Electric power converting device

Abstract

The objective of the present invention is to reduce current distortion in an electric power converting device. In an electric power converting device (100), a control device (50) performs one of a first control operation and a second control operation on the basis of an overlap period in which resonance currents that pass through each of two or more switches (8) flow simultaneously. The first control operation shifts a high-level period of a control signal to at least one switch (8) among the two or more switches (8) such that the resonance currents passing through each of the two or more switches (8) do not flow simultaneously to a resonance inductor (L1). The second control operation shifts the high-level period of the control signal to at least one switch (8) among the two or more switches (8) such that the high-level periods of the control signals to each of the two or more switches (8) overlap for at least a predetermined period in a dead time period corresponding to each of two or more switching circuits (10) that are connected to the two or more switches (8), among a plurality of switching circuits (10).

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] The power conversion device disclosed in Patent Document 1 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 capacitor, and a control device. The power conversion circuit has multiple first switching elements and multiple second switching elements. In the power conversion circuit, multiple switching circuits, each of which has multiple first switching elements and multiple second switching elements connected in series in a one-to-one relationship, are connected in parallel to each other. The multiple switches correspond one-to-one to the multiple switching circuits, and have first ends connected to junctions between the first switching elements and the second switching elements in the corresponding switching circuits and second ends connected in common to a common junction. The first end of the resonant inductor is connected to the common junction. When the control device determines that two-phase resonant currents corresponding to two of the multiple switching circuits simultaneously flow through one resonant inductor, the control device controls to shift the on-periods of the first switching elements and the second switching elements in one of the two switching circuits.

[0003] The power conversion device disclosed in Patent Document 1 can perform soft switching more reliably.

[0004] International Publication No. 2023 / 074636

[0005] In the power conversion device disclosed in Patent Document 1, current distortion of the load current flowing through the AC terminal connected to the connection point of the first switching element and the second switching element, which have been controlled to shift the on-period, may become large.

[0006] An object of the present disclosure is to provide a power conversion device capable of reducing current distortion.

[0007] A power conversion device according to one aspect of the present disclosure includes a first DC terminal and a second DC terminal, a power conversion circuit, multiple AC terminals, multiple switches, multiple resonant capacitors, a resonant inductor, a regenerative capacitor, and a control device. The power conversion circuit includes multiple first switching elements and multiple second switching elements. In the power conversion circuit, multiple switching circuits are connected in parallel, with the multiple first switching elements and the multiple second switching elements connected in series in a one-to-one relationship. In the power conversion circuit, the multiple first switching elements are connected to the first DC terminals, and the multiple second switching elements are connected to the second DC terminals. The multiple AC terminals correspond one-to-one to the multiple switching circuits. Each of the multiple AC terminals is connected to a connection point between the first switching element and the second switching element in the corresponding switching circuit. The multiple switches correspond one-to-one to the multiple switching circuits. Each of the multiple switches has a first end connected to the connection point between the first switching element and the second switching element in the corresponding switching circuit, and a second end connected in common to a common connection point. The resonant capacitors correspond one-to-one to the multiple switches. Each of the multiple resonant capacitors is connected between the first end and the second DC terminal of the corresponding switch. The resonant inductor has a first end and a second end. The first end of the resonant inductor is connected to the common connection point. The regenerative capacitor has a third end and a fourth end. The third end of the regenerative capacitor is connected to the first DC terminal or the second DC terminal. The control device provides a control signal whose potential changes between a high level and a low level to each of the multiple first switching elements, the multiple second switching elements, and the multiple switches.The control device sets a dead time period between a high-level period of a control signal to the first switching element and a high-level period of a control signal to the second switching element for each of the multiple switching circuits, and sets the high-level period of the control signal to each of the multiple switches based on the dead time period for a corresponding switching circuit among the multiple switching circuits. The control device is configured to perform one of a first control action and a second control action based on an overlap period during which the resonant currents flow simultaneously through the two or more switches when it determines that resonant currents passing through two or more switches of the multiple switches simultaneously flow through the resonant inductor. The first control action is an action of shifting 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 do not simultaneously flow through the resonant inductor. The second control action is an action of shifting the high-level period of the control signal to at least one of the two or more switches so that the high-level period of the control signal to each of the two or more switches overlaps by a predetermined period or more with dead time periods corresponding to two or more switching circuits connected to the two or more switches of the multiple switching circuits.

[0008] The power conversion device of the present disclosure has the effect of making it possible to reduce current distortion.

[0009] FIG. 1 is a circuit diagram of a system including a power conversion device according to a first embodiment. FIG. 2 is an explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being charged. FIG. 3 is another explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being charged. FIG. 4 is another explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being charged. FIG. 5 is a diagram illustrating a time change in duty and a time change in load current corresponding to voltage commands for each of three phases in an AC load connected to multiple AC terminals of the power conversion device according to the first embodiment. FIG. 6 is an explanatory diagram illustrating a first current threshold and a second current threshold used by the control device in the power conversion device according to the first embodiment. FIG. 7 is an explanatory diagram illustrating an operation when a control device in the power conversion device according to the first embodiment performs a basic operation when a load current is greater than 0 and a resonant capacitor is being discharged. FIG. 8 is an explanatory diagram of an operation when the control device in the power conversion device of the first embodiment performs a basic operation when the load current is less than 0 and the resonant capacitor is discharging. FIG. 9 is an explanatory diagram of an operation when the control device in the power conversion device of the first embodiment performs a basic operation when the load current is less than 0 and the resonant capacitor is charging. FIG. 10 is a timing chart for explaining an example of a boundary condition between a case where currents flowing through the resonant inductors in the power conversion device of the first embodiment do not overlap (do not flow simultaneously) and a case where they overlap (flow simultaneously). FIG. 11 is a timing chart for explaining an example of a boundary condition between a case where the U-phase resonant current and the V-phase resonant current do not overlap (do not flow simultaneously) and a case where they overlap (flow simultaneously) in the power conversion device of the first embodiment. FIG. 12 is a timing chart for explaining an operation when the control device in the power conversion device of the first embodiment performs a first control operation. FIG. 13 is a waveform diagram of a load current in the power conversion device of the first embodiment. FIG. 14 is a timing chart for explaining an operation when the control device in the power conversion device of the first embodiment performs a second control operation.FIG. 15 is a timing chart for explaining an operation when the control device performs a first control operation in the power conversion device according to the second embodiment. FIG. 16 is a timing chart for explaining an operation when the control device performs a second control operation in the power conversion device according to the second embodiment. FIG. 17 is a timing chart for explaining an operation when the control device performs a first control operation in the power conversion device according to the third embodiment. FIG. 18 is a timing chart for explaining an operation when the control device performs a second control operation in the power conversion device according to the third embodiment. FIG. 19 is a circuit diagram of a system including a power conversion device according to the fourth embodiment. FIG. 20 is a circuit diagram of a system including a power conversion device according to the fifth embodiment. FIG. 21 is a circuit diagram of a system including a power conversion device according to the sixth embodiment. FIG. 22 is a circuit diagram of a system including a power conversion device according to the seventh embodiment. FIG. 23 is a circuit diagram of a system including a power conversion device according to the eighth embodiment. FIG. 24 is a circuit diagram of a system including a power conversion device according to the ninth embodiment. FIG. 25 is a circuit diagram of a system including a power conversion device according to the tenth embodiment. FIG. 26 is a circuit diagram of a system including a power conversion device according to the eleventh embodiment. Fig. 27 is a timing chart for explaining the operation when the control device in the power conversion apparatus of Example 1 performs the second control operation. Fig. 28 is a timing chart for explaining the operation when the control device in the power conversion apparatus of Example 1 performs the first control operation. Fig. 29 is a timing chart for explaining the operation when the control device in the power conversion apparatus of Example 2 performs the second control operation. Fig. 30 is a timing chart for explaining the operation when the control device in the power conversion apparatus of Example 2 performs the first control operation. Fig. 31 is a timing chart for explaining the operation when the control device in the power conversion apparatus of Example 3 performs the second control operation. Fig. 32 is a timing chart for explaining the operation when the control device in the power conversion apparatus of Example 3 performs the first control operation.

[0010] First Embodiment A power conversion device 100 according to a first embodiment will be described below with reference to FIGS. 1 to 14. FIG.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0026] The control device 50 controls a plurality of first switching elements 1, a plurality of second switching elements 2, and a plurality of switches 8. The execution entity of the control device 50 includes a computer system. The computer system has one or more computers. The computer system is primarily composed of a processor and memory as hardware. The processor executes a program 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-wave carrier signal (see FIGS. 2 and 11 ) 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 values ​​(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 with the left and right reversed as shown in FIG. 11 . The U-phase voltage command, V-phase voltage command, and W-phase voltage command each have the same cycle length. Furthermore, 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. 5, described later, 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. 5, the duty of the control signal SV1 is shown as the V-phase duty. The control device 50 (see FIG. 1) compares the V-phase voltage command with a carrier signal to generate the control signal SV1 provided to the first switching element 1V. The control device 50 also inverts the control signal SV1 provided to the first switching element 1V to generate the control signal SV2 provided to the second switching element 2V. The control device 50 also sets a dead time Td (see FIG. 3) between the high-level period of the control signal SV1 and the high-level period of the control signal SV2 so that the on periods of the first switching element 1V and the second switching element 2V do not overlap.

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

[0032] The U-phase voltage command, 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 values ​​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. 5 . 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 servo motor, the information about the state of the AC load RA1 includes, for example, detection values ​​from a plurality of current sensors that detect output currents (hereinafter also referred to as load currents) iU, iV, and iW flowing through the U-phase, V-phase, and W-phase of the AC load RA1, respectively.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0048] (3.1.1) Operation for Soft-Switching 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 the resonant inductor L1 and the resonant capacitor 9 connected to the target first switching element 1 to resonate, charging the resonant capacitor 9 from the regenerative capacitor 15 and setting the voltage across the target first switching element 1 to zero. This allows the power conversion device 100 to achieve zero-voltage soft switching of the target first switching element 1.

[0049] FIG. 2 is an explanatory diagram illustrating the basic operation of the control device 50 in the power conversion device 100 of the first embodiment when the load current is greater than 0 and the resonant capacitor 9 is charging. FIG. 2 illustrates control signals SU1 and SU2 provided by the control device 50 to the first switching element 1U and the second switching element 2U of the switching circuit 10U, respectively, for the case where the first switching element in question is the first switching element 1U of the switching circuit 10U. Also illustrated in FIG. 2 are the control signal SU6 provided by the control device 50 to the first IGBT 6U of the switch 8U, the load current iU flowing through the U-phase of the AC load RA1, the current iL1 flowing through the resonant inductor L1, the voltage V1u across the first switching element 1U, and the voltage V2u across the second switching element 2U. In FIG. 2, the voltage value of the DC power source E1 is indicated as Vd.

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

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

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

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

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

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

[0056] As described above, the control device 50 determines the additional time Tau based on the load current iU so that iL1 = iU at time t2 when the first period starts, in order to start LC resonance at time t2 when the first period starts and end the resonance half cycle at time t3 when the dead time period Td ends. More specifically, the control device 50 determines the additional time Tau by calculating Tau = iU × (L / V15) using, for example, the detection result of the load current iU by a current sensor or its signal processed value, or an estimated value of the load current iU, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 of the regenerative capacitor 15 (the potential V15 at the fourth end 154 of the regenerative capacitor 15). In this case, the detection result of the load current iU or its signal processed value is the detection value at the carrier period to which the additional time Tau is added, or at the timing closest to that carrier period. The estimated value of the load current iU at this time is, for example, a value obtained by estimating the load current iU in the carrier period to which the additional time Tau is added. The resonance half period in the case of basic operation is half the resonance period, which is the reciprocal of the resonance frequency of the resonance circuit including the resonance inductor L1 and one resonance capacitor 9. Therefore, if the inductance of the resonance inductor L1 is L and the capacitance of the resonance capacitor 9 is C, then the resonance half period is π × (L C) １／２ In the control device 50, the length of the dead time period Td is set to be longer than the length of the resonance half cycle, but the length of the dead time period Td may be set to the same value as the length of the resonance half cycle.

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

[0058] As described above, the control device 50 determines the additional time Tav based on the load current iV so that iL1 = iV at time t12, when the first period starts, in order to start LC resonance at time t12, when the first period starts, and end the resonant half cycle at time t13, when the dead time period Td ends. More specifically, the control device 50 determines the additional time Tav by calculating Tav = iV × (L / V15) using, for example, the detection result of the load current iV by a current sensor, its signal-processed value, or an estimated value of the load current iV, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 of the regenerative capacitor 15. The detection result of the load current iV or its signal-processed value 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 is, for example, an estimated value of the load current iV at the carrier cycle to which the additional time Tav is added.

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

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

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

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

[0063] When the current value of the load current iU is greater than the first current threshold I1, the control device 50 does not provide a high-level period for the control signal SU7. In this case, in the power conversion device 100, the current i9U starts to flow from the resonant capacitor 9U at time t31 when the control signal SU1 changes from high to low. The current i9U decreases to zero before time t33 when the dead time period Td ends, and the voltage V2u across the second switching element 2U becomes zero before time t33 when the dead time period Td ends. As a result, in the power conversion device 100, the second switching element 2U is subjected to zero-voltage soft switching when the control signal SU2 changes from low to high at time t33 when the dead time period Td ends.

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

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

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

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

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

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

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

[0071] When the load current value is smaller than the second current threshold I2 (in other words, when the absolute value of the load current is greater than the absolute value of the second current threshold I2), the control device 50 does not provide a high-level period for the control signal SU6. In this case, in the power conversion device 100, the current i9U begins to flow through the resonant capacitor 9U at time t50 when the control signal SU2 changes from high to low. As a result, in the power conversion device 100, the resonant capacitor 9U is charged, and the voltage V2u across the second switching element 2U increases. The current i9U becomes zero before time t52 when the dead time period Td ends, and the voltage V1u across the first switching element 1U becomes zero before time t52 when the dead time period Td ends. As a result, in the power conversion device 100, the first switching element 1U is zero-voltage soft-switched when the control signal SU1 changes from low to high at time t52 when the dead time period Td ends.

[0072] When the load current value is greater than the second current threshold I2 (in other words, when the absolute value of the load current is less than the absolute value of the second current threshold), the control device 50 sets a high-level period for the control signal SU6, as shown by the two-dot chain line in FIG. 9 . In this case, the high-level period of the control signal SU6 starts at time t51, which is later than time t50, when the dead time period Td starts. The high-level period of the control signal SU6 ends at time t52, when the dead time period Td ends. As a result, in the power conversion device 100, the voltage V1u across the first switching element 1U becomes zero before time t52, when the dead time period Td ends. Therefore, in the power conversion device 100, when the control signal SU1 changes from low to high at time t52, when the dead time period Td ends, the first switching element 1U is subjected to zero-voltage soft switching. Furthermore, the relationship between the load current threshold and the method for determining the high-level period of the control signal to the switch 8 described above is an ideal design example and is not limited to the above design example. For example, the control device 50 may set the high-level period of the control signal to the switch 8 so that the switch 8 is turned on, for example, during the first period, even when the current value of the load current is smaller than the second current threshold I2. The control device 50 may not turn on the switch 8 during the first period even when the current value of the load current is larger than the second current threshold I2. The control device 50 may set the high-level period of the control signal to the switch 8 so that the switch 8 is always turned on, for example, during the first period, regardless of the second current threshold I2. The control device 50 may also keep the switch 8 always in the off state regardless of the second current threshold I2. The control device 50 may also appropriately combine the operations described in (3.1.4). The control device 50 may not necessarily set the high-level period of the control signal to the switch 8 to coincide with the first period as described above. For example, depending on the design time of the resonance half cycle, the control device 50 may set the high level period of the control signal to the switch 8 to a length other than the first period.

[0073] (3.2) First Control Operation and Second Control Operation When the control device 50 determines that a resonant current passing through each of two or more switches 8 out of the multiple switches 8 flows simultaneously through the resonant inductor L1, the control device 50 performs one of a first control operation and a second control operation. The control device 50 is configured to perform one of the first control operation and the second control operation based on an overlap period Tov (see FIGS. 12 and 14 ) during which a resonant current passing through each of the two or more switches 8 flows simultaneously. The phrase “when it is determined that a resonant current passing through each of two or more switches 8 out of the multiple switches 8 flows simultaneously” refers to a case where it is estimated in advance that a resonant current passing through each of two or more switches 8 will flow simultaneously through the resonant inductor L1.

[0074] The first control operation is an operation of shifting the high-level period of the control signal to at least one of the two or more switches 8 so that the resonant currents passing through each of the two or more switches 8 do not flow simultaneously through the resonant inductor L1.

[0075] The second control operation is an operation of shifting the high-level period of the control signal to at least one of the two or more switches 8 so that the high-level period of the control signal to each of the two or more switches 8 overlaps for at least a predetermined period during a dead time period corresponding to each of the two or more switching circuits 10 connected to the two or more switches 8 among the multiple switching circuits 10. The predetermined period is, for example, the entire resonant half cycle of a resonant circuit including the resonant inductor L1 and two or more resonant capacitors 9 connected to each of the two or more switches 8. The control device 50 can execute a third control operation of extending the high-level period of the control signal to each of the two or more switches 8 by an additional time beyond the predetermined period. The control device 50 executes the third control operation when executing the second control operation.

[0076] (3.2.1) Determination of whether two-phase resonant currents flow simultaneously. Figure 5 is a diagram showing time variations in duties corresponding to voltage commands for each of three phases and time variations in load current in an AC load connected to multiple AC terminals of the power conversion device 100 of embodiment 1. In the power conversion device 100, the phases of the voltage commands for the 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 the first region A1 and the second region A2 in Figure 5). In the first region A1 in Figure 5, the duties of the U-phase control signal and the V-phase control signal are approximately 0.75. In the second region A2 in Figure 5, the duties of the U-phase control signal and the V-phase control signal are approximately 0.25. In the power conversion device 100, the polarity of the resonant current is the same as the polarity of the current iL1, and in the first region A1, the polarity of the resonant current is positive, and in the second region A2, the polarity of the resonant current is negative. In the case of the first region A1 of the power conversion device 100, for example, during one cycle of the carrier signal, the time difference between time t1 (see FIG. 2 ) at which the high-level period of the control signal SU6 applied to the first IGBT 6U starts and time t11 (see FIG. 3 ) at which the high-level period of the control signal SV6 applied to the first IGBT 6V starts is short, and there is a possibility that the U-phase resonant current and the V-phase resonant current will simultaneously flow through the resonant inductor L1. In the power conversion device 100, in the second region A2, the direction of the resonant current is opposite to that in the first region A1, but there is a possibility that the U-phase resonant current and the V-phase resonant current will simultaneously flow through the resonant inductor L1.

[0077] 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.

[0078] (3.2.1.1) Case of Charging Operation of Resonant Capacitor FIG. 10 is a timing chart illustrating an example of a boundary condition between a case where currents flowing through the resonant inductor L1 do not overlap (do not flow simultaneously) and a case where currents flow through the resonant inductor L1 overlap (flow simultaneously) in the power conversion device 100 of the first embodiment. Specifically, FIG. 10 is a diagram illustrating an example of a boundary condition between a case where a U-phase current (current passing through the switch 8U) and a V-phase current (current passing through the switch 8V) do not overlap (do not flow simultaneously) and a case where currents flow through the resonant inductor L1 overlap (flow simultaneously). The boundary condition will be described with reference to FIG. 10 . FIG. 11 is a timing chart illustrating an example of a boundary condition between a case where a U-phase resonant current and a V-phase resonant current do not overlap (do not flow simultaneously) and a case where currents flow through the resonant inductor L1 overlap (flow simultaneously) in the power conversion device 100 of the first embodiment. 11 is a diagram showing an example of boundary conditions between a case where the U-phase resonant current and the V-phase resonant current do not overlap (flow simultaneously) in the resonant inductor L1 and a case where they overlap (flow simultaneously). These boundary conditions will be described with reference to FIG.

[0079] 10, in the power conversion device 100, if the time difference ΔTuv is equal to or greater than Tau+Tav+(Tres / 2), the U-phase current and the V-phase current do not overlap in the resonant inductor L1, and if the time difference ΔTuv is less than Tau+Tav+(Tres / 2), the U-phase current and the V-phase current overlap. The time difference ΔTuv is the time difference between time t3 when the high-level period of the control signal SU1 starts and time t13 when the high-level period of the control signal SV1 starts.

[0080] In the power conversion device 100, as shown in FIG. 11, if the time difference ΔTuv is Tres / 2 or more, the U-phase resonant current and the V-phase resonant current do not overlap in the resonant inductor L1, and if the time difference ΔTuv is less than Tres / 2, the U-phase resonant current and the V-phase resonant current overlap in the resonant inductor L1.

[0081] The control device 50 sets a threshold for the time difference ΔTuv to Tres / 2. If the time difference ΔTuv is less than Tres / 2, the control device 50 estimates that resonant currents corresponding to two phases, the switching circuit 10U and the switching circuit 10V, among the multiple switching circuits 10, flow simultaneously through the resonant inductor L1. The above threshold setting is merely an example, and other values ​​may be used. For example, the threshold may be set to a value greater than Tres / 2, taking into account errors in the value of Tres / 2. Furthermore, the method for calculating the time difference ΔTuv used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example, and other calculation methods may be used as long as they can calculate a time difference equivalent to the time difference ΔTuv. For example, the time difference ΔTuv used to determine whether two-phase resonant currents flow simultaneously may be the time difference between the end point t0 (see FIG. 2 ) of the high-level period of the control signal SU2 and the end point t10 (see FIG. 3 ) of the high-level period of the control signal SV2.

[0082] In the following description, the time difference between time t3 (see FIG. 2) when the high-level period of control signal SU1 starts and time t23 (see FIG. 4) when the high-level period of control signal SW1 starts is referred to as ΔTuw. In power conversion device 100, if the time difference ΔTuw is Tres / 2 or more, the U-phase resonant current and the W-phase resonant current do not overlap in resonant inductor L1, and if the time difference ΔTuw is less than Tres / 2, the U-phase resonant current and the W-phase resonant current overlap in resonant inductor L1.

[0083] The control device 50 sets a threshold for the time difference ΔTuw to Tres / 2. If the time difference ΔTuw is less than the threshold, the control device 50 estimates that resonant currents corresponding to two phases, the switching circuit 10U and the switching circuit 10W, among the multiple switching circuits 10, flow simultaneously through the resonant inductor L1. The above threshold setting is merely an example, and other values ​​may be used. For example, the threshold may be set to a value greater than Tres / 2, taking into account errors in the value of Tres / 2. Furthermore, the method for calculating the time difference ΔTuw used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example, and other calculation methods may be used as long as they can calculate a time difference equivalent to the time difference ΔTuw. For example, the time difference ΔTuw used to determine whether two-phase resonant currents flow simultaneously may be the time difference between the end point t0 (see FIG. 2 ) of the high-level period of the control signal SU2 and the end point t20 (see FIG. 4 ) of the high-level period of the control signal SW2.

[0084] In the following description, the time difference between time t13 (see FIG. 3 ) at which the high-level period of the control signal SV1 given to the first switching element 1V of the switching circuit 10V starts and time t23 (see FIG. 4 ) at which the high-level period of the control signal SW1 given to the first switching element 1W of the switching circuit 10W starts will be referred to as ΔTvw. In the power conversion device 100, if the time difference ΔTvw is Tres / 2 or more, the V-phase resonant current and the W-phase resonant current do not overlap in the resonant inductor L1, and if the time difference ΔTvw is less than Tres / 2, the V-phase resonant current and the W-phase resonant current overlap in the resonant inductor L1.

[0085] The control device 50 sets a threshold value for the time difference ΔTvw to Tres / 2. If the time difference ΔTvw is less than the threshold value, the control device 50 estimates that resonant currents corresponding to two phases, the switching circuit 10V and the switching circuit 10W, among the multiple switching circuits 10, flow simultaneously through the resonant inductor L1. The above threshold value is merely an example, and other values ​​may be used. For example, the threshold value may be set to a value greater than Tres / 2, taking into account errors in the value of Tres / 2. Furthermore, the method for calculating the time difference ΔTvw used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example. Any other calculation method may be used as long as it can calculate a time difference equivalent to the time difference ΔTvw. For example, the time difference ΔTvw used to determine whether two-phase resonant currents flow simultaneously may be the time difference between the end point t10 (see FIG. 3 ) of the high-level period of the control signal SV2 and the end point t20 (see FIG. 4 ) of the high-level period of the control signal SW2.

[0086] (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.

[0087] 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 of the high-level period of the control signal SU2 and the start of the high-level period of the control signal SV2 is less than a threshold value (e.g., Tres / 2).

[0088] 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 of the high-level period of the control signal SU2 and the start of the high-level period of the control signal SW2 is less than a threshold value (e.g., Tres / 2).

[0089] 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., Tres / 2).

[0090] (3.2.2) First control action and second control action when it is determined that two-phase resonant currents flow simultaneously When the control device 50 determines, for example, that the resonant currents passing through the two switches 8 respectively flow simultaneously in the resonant inductor L1, it performs one of the first control action and the second control action based on the overlap period Tov during which the resonant currents passing through the two switches 8 respectively flow simultaneously.

[0091] In the first control operation, the control device 50 shifts the high-level period of the control signal to one of the two switches 8 so that the resonant currents passing through the two switches 8 do not simultaneously flow through the resonant inductor L1. When performing the first control operation, the control device 50 performs the first control operation so that the length of the dead time period Td between the high-level period of the control signal provided to the first switching element 1 and the high-level period of the control signal provided to the second switching element 2 of the two switching circuits 10 corresponding to the two switches 8 does not change.

[0092] In the second control operation, the control device 50 shifts the high-level period of the control signal to one of the two switches 8 so that the high-level period of the control signal to each of the two switches 8 overlaps by a predetermined period or more in the dead-time period Td corresponding to each of the two switching circuits 10 connected to the two switches 8 out of the multiple (three) switching circuits 10. When performing the second control operation, the control device 50 performs the second control operation so that the length of the dead-time period Td between the high-level period of the control signal provided to the first switching element 1 and the high-level period of the control signal provided to the second switching element 2 of the two switching circuits 10 corresponding to the two switches 8 does not change.

[0093] For ease of explanation, the shift time of the high-level period of the control signal SU6 or SU7 to the switch 8U when the high-level period of the control signal SU6 or SU7 is shifted will be referred to as Tsu. Furthermore, the shift time of the high-level period of the control signal SV6 or SV7 to the switch 8V when the high-level period of the control signal SV6 or SV7 is shifted will be referred to as Tsv. Furthermore, the shift time of the high-level period of the control signal SW6 or SW7 to the switch 8W when the high-level period of the control signal SW6 or SW7 is shifted will be referred to as Tws. Note that the following description will be given assuming that the resonant current flowing through the U-phase switch 8U and the V-phase switch 8V overlap in the resonant inductor L1, but the same applies to the cases of the U and W phases and the V and W phases.

[0094] (3.2.2.1) Operation for Soft-Switching the First Switching Element FIG. 12 is a timing chart illustrating the operation when the control device 50 performs the first control operation in the power conversion device 100 of embodiment 1. The upper part of FIG. 12 illustrates a timing chart in the case where the control device 50 has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously during a period corresponding to the first region A1 of FIG. 5 . The upper part of FIG. 12 illustrates timing charts of the control signals SU1, SU2, SV1, SV2, control signals SU6, SV6, load currents iU, iV, and current iL1 before the shift. In the upper part of FIG. 12 , the length of the overlap period Tov during which the resonant current (U-phase resonant current) passing through switch 8U and the resonant current (V-phase resonant current) passing through switch 8V simultaneously flows is shorter than the length of the time difference ΔTuv.

[0095] The lower part of Fig. 12 shows a timing chart for when the control device 50 performs the first control operation (hereinafter also referred to as "after shift"). The lower part of Fig. 12 shows timing charts of the control signals SV1, SV2, control signal SV6, load currents iU, iV, and current iL1 after the shift. In the example of Fig. 12, the polarities of the load currents iU, iV flowing through the two AC terminals 41U, 41V connected to the two switches 8U, 8V are positive, and the absolute values ​​of the load current iU and the load current iV are the same. Note that Fig. 12 shows a timing chart for one cycle of the carrier signal.

[0096] In the example of FIG. 12 , when performing the first control operation, the control device 50 shifts the high-level period of the control signal SV6 by a shift time Tsv in a direction that delays the high-level period. The length of the shift time Tsv is the same as the length of the overlap period Tov during which the resonant current (U-phase resonant current) passing through the switch 8U and the resonant current (V-phase resonant current) passing through the switch 8V simultaneously. Tov = Tres / 2 - ΔTuv. Therefore, as shown in the lower part of FIG. 12 , the resonant current passing through the switch 8U and the resonant current passing through the switch 8V do not overlap, enabling zero-voltage soft switching of the first switching element 1U.

[0097] 13 is a waveform diagram of the load current in the power conversion device 100 of embodiment 1. In the power conversion device 100, when the control device 50 performs the first control operation when the condition Tov<ΔTuv is satisfied, the shift time Tsv can be shortened compared to when the control device 50 shifts the high-level period of the control signal SV6 by the shift time Tsv in the direction of advancing the high-level period of the control signal SV6 when the condition Tov<ΔTuv is satisfied, and the current distortion of the output current iV within one period of the carrier signal (see FIG. 13 ) is reduced. Note that one period of the carrier signal is shorter than one period of the output current iV and shorter than half the period of the output current iV.

[0098] Fig. 14 is a timing chart for explaining the operation when the control device 50 performs the second control operation in the power conversion device 100 of embodiment 1. The upper part of Fig. 14 illustrates a timing chart in the case where the control device 50 has determined in advance that two-phase resonant currents, U-phase and V-phase, will flow simultaneously during a period corresponding to the first region A1 of Fig. 5. The upper part of Fig. 14 illustrates timing charts of the control signals SU1, SU2, SV1, SV2, control signals SU6, SV6, load currents iU, iV, and current iL1 before the shift. In the upper part of Fig. 14, the length of the overlap period Tov during which the resonant current (U-phase resonant current) passing through switch 8U and the resonant current (V-phase resonant current) passing through switch 8V simultaneously flows is longer than the length of the time difference ΔTuv.

[0099] The lower part of Fig. 14 shows a timing chart when the control device 50 performs the second control operation (hereinafter also referred to as "post-shift"). The lower part of Fig. 14 shows timing charts of the control signals SV1, SV2, control signal SV6, load currents iU, iV, and current iL1 after the shift. In the example of Fig. 14, the polarities of the load currents iU, iV flowing through the two AC terminals 41U, 41V connected to the two switches 8U, 8V are positive, and the absolute values ​​of the load current iU and the load current iV are the same. Note that Fig. 14 shows a timing chart for one cycle of the carrier signal.

[0100] When the length of the overlap period Tov is longer than the length of the time difference ΔTuv, the control device 50 executes the first step, the second step, and the third step in this order.

[0101] In the first step, the U-phase control signals SU1 and SU2 are synchronized with the V-phase control signals SV1 and SV2. In the example shown in the lower part of FIG. 14 , the start point of the high-level period of the V-phase control signal SV1 and the end point of the high-level period of the V-phase control signal SV2 are shifted forward by a shift time Tsv in the example shown in the upper part of FIG. 14 , thereby synchronizing the U-phase control signals SU1 and SU2 with the V-phase control signals SV1 and SV2. In the example shown in FIG. 14 , the shift time Tsv is the time difference ΔTuv between the start point of the high-level period of the control signal SU1 before the shift and the start point of the high-level period of the control signal SV1. The time difference ΔTuv may also be the time difference between the end point of the high-level period of the control signal SU2 and the end point of the control signal SV2. In addition, in the first step, the high-level periods of the U-phase control signals SU1 and SU2 may be shifted in a direction that delays them by the shift time Tsu, thereby synchronizing the U-phase control signals SU1 and SU2 with the V-phase control signals SV1 and SV2.

[0102] In the second step, an additional time Tad corresponding to the current value |iU+iV| of the total current of the two-phase load currents iU and iV is added to the high-level periods of the control signals SU6 and SV6 sent to the switches 8 for two phases, corresponding to the resonant half-cycle Tres / 2 of each of the two phases. The control device 50 determines the additional time Tad by calculating Tad=L×|iU+iV| / V15 using, for example, the detection results of the load currents iU and iV by the current sensors, or their signal-processed values, or the estimated values ​​of the load currents iU and iV, the inductance L of the resonant inductor L1 that is stored in advance, and the detection result of the potential V15 of the regenerative capacitor 15. The second step corresponds to the third control operation described above.

[0103] In the third step, the lengths of the high level periods and dead time periods of the control signals SU6 and SV6 are set to 2, which is the resonant half period of the resonant circuit. １／２ More specifically, the control device 50 changes the length of the period obtained by subtracting the additional time Tad from the high level period of each of the control signals SU6 and SV6 in accordance with 2 × Tres / 2, which is the resonant half period of the resonant circuit. １／２×Tres / 2, and the dead time period is changed to a dead time period longer than the resonant half cycle of the resonant circuit. Note that the end point of the high level period of the control signals SU6 and SV6 may be any point after the end point of the resonant half cycle.

[0104] In the power conversion device 100, unless the control device 50 executes the second control operation, the voltages V2u and V2v across the second switching elements 2U and 2V do not rise to Vd when the control signals SU1 and SV1 change from a low level to a high level (the end of the dead time period corresponding to the U phase and the V phase, respectively). In other words, unless the control device 50 executes the second control operation, the charging of the resonant capacitors 9U and 9V does not finish when the dead time period Td corresponding to the U phase and the V phase, respectively, ends. Therefore, unless the second control operation is executed, the voltages across the first switching elements 1U and 1V do not decrease to zero when the dead time period corresponding to the U phase and the V phase, respectively, ends. As a result, in the power conversion device 100, the switching of the first switching elements 1U and 1V becomes hard switching.

[0105] In contrast, when the control device 50 executes the second control operation, 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 (at the end of the dead time period corresponding to the U phase and the V phase, respectively). In other words, when the control device 50 executes the second control operation, charging of the resonant capacitors 9U and 9V ends at the end of the dead time period 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 the second control operation, the switching of the first switching elements 1U and 1V becomes zero-voltage soft switching.

[0106] Furthermore, when it is determined that the two-phase resonant currents overlap, it is sufficient that at least a portion of the control signals to the two switches 8 overlap, and 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, as shown in Fig. 14, the start point of the high-level period of the control signal SV1 may be earlier than the start point of the high-level period of the control signal SU1, and 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.

[0107] 14, when the control device 50 performs the second control operation, it shifts the high-level period of the control signal SV6 forward by a shift time Tsv. The length of the shift time Tsv is the same as the length of the time difference ΔTuv.

[0108] In the power conversion device 100, when the control device 50 performs the second control operation when the condition Tov > ΔTuv is satisfied, the shift time Tsv can be shortened compared to when the control device 50 shifts the high-level period of the control signal SV6 in the direction of delaying it by the length of the overlap period Tov when the condition Tov > ΔTuv is satisfied, and the current distortion of the output current iV within one period of the carrier signal (see Figure 13) is reduced.

[0109] (3.2.2.2) Operation for Soft-Switching the Second Switching Element When the control device 50 soft-switches the second switching element 2, the high-level periods and low-level periods of the control signals SU1, SU2, SV1, and SV2 before the shift are reversed compared to when soft-switching the first switching element 1, and the polarities of the load currents iU, iV and current iL1 are reversed. Furthermore, when soft-switching the second switching element 2, the control device 50 shifts the high-level period of one of the control signals SU7 and SV7 by the shift time instead of the control signals SU6 and SV6.

[0110] The control device 50 performs a first control action when the length of the overlap period Tov is shorter than the length of the time difference ΔTuv, and performs a second control action when the length of the overlap period Tov is longer than the length of the time difference ΔTuv.

[0111] In the power conversion device 100, when the control device 50 performs neither the first control operation nor the second control operation, the voltages V2u and V2v across the second switching elements 2U and 2V do not decrease to zero 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 performs neither the first control operation nor the second control operation, the discharge of the resonant capacitors 9U and 9V does not finish when the dead time period Td corresponding to the U phase and the V phase, respectively, ends. Therefore, when the control device 50 performs neither the first control operation nor the second control operation, the voltages across the second switching elements 2U and 2V do not decrease to zero when the dead time period Td corresponding to the U phase and the V phase, respectively, ends. As a result, in the power conversion device 100, the switching of the second switching elements 2U and 2V becomes hard switching.

[0112] In contrast, when the control device 50 performs the first control operation or the second control operation, the voltages V2u and V2v across the second switching elements 2U and 2V decrease to zero at the point in time when the control signals SU2 and SV2 change from a low-level period to a high-level period (the point in time when the dead time periods Td corresponding to the U phase and the V phase, respectively, end). In other words, when the control device 50 performs the first control operation or the second control operation, the discharge of the resonant capacitors 9U and 9V ends at the point in time when the dead time periods 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 performs the first control operation or the second control operation, the switching of the second switching elements 2U and 2V becomes zero-voltage soft switching.

[0113] (3.3) Operation when it is determined that three-phase resonant currents flow simultaneously The power conversion device 100 according to the first embodiment performs either the first control operation or the second control operation even when the control device 50 determines that three-phase resonant currents overlap, for both the operation for soft switching the first switching element 1 and the operation for soft switching the second switching element 2.

[0114] Below, we will explain the operation for soft switching the first switching element 1 when it is determined in advance that resonant currents passing through three of the multiple switches 8 simultaneously flow through the resonant inductor L1.

[0115] When the control device 50 determines that resonant currents passing through three switches 8 out of the multiple switches 8 simultaneously flow through the resonant inductor L1, it performs either a first control operation or a second control operation on two switches 8 having the same load current polarity based on the overlap period Tov. Furthermore, it stops the operation of the switch 8 corresponding to the AC terminal 41 of one phase having the opposite load current polarity (the high-level period of the control signal within one carrier cycle is set to zero). The phrase "when it is determined that resonant currents passing through three switches 8 out of the multiple switches 8 simultaneously flow" refers to a case where it has been estimated in advance that resonant currents passing through the three switches 8 simultaneously flow through the resonant inductor L1. Control device 50 determines that three-phase resonant currents are flowing simultaneously when, for example, the time difference between the start point of a high-level period of control signal SU6 corresponding to the U phase and the start point of a high-level period of control signal SV6 corresponding to the V phase, the time difference between the start point of a high-level period of control signal SV6 corresponding to the V phase and the start point of a high-level period of control signal SW6 corresponding to the W phase, and the time difference between the start point of a high-level period of control signal SW6 corresponding to the W phase and the start point of a high-level period of control signal SU6 corresponding to the U phase are all less than a threshold. When it is determined that resonant currents are flowing simultaneously through three switches 8 out of the multiple switches 8, one of two switches 8 that correspond one-to-one to two AC terminals 41 having the same load current polarity is designated as a first switch, and the other is designated as a second switch.

[0116] 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.

[0117] In contrast, in the power conversion device 100, if the control device 50 determines in advance that three-phase resonant currents of the U phase, V phase, and W phase will flow simultaneously, it performs the first control operation or the second control operation, thereby realizing soft switching of the two target switching elements corresponding to the two switches 8 and reducing current distortion in the load current (output current) flowing through the AC terminal 41 of one of the two switches 8.

[0118] In the case of an operation for soft-switching the second switching element 2, the control device 50 also performs either the first control operation or the second control operation on the two switches 8 having the same load current polarity based on the overlap period Tov, and further stops the operation of the switch 8 corresponding to the AC terminal 41 of one phase having the different load current polarity.

[0119] (4) Advantages In the power conversion device 100 according to the first embodiment, the control device 50 performs one of a first control operation and a second control operation based on an overlap period Tov during which resonant currents flow simultaneously through the two or more switches 8. The first control operation shifts the high-level period of a control signal to at least one switch 8 among the two or more switches 8 so that the resonant currents flowing simultaneously through the resonant inductor L1 do not flow simultaneously through the resonant inductor L1. The second control operation shifts the high-level period of a control signal to at least one switch 8 among the two or more switches 8 so that the high-level periods of the control signal to the two or more switches 8 overlap by a predetermined period or more in dead-time periods corresponding to two or more switching circuits 10 connected to the two or more switches 8 among the multiple switching circuits 10.

[0120] The above configuration makes it possible to reduce current distortion. More specifically, the above configuration makes it possible for the control device 50 to perform one of the first control operation and the second control operation based on the overlap period Tov during which the resonant currents flow simultaneously through the two or more switches 8, thereby realizing soft switching and reducing current distortion in the load current (output current).

[0121] Furthermore, in the power conversion device 100 according to the first embodiment, the first switching element 1 or the second switching element 2 that is a target of zero voltage soft switching in one of the two or more switching circuits 10 is designated as the first target switching element. The first switching element 1 or the second switching element 2 that is a target of zero voltage soft switching in one of the two or more switching circuits 10 that is different from the switching circuit 10 that includes the first target switching element is designated as the second target switching element. The time difference between the start of the high-level period of the control signal to the first target switching element and the start of the high-level period of the control signal to the second target switching element is designated as ΔT. In this case, the control device 50 performs a first control operation when the length of the overlap period Tov of the resonant current is less than ΔT, and performs a second control operation when the length of the overlap period Tov of the resonant current is equal to or greater than ΔT.

[0122] According to the above configuration, it is possible to reduce the amount of shift in the high level period of the control signal, and to reduce current distortion.

[0123] Furthermore, in the power conversion device 100 according to the first embodiment, one of the two or more switches 8 is designated as a first switch, and the other switch different from the first switch is designated as a second switch. In this case, when performing the first control operation, the control device 50 shifts the high-level period of the control signal to one of the first switch and the second switch so that the resonant current flows through the second switch from the time point when the current value of the resonant current passing through the first switch reaches an extreme value and coincides with the current value of the load current flowing through the AC terminal 41 corresponding to the first switch among the two or more AC terminals 41.

[0124] According to the above configuration, the amount of shift in the high level period of the control signal can be reduced, and higher frequencies can be achieved.

[0125] Second Embodiment (1) Circuit Configuration of Power Conversion Device The circuit configuration of the power conversion device 100 according to the second embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0126] (2) Operation of the power conversion device In the following, an example is given of a case where it is estimated that the resonant current flowing through the U-phase switch 8U and the resonant current flowing through the V-phase switch 8V overlap in the resonant inductor L1, but the same is true for the cases of the U-phase and W-phase, and the V-phase and W-phase.

[0127] (2.1) Operation for Soft-Switching the First Switching Element With regard to the operation of the power conversion device 100 according to the second embodiment, the operation for soft-switching the first switching element 1 will be described with reference to Figs. 15 and 16. Fig. 15 is a timing chart for describing the operation when the control device 50 performs the first control operation in the power conversion device 100 according to the second embodiment. Fig. 16 is a timing chart for describing the operation when the control device 50 performs the second control operation in the power conversion device 100 according to the second embodiment. Figs. 15 and 16 can be interpreted in the same way as Figs. 12 and 14, respectively, and therefore will not be described again.

[0128] The control device 50 performs the first control operation when the length of the overlap period Tov is shorter than the length of the time difference ΔTuv. In the example of FIG. 15, the length of the time difference ΔTuv is longer than the length of the overlap period Tov. When performing the first control operation, the control device 50 shifts the high-level period of the control signal SU6 and the high-level period of the control signal SV6 in opposite directions. In the example of FIG. 15, the control device 50 shifts the end point of the high-level period of the control signal SU6 by a shift time Tsu in an earlier direction, and shifts the end point of the high-level period of the control signal SV6 by a shift time Tsv in a later direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 by a shift time Tsu in an earlier direction. Furthermore, the control device 50 shifts the start point of the high-level period of the control signal SV1 by a shift time Tsv in a later direction. The length of the shift time Tsu is equal to the length of Tov / 2. The length of the shift time Tsv is the same as the length of Tov / 2.

[0129] In the power conversion device 100 according to the second embodiment, the control device 50 performs the first control operation, thereby eliminating the overlap period Tov between the U-phase resonant current and the V-phase resonant current as shown in FIG. 15, and enabling zero-voltage soft switching of the first switching elements 1U and 1V.

[0130] The control device 50 performs the second control action when the length of the overlap period Tov is longer than the length of the time difference ΔTuv.

[0131] In the example of FIG. 16 , the length of the time difference ΔTuv is shorter than the length of the overlap period Tov. When performing the second control operation, the control device 50 shifts the high-level period of the control signal SU6 and the high-level period of the control signal SV6 in opposite directions. In the example of FIG. 16 , the control device 50 shifts the end point of the high-level period of the control signal SU6 by a shift time Tsu in a later direction, and shifts the end point of the high-level period of the control signal SV6 by a shift time Tsv in an earlier direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 by a shift time Tsu in a later direction. Furthermore, the control device 50 shifts the start point of the high-level period of the control signal SV1 by a shift time Tsv in an earlier direction. The length of the shift time Tsu is equal to the length of ΔTuv / 2. Furthermore, the length of the shift time Tsv is equal to the length of ΔTuv / 2.

[0132] In the power conversion device 100 of embodiment 2, the control device 50 performs the second control operation, thereby enabling zero-voltage soft switching of the first switching elements 1U, 1V, similar to when the control device 50 performs the second control operation in the power conversion device 100 of embodiment 1.

[0133] (2.2) Operation for Soft-Switching the Second Switching Element When the control device 50 soft-switches the second switching element 2, the high-level periods and low-level periods of the control signals SU1, SU2, SV1, and SV2 before shifting are reversed compared to when soft-switching the first switching element 1, and the polarities of the load currents iU, iV and current iL1 are reversed. Furthermore, when the control device 50 soft-switches the second switching element 2, it shifts the high-level periods of the control signals SU7 and SV7 in opposite directions to each other instead of the control signals SU6 and SV6.

[0134] In the power conversion device 100 according to the second embodiment, the control device 50 performs either the first control operation or the second control operation, thereby completing the discharge of the resonant capacitors 9U, 9V at the end of the dead time period corresponding to each of the U phase and the V phase. Therefore, in the power conversion device 100, when the control device 50 performs the first control operation or the second control operation, the switching of the second switching elements 2U, 2V becomes zero-voltage soft switching.

[0135] (3) Advantages In the power conversion device 100 according to the second embodiment, similarly to the power conversion device 100 according to the first embodiment, the control device 50 performs one of the first control operation and the second control operation based on the overlap period Tov during which the resonant currents flow simultaneously through the two or more switches 8, respectively, and therefore it is possible to reduce current distortion.

[0136] Furthermore, in the power conversion device 100 according to the second embodiment, when the control device 50 performs the first control operation, the high-level periods of the control signals to the two switches 8 are shifted in opposite directions, and the lengths of the shift times for the high-level periods of the two control signals are made the same. Therefore, compared to the first embodiment, current distortion in the load current of each phase can be reduced, and variation can also be reduced.

[0137] Furthermore, in the power conversion device 100 according to the second embodiment, when the control device 50 performs the second control operation, the high-level periods of the control signals to the two switches 8 are shifted in opposite directions, and the lengths of the shift times for the high-level periods of the two control signals are made the same. Therefore, compared to the first embodiment, current distortion in the load current of each phase can be reduced, and variations can also be reduced.

[0138] Third Embodiment (1) Circuit Configuration of Power Conversion Device The circuit configuration of the power conversion device 100 according to the third embodiment is the same as that of the power conversion device 100 according to the first embodiment (see FIG. 1), and therefore will not be illustrated or described again.

[0139] (2) Operation of the power conversion device In the following, an example is given of a case where it is estimated that the resonant current flowing through the U-phase switch 8U and the resonant current flowing through the V-phase switch 8V overlap in the resonant inductor L1, but the same is true for the cases of the U-phase and W-phase, and the V-phase and W-phase.

[0140] (2.1) Operation for Soft-Switching the First Switching Element With regard to the operation of the power conversion device 100 according to the third embodiment, the operation for soft-switching the first switching element 1 will be described with reference to Figs. 17 and 18. Fig. 17 is a timing chart for describing the operation when the control device 50 performs the first control operation in the power conversion device 100 according to the third embodiment. Fig. 18 is a timing chart for describing the operation when the control device 50 performs the second control operation in the power conversion device 100 according to the third embodiment. Figs. 17 and 18 can be interpreted in the same way as Figs. 12 and 14, respectively, and therefore will not be described again.

[0141] The control device 50 performs the first control operation when the length of the overlap period Tov is shorter than the length of the time difference ΔTuv. In the example of FIG. 17 , the length of the time difference ΔTuv is longer than the length of the overlap period Tov. When performing the first control operation, the control device 50 shifts the high-level period of the control signal SU6 and the high-level period of the control signal SV6 in opposite directions. In the example of FIG. 17 , the control device 50 shifts the end point of the high-level period of the control signal SU6 by a shift time Tsu in an earlier direction, and shifts the end point of the high-level period of the control signal SV6 by a shift time Tsv in a later direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 by a shift time Tsu in an earlier direction. Furthermore, the control device 50 shifts the start point of the high-level period of the control signal SV1 by a shift time Tsv in a later direction. When Tov=Ta+Tb, the length of the shift time Tsu is Tov·Tb / (Ta+Tb). The length of the shift time Tsv is the same as the length of Tov·Ta / (Ta+Tb). Each of Ta and Tb can be set to any value.

[0142] In the power conversion device 100 according to the third embodiment, the control device 50 performs the first control operation, thereby eliminating the overlap period Tov between the U-phase resonant current and the V-phase resonant current before the shift, as shown in the waveform of the post-shift current iL1 in the lower part of Figure 17, and enabling zero-voltage soft switching of the first switching elements 1U and 1V.

[0143] The control device 50 performs the second control action when the length of the overlap period Tov is longer than the length of the time difference ΔTuv.

[0144] In the example of FIG. 18 , the length of the time difference ΔTuv is shorter than the length of the overlap period Tov. When performing the second control operation, the control device 50 shifts the high-level periods of the control signals SU6 and SV6 in opposite directions. The control device 50 shifts the end point of the high-level period of the control signal SU6, whose high-level period has an earlier phase than the other of the two control signals SU6 and SV6, by a shift time Tsu in a direction to delay the end point of the high-level period of the control signal SU6, whose high-level period has an earlier phase, by the shift time Tsv in a direction to advance the end point of the high-level period of the control signal SV6, whose high-level period has a later phase, by the shift time Tsv. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 in a direction to delay the start point of the high-level period by the shift time Tsu. The control device 50 also shifts the start point of the high-level period of the control signal SV1 in a direction to advance the start point of the high-level period by the shift time Tsv. The length of the shift time Tsu is (√2 / 4) Tres - Tov Tb / (Ta + Tb). The length of the shift time Tsv is Tov Tb / (Ta + Tb) + ΔTuv - (√2 / 4) Tres. Note that √2 represents the square root of 2 (i.e., √2 = 1.41421356...).

[0145] In the power conversion device 100 of embodiment 3, the control device 50 performs the second control operation, thereby enabling zero-voltage soft switching of the first switching elements 1U, 1V, similar to when the control device 50 performs the second control operation in the power conversion device 100 of embodiment 1.

[0146] (2.2) Operation for Soft-Switching the Second Switching Element When the control device 50 soft-switches the second switching element 2, the high-level periods and low-level periods of the control signals SU1, SU2, SV1, and SV2 before shifting are reversed compared to when soft-switching the first switching element 1, and the polarities of the load currents iU, iV and current iL1 are reversed. Furthermore, when the control device 50 soft-switches the second switching element 2, it shifts the high-level periods of the control signals SU7 and SV7 in opposite directions to each other instead of the control signals SU6 and SV6.

[0147] In the power conversion device 100 according to the third embodiment, the control device 50 performs either the first control operation or the second control operation, thereby completing the discharge of the resonant capacitors 9U, 9V at the end of the dead time period corresponding to each of the U phase and the V phase. Therefore, in the power conversion device 100, when the control device 50 performs the first control operation or the second control operation, the switching of the second switching elements 2U, 2V becomes zero-voltage soft switching.

[0148] (3) Advantages In the power conversion device 100 according to the third embodiment, similarly to the power conversion device 100 according to the first embodiment, the control device 50 performs one of the first control operation and the second control operation based on the overlap period Tov during which the resonant currents flow simultaneously through the two or more switches 8, respectively, and therefore it is possible to reduce current distortion.

[0149] Furthermore, in the power conversion device 100 according to the third embodiment, when the control device 50 performs the first control operation, the high-level periods of the control signals to the two switches 8 are shifted in opposite directions to each other, thereby reducing current distortion and variation in the load current of each phase compared to the first embodiment.

[0150] Furthermore, in the power conversion device 100 according to the second embodiment, when the control device 50 performs the second control operation, the high-level periods of the control signals to the two switches 8 are shifted in opposite directions to each other, thereby reducing current distortion and variation in the load current of each phase compared to the first embodiment.

[0151] (Embodiment 4) A power conversion device 100A according to embodiment 4 will be described with reference to Fig. 19. Fig. 19 is a circuit diagram of a system including the power conversion device 100A according to embodiment 4. Regarding the power conversion device 100A according to embodiment 4, components similar to those of the power conversion device 100 according to embodiment 1 are denoted by the same reference numerals, and description thereof will be omitted.

[0152] In the power conversion device 100A according to the fourth 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 fourth 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.

[0153] In the power conversion device 100A according to the fourth embodiment, each of the first IGBT 6 and the second IGBT 7 may be replaced with a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or a bipolar transistor. In this case, the diode 61 and the diode 71 in FIG. 19 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 fourth 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 also be built into the chip.

[0154] The operation of the control device 50 is, for example, the same as the operation of the control device 50 in embodiment 1, but is not limited to this and may be the same as the operation of the control device 50 in embodiment 2 or embodiment 3, or these operations may be performed in an appropriate combination.

[0155] The power conversion device 100A according to the fourth embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0156] Fifth Embodiment A power conversion device 100B according to a fifth embodiment will be described with reference to Fig. 20. Fig. 20 is a circuit diagram of a system including the power conversion device 100B according to the fifth embodiment. Constituent elements of the power conversion device 100B according to the fifth embodiment that are the same as those of the power conversion device 100 according to the first embodiment will be denoted by the same reference numerals, and description thereof will be omitted.

[0157] In a power conversion device 100B according to the fifth 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 100B according to the fifth 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.

[0158] In the power conversion device 100B according to the fifth 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. 20 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 100B according to the fifth 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 also be an element built into the chip.

[0159] The operation of the control device 50 is the same as that of the control device 50 of the first embodiment, for example, but is not limited to this and may be the same as that of the control device 50 of the second or third embodiment.

[0160] The power conversion device 100B according to the fifth embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0161] Sixth Embodiment A power conversion device 100C according to a sixth embodiment will be described with reference to Fig. 21. Fig. 21 is a circuit diagram of a system including the power conversion device 100C according to the sixth embodiment. Regarding the power conversion device 100C according to the sixth embodiment, components similar to those of the power conversion device 100 according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

[0162] In a power conversion device 100C according to the sixth 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 100C according to the sixth 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 the 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.

[0163] 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 the control device 50 of embodiment 2 or embodiment 3, or these operations may be performed in an appropriate combination.

[0164] The power conversion device 100C according to the sixth embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0165] Seventh Embodiment A power conversion device 100D according to a seventh embodiment will be described with reference to Fig. 22. Fig. 22 is a circuit diagram of a system including the power conversion device 100D according to the seventh embodiment. Regarding the power conversion device 100D according to the seventh embodiment, components similar to those of the power conversion device 100 according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

[0166] In the power conversion device 100D according to the seventh 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 100D according to the seventh 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.

[0167] 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 the control device 50 of embodiment 1 or embodiment 2, or these operations may be performed in an appropriate combination.

[0168] The power conversion device 100D according to the seventh embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0169] Eighth Embodiment A power conversion device 100E according to an eighth embodiment will be described with reference to Fig. 23. Fig. 23 is a circuit diagram of a system including the power conversion device 100E according to the eighth embodiment. Regarding the power conversion device 100E according to the eighth embodiment, components similar to those of the power conversion device 100 according to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

[0170] In the power conversion device 100E according to the eighth 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, when the MOSFET 80 is in an on state, the switch 8 is in an on state, and when the MOSFET 80 is in an off state, the switch 8 is in an off state.

[0171] 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.

[0172] 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 100E, 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 100E, 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.

[0173] In the power conversion device 100E according to the eighth embodiment, each of the plurality of MOSFETs 80 may be replaced with an IGBT. Furthermore, in the power conversion device 100E according to the eighth 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.

[0174] 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 the control device 50 of embodiment 2 or embodiment 3, or these operations may be performed in an appropriate combination.

[0175] The power conversion device 100E according to the eighth embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0176] (Embodiment 9) A power conversion device 100F according to embodiment 9 will be described with reference to Fig. 24. Fig. 24 is a circuit diagram of a system including the power conversion device 100F according to embodiment 9. Regarding the power conversion device 100F according to embodiment 9, components similar to those of the power conversion device 100 according to embodiment 1 are denoted by the same reference numerals, and description thereof will be omitted.

[0177] In a power conversion device 100F according to the ninth 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 100F according to the ninth 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.

[0178] 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 the control device 50 of embodiment 2 or embodiment 3, or these operations may be performed in an appropriate combination.

[0179] The power conversion device 100F according to the ninth embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0180] (Tenth Embodiment) A power conversion device 100G according to a tenth embodiment will be described below with reference to Fig. 25. Fig. 25 is a circuit diagram of a system including the power conversion device 100G according to the tenth embodiment. The power conversion device 100G according to the tenth 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 100G according to the tenth 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.

[0181] The power conversion device 100G 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 100G, 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.

[0182] In the power conversion device 100G according to the tenth embodiment, the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is equal to the voltage value Vd of the DC power supply E1 divided by the capacitor 16 and the regenerative capacitor 15. Therefore, the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is approximately Vd / 2. In the power conversion device 100G according to the tenth 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.

[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 the control device 50 of embodiment 2 or embodiment 3, or these operations may be performed in an appropriate combination.

[0184] The power conversion device 100G according to the tenth embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0185] (Embodiment 11) A power conversion device 100H according to embodiment 11 will be described below with reference to Fig. 26. Fig. 26 is a circuit diagram of a system including the power conversion device 100H according to embodiment 11. The power conversion device 100H according to embodiment 11 differs from the power conversion device 100 according to embodiment 1 in that a regenerative capacitor 15 is connected between the second end of the resonance inductor L1 and the first DC terminal 31. With regard to the power conversion device 100H 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 description thereof will be omitted.

[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 the control device 50 of embodiment 2 or embodiment 3, or these operations may be performed in an appropriate combination.

[0187] The power conversion device 100H according to the eleventh embodiment can reduce current distortion, similarly to the power conversion device 100 according to the first embodiment.

[0188] (Modifications) The above-described first to eleventh embodiments are merely examples of various embodiments of the present disclosure. The above-described first to eleventh embodiments can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved.

[0189] 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.

[0190] For example, the control device 50 may determine that two-phase resonant currents are flowing simultaneously when any one of the current difference between the U-phase load current iU and the V-phase load current iV, the current difference between the V-phase load current iV and the W-phase load current iW, and the current difference between the W-phase load current iW and the U-phase load current iU is less than a current difference threshold.

[0191] 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.

[0192] 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).

[0193] 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.

[0194] Furthermore, in the power conversion devices 100, 100A to 100H, 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.

[0195] The dead time period may be set by a dead time generation circuit included in a gate driver IC (Integrated Circuit) or the like that is provided separately from the control device 50. Alternatively, the control device 50 may include a gate driver IC, and the dead time period may be set by a dead time generation circuit included in the gate driver IC.

[0196] Furthermore, the power conversion devices 100, 100A-100H are not limited to a configuration that outputs three-phase AC, but may be configured to output three or more phases of polyphase AC. Regarding the method for determining the additional times Tau, Tav, and Taw described in the "(3.1) Basic Example" section of the first embodiment, the formulas used are ideal design examples, and the invention is not limited to always calculating using such formulas. In some cases, the additional times Tau, Tav, and Taw may be set to 0 or another fixed time. Furthermore, values ​​calculated using another formula may be used as long as the purpose of the additional times Tau, Tav, and Taw can be achieved. For example, in the basic example, Tau = iU × (L / V15) is calculated, but this is not limiting. Tau may be set to 0, set between 0 and iU × (L / V15), or always set to a constant additional time. It may also be calculated using another formula, or a combination of these.

[0197] (Other Examples) (Example 1) (1) Circuit Configuration of Power Conversion Device The circuit configuration of the power conversion device 100 according to Example 1 is the same as that of the power conversion device 100 according to embodiment 1 (see FIG. 1), and therefore will not be illustrated or described.

[0198] (2) Operation of the power conversion device In the following, an example is given of a case where it is estimated that the resonant current flowing through the U-phase switch 8U and the resonant current flowing through the V-phase switch 8V overlap in the resonant inductor L1, but the same is true for the cases of the U-phase and W-phase, and the V-phase and W-phase.

[0199] (2.1) Operation for Soft-Switching First Switching Element With regard to the operation of the power conversion device 100 according to Example 1, the operation for soft-switching the first switching element 1 will be described with reference to Figs. 27 and 28. Fig. 27 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to Example 1 performs the second control operation. Fig. 28 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of Example 1 performs the first control operation. Figs. 27 and 28 can be interpreted in the same way as Figs. 14 and 12, respectively, and therefore will not be described again.

[0200] The control device 50 performs the second control action when the length of the overlap period Tov is shorter than the length of the time difference ΔTuv.

[0201] In the example of Figure 27, the length of the time difference ΔTuv is longer than the length of the overlap period Tov. When performing the second control operation, the control device 50 shifts the end points of the high-level periods of the control signals SV1, SV2, SV6, and SV7 by the shift time Tsv in an earlier direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SV1 in an earlier direction by the shift time Tsv. The length of the shift time Tsv is the same as the length of ΔTuv.

[0202] In the power conversion device 100 of Example 1, the control device 50 performs the second control operation, thereby enabling zero-voltage soft switching of the first switching elements 1U, 1V, similar to when the control device 50 performs the second control operation in the power conversion device 100 of Embodiment 1.

[0203] The control device 50 performs the first control operation when the length of the overlap period Tov is longer than the length of the time difference ΔTuv. In the example of FIG. 28, the length of the time difference ΔTuv is shorter than the length of the overlap period Tov. When performing the first control operation, the control device 50 shifts the end point of the high-level period of the control signal SV6 by the shift time Tsv in a direction that delays the end point. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SV1 in a direction that delays the start point of the high-level period of the control signal SV1 by the shift time Tsv. The length of the shift time Tsv is the same as the length of the overlap period Tov.

[0204] In the power conversion device 100 of Example 1, the control device 50 performs the first control operation, thereby eliminating the overlap period Tov between the U-phase resonant current and the V-phase resonant current as shown in Figure 28, and enabling zero-voltage soft switching of the first switching elements 1U and 1V.

[0205] (2.2) Operation for Soft-Switching the Second Switching Element When the control device 50 soft-switches the second switching element 2, the high-level periods and low-level periods of the control signals SU1, SU2, SV1, and SV2 before the shift are reversed compared to when soft-switching the first switching element 1, and the polarities of the load currents iU, iV, and current iL1 are reversed. Furthermore, when the control device 50 soft-switches the second switching element 2, it shifts the high-level periods of the control signals SV7, instead of the control signal SV6, by the shift time Tsv in the direction of delaying them.

[0206] In the power conversion device 100 according to Example 1, the control device 50 performs either the first control operation or the second control operation, thereby completing the discharge of the resonant capacitors 9U and 9V at the end of the dead time period corresponding to each of the U phase and the V phase. Therefore, in the power conversion device 100, when the control device 50 performs the first control operation or the second control operation, the switching of the second switching elements 2U and 2V becomes zero-voltage soft switching.

[0207] (3) Advantages The power conversion device 100 according to Example 1 can reduce the peak value of the current iL1 flowing through the resonant inductor L1 when the control device 50 performs the first control operation, and can achieve higher frequencies when the control device 50 performs the second control operation.

[0208] (Example 2) (1) Circuit Configuration of Power Conversion Device The circuit configuration of the power conversion device 100 according to Example 2 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.

[0209] (2) Operation of the power conversion device In the following, an example is given of a case where it is estimated that the resonant current flowing through the U-phase switch 8U and the resonant current flowing through the V-phase switch 8V overlap in the resonant inductor L1, but the same is true for the cases of the U-phase and W-phase, and the V-phase and W-phase.

[0210] (2.1) Operation for Soft-Switching First Switching Element With regard to the operation of the power conversion device 100 according to Example 2, the operation for soft-switching the first switching element 1 will be described with reference to Figs. 29 and 30. Fig. 29 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 according to Example 2 performs the second control operation. Fig. 30 is a timing chart for explaining the operation when the control device 50 in the power conversion device 100 of Example 2 performs the first control operation. Figs. 29 and 30 can be interpreted in the same way as Figs. 14 and 12, respectively, and therefore will not be described again.

[0211] In the example of FIG. 29 , the length of the time difference ΔTuv is longer than the length of the overlap period Tov. When performing the second control operation, the control device 50 shifts the high-level period of the control signal SU6 and the high-level period of the control signal SV6 in opposite directions. The control device 50 shifts the end point of the high-level period of the control signal SU6 by a shift time Tsu in a later direction, and shifts the end point of the high-level period of the control signal SV6 by a shift time Tsv in an earlier direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 by a shift time Tsu in a later direction. The control device 50 also shifts the start point of the high-level period of the control signal SV1 by a shift time Tsv in an earlier direction. The length of the shift time Tsu is ΔTuv / 2. The length of the shift time Tsv is also ΔTuv / 2.

[0212] In the power conversion device 100 of Example 2, the control device 50 performs the second control operation, thereby enabling zero-voltage soft switching of the first switching elements 1U, 1V, similar to when the control device 50 performs the second control operation in the power conversion device 100 of Embodiment 1.

[0213] The control device 50 performs the first control operation when the length of the overlap period Tov is longer than the length of the time difference ΔTuv. In the example of FIG. 30, the length of the time difference ΔTuv is shorter than the length of the overlap period Tov. When performing the first control operation, the control device 50 shifts the high-level period of the control signal SU6 and the high-level period of the control signal SV6 in opposite directions. The control device 50 shifts the end point of the high-level period of the control signal SU6 by a shift time Tsu in an earlier direction, and shifts the end point of the high-level period of the control signal SV6 by a shift time Tsv in a later direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 by the shift time Tsu in an earlier direction. The control device 50 also shifts the start point of the high-level period of the control signal SV1 by a shift time Tsv in a later direction. The length of the shift time Tsu is equal to the length of Tov / 2. The length of the shift time Tsv is also equal to the length of Tov / 2.

[0214] In the power conversion device 100 of Example 2, the control device 50 performs the first control operation, thereby eliminating the overlap period Tov between the U-phase resonant current and the V-phase resonant current as shown in Figure 30, and enabling zero-voltage soft switching of the first switching elements 1U and 1V.

[0215] (2.2) Operation for Soft-Switching the Second Switching Element When the control device 50 soft-switches the second switching element 2, the high-level periods and low-level periods of the control signals SU1, SU2, SV1, and SV2 before shifting are reversed compared to when soft-switching the first switching element 1, and the polarities of the load currents iU, iV and current iL1 are reversed. Furthermore, when the control device 50 soft-switches the second switching element 2, it shifts the high-level periods of the control signals SU7 and SV7 in opposite directions to each other instead of the control signals SU6 and SV6.

[0216] In the power conversion device 100 according to Example 2, the control device 50 performs either the first control operation or the second control operation, thereby completing the discharge of the resonant capacitors 9U and 9V at the end of the dead time period corresponding to each of the U phase and the V phase. Therefore, in the power conversion device 100, when the control device 50 performs the first control operation or the second control operation, the switching of the second switching elements 2U and 2V becomes zero-voltage soft switching.

[0217] (3) Advantages The power conversion device 100 according to Example 2 can reduce the peak value of the current iL1 flowing through the resonant inductor L1 when the control device 50 performs the first control operation, and can achieve higher frequencies when the control device 50 performs the second control operation.

[0218] (Example 3) (1) Circuit Configuration of Power Conversion Device The circuit configuration of the power conversion device 100 according to Example 3 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.

[0219] (2) Operation of the power conversion device In the following, an example is given of a case where it is estimated that the resonant current flowing through the U-phase switch 8U and the resonant current flowing through the V-phase switch 8V overlap in the resonant inductor L1, but the same is true for the cases of the U-phase and W-phase, and the V-phase and W-phase.

[0220] (2.1) Operation for Soft-Switching First Switching Element With regard to the operation of the power conversion device 100 according to Example 3, the operation for soft-switching the first switching element 1 will be described with reference to Figs. 31 and 32. Fig. 31 is a timing chart for describing the operation when the control device 50 in the power conversion device 100 according to Example 3 performs the second control operation. Fig. 32 is a timing chart for describing the operation when the control device 50 in the power conversion device 100 of Example 3 performs the first control operation. Figs. 31 and 32 can be interpreted in the same way as Figs. 14 and 12, respectively, and therefore will not be described again.

[0221] The control device 50 performs the second control operation when the length of the overlap period Tov is shorter than the length of the time difference ΔTuv. In the example of FIG. 31 , the length of the time difference ΔTuv is longer than the length of the overlap period Tov. When performing the second control operation, the control device 50 shifts the high-level period of the control signal SU6 and the high-level period of the control signal SV6 in opposite directions. The control device 50 shifts the end point of the high-level period of the control signal SU6 by a shift time Tsu in a later direction, and shifts the end point of the high-level period of the control signal SV6 by a shift time Tsv in an earlier direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 by a shift time Tsu in a later direction. The control device 50 also shifts the start point of the high-level period of the control signal SV1 by a shift time Tsv in an earlier direction. When Tov=Ta+Tb, the length of the shift time Tsu is (√2 / 4)·Tres−Tov·Tb / (Ta+Tb). Also, the length of the shift time Tsv is Tov·Tb / (Ta+Tb)+ΔTuv−(√2 / 4)·Tres. Each of Ta and Tb can be set to any value.

[0222] In the power conversion device 100 of Example 3, the control device 50 performs the second control operation, thereby enabling zero-voltage soft switching of the first switching elements 1U and 1V, similar to when the control device 50 performs the second control operation in the power conversion device 100 of Embodiment 1.

[0223] The control device 50 performs a first control action when the length of the overlap period Tov is longer than the length of the time difference ΔTuv.

[0224] In the example of FIG. 32 , the length of the time difference ΔTuv is shorter than the length of the overlap period Tov. When performing the first control operation, the control device 50 shifts the high-level periods of the control signals SU6 and SV6 in opposite directions. The control device 50 shifts the end point of the high-level period of the control signal SU6, whose high-level period has an earlier phase than the other of the two control signals SU6 and SV6, by a shift time Tsu in an earlier direction within one cycle of the carrier signal, and shifts the end point of the high-level period of the control signal SV6, whose high-level period has a later phase, by a shift time Tsv in a later direction. Therefore, the control device 50 shifts the start point of the high-level period of the control signal SU1 in an earlier direction by the shift time Tsu. The control device 50 also shifts the start point of the high-level period of the control signal SV1 in a later direction by the shift time Tsv. The length of the shift time Tsu is Tov·Tb / (Ta+Tb). The length of the shift time Tsv is Tov·Ta / (Ta+Tb).

[0225] In the power conversion device 100 of Example 3, the control device 50 performs the first control operation, thereby eliminating the overlap period Tov between the U-phase resonant current and the V-phase resonant current before the shift, as shown in the waveform of the shifted current iL1 in the lower part of Figure 32, and enabling zero-voltage soft switching of the first switching elements 1U and 1V.

[0226] (2.2) Operation for Soft-Switching the Second Switching Element When the control device 50 soft-switches the second switching element 2, the high-level periods and low-level periods of the control signals SU1, SU2, SV1, and SV2 before shifting are reversed compared to when soft-switching the first switching element 1, and the polarities of the load currents iU, iV and current iL1 are reversed. Furthermore, when the control device 50 soft-switches the second switching element 2, it shifts the high-level periods of the control signals SU7 and SV7 in opposite directions to each other instead of the control signals SU6 and SV6.

[0227] In the power conversion device 100 according to Example 3, the control device 50 performs either the first control operation or the second control operation, thereby completing the discharge of the resonant capacitors 9U and 9V at the end of the dead time period corresponding to each of the U phase and the V phase. Therefore, in the power conversion device 100, when the control device 50 performs the first control operation or the second control operation, the switching of the second switching elements 2U and 2V becomes zero-voltage soft switching.

[0228] (3) Advantages The power conversion device 100 according to Example 3 can reduce the peak value of the current iL1 flowing through the resonant inductor L1 when the control device 50 performs the first control operation, and can achieve higher frequencies when the control device 50 performs the second control operation.

[0229] (Aspects) The following aspects are disclosed in this specification.

[0230] A power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F; 100G; 100H) 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) in which a plurality of first switching elements (1) and a plurality of second switching elements (2) are connected in series in a one-to-one relationship are connected in parallel to each other. In the power conversion circuit (11), a plurality of first switching elements (1) are connected to a first DC terminal (31), and a plurality of second switching elements (2) are connected to a 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 the first switching element (1) and the 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 the 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 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 a first end (81) of a 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), the first end of the resonant inductor (L1) is connected to the common connection point (25). The regenerative capacitor (15) has a third end (153) and a fourth end (154). In the regenerative capacitor (15), the third end (153) is connected to the first DC terminal (31) or the second DC terminal (32).A control device (50) provides a control signal whose potential changes between a high level and a low level to each of a plurality of first switching elements (1), a plurality of second switching elements (2), and a plurality of switches (8). The control device (50) sets, for each of a plurality of switching circuits (10), a dead time period (Td) between a high level period of the control signal to the first switching element (1) and a high level period of the control signal to the second switching element (2), and sets the high level period of the control signal to each of the plurality of switches (8) based on the dead time period (Td) for a corresponding switching circuit (10) among the plurality of switching circuits (10). The control device (50) is configured to perform one of a first control operation and a second control operation based on an overlap period (Tov) during which the resonant currents flow simultaneously through the two or more switches (8) when it is determined that resonant currents flow simultaneously through the resonant inductor (L1). The first control operation is an operation of shifting a high-level period of a control signal to at least one switch (8) of the two or more switches (8) so that resonant currents passing through the two or more switches (8) do not simultaneously flow through the resonant inductor (L1). The second control operation is an operation of shifting a high-level period of a control signal to at least one switch (8) of the two or more switches (8) so that the high-level periods of the control signal to the two or more switches (8) overlap by a predetermined period or more in dead time periods corresponding to two or more switching circuits (10) connected to the two or more switches (8) out of the plurality of switching circuits (10).

[0231] According to this aspect, it is possible to reduce current distortion.

[0232] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F; 100G; 100H) according to a second aspect, in the first aspect, a first switching element (1) or a second switching element (2) that is a target of zero voltage soft switching in one switching circuit (10) of the two or more switching circuits (10) is defined as a first target switching element. In one switching circuit (10) of the two or more switching circuits (10) different from the switching circuit (10) that includes the first target switching element, the first switching element (1) or the second switching element (2) that is a target of zero voltage soft switching is defined as a second target switching element. The time difference between the start of a high-level period of a control signal to the first target switching element and the start of a high-level period of a control signal to the second target switching element is defined as ΔT. At this time, the control device (50) performs a first control operation when the length (Tov) of the overlap period of the resonant current is less than ΔT, and performs a second control operation when the length (Tov) of the overlap period of the resonant current is equal to or greater than ΔT.

[0233] According to this embodiment, it is possible to reduce the amount of shift in the high level period of the control signal, and to reduce current distortion.

[0234] A power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F; 100G; 100H) according to a third aspect is configured in the second aspect such that one of the two or more switches (8) is a first switch and the other switch different from the first switch is a second switch. In this case, when performing a first control operation, the control device (50) shifts a high-level period of a control signal to at least one of the first switch and the second switch so that the resonant current flows through the second switch from a time point when the current value of the resonant current passing through the first switch reaches an extreme value and coincides with the current value of a load current flowing through an AC terminal (41) corresponding to the first switch among the two or more AC terminals (41).

[0235] According to this embodiment, the amount of shift in the high level period of the control signal can be reduced, and higher frequencies can be achieved.

[0236] In the power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F; 100G; 100H) according to the fourth aspect, in the third aspect, when performing the first control operation, the control device (50) shifts the high-level period of the control signal to the first switch and the high-level period of the control signal to the second switch in directions different from each other.

[0237] According to this aspect, the amount of shift in the high level period of the control signal can be further reduced, and current distortion can be further reduced.

[0238] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F; 100G; 100H) according to a fifth aspect, in the first or second aspect, the predetermined period is the entire period of a resonant half cycle of a resonant circuit including a resonant inductor (L1) and two or more resonant capacitors (9) connected to the two or more switches (8), respectively. The control device (50) is capable of executing a third control action that extends the high-level period of a control signal to each of the two or more switches (8) by an additional time (Tad) longer than the predetermined period. The control device (50) executes the third control action when executing the second control action.

[0239] The power conversion device of the present disclosure can reduce current distortion. Therefore, when DC power is converted to AC power using the power conversion device of the present disclosure, AC power with small current distortion can be obtained. In this way, the power conversion device of the present disclosure is industrially useful.

[0240] 1, 1U, 1V, 1W First switching element 2, 2U, 2V, 2W Second switching element 3, 3U, 3V, 3W Connection point 8, 8U, 8V, 8W Switch 9, 9U, 9V, 9W Resonant capacitor 10, 10U, 10V, 10W Switching circuit 11 Power conversion circuit 15 Regenerative capacitor 153 Third terminal 154 Fourth terminal 31 First DC terminal 32 Second DC terminal 41, 41U, 41V, 41W AC terminal 50 Control device 100, 100A, 100B, 100C, 100D, 100E, 100F, 100G, 100H Power conversion device iU, iV, iW Output current (load current) L1 Resonant inductor RA1 AC load SU1, SU2, SU6, SU7, SU8 Control signals SV1, SV2, SV6, SV7, SV8 Control signals SW1, SW2, SW6, SW7, SW8 Control signals Tad, Tau, Tav, Taw Addition time Tres Resonant half cycle Tov Overlap period V15 Voltage (potential)

Claims

1. A power conversion circuit having a first DC terminal and a second DC terminal, the power conversion circuit having a plurality of first switching elements and a plurality of second switching elements, the plurality of first switching elements and the plurality of second switching elements being connected in series in a one-to-one relationship, the plurality of first switching elements being connected to the first DC terminal, and the plurality of second switching elements being connected to the second DC terminal; a plurality of AC terminals corresponding one-to-one to the plurality of switching circuits, each of which is connected to a connection point of the first switching elements and the second switching elements in the corresponding switching circuit; a plurality of switches corresponding one-to-one to the plurality of switching circuits, each of which has a first end connected to the connection point of the first switching elements and the second switching elements in the corresponding switching circuit and a second end commonly connected to a common connection point; a plurality of resonant capacitors corresponding one-to-one to the plurality of switches, each of which is connected between the first end and the second DC terminal of the corresponding switch; and a resonant inductor having a first end and a second end, the first end being connected to the common connection point. a regenerative capacitor having a third end and a fourth end, the third end being connected to the first DC terminal or the second DC terminal; and a control device providing a control signal whose potential changes between a high level and a low level to each of the plurality of first switching elements, the plurality of second switching elements and the plurality of switches, wherein the control device sets a dead time period between a high level period of a control signal to the first switching element and a high level period of a control signal to the second switching element for each of the plurality of switching circuits, and sets the high level period of the control signal to each of the plurality of switches based on the dead time period for a corresponding one of the plurality of switching circuits, and when the control device determines that a resonant current passing through each of two or more switches of the plurality of switches simultaneously flows through the resonant inductor, the control device is configured to perform one of a first control operation and a second control operation based on an overlap period during which a resonant current passing through each of the two or more switches simultaneously flows, and the first control operation isa second control operation is an operation of shifting a high-level period of a control signal to at least one of the two or more switches so that resonant currents passing through each of the two or more switches do not flow simultaneously through the resonant inductor, and the second control operation is an operation of shifting a high-level period of a control signal to at least one of the two or more switches so that the high-level periods of the control signal to each of the two or more switches overlap for a predetermined period or more in dead time periods corresponding to each of two or more switching circuits connected to the two or more switches among the plurality of switching circuits.

2. The power conversion device according to claim 1, wherein the first switching element or the second switching element that is a target of zero voltage soft switching in one switching circuit of the two or more switching circuits is a first target switching element, and the first switching element or the second switching element that is a target of zero voltage soft switching in one switching circuit of the two or more switching circuits other than the switching circuit including the first target switching element is a second target switching element, and when a time difference between a point in time when a high level period of a control signal to the first target switching element starts and a point in time when a high level period of a control signal to the second target switching element starts is ΔT, the control device performs the first control operation when a length of the overlap period of the resonant current is less than ΔT, and performs the second control operation when a length of the overlap period of the resonant current is equal to or greater than ΔT.

3. The power conversion device according to claim 2, wherein, when one of the two or more switches is designated as a first switch and the other switch different from the first switch is designated as a second switch, when performing the first control operation, the control device shifts a high level period of a control signal to at least one of the first switch and the second switch so that the resonant current flows through the second switch from a point in time when the current value of the resonant current passing through the first switch reaches an extreme value and coincides with the current value of a load current flowing through an AC terminal among the two or more AC terminals corresponding to the first switch.

4. The power conversion device according to claim 3, wherein the control device, when performing the first control operation, shifts a high-level period of a control signal to the first switch and a high-level period of a control signal to the second switch in directions different from each other.

5. The power conversion device according to claim 1 or 2, wherein the predetermined period is the entire period of a resonant half cycle of a resonant circuit including the resonant inductor and two or more resonant capacitors connected to the two or more switches, respectively, and the control device is capable of executing a third control action of making a high-level period of a control signal to each of the two or more switches longer by an additional time than the predetermined period, and executes the third control action when performing the second control action.

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