switching elements (1) includes three first switching elements (1). The plurality of second switching elements (2) includes three second switching elements (2). The plurality of switches (8) includes three switches (8). When the control device (50) determines that a resonant current passing through each of the three switches (8) simultaneously flows through the resonant inductor (L1), in the case of a charging operation for charging the plurality of resonant capacitors (9), the control device (50) performs a first shift control when a first condition is satisfied, and in the case of a discharging operation for discharging the plurality of resonant capacitors (9), the control device (50) performs a second shift control when a second condition is satisfied. The first condition is that the time difference between the start points of the high-level periods of the control signals (SU1, SV1, SW1) having the longest high-level period and the control signal having the shortest high-level period, among the control signals (SU1, SV1, SW1) given to the three first switching elements (1), is longer than a resonant half period (Tres). The resonant half period (Tres) is half the resonant period determined by the reciprocal of the resonant frequency of a resonant circuit including a resonant inductor (L1) and one of the multiple resonant capacitors (9). The first shift control includes control to shift the high-level period of the control signal given to the switch (8) corresponding to the first switching element (1) that gives the control signal having the second longest high-level period, among the control signals (SU1, SV1, SW1) given to the three first switching elements (1). The second condition is that the time difference between the start points of the high-level periods of the control signal having the longest high-level period and the control signal having the shortest high-level period among the control signals (SU2, SV2, SW2) given to the three second switching elements (2) is longer than the resonance half period (Tres). The second shift control includes control to shift the high-level period of the control signal to the switch (8) corresponding to the second switching element (2) that gives the control signal having the second longest high-level period among the control signals (SU2, SV2, SW2) given to the three second switching elements (2).

[0228] According to this aspect, in the case of a charging operation, it is possible to realize soft switching of the three first switching elements (1) by shifting the high-level period of the control signal to one switch (8) of the multiple switches (8), and in the case of a discharging operation, it is possible to realize soft switching of the three second switching elements (2) by shifting the high-level period of the control signal to one switch (8) of the multiple switches (8).

[0229] The power conversion device of the present disclosure is capable of performing soft switching more reliably, thereby improving the reliability of the power conversion device. Thus, the power conversion device of the present disclosure is industrially useful.

[0230] 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 153 Third terminal 154 Fourth terminal 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 Tres Resonant half cycle

Claims

1. a first DC terminal and a second DC terminal; a power conversion circuit including a plurality of first switching elements and a plurality of second switching elements, wherein a plurality of switching circuits, each having the plurality of first switching elements and the plurality of second switching elements connected in series in a one-to-one relationship, are connected in parallel with each other, the plurality of first switching elements are connected to the first DC terminal, and the plurality of second switching elements are connected to the second DC terminal; a plurality of AC terminals corresponding one-to-one to the plurality of switching circuits, each AC terminal being connected to a connection point between the first switching element and the second switching element in the corresponding switching circuit; a plurality of switches corresponding one-to-one to the plurality of switching circuits, each having a first end connected to the connection point of the first switching element and the second switching element in the corresponding switching circuit and a second end commonly connected to a common connection point; a plurality of resonance 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; 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; a control device that applies 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; The control device when it is determined that a resonant current passing through each of two or more switches among the plurality of switches simultaneously flows through the resonant inductor, a shift control is performed to shift a high level period of a control signal to at least one switch among the two or more switches so that the resonant current passing through each of the two or more switches does not simultaneously flow through the resonant inductor; Power conversion device.

2. When performing the shift control, the control device shifts a high-level period of a control signal to the at least one switch so that a length of a high-level period of the control signal to be applied to each of the first switching element and the second switching element of a switching circuit connected to the at least one switch among the plurality of switching circuits does not change. The power conversion device according to claim 1 .

3. When performing the shift control, the control device shifts high-level periods of the control signals to two of the two or more switches in directions different from each other. The power conversion device according to claim 1 or 2.

4. When the control device determines that a resonant current flows through each of two of the plurality of switches simultaneously in the resonant inductor, when the polarity of the load current flowing through each of two AC terminals connected to the two switches among the plurality of AC terminals is positive, the control signals for two first switching elements among the plurality of first switching elements corresponding to the two switches are compared, and a high-level period of the control signal to be provided to the switch corresponding to the first switching element that provides a control signal with a relatively large duty is shifted in an earlier direction, and a high-level period of the control signal to be provided to the switch corresponding to the first switching element that provides a control signal with a relatively small duty is shifted in a later direction; when the polarity of the load current flowing through each of two AC terminals connected to the two switches among the plurality of AC terminals is negative, the control circuit compares duties of control signals for two first switching elements among the plurality of first switching elements corresponding to the two switches, and shifts in a later direction a high-level period of the control signal to be given to the switch corresponding to the first switching element that gives a control signal of a relatively large duty, and shifts in a later direction a high-level period of the control signal to be given to the switch corresponding to the first switching element that gives a control signal of a relatively small duty; The power conversion device according to claim 1 or 2.

5. When the control device determines that a resonant current flows through each of two of the plurality of switches simultaneously in the resonant inductor, when the polarity of the load current flowing through each of two AC terminals connected to the two switches among the plurality of AC terminals is positive, the control signal for two first switching elements among the plurality of first switching elements corresponding to the two switches is compared, and a high-level period of the control signal to be provided to the switch corresponding to the first switching element that provides a control signal with a relatively large duty is shifted in a delaying direction, and a high-level period of the control signal to be provided to the switch corresponding to the first switching element that provides a control signal with a relatively small duty is shifted in an advanceing direction, when the polarity of the load current flowing through each of two AC terminals connected to the two switches among the plurality of AC terminals is negative, the control circuit compares duties of control signals for two first switching elements among the plurality of first switching elements corresponding to the two switches, and shifts in an earlier direction a high-level period of the control signal to be given to the switch corresponding to the first switching element that gives a control signal of a relatively large duty, and shifts in a later direction a high-level period of the control signal to be given to the switch corresponding to the first switching element that gives a control signal of a relatively small duty; The power conversion device according to claim 1 or 2.

6. When the control device determines that a resonant current flows through each of three switches of the plurality of switches simultaneously in the resonant inductor, In the shift control, high level periods of control signals applied to two of the three switches are shifted in the same direction. The power conversion device according to claim 1 or 2.

7. the plurality of first switching elements include three of the first switching elements, the plurality of second switching elements include three of the second switching elements, the plurality of switches includes three of the switches; When the control device determines that a resonant current passing through each of the three switches simultaneously flows through the resonant inductor, In the case of a charging operation for charging the plurality of resonance capacitors, a first shift control is performed when a first condition is satisfied; The first condition is a condition that a time difference between the start points of the high level periods of the control signal having the longest high level period and the control signal having the shortest high level period among the control signals to be applied to the three first switching elements is longer than a resonance half period; the resonance half period is half the value of a resonance period determined by the reciprocal of a resonance frequency of a resonance circuit including the resonance inductor and one of the plurality of resonance capacitors, The first shift control includes: a control for shifting a high level period of a control signal to a switch corresponding to a first switching element that supplies a control signal having a second longest high level period among the control signals supplied to the three first switching elements, In the case of a discharging operation for discharging the plurality of resonance capacitors, a second shift control is performed when a second condition is satisfied; The second condition is: a condition that a time difference between the start points of the high level periods of the control signal having the longest high level period and the control signal having the shortest high level period among the control signals to be applied to the three second switching elements is longer than the resonance half period; The second shift control includes: and control for shifting a high level period of a control signal to a switch corresponding to a second switching element that supplies a control signal having the second longest high level period among the control signals supplied to the three second switching elements, respectively. The power conversion device according to claim 1 or 2.