Power conversion device

Abstract

In the present invention, a control device is configured so that power conversion efficiency is improved without direct control of a plurality of switches for zero-voltage soft switching. In the present invention, a control device (51) sets, between a high-level period of a first control signal to a first switching element (1) and a high-level period of a first control signal to a second switching element (2), a second dead time period resulting from the addition of a variable time to a first dead time period. A signal generation circuit (52) generates, for each of a plurality of switches (8), a second control signal that has a high-level period corresponding to the second dead time period. A control system (50), on the basis of the potential (V15) of a fourth end (154) of a regenerative capacitor (15) and the polarities of a plurality of load currents (iU, iV, iW), toggles between a first control operation that does not cause overlap of the high-level periods of the second control signal for the plurality of switches (8) and a second control operation that causes overlap of the high-level periods of the second control signal for two switches (8) among the plurality of switches (8).

Description

Power Conversion Device

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

[0002] Patent Document 1 discloses a power conversion device that converts direct current to multi-phase alternating current. The power conversion device disclosed in Patent Document 1 includes a main switching means (power conversion circuit), two capacitors, one coil (resonant inductor), multiple auxiliary switching elements, and a controller. The main switching means is composed of a pair of main switching elements connected in series between both terminals of a DC power supply. A main switching circuit is provided for each phase of the multi-phase alternating current, with the interconnection point of the pair of main switching elements serving as an output point for each phase. The two capacitors divide the voltage of the DC power supply. One end of the coil is connected to the voltage division point between the two capacitors. The multiple auxiliary switching elements connect the other end of the coil to the output points for each phase. The controller generates a control signal (first control signal) that PWM controls each main switching element and outputs it to the gate of each main switching element. The controller also generates a control signal (second control signal) that controls the on / off of each auxiliary switching element and outputs it to the gate of each auxiliary switching element. When the controller determines that multiple phase currents flow through the coil, it controls the multiple auxiliary switch elements so that the current flowing through at least one phase is smaller than a preset value.

[0003] In the power conversion device disclosed in Patent Document 1, the controller needs to generate and output a plurality of first control signals and a plurality of second control signals, which results in an increase in the size of the controller.

[0004] Furthermore, in a power conversion device, the power conversion efficiency may decrease due to changes in the load state.

[0005] JP 2010-233306 A

[0006] An object of the present disclosure is to provide a power conversion device that can improve power conversion efficiency without directly controlling a plurality of switches for zero voltage soft switching in a control device.

[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 system. 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, each of which has 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 a corresponding one of the multiple switching circuits. The multiple switches correspond one-to-one to the multiple switching circuits. Each of the plurality of switches has a first end connected to the connection point between the first switching element and the second switching element in a corresponding switching circuit among the plurality of switching circuits, and a second end connected to a common connection point. The plurality of resonant capacitors correspond one-to-one to the plurality of switches. Each of the plurality of resonant capacitors is connected between the first end and the second DC terminal of a corresponding switch among the plurality of switches. The resonant inductor has a first end and a second end. With respect to the resonant inductor, 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. With respect to the regenerative capacitor, the third end is connected to the first DC terminal or the second DC terminal. With respect to the regenerative capacitor, the fourth end is connected to the second end of the resonant inductor. The control system includes a control device and a signal generating circuit. The control device provides each of the plurality of first switching elements and the plurality of second switching elements with a first control signal whose potential changes between a high level and a low level.The signal generating circuit provides each of the plurality of switches with a second control signal whose potential changes between a high level and a low level. The control device sets, for each of the plurality of switching circuits, a second dead time period obtained by adding a variable time to a predetermined first dead time period between a high level period of the first control signal to the first switching element and a high level period of the first control signal to the second switching element so that on periods of the first switching element and the second switching element do not overlap. The variable time period is determined based on a current value of a load current corresponding to each of the plurality of switching circuits among a plurality of load currents flowing through each of the plurality of AC terminals, an inductance of the resonance inductor, and a potential at the fourth end of the regenerative capacitor. The signal generating circuit generates, for each of the plurality of switches, the second control signal having a high level period corresponding to the second dead time period for a corresponding switching circuit among the plurality of switching circuits. The control system switches between a first control operation that does not overlap the high-level periods of the second control signals of the multiple switches and a second control operation that overlaps the high-level periods of the second control signals of two of the multiple switches, based on the potential of the fourth end of the regenerative capacitor and the polarities of the multiple load currents.

[0008] FIG. 1 is a circuit diagram of a system including a power conversion device according to a first embodiment. FIG. 2 is a circuit block diagram of a signal generating circuit in the power conversion device. FIG. 3 is a diagram illustrating time variations in duty and 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. FIG. 4 is a timing chart illustrating the operation of the power conversion device. FIG. 5 is a timing chart illustrating the operation of the power conversion device. FIG. 6 is a timing chart illustrating the operation of the power conversion device. FIG. 7 is a diagram illustrating the operation of the power conversion device when the U-phase load current is positive. FIG. 8 is a diagram illustrating the operation of the power conversion device during a first period. FIG. 9 is a diagram illustrating the operation of the power conversion device during a second period. FIG. 10 is a diagram illustrating the operation of the power conversion device during a third period. FIG. 11 is a diagram illustrating the operation of the power conversion device when the polarity of the U-phase load current is negative. FIG. 12 is a timing chart illustrating the overlap time when two-phase resonant currents overlap in the power conversion device. Fig. 13 is a timing chart showing a case where a control system in the power conversion apparatus performs a first control operation. Fig. 14 is a timing chart showing a case where a control system in the power conversion apparatus performs a second control operation on two switches. Fig. 15 is a timing chart showing a case where a control system in the power conversion apparatus performs a second control operation on two switches. Fig. 16 is a timing chart showing a case where a control device in the power conversion apparatus performs a first control operation on two switches. Fig. 17 is a timing chart showing a case where a control device in the power conversion apparatus performs a second control operation on two switches. Fig. 18 is a timing chart showing a case where a control device in the power conversion apparatus performs a first control operation on two switches. Fig. 19 is a timing chart showing another example where a control device in the power conversion apparatus performs a second control operation on two switches. Fig. 20 is a circuit diagram of a system including a power conversion apparatus according to a second embodiment. Fig. 21 is an explanatory diagram showing an operation of the power conversion apparatus when the U-phase load current is positive.Fig. 22 is a diagram illustrating operation of the power conversion device in a third period of the above embodiment. Fig. 23 is a diagram illustrating operation of the power conversion device in the case where the polarity of the U-phase load current is negative in the above embodiment. Fig. 24 is a circuit diagram of a system including the power conversion device of embodiment 3. Fig. 25 is a diagram illustrating operation of the power conversion device in the case where the polarity of the U-phase load current is positive in the above embodiment. Fig. 26 is a diagram illustrating operation of the power conversion device in the case where the polarity of the U-phase load current is negative in the above embodiment. Fig. 27 is a circuit diagram of a system including the power conversion device of embodiment 4. Fig. 28 is a diagram illustrating operation of the power conversion device in the case where the polarity of the U-phase load current is positive in the above embodiment. Fig. 29 is a diagram illustrating operation of the power conversion device in the case where the polarity of the U-phase load current is negative in the above embodiment. Fig. 30 is a circuit diagram of a system including the power conversion device of embodiment 5. Fig. 31 is a circuit diagram of a system including the power conversion device of embodiment 6. Fig. 32 is a circuit diagram of a system including the power conversion device of embodiment 7. Fig. 33 is a circuit diagram of a system including the power conversion device of embodiment 8. Fig. 34 is a circuit diagram of a system including a power conversion device according to embodiment 9. Fig. 35 is a circuit diagram of a system including a power conversion device according to embodiment 10. Fig. 36 is a circuit diagram of a system including a power conversion device according to embodiment 11.

[0009] First Embodiment A power conversion device 100 according to a first embodiment will be described below with reference to FIGS.

[0010] (1) Overall Configuration of the Power Conversion Device As shown in FIG. 1 , the power conversion device 100 includes a first DC terminal 31, a second DC terminal 32, and multiple (e.g., three) AC terminals 41. A DC power source E1 is connected between the first DC terminal 31 and the second DC terminal 32, and an AC load RA1 is connected to the multiple AC terminals 41. The AC load RA1 is, for example, a three-phase 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.

[0011] 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 system 50. The control system 50 includes a control device 51 and a signal generating circuit 52. Each of the plurality of switches 8 is, for example, a bidirectional switch. The power conversion device 100 also includes a first clamp diode 13 and a second clamp diode 14. The power conversion device 100 also includes a capacitor C10.

[0012] 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 each other. 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.

[0013] 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 of a first switching element 1 and a second switching element 2 in a corresponding one of the plurality of switching circuits 10.

[0014] The multiple switches 8 correspond one-to-one to the multiple switching circuits 10. Each of the multiple switches 8 has a first end 81 and a second end 82. The first end 81 of each of the multiple switches 8 is connected to a connection point 3 between a first switching element 1 and a second switching element 2 in a corresponding one of the multiple switching circuits 10.

[0015] The plurality of resonance capacitors 9 correspond one-to-one to the plurality of switches 8. Each of the plurality of resonance capacitors 9 is connected between the first end 81 of the corresponding switch 8 among the plurality of switches 8 and the second DC terminal 32.

[0016] The resonant inductor L1 has a first end and a second end, and the first end of the resonant inductor L1 is connected to the second end 82 of a corresponding one of the multiple switches 8 (three switches 8 in this embodiment).

[0017] 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 second end of the resonant inductor L1.

[0018] The control device 51 provides a first control signal to each of the plurality of first switching elements 1 and the plurality of second switching elements 2 .

[0019] The signal generating circuit 52 provides a second control signal to each of the plurality of switches 8 .

[0020] (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.

[0021] In the power conversion device 100, for example, 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 the AC load RA1 are connected to three AC terminals 41U, 41V, and 41W, respectively.

[0022] In the power conversion circuit 11, each of a plurality of (three in the example of FIG. 1 ) first switching elements 1 and a plurality of (three in the example of FIG. 1 ) 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 51. 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.

[0023] 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.

[0024] A connection point 3U between the first switching element 1U and the second switching element 2U is connected to, for example, a U-phase terminal of an 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, a V-phase terminal 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, a W-phase terminal of the AC load RA1 via an AC terminal 41W.

[0025] The plurality of resonant capacitors 9 correspond one-to-one to the plurality of switches 8. Each of the plurality of resonant capacitors 9 is connected between a first end 81 and a second DC terminal 32 of a corresponding switch 8 among the plurality of switches 8. The power conversion device 100 has a plurality of resonant circuits. Each of the plurality of resonant circuits includes a resonant capacitor 9 and a resonant inductor L1.

[0026] In this embodiment, each of the multiple switches 8 has a third switching element 6 and a fourth switching element 7. Each of the multiple (three in the example of FIG. 1 ) third switching elements 6 and the multiple (three in the example of FIG. 1 ) fourth switching elements 7 has a control terminal, a first main terminal, and a second main terminal. The control terminals of the multiple third switching elements 6 and the multiple fourth switching elements 7 are connected to the signal generating circuit 52. Each of the multiple third switching elements 6 and the multiple fourth switching elements 7 is, for example, an IGBT. Therefore, the control terminal, the first main terminal, and the second main terminal of each of the multiple third switching elements 6 and the multiple fourth switching elements 7 are a gate terminal, a collector terminal, and an emitter terminal, respectively. In each of the multiple switches 8, the third switching element 6 and the fourth switching element 7 are connected in anti-series. In each of the multiple switches 8, the first main terminal (collector terminal) of the third switching element 6 and the first main terminal (collector terminal) of the fourth switching element 7 are connected. In each of the switches 8, the second main terminal (emitter terminal) of the third switching element 6 is connected to the connection point 3 of a corresponding one of the switching circuits 10, and the second main terminal (emitter terminal) of the fourth switching element 7 is connected to the resonance inductor L1. Each of the switches 8 further includes a diode 61 connected in anti-parallel to the third switching element 6 and a diode 71 connected in anti-parallel to the fourth switching element 7.

[0027] In the power conversion device 100, the switch 8U is connected to a connection point 3U between the first switching element 1U and the second switching element 2U. The switch 8V is connected to a 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 third switching element 6 and the fourth switching element 7 of the switch 8U will be referred to as the third switching element 6U and the fourth switching element 7U, the third switching element 6 and the fourth switching element 7 of the switch 8V will be referred to as the third switching element 6V and the fourth switching element 7V, and the third switching element 6 and the fourth switching element 7 of the switch 8W will be referred to as the third switching element 6W and the fourth switching element 7W, respectively.

[0028] The multiple switches 8 are controlled by the signal generating circuit 52. In other words, the third switching element 6U, the fourth switching element 7U, the third switching element 6V, the fourth switching element 7V, the third switching element 6W, and the fourth switching element 7W are controlled by the signal generating circuit 52.

[0029] The resonant inductor L1 has a first end and a second end. The first end of the resonant inductor L1 is connected to the second ends 82 of the multiple switches 8. The first end of the resonant inductor L1 is connected to a common connection point 25 to which the second ends 82 of the multiple switches 8 are connected. The second end of the resonant inductor L1 is connected to the fourth end 154 of the regenerative capacitor 15.

[0030] The regenerative capacitor 15 is connected between the second end of the resonance inductor L1 and the second DC terminal 32. The regenerative capacitor 15 is, for example, a film capacitor.

[0031] The control device 51 controls a plurality of first switching elements 1 and a plurality of second switching elements 2. The control device 51 includes a computer system. The computer system is primarily composed of a processor and a memory as hardware. The processor executes a program recorded in the memory of the computer system to realize the functions of the control device 51 in the present disclosure. 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 integrated circuits, such as ICs and LSIs, are referred to by different names depending on the degree of integration, and include integrated circuits called system LSIs, very large-scale integrations (VLSIs), or ultra-large-scale integrations (ULSIs). Furthermore, field-programmable gate arrays (FPGAs), which are programmed after the LSI is manufactured, or logic devices that can reconfigure the connections within the LSI or reconfigure the circuit partitions within the LSI, can also be used as processors. The electronic circuits may be integrated into a single chip or distributed across multiple chips. The multiple chips may be integrated into one device or distributed across multiple devices. The computer system referred to here includes a microcontroller having one or more processors and one or more memories. Therefore, the microcontroller is also composed of one or more electronic circuits including semiconductor integrated circuits or large-scale integrated circuits.

[0032] The control device 51 outputs first control signals SU1, SV1, and SW1 that control the on / off states of the multiple first switching elements 1U, 1V, and 1W, respectively. Each of the first 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 first control signals SU1, SV1, and SW1 are at a high level, and turned off when the first control signals SU1, SV1, and SW1 are at a low level. The control device 51 also outputs first control signals SU2, SV2, and SW2 that control the on / off states of the multiple second switching elements 2U, 2V, and 2W, respectively. Each of the first 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 first control signals SU2, SV2, and SW2 are at a high level, and turned off when the first control signals SU2, SV2, and SW2 are at a low level.

[0033] The control device 51 uses a sawtooth-wave carrier signal to generate first control signals SU1, SV1, SW1 corresponding to the multiple first switching elements 1U, 1V, and 1W, respectively, and first control signals SU2, SV2, and SW2 corresponding to the multiple second switching elements 2U, 2V, and 2W, respectively. More specifically, the control device 51 generates the first control signals SU1 and SU2 to be applied 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 51 also generates the first control signals SV1 and SV2 to be applied 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 51 also generates the first control signals SW1 and SW2 to be applied 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°, 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. The U-phase voltage command, V-phase voltage command, and W-phase voltage command each have the same cycle length. The U-phase voltage command, V-phase voltage command, and W-phase voltage command each have a longer cycle length T10 (see FIG. 12 ) than the carrier signal cycle length.

[0034] The duty of the first control signals SU1 and SU2 provided by the control device 51 to the first switching element 1U and the second switching element 2U, respectively, varies based on the U-phase voltage command. In FIG. 3, the duty of the first control signal SU1 is shown as the U-phase duty. The control device 51 (see FIG. 1) compares the U-phase voltage command with a carrier signal to generate the first control signal SU1 provided to the first switching element 1U. The control device 51 also inverts the first control signal SU1 provided to the first switching element 1U to generate the first control signal SU2 provided to the second switching element 2U. The control device 51 also sets a first dead time period Td1 (see FIG. 4) between the high-level period of the first control signal SU1 and the high-level period of the first control signal SU2 so that the on periods of the first switching element 1U and the second switching element 2U do not overlap.

[0035] The duty of the first control signals SV1 and SV2 provided by the control device 51 to the first switching element 1V and the second switching element 2V, respectively, varies based on the V-phase voltage command. In FIG. 3, the duty of the first control signal SV1 is shown as the V-phase duty. The control device 51 (see FIG. 1) compares the V-phase voltage command with a carrier signal to generate the first control signal SV1 provided to the first switching element 1V. The control device 51 also inverts the first control signal SV1 provided to the first switching element 1V to generate the first control signal SV2 provided to the second switching element 2V. The control device 51 also sets a first dead time period Td1 (see FIG. 5) between the high-level period of the first control signal SV1 and the high-level period of the first control signal SV2 so that the on periods of the first switching element 1V and the second switching element 2V do not overlap.

[0036] The duty of the first control signals SW1 and SW2 provided by the control device 51 to the first switching element 1W and the second switching element 2W, respectively, varies based on the W-phase voltage command. In FIG. 3, the duty of the first control signal SW1 is shown as the W-phase duty. The control device 51 (see FIG. 1) compares the W-phase voltage command with a carrier signal to generate the first control signal SW1 provided to the first switching element 1W. The control device 51 also inverts the first control signal SW1 provided to the first switching element 1W to generate the first control signal SW2 provided to the second switching element 2W. The control device 51 also sets a first dead time period Td1 (see FIG. 6) between the high-level period of the first control signal SW1 and the high-level period of the first control signal SW2 so that the on periods of the first switching element 1W and the second switching element 2W do not overlap.

[0037] 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 first control signal SU1 (U-phase duty), the duty of the first control signal SV1 (V-phase duty), and the duty of the first control signal SW1 (W-phase duty) change like sinusoidal waves whose phases are different from each other by 120°, as shown in FIG. 3 . Similarly, the duty of the first control signal SU2, the duty of the first control signal SV2, and the duty of the first control signal SW2 change like sinusoidal waves whose phases are different from each other by 120°.

[0038] The control device 51 generates the first control signals SU1, SU2, SV1, SV2, SW1, and SW2 based on the carrier signal, the voltage commands, and information about the state of the AC load RA1. For example, if the AC load RA1 is a three-phase motor, the information about the state of the AC load RA1 includes, for example, detection values ​​from a plurality of current sensors that detect load currents (output currents) iU, iV, and iW flowing through the U-phase, V-phase, and W-phase of the AC load RA1, respectively.

[0039] 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.

[0040] When the third switching element 6U is in the ON state and the fourth switching element 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-resonant capacitor 9U. The charging current is a current that charges the resonant capacitor 9U. When the third switching element 6U is in the OFF state and the fourth switching element 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-regenerative capacitor 15. The discharging current is a current that discharges the charge in the resonant capacitor 9U.

[0041] When the third switching element 6V is in the ON state and the fourth switching element 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 third switching element 6V is in the OFF state and the fourth switching element 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.

[0042] When the third switching element 6W is in the ON state and the fourth switching element 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 - resonant capacitor 9W. The charging current is a current that charges the resonant capacitor 9W. When the third switching element 6W is in the OFF state and the fourth switching element 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 - regenerative capacitor 15. The discharging current is a current that discharges the charge in the resonant capacitor 9W.

[0043] In this embodiment, the control device 51 sets the dead time period between the high-level period of the first control signal to the first switching element 1 and the high-level period of the first control signal to the second switching element 2 for each of the multiple switching circuits 10 to a second dead time period Td2 (see FIGS. 4 to 6) determined by the first dead time period Td1 (see FIGS. 4 to 6) and the variable time Tp1 (see FIGS. 4 to 6). The second dead time period Td2 includes the first dead time period Td1. The second dead time period Td2 has a length obtained by adding the variable time Tp1 (= additional time Tad) to the length of the first dead time period Td1 (the total length of the first dead time period Td1 and the variable time Tp1). From another perspective, the control device 51 sets the second dead time period Td2, which is the first dead time period Td1 extended by the variable time Tp1, as the dead time period.

[0044] The first dead time period Td1 is a period set for each of the multiple switching circuits 10 between a high-level period of the first control signal to the first switching element 1 and a high-level period of the first control signal to the second switching element 2, during which both the first control signal to the first switching element 1 and the first control signal to the second switching element 2 are low so that the on-periods of the first switching element 1 and the second switching element 2 do not overlap (i.e., there is no period in which both the first switching element 1 and the second switching element 2 are on). The first dead time period Td1 is determined in advance for each of the multiple switching circuits 10 based on the switching characteristics (specified turn-on time and turn-off time) of the first switching element 1 and the second switching element 2, and is stored in the control device 51. In this embodiment, for example, the length of the resonance half cycle corresponding to each of the multiple switches 8 is designed according to the length of the first dead time period Td1 of each of the multiple switching circuits 10. The resonance half period corresponding to each of the multiple switches 8 is half the resonance period, which is the reciprocal of the resonance frequency of the resonance circuit including the switch 8, 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 this embodiment, the length of the resonance half cycle is set to be the same as the length of the first dead time period Td1, for example.

[0045] The length of the resonant half cycle of the resonant circuit including the resonant inductor L1 and the resonant capacitor 9U is set to, for example, the length of the first dead time period Td1 (see FIG. 4) set between the high-level period of the first control signal SU1 and the high-level period of the first control signal SU2. The length of the resonant half cycle of the resonant circuit including the resonant inductor L1 and the resonant capacitor 9V is set to, for example, the length of the first dead time period Td1 (see FIG. 5) set between the high-level period of the first control signal SV1 and the high-level period of the first control signal SV2. The length of the resonant half cycle of the resonant circuit including the resonant inductor L1 and the resonant capacitor 9W is set to the length of the first dead time period Td1 (see FIG. 6) set between the high-level period of the first control signal SW1 and the high-level period of the first control signal SW2.

[0046] If the resonant period is Tres, Tres / 2, which is the resonant half period, is the length of the first dead time period Td1. The end of the resonant half period preferably coincides with the end of the first dead time period Td1 for the switching circuit 10 corresponding to the switch 8. FIG. 4 illustrates the first control signals SU1 and SU2, the second control signals SU6 and SU7, the first dead time period Td1, the variable time Tp1 (additional time Tad), and the second dead time period Td2. In the example of FIG. 4, the start and end of the first dead time period Td1 are time t12 and time t13, respectively. FIG. 5 also illustrates the first control signals SV1 and SV2, the second control signals SV6 and SV7, the first dead time period Td1, the variable time Tp1 (additional time Tad), and the second dead time period Td2. In the example of Fig. 5, the start and end points of the first dead time period Td1 are time t22 and time t23, respectively. Fig. 6 also illustrates the first control signals SW1 and SW2, the second control signals SW6 and SW7, the first dead time period Td1, the variable time Tp1 (additional time Tad), and the second dead time period Td2. In the example of Fig. 6, the start and end points of the first dead time period Td1 are time t32 and time t33, respectively.

[0047] The length of the resonant half cycle described above is an ideal design example, and may be 90% to 110% of the length of the first dead time period Td1. The resonant half cycle and the first dead time period Td1 may have different lengths.

[0048] The second dead time period Td2 is a period obtained by extending the first dead time period Td1 by a variable time Tp1 that depends on the current value of the load current and the potential V15 of the fourth end 154 of the regenerative capacitor 15. The variable time Tp1 is determined, for example, based on the current value of the load current, the potential V15 of the fourth end 154 of the regenerative capacitor 15, and the inductance L of the resonant inductor L1. In the case of basic operation described below, the control device 51 determines, for example, the variable time Tp1 as the additional time Tad. The additional time Tad is determined using the current value of the load current, the potential V15 of the fourth end 154 of the regenerative capacitor 15, and the inductance L of the resonant inductor L1. The current value of the load current is, for example, the result of load current detection by a current sensor or a signal processed value thereof, or an estimated value of the load current iU. The load current detection result or its signal processing value at this time is the detection value at the carrier cycle in which the additional time Tad is added to the first dead time period Td1, or the detection value at the timing closest to that carrier cycle. The estimated load current value at this time is the load current estimated value at the carrier cycle in which the additional time Tad is added to the first dead time period Td1. The inductance L of the resonance inductor L1 is a value pre-stored in the control device 51. The potential V15 at the fourth end 154 of the regenerative capacitor 15 is, for example, a detected value by a voltage detection circuit. In the example of FIG. 4, the additional time Tad is a value calculated by Tad = iU × (L / V15). In the example of FIG. 5, the additional time Tad is a value calculated by Tad = iV × (L / V15). In the example of FIG. 6, the additional time Tad is a value calculated by Tad = iW × (L / V15).

[0049] The length of the variable time Tp1 in the case of basic operation is an ideal design example, and may be 90% or more and 110% or less of the additional time Tad.

[0050] In the power conversion device 100 , a signal generating circuit 52 separate from the control device 51 controls the multiple switches 8 .

[0051] The signal generating circuit 52 generates second control signals SU6, SU7, SV6, SV7, SW6, and SW7 that control the on / off of the third switching element 6U, the fourth switching element 7U, the third switching element 6V, the fourth switching element 7V, the third switching element 6W, and the fourth switching element 7W, respectively, and outputs these signals to the gate terminals of the third switching element 6U, the fourth switching element 7U, the third switching element 6V, the fourth switching element 7V, the third switching element 6W, and the fourth switching element 7W, respectively.

[0052] The signal generating circuit 52 generates, for each of the plurality of switches 8, a second control signal having a high-level period corresponding to the second dead time period Td2 for the corresponding switching circuit 10 among the plurality of switching circuits 10.

[0053] The signal generation circuit 52 synchronizes the start of the high-level period of the second control signal generated for each of the multiple switches 8 with the start of the second dead time period Td2. In the present disclosure, "synchronizing the start of the high-level period of the second control signal with the start of the second dead time period Td2" means that the start of the high-level period of the second control signal occurs between the start and end of the second dead time period Td2, and the length of time between the start of the high-level period of the second control signal and the end of the second dead time period Td2 is equal to or longer than the length of the first dead time period Td1. In the first embodiment, the signal generation circuit 52 synchronizes the start of the high-level period of the second control signal for each of the multiple switches 8 with the start of the second dead time period Td2. In the first embodiment, the length of the high-level period of the second control signal for each of the multiple switches 8 is longer than the length of the second dead time period Td2.

[0054] The signal generation circuit 52 generates a second control signal for each of the multiple switches 8 using a first control signal to the first switching element 1 and a first control signal to the second switching element 2 of a corresponding switching circuit 10 among the multiple switching circuits 10. In this embodiment, the signal generation circuit 52 has multiple (six) logic circuits 521 to 526 and multiple (six) gate drive circuits 531 to 536, as shown in Fig. 2. In the signal generation circuit 52, the multiple logic circuits 521 to 526 correspond one-to-one to the multiple gate drive circuits 531 to 536.

[0055] The logic circuit 521 is configured to generate a second control signal SU6 using the first control signal SU1 and the first control signal SU2. The logic circuit 521 is, for example, a two-input, one-output logic circuit, to which the first control signal SU1 and the first control signal SU2 output from the control device 51 are input, and which outputs a second control signal SU6. The second control signal SU6 is provided to the third switching element 6U via the gate drive circuit 531.

[0056] The logic circuit 522 is configured to generate a second control signal SU7 using the first control signal SU1 and the first control signal SU2. The logic circuit 522 is, for example, a two-input, one-output logic circuit, to which the first control signal SU1 and the first control signal SU2 output from the control device 51 are input, and which outputs a second control signal SU7. The second control signal SU7 is provided to the fourth switching element 7U via the gate drive circuit 532.

[0057] The logic circuit 523 is configured to generate a second control signal SV6 using the first control signal SV1 and the first control signal SV2. The logic circuit 523 is, for example, a two-input, one-output logic circuit, to which the first control signal SV1 and the first control signal SV2 output from the control device 51 are input, and which outputs a second control signal SV6. The second control signal SV6 is provided to the third switching element 6V via the gate drive circuit 533.

[0058] The logic circuit 524 is configured to generate a second control signal SV7 using the first control signal SV1 and the first control signal SV2. The logic circuit 524 is, for example, a two-input, one-output logic circuit, to which the first control signal SV1 and the first control signal SV2 output from the control device 51 are input, and which outputs a second control signal SV7. The second control signal SV7 is provided to the fourth switching element 7V via the gate drive circuit 534.

[0059] The logic circuit 525 is configured to generate a second control signal SW6 using the first control signal SW1 and the first control signal SW2. The logic circuit 525 is, for example, a two-input, one-output logic circuit, to which the first control signal SW1 and the first control signal SW2 output from the control device 51 are input, and which outputs a second control signal SW6. The second control signal SW6 is provided to the third switching element 6W via the gate drive circuit 535.

[0060] The logic circuit 526 is configured to generate a second control signal SW7 using the first control signal SW1 and the first control signal SW2. The logic circuit 526 is, for example, a two-input, one-output logic circuit, to which the first control signal SW1 and the first control signal SW2 output from the control device 51 are input, and which outputs a second control signal SW7. The second control signal SW7 is provided to the fourth switching element 7W via the gate drive circuit 536.

[0061] The first clamp diode 13 has an anode connected to the first end of the resonance inductor L1 and a cathode connected to the first DC terminal 31. The second clamp diode 14 has a cathode connected to the first end of the resonance inductor L1 and an anode connected to the second DC terminal 32. Therefore, the second clamp diode 14 is connected in series with the first clamp diode 13.

[0062] 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.

[0063] (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.

[0064] In the following description, the switching element (the first switching element 1 or the second switching element 2) that is the target of zero voltage soft switching will also be referred to as the target switching element.

[0065] 1 to 11, 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 51 determines that a resonant current passing through two of the plurality of switches 8 will flow simultaneously will be described.

[0066] (3.1) Basic Operation In the power conversion device 100, the target switching element is the first switching element 1 (hereinafter also referred to as the target first switching element 1), and when the polarity of the load current flowing through the AC terminal 41 connected to the target first switching element 1 is positive, the signal generation circuit 52 turns on the third switching element 6 corresponding to the target first switching element 1. As a result, the power conversion device 100 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. As a result, the power conversion device 100 can achieve zero-voltage soft switching of the target first switching element 1.

[0067] Furthermore, in the power conversion device 100, when the target switching element is the second switching element 2 (hereinafter also referred to as the target second switching element 2) and the polarity of the load current flowing through the AC terminal 41 connected to the target second switching element 2 is negative, the signal generating circuit 52 turns on the fourth switching element 7 corresponding to the target second switching element 2. As a result, the power conversion device 100 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. As a result, the power conversion device 100 can achieve zero-voltage soft switching of the target second switching element 2.

[0068] When the control device 51 sets the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is positive, it adds a variable time Tp1 (additional time Tad) to the first dead time period Td1 by shortening the high-level period of the first control signal to the second switching element 2.

[0069] For example, when the control device 51 sets the second dead time period Td2 for the switching circuit 10U, if the polarity of the load current iU is positive, as shown in Figure 7, the control device 51 adds a variable time Tp1 (additional time Tad) to the first dead time period Td1 by shortening the high-level period of the first control signal SU2 to the second switching element 2U.

[0070] 7 illustrates the first control signals SU1 and SU2, the second control signal SU6, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V1u across the first switching element 1U when the target switching element is the first switching element 1U of the switching circuit 10U. Also, in FIG. 7, the voltage value of the DC power supply E1 is illustrated as Vd.

[0071] When the target switching element is the first switching element 1U, in the switching circuit 10U, the voltage V2u across the second switching element 2U becomes Vd at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends, and the voltage V1u across the first switching element 1U becomes zero at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends. Therefore, when the first control signal SU1 changes from low level to high level at time t13, the first switching element 1U is subjected to zero-voltage soft switching. 7 , the current iL1 flowing through the resonance inductor L1 starts at time t11, when the high-level period of the second control signal SU6 begins, reaches the same value as the load current iU at time t12, when a variable time Tp1 (additional time Tad) has elapsed since time t11, reaches the same value as the load current iU at time t13, when the second dead-time period Td2 ends, and reaches zero at time t14, when a time Tad2 equal to the additional time Tad has elapsed since time t13. In the signal generating circuit 52, the second control signal SU6 changes from low to high at time t11, when the first control signal SU2 changes from high to low, and then changes from high to low at time t14, when the sum of the second dead-time period Td2 and the time Tad2 has elapsed. The current iL1 that flows between time t12 and time t13 is a resonant current (a charging current for the resonant capacitor 9U) that flows from the regenerative capacitor 15 through the resonant inductor L1 to the resonant capacitor 9U.

[0072] Hereinafter, a period T01 in which the absolute value of the current iL1 increases from zero to the absolute value of the load current (load current iU in the example of FIG. 7 ) will be referred to as a first period T01, a period T02 in which the absolute value of the current iL1 is greater than the load current (load current iU in the example of FIG. 7 ) will be referred to as a second period T02, and a period T03 in which the absolute value of the current iL1 decreases from the absolute value of the load current (load current iU in the example of FIG. 7 ) to zero will be referred to as a third period T03. In the power conversion device 100, when the current iL1 increases from zero to iL1=iU in the first period T01, the first period T01 transitions to a second period T02, and when a resonant current flows in the second period T02 and the first control signal to the target switching element changes from low level to high level, the second period T02 transitions to a third period T03.

[0073] In FIG. 7, the first period T01 is the period from time t11 to time t12. The length of the first period T01 is the same as the additional time Tad. In FIG. 8, the current path of the current iL1 flowing through the resonant inductor L1 during the first period T01 in FIG. 7 is indicated by a bold solid line, and the current path of the load current iU flowing during the first period T01 is indicated by a bold dashed line. During the first period T01, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are all in an OFF state, and the third switching element 6U is in an ON state. During the first period T01, the current iL1 flows through the regenerative capacitor 15, the resonant inductor L1, the diode 71, the third switching element 6U, and the AC terminal 41U. Furthermore, during the first period T01, the load current iU flows through the second diode 5 connected in antiparallel to the second switching element 2U, the AC terminal 41U, and the AC load RA1.

[0074] In FIG. 7 , the second period T02 is the period from time t12 to time t13. The length of the second period T02 is the same as the length of the resonant half cycle. In FIG. 9 , the current path of the current iL1 flowing through the resonant inductor L1 during the second period T02 in FIG. 7 is shown by a bold solid line. During the second period T02, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are all in the OFF state, and the third switching element 6U is in the ON state. During the second period T02, the current iL1 flows through the regenerative capacitor 15, the resonant inductor L1, the diode 71, and the third switching element 6U, and is divided into a current flowing to the AC terminal 41U and a current flowing to the resonant capacitor 9U (resonant current). During the second period T02, the resonant capacitor 9U is charged by the current flowing through the resonant capacitor 9.

[0075] In FIG. 7, the third period T03 is the period from time t13 to time t14. The length of the third period T03 is the same as the time Tad2. In FIG. 10, the current path of the current iL1 flowing through the resonant inductor L1 during the third period T03 in FIG. 7 is shown by a thick solid line, and the current path of the load current iU flowing during the third period T03 is shown by a thick dashed line. During the third period T03, the first switching element 1U and the third switching element 6U are each in an ON state, and the second switching element 2U and the fourth switching element 7U are each in an OFF state. During the third period T03, the current iL1 flows through the regenerative capacitor 15, the resonant inductor L1, the diode 71, the third switching element 6U, the connection point 3U, and the AC terminal 41U. This reduces the current iL1 to zero. Furthermore, during the third period T03, the load current iU flows through a path passing through the first DC terminal 31, the first switching element 1U, the AC terminal 41U and the AC load RA1.

[0076] When the control device 51 sets the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is negative, it adds a variable time Tp1 (additional time Tad) to the first dead time period Td1 by shortening the high-level period of the first control signal to the first switching element 1.

[0077] For example, when the control device 51 sets the second dead time period Td2 for the switching circuit 10U, if the polarity of the load current iU is negative, as shown in Figure 11, the control device 51 adds a variable time Tp1 (additional time Tad) to the first dead time period Td1 by shortening the high-level period of the first control signal SU1 to the first switching element 1U.

[0078] 11 illustrates the first control signals SU1 and SU2, the second control signal SU7, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V2u across the second switching element 2U when the target switching element is the second switching element 2U of the switching circuit 10U. Also, in FIG. 11, the voltage value of the DC power supply E1 is illustrated as Vd.

[0079] When the target switching element is the second switching element 2U, in the switching circuit 10U, the voltage V1u across the first switching element 1U becomes Vd at time t43 when the second dead time period Td2 immediately before the high-level period of the first control signal SU2 ends, and the voltage V2u across the second switching element 2U becomes zero at time t43 when the second dead time period Td2 ends. Therefore, when the first control signal SU2 changes from low level to high level at time t43, the second switching element 2U is subjected to zero-voltage soft switching. 11 , the current iL1 flowing through the resonant inductor L1 begins at time t41, when the high-level period of the second control signal SU7 begins, reaches the same value as the load current iU at time t42, when the variable time Tp1 (additional time Tad) has elapsed, reaches the same value as the load current iU at time t43, when the second dead time period Td2 ends, and reaches zero at time t44, when the time Tad2, which is the same length as the variable time Tp1, has elapsed from time t43. In the signal generating circuit 52, the second control signal SU7 changes from low to high when the first control signal SU1 changes from high to low. The current iL1 flowing between time t42 and time t43 is the resonant current (the discharge current of the resonant capacitor 9U) flowing from the resonant capacitor 9U to the resonant inductor L1.

[0080] 11, the first period T01 is the period from time t41 to time t42. During the first period T01, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the first period T01, the current iL1 flows through a path that passes through the AC terminal 41U, the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. Also, during the first period T01, the load current iU flows through a path that passes through the AC load RA1, the AC terminal 41U, and the first diode 4 connected in anti-parallel to the first switching element 1U.

[0081] In FIG. 11 , the second period T02 is the period from time t42 to time t43. During the second period T02, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the second period T02, the current iL1 flowing through the resonant inductor L1 is a combined current of a current flowing from the AC terminal 41U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15, and a current (resonant current) flowing from the resonant capacitor 9U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. During the second period T02, the resonant capacitor 9 is discharged.

[0082] In FIG. 11 , the third period T03 is the period from time t43 to time t44. During the third period T03, the second switching element 2U and the fourth switching element 7U are each in an ON state, and the first switching element 1U and the third switching element 6U are each in an OFF state. During the third period T03, the current iL1 flows through the AC terminal 41U, the connection point 3U, the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. This reduces the absolute value of the current iL1 to zero. Furthermore, during the third period T03, the load current iU flows through the AC load RA1, the AC terminal 41U, the second switching element 2U, and the second DC terminal 32.

[0083] Although an example of setting the second dead time period Td2 for the switching circuit 10U has been described above, the same applies to the switching circuit 10V and the switching circuit 10W.

[0084] The power conversion device 100 resonates a resonant capacitor 9 associated with a target switching element among the plurality of first switching elements 1 and the plurality of second switching elements 2, and a resonant inductor L1. At this time, the voltage across the resonant capacitor 9 associated with the target switching element varies depending on the amplitude of a resonant voltage centered on a potential V15 at a fourth terminal 154 of the regenerative capacitor 15. The voltage across the target switching element varies from the voltage value Vd (see FIGS. 7 and 11 ) of the DC power supply E1 to zero, thereby achieving zero-voltage soft switching. The potential V15 at the fourth terminal 154 of the regenerative capacitor 15 varies depending on the amount of charge charged or discharged in the regenerative capacitor 15 for each period during which a resonant current flows in a resonant circuit including the resonant capacitor 9 and the resonant inductor L1. Furthermore, the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 varies depending on the amount of charge charged or discharged in the regenerative capacitor 15 for each period T10 of the carrier signal. In the regenerative capacitor 15, the charge amount or discharge amount associated with the load currents iU, iV, and iW of the U, V, and W phases is determined for each carrier signal cycle T10. The largest absolute value of the load currents iU, iV, and iW produces the largest charge. Therefore, in the power conversion device 100, the charge amount of the regenerative capacitor 15 changes for each carrier signal cycle T10. However, in the power conversion device 100, when the three-phase servo motor serving as the AC load RA1 is rotating normally, the load currents iU, iV, and iW of each phase are sinusoidal and out of phase with each other by 120°. This balances the charge and discharge of the regenerative capacitor 15, thereby suppressing fluctuations in the potential V15 at the fourth terminal 154 of the regenerative capacitor 15.

[0085] In the power conversion apparatus 100, if the control device 51 performs only the basic operation described above, for example, if the three-phase servo motor serving as the AC load RA1 locks due to a load state change, the load currents iU, iV, and iW of the U, V, and W phases will each have a different constant value. This results in a large difference between the potential V15 at the fourth end 154 of the regenerative capacitor 15 and half the voltage Vd (Vd / 2) of the DC power supply E1. This reduces the amplitude of the resonant voltage when resonating the resonant capacitors 9U, 9V, and 9W with the resonant inductor L1 to achieve zero-voltage soft switching in the power conversion apparatus 100, potentially making it impossible to achieve zero-voltage soft switching.

[0086] (3.2) First Control Operation and Second Control Operation The control system 50 is configured to be able to perform either a first control operation (hereinafter also referred to as an independent control operation) or a second control operation (hereinafter also referred to as a simultaneous control operation) when, for example, the control device 51 determines that two-phase resonant currents corresponding to two of the multiple switching circuits 10 flow simultaneously through the resonant inductor L1. "When it is determined that two-phase resonant currents flow simultaneously" means that the control system 50 has estimated in advance that two-phase resonant currents will flow simultaneously through the resonant inductor L1.

[0087] (3.2.1) Determination of whether two-phase resonant currents flow simultaneously In the power conversion device 100, the phases of the voltage commands for three phases (U phase, V phase, and W phase) are shifted by 120° from one another, 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 first control signals for the two phases approach each other (see regions A1 and A2 in FIG. 3 ). In region A1 in FIG. 3 , the duties of the first control signal for the U phase and the first control signal for the V phase are approximately 0.75. In region A2 in FIG. 3 , the duties of the first control signal for the U phase and the first control signal for the V phase are approximately 0.25. The polarity of the resonant current is the same as the polarity of the current iL1; in region A1, the polarity of the resonant current is positive, and in region A2, the polarity of the resonant current is negative. In the case of region A1, for example, during one cycle T10 (see FIG. 12 ) of the carrier signal, the time difference between the start of the high-level period of the second control signal SU6 supplied to the third switching element 6U (time t11 in FIG. 4 ) and the start of the high-level period of the second control signal SV6 supplied to the third switching element 6V (time t21 in FIG. 5 ) becomes short, and there is a possibility that the U-phase resonant current and the V-phase resonant current will flow simultaneously through the resonant inductor L1. In the power conversion device 100, in the case of region A2, the direction of the resonant current is opposite to that in region A1, but there is a possibility that the U-phase resonant current and the V-phase resonant current will flow simultaneously through the resonant inductor L1.

[0088] 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 changes compared to when a single-phase current flows through the resonant inductor L1.

[0089] 12, the length of the period corresponding to the first period T01 during the high-level period of the second control signal SU6 is denoted by Tau, the length of the period corresponding to the second period T02 is denoted by Tres / 2, and the length of the period corresponding to the third period T03 is denoted by Tau. Tau is a value determined in the control device 51 by calculating Tau=iU×(L / V15) using, for example, the detection result of the load current iU by the current sensor or its signal processed value, or the estimated value of the load current iU, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the potential V15 at the fourth end 154 of the regenerative capacitor 15 (in this embodiment, the detected value of the voltage across the regenerative capacitor 15).

[0090] 12, the length of the period corresponding to the first period T01 during the high-level period of the second control signal SV6 is denoted as Tav, the length of the period corresponding to the second period T02 is denoted as Tres / 2, and the length of the period corresponding to the third period T03 is denoted as Tav. Tav is a value determined in the control device 51 by, for example, calculating Tav=iV×(L / V15) using the detection result of the load current iV by the current sensor or its signal processed value, or the estimated value of the load current iV, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the potential V15 at the fourth end 154 of the regenerative capacitor 15.

[0091] In the following description, the length of the period corresponding to the first period T01 during the high-level period of the second control signal SW6 is denoted as Taw, the length of the period corresponding to the second period T02 is denoted as Tres / 2, and the length of the period corresponding to the third period T03 is denoted as Taw. Taw is a value determined in the control device 51 by, for example, calculating Taw=iW×(L / V15) using the detection result of the load current iW by the current sensor or its signal processed value, or the estimated value of the load current iW, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the potential V15 of the fourth end 154 of the regenerative capacitor 15.

[0092] The on-period of the first switching element 1U corresponds one-to-one to the high-level period of the first control signal SU1. The on-period of the first switching element 1V corresponds one-to-one to the high-level period of the first control signal SV1. The on-period of the first switching element 1W corresponds one-to-one to the high-level period of the first control signal SW1. Furthermore, the on-period of the second switching element 2U corresponds one-to-one to the high-level period of the first control signal SU2. The on-period of the second switching element 2V corresponds one-to-one to the high-level period of the first control signal SV2. The on-period of the second switching element 2W corresponds one-to-one to the high-level period of the first control signal SW2.

[0093] The control device 51 generates the first control signals SU1, SU2, SV1, SV2, SW1, and SW2, for example, every one cycle T10 of the carrier signal (see Figure 12), and then determines whether or not there is overlap in the resonant currents before controlling the first switching element 1U, the second switching element 2U, the first switching element 1V, the second switching element 2V, the first switching element 1W, and the second switching element 2W.

[0094] (3.2.1.1) In the case of charging operation of the resonance capacitor FIG. 12 is an explanatory diagram of how to calculate the overlap time when the U-phase resonance current and the V-phase resonance current overlap.

[0095] In the power conversion device 100, if the time difference ΔTuv is equal to or greater than Tau + Tav + (Tres / 2), the U-phase resonant current and the V-phase resonant current do not overlap, and if the time difference ΔTuv is less than Tau + Tav + Tres / 2, the U-phase resonant current and the V-phase resonant current overlap. The time difference ΔTuv is the time difference between the start of a high-level period of the first control signal SU1 provided to the first switching element 1U of the switching circuit 10U and the start of a high-level period of the first control signal SV1 provided to the first switching element 1V of the switching circuit 10V. The control device 51 determines the overlap time Tov_uv, during which the U-phase resonant current and the V-phase resonant current overlap, by calculating Tov_uv = Tau + Tav + (Tres / 2) - ΔTuv. The control device 51 estimates that the U-phase resonant current and the V-phase resonant current overlap if Tov_uv > 0. The method for calculating the time difference ΔTuv is not limited to the above example, and may use, for example, the time difference between the end point of the high-level period of the first control signal SU2 given to the second switching element 2U of the switching circuit 10U and the end point of the high-level period of the first control signal SV2 given to the second switching element 2V of the switching circuit 10V.

[0096] Furthermore, in the power conversion device 100, if the time difference ΔTuw is equal to or greater than Tau + Taw + (Tres / 2), the U-phase resonant current and the W-phase resonant current do not overlap, and if the time difference ΔTuw is less than Tau + Taw + (Tres / 2), the U-phase resonant current and the W-phase resonant current overlap. The time difference ΔTuw is the time difference between the start of the high-level period of the first control signal SU1 and the start of the high-level period of the first control signal SW1. The control device 51 determines the overlap time Tov_uw by calculating Tov_uw = Tau + Taw + (Tres / 2) - ΔTuw, where Tov_uw is the overlap time during which the U-phase resonant current and the W-phase resonant current overlap. If Tov_uw > 0, the control device 51 estimates that the U-phase resonant current and the W-phase resonant current overlap. The method for calculating the time difference ΔTuw is not limited to the above example, and for example, the time difference between the end point of the high-level period of the first control signal SU2 and the end point of the high-level period of the first control signal SW2 may be used.

[0097] Furthermore, in the power conversion device 100, if the time difference ΔTvw is equal to or greater than Tav + Taw + (Tres / 2), the V-phase resonant current and the W-phase resonant current do not overlap, and if the time difference ΔTvw is less than Tav + Taw + (Tres / 2), the V-phase resonant current and the W-phase resonant current overlap. The time difference ΔTvw is the time difference between the start of a high-level period of the first control signal SV1 provided to the first switching element 1V of the switching circuit 10V and the start of a high-level period of the first control signal SW1 provided to the first switching element 1W of the switching circuit 10W. The control device 51 determines the overlap time Tov_vw, during which the V-phase resonant current and the W-phase resonant current overlap, by calculating Tov_vw = Tav + Taw + (Tres / 2) - ΔTvw. The control device 51 estimates that the V-phase resonant current and the W-phase resonant current overlap if Tov_vw > 0. The method for calculating the time difference ΔTvw is not limited to the above example, and may use, for example, the time difference between the end point of the high-level period of the first control signal SV2 and the end point of the high-level period of the first control signal SW2.

[0098] (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 51 determines whether two-phase resonant currents flow simultaneously using the same time difference and overlap time as in the case of charging operation of the resonant capacitor 9.

[0099] For example, if the time difference ΔTuv between the start of the high-level period of the first control signal SU2 and the start of the high-level period of the first control signal SV2 is less than Tau + Tav + (Tres / 2), the control device 51 determines that the U-phase resonant current and the V-phase resonant current overlap.

[0100] In addition, the control device 51 determines that the U-phase resonant current and the W-phase resonant current overlap, for example, if the time difference ΔTuw between the start of the high-level period of the first control signal SU2 and the start of the high-level period of the first control signal SW2 is less than Tau + Taw + (Tres / 2).

[0101] In addition, the control device 51 determines that the V-phase resonant current and the W-phase resonant current overlap, for example, if the time difference ΔTvw between the start of the high-level period of the first control signal SV2 and the start of the high-level period of the first control signal SW2 is less than Tav + Taw + (Tres / 2).

[0102] (3.2.2) First Control Operation The first control operation (independent control operation) includes a control operation for shifting the high-level period of at least one of two second control signals to two switches 8 through which a resonant current flows among the multiple switches 8 when it is determined that a two-phase resonant current flows through the resonant inductor L1, so as to eliminate overlapping periods between the high-level periods of the two second control signals. In other words, in the first control operation, the control system 50 generates second control signals to the two switches 8 so that the resonant currents passing through the two switches 8 do not flow simultaneously through the resonant inductor L1 but flow independently in different periods.

[0103] 13 shows the second control signals SV7 and SW7 when the high-level periods of the two second control signals SV7 and SW7 for two of the three switches 8U, 8V, and 8W are shifted so as to eliminate any overlap between the high-level periods of the two second control signals SV7 and SW7 for two of the three switches 8V and 8W within one period T10 of the carrier signal (see FIG. 12). Figure 13 shows the second control signals SU6, SV7, and SW7, the load currents iU, iV, and iW, the current iL1 flowing through the resonant inductor L1, and the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 for an example case where the three-phase servo motor serving as the AC load RA1 is locked and the load currents iU, iV, and iW each assume a different constant value. 13, the absolute value of load current iU > the absolute value of load current iW > the absolute value of load current iV, and the high-level period of second control signal SU6 > the high-level period of second control signal SW7 > the high-level period of second control signal SV7. The absolute value of current iL1 is greatest during the high-level period of second control signal SU6 of switch 8U, which corresponds to load current iU having the largest absolute value among load currents iU, iV, and iW. As a result, the fluctuation range of potential V15 at fourth terminal 154 of regenerative capacitor 15 is greatest during the high-level period of second control signal SU6 among the high-level periods of the three second control signals SU6, SV7, and SW7.

[0104] (3.2.3) Second Control Operation The second control operation (simultaneous control operation) includes a control operation for overlapping the high-level periods of two second control signals for two switches 8 through which a resonant current flows among the multiple switches 8 when it is determined that a two-phase resonant current flows through the resonant inductor L1. "Overlapping the high-level periods of two second control signals" is not limited to overlapping the entire high-level period of one of the two second control signals with the entire high-level period of the remaining second control signal.

[0105] 14 and 15 show the second control signals SV7 and SW7 when the high-level periods of the two second control signals SV7 and SW7 of two of the three switches 8U, 8V, and 8W are shifted so that the high-level periods of the two second control signals SV7 and SW7 of two of the three switches 8V and 8W overlap within one cycle T10 of the carrier signal (see FIG. 12). More specifically, FIG. 14 shows the second control signals SV7 and SW7 when the high-level periods of the two second control signals SV7 and SW7 of two of the three switches 8U, 8V, and 8W are shifted so that the entire high-level periods of the two second control signals SV7 and SW7 of two of the three switches 8V and 8W overlap. It is not necessary to overlap the high-level periods of the two second control signals SV7 and SW7 for the two switches 8V and 8W, as shown in Figure 14. For example, as shown in Figure 15, it is sufficient that the second control signal for one of the three switches 8U, 8V, and 8W remains high until the current iL1 reaches the V-phase and W-phase load currents iV and iW, respectively. Figures 14 and 15 illustrate the second control signals SU6, SV7, and SW7, the load currents iU, iV, and iW, and the current iL1 flowing through the resonant inductor L1 for an example in which the three-phase servo motor serving as the AC load RA1 is locked and the load currents iU, iV, and iW each assume a different constant value. Figure 14 also illustrates the potential V15 at the fourth terminal 154 of the regenerative capacitor 15. 14 and 15, the absolute value of the load current iU > the absolute value of the load current iW > the absolute value of the load current iV, and the high-level period of the second control signal SU6 > the high-level period of the second control signal SW7 > the high-level period of the second control signal SV7. The absolute value of the current iL1 is greatest during the period when the high-level periods of the two second control signals SV7 and SW7 overlap. As a result, the fluctuation range of the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is greatest during the period when the high-level periods of two of the three second control signals SU6, SV7, and SW7 overlap.

[0106] (3.2.4) Details of the First Control Operation and the Second Control Operation In the control system 50, the control device 51 acquires a detected potential of the potential V15 at the fourth terminal 154 of the regenerative capacitor 15. The control device 51 acquires the detected potential, for example, for each cycle T10 of the carrier signal. For example, the control device 51 may store the voltage value Vd of the DC power supply E1 in advance, or may acquire the detected result of the voltage value Vd. In the control system 50, the control device 51 determines the content of the control operation based on the detected potential at the fourth terminal 154 of the regenerative capacitor 15, the value of Vd / 2, and the detected results of the load currents iU, iV, and iW.

[0107] In the control system 50, the control device 51 switches between a first control operation and a second control operation based on the detected potential of the fourth terminal 154 of the regenerative capacitor 15 and the polarities of the multiple load currents iU, iV, iW output from the multiple AC terminals 41. The "detected potential of the fourth terminal 154 of the regenerative capacitor 15" is the detected potential of the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 relative to the ground potential.

[0108] In the first control operation (independent control operation), the control system 50 shifts the high-level periods of two second control signals sent to two of the multiple switches 8 to prevent overlap between the high-level periods. In the first control operation, for example, as shown in FIG. 13 , the end point of the high-level period of one of the two second control signals is the same as the start point of the high-level period of the remaining second control signal. However, this is not limited to this, and the high-level period of the remaining second control signal may start after the end of the high-level period of one second control signal. In the independent control operation, the control system 50 prevents overlap between the high-level periods of the second control signals sent to the multiple switches 8.

[0109] In the second control operation (simultaneous control operation), the control system 50 shifts the high-level periods of two second control signals to two of the multiple switches 8 so that the high-level periods of the two second control signals overlap.

[0110] When the control system 50 determines that resonant currents passing through two or more switches 8 of the multiple switches 8 simultaneously flow through the resonant inductor L1, and when performing the second control operation, the control system 50 determines the variable time Tp1 to be the sum of two or more additional times Tad corresponding one-to-one to the two or more switches 8, and the difference (ΔTres / 2) between a first resonant half cycle when the resonant currents do not flow simultaneously and a second resonant half cycle when the resonant currents flow simultaneously. Each of the two or more additional times Tad is determined using the current value of the load current flowing through the corresponding switch 8 of the two or more switches 8, the inductance of the resonant inductor L1, and the potential V15 at the fourth end 154 of the regenerative capacitor 15. The first resonant half cycle is half the length (Tres / 2) of the resonant cycle (Tres) of a first resonant circuit including the resonant inductor L1 and only one of the multiple resonant capacitors 9. The second resonant half period is half the length of the resonant period (Tres2) of the second resonant circuit including the resonant inductor L1 and two or more resonant capacitors 9 corresponding to two or more switches 8 among the plurality of resonant capacitors 9. When the second resonant circuit includes two resonant capacitors 9, the second resonant half period is Tres2 / 2=2 １／２ When the second resonant circuit includes three resonant capacitors 9, the second resonant half period is Tres2 / 2=3 １／２ ×(Tres / 2).

[0111] The control system 50 performs a first control action when the first condition or the second condition is satisfied, and performs a second control action when the third condition or the fourth condition is satisfied.

[0112] The first condition is that the detected potential at the fourth terminal 154 of the regenerative capacitor 15 is greater than an upper threshold Vth2, and the product of the multiple load currents iU, iV, and iW is positive. The upper threshold Vth2 is greater than Vd / 2. For example, the upper threshold Vth2 is 110% of Vd / 2. When calculating iU x iV x iW, iU, iV, and iW are instantaneous values ​​and are positive, zero, or negative.

[0113] The second condition is that the detected potential at the fourth terminal 154 of the regenerative capacitor 15 is smaller than the lower threshold Vth1, and the product of the multiple load currents iU, iV, and iW (iU x iV x iW) is positive. The lower threshold Vth1 is smaller than Vd / 2. Therefore, the lower threshold Vth1 is smaller than the upper threshold Vth2. The lower threshold Vth1 is, for example, 90% of Vd / 2.

[0114] The third condition is that the detected potential at the fourth terminal 154 of the regenerative capacitor 15 is smaller than the lower limit threshold Vth1, and the product of the multiple load currents iU, iV, iW is negative.

[0115] The fourth condition is that the detected potential at the fourth terminal 154 of the regenerative capacitor 15 is greater than the upper limit threshold Vth2, and the product of the multiple load currents iU, iV, iW is negative.

[0116] The control system 50 performs the basic operation described above when the detected potential at the fourth terminal 154 of the regenerative capacitor 15 is equal to or greater than the lower threshold Vth1 and equal to or less than the upper threshold Vth2.

[0117] The relationship between the first, second, third and fourth conditions and the first control action (independent control action) and the second control action (simultaneous control action) is as shown in Table 1.

[0118]

[0119] Note that "iU x iV x iW" is the product of the load currents iU, iV, and iW, where the polarity is positive when the current flows in the direction of the arrow in Fig. 1 and negative when the current flows in the opposite direction. Instead of "iU x iV x iW", it is also possible to determine that the product of the multiple load currents iU, iV, and iW is positive when the value of sgn(iU) x sgn(iV) x sgn(iW) is positive "1", and to determine that the product of the multiple load currents iU, iV, and iW is negative when the value of sgn(iU) x sgn(iV) x sgn(iW) is "-1".

[0120] In the control system 50, the control device 51 performs a first control operation (independent control operation) when it determines that the first condition or the third condition is satisfied, and performs a second control operation (simultaneous control operation) when it determines that the second condition or the fourth condition is satisfied. Examples of the first control operation and the second control operation will be described below with reference to FIGS.

[0121] Fig. 16 shows a time chart of an example when the control device 51 determines that the first condition is satisfied and performs the first control operation (independent control operation). Fig. 17 shows a time chart of an example when the control device 51 determines that the second condition is satisfied and performs the second control operation (simultaneous control operation). Fig. 18 shows a time chart of an example when the control device 51 determines that the third condition is satisfied and performs the first control operation (independent control operation). Fig. 19 shows a time chart of an example when the control device 51 determines that the fourth condition is satisfied and performs the second control operation (simultaneous control operation).

[0122] 16 to 19 illustrate six first control signals SU1, SU2, SV1, SV2, SW1, and SW2, and three second control signals out of six second control signals SU6, SU7, SV6, SV7, SW6, and SW7. Also illustrated in Figures 16 to 19 are current iL1, three load currents iU, iV, and iW, voltages V2u, V2v, and V2w across three second switching elements 2U, 2V, and 2W, respectively, and potential V15 at the fourth end 154 of regenerative capacitor 15. In Figures 16 to 19, when the high-level periods of each of the six first control signals SU1, SU2, SV1, SV2, SW1, and SW2 are shifted, an arrow Ts is illustrated indicating the shift direction and shift time relative to the high-level period before the shift. In the control system 50, when the control device 51 shifts the high-level periods of the first control signals SU1, SU2, SV1, SV2, SW1, and SW2, the high-level periods of the second control signals SU6, SU7, SV6, SV7, SW6, and SW7 generated by the signal generation circuit 52 are shifted. Also, in Fig. 17, the currents flowing through the different resonant inductors are shown by two-dot chain lines assuming that different resonant inductors are connected to the switches 8V and 8W before the high-level periods of two of the three second control signals SU6, SV7, and SW7 are shifted. 19, the dashed-double-dot lines indicate the currents flowing through the different resonant inductors when it is assumed that different resonant inductors are connected to the switches 8V and 8W before the high-level periods of two of the three second control signals SU7, SV6, and SW6 are shifted. In FIGS. 16 to 19, the arrows indicate the timing at which the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is detected. The potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is detected every carrier signal cycle T10 (see FIG. 12). Thus, the control device 51 obtains the detected potential at the fourth terminal 154 of the regenerative capacitor 15 every carrier signal cycle T10. The timing at which the control device 51 obtains the detected potential at the fourth terminal 154 of the regenerative capacitor 15 is, for example, the same as the timing at which the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is detected, but is not limited to this.

[0123] 16 , when the detected potential at the fourth end 154 of the regenerative capacitor 15 at the timing indicated by the arrow is greater than the upper limit threshold Vth2, the product of the three load currents iU, iV, and iW is positive, and therefore the control system 50 performs a first control operation (independent control operation) so that the high-level period of the second control signal SV7 does not overlap with the high-level period of the second control signal SW7. The variable time Tp1 corresponding to the switching circuit 10U is an additional time Tau, the variable time Tp1 corresponding to the switching circuit 10V is an additional time Tav, and the variable time Tp1 corresponding to the switching circuit 10W is an additional time Taw. In the power conversion device 100, the control system 50 performs the first control operation, thereby reducing the amplitude of the current iL1 when the polarity of the current iL1 flowing through the resonance inductor L1 is negative, and therefore, for example, the potential V15 of the fourth end 154 of the regenerative capacitor 15 one cycle T10 of the carrier signal after the timing indicated by the arrow in Fig. 16 can be set to be equal to or lower than the upper limit threshold Vth2. As a result, the power conversion device 100 achieves zero-voltage soft switching of each of the three first switching elements 1 and the three second switching elements 2. In the example of Figure 16, the potential V15 of the fourth end 154 of the regenerative capacitor 15 can be set to be equal to or lower than the upper limit threshold Vth2 one cycle T10 of the carrier signal after the timing of the arrow, but since the voltage fluctuation value of the potential V15 that can be varied in one cycle T10 of the carrier signal varies depending on the magnitude of the resonant current, it is sufficient that the potential V15 of the fourth end 154 of the regenerative capacitor 15 be set to be equal to or lower than the upper limit threshold Vth2 one cycle T10 of the carrier signal after the timing of the arrow in Figure 16.

[0124] In the example of Fig. 17 , when the detected potential at the fourth terminal 154 of the regenerative capacitor 15 at the timing indicated by the arrow in Fig. 17 is smaller than the lower threshold Vth1, the product of the three load currents iU, iV, and iW is positive, so the control system 50 performs the second control operation (simultaneous control operation) so that the high-level period of the second control signal SV7 and the high-level period of the second control signal SW7 overlap. In the example of Fig. 17 , the variable time Tp1 corresponding to the switching circuit 10U is the additional time Tau. Also, in the example of Fig. 17 , the control system 50 determines the variable time Tp1 of each of the two switching circuits 10V and 10W to be the sum of Tav + Taw + (½) × ΔTres.

[0125] The control system 50 lengthens the length of the second dead time period Td2, thereby lengthening the high-level period of each of the second control signals SV7 and SW7. The end points of each of the second control signals SV7 and SW7 may be any time after the end point of the second resonant half cycle. When the control system 50 performs the second control operation, the amplitude of the current iL1 flowing through the resonant inductor L1 increases and the amount of charge stored in the regenerative capacitor 15 increases. Therefore, for example, the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 can be set to or above the lower threshold Vth1 one cycle T10 after the timing indicated by the arrow in FIG. 17 . This allows the power conversion device 100 to achieve zero-voltage soft switching of each of the three first switching elements 1 and the three second switching elements 2. In the example of Figure 17, the potential V15 of the fourth end 154 of the regenerative capacitor 15 can be made equal to or greater than the lower threshold value Vth1 one cycle T10 of the carrier signal after the timing of the arrow, but since the voltage fluctuation value of the potential V15 that can be varied in one cycle T10 of the carrier signal varies depending on the magnitude of the resonant current, it is sufficient that the potential V15 of the fourth end 154 of the regenerative capacitor 15 becomes equal to or greater than the lower threshold value Vth1 one cycle T10 of the carrier signal after the timing of the arrow in Figure 17.

[0126] 18 , when the detected potential at the fourth terminal 154 of the regenerative capacitor 15 at the timing indicated by the arrow in FIG. 18 is smaller than the lower limit threshold Vth1, the control system 50 performs a first control operation (independent control operation) so that the high-level period of the second control signal SV7 does not overlap with the high-level period of the second control signal SW7 because the product of the three load currents iU, iV, and iW is negative. The variable time Tp1 corresponding to the switching circuit 10U is the additional time Tau, the variable time Tp1 corresponding to the switching circuit 10V is the additional time Tav, and the variable time Tp1 corresponding to the switching circuit 10W is the additional time Taw. As a result, in the power conversion device 100, the amount of charge stored in the regenerative capacitor 15 is increased, so that the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 one carrier signal cycle T10 after the timing indicated by the arrow in FIG. 18 can be set to be equal to or greater than the lower limit threshold Vth1. This achieves zero-voltage soft switching of the three first switching elements 1 and the three second switching elements 2 in the power conversion device 100. In the example of Fig. 18 , the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 can be made equal to or greater than the lower limit threshold Vth1 one cycle T10 of the carrier signal after the timing indicated by the arrow, but it is sufficient that the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 is made equal to or greater than the lower limit threshold Vth1 one cycle T10 of the carrier signal after the timing indicated by the arrow in Fig. 18 .

[0127] In the example of Figure 19, when the detected potential at the fourth terminal 154 of the regenerative capacitor 15 at the timing indicated by the arrow in Figure 19 is greater than the upper threshold value Vth2, the product of the three load currents iU, iV, and iW is negative, so the control system 50 performs a second control operation (simultaneous control operation) so that the high-level period of the second control signal SV6 and the high-level period of the second control signal SW6 overlap. In the example of Figure 19, the variable time Tp1 corresponding to the switching circuit 10U is the additional time Tau. Also, in the example of Figure 19, the control system 50 determines the variable time Tp1 of each of the two switching circuits 10V and 10W to be the sum of Tav + Taw + (1 / 2) x ΔTres.

[0128] The control system 50 lengthens the length of the second dead time period Td2, thereby lengthening the high-level period of each of the second control signals SV6 and SW6. The end points of the second control signals SV6 and SW6 may be any time after the end point of the second resonant half cycle. When the control system 50 performs the second control operation, the amplitude of the current iL1 flowing through the resonant inductor L1 increases and the amount of charge discharged to the regenerative capacitor 15 increases. Therefore, for example, the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 one cycle T10 after the timing indicated by the arrow in FIG. 19 can be set to or below the upper threshold Vth2. This allows the power conversion device 100 to achieve soft switching of the three first switching elements 1 and the three second switching elements 2. In the example of Figure 19, the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 one cycle T10 of the carrier signal after the timing of the arrow can be made equal to or lower than the upper limit threshold Vth2, but it is sufficient that the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 is made equal to or lower than the upper limit threshold Vth2 one cycle T10 of the carrier signal after the timing of the arrow in Figure 19.

[0129] The control system 50 may include control that does not shift the high-level periods of the two second control signals to two of the multiple switches 8 in the first control operation and the second control operation.

[0130] The above describes an example of the first control operation and the second control operation when the control device 51 determines that two-phase resonant currents, V phase and W phase, flow simultaneously (overlap). However, in both cases where two-phase resonant currents, U phase and V phase, flow simultaneously and where two-phase resonant currents, U phase and W phase, flow simultaneously, the first control operation and the second control operation are performed using the same concept (algorithm) as when it is determined that two-phase resonant currents, V phase and W phase, flow simultaneously.

[0131] (4) Advantages In the power conversion device 100 according to the first embodiment, the control system 50 includes a control device 51 and a signal generating circuit 52. The control device 51 provides a first control signal, the potential of which changes between high and low levels, to each of the plurality of first switching elements 1 and the plurality of second switching elements 2. The signal generating circuit 52 provides a second control signal, the potential of which changes between high and low levels, to each of the plurality of switches 8. The control device 51 sets a second dead time period Td2, which is obtained by adding a variable time Tp1 to a predetermined first dead time period Td1, between the high-level period of the first control signal to the first switching element 1 and the high-level period of the first control signal to the second switching element 2 for each of the plurality of switching circuits 10, so that the on-periods of the first switching element 1 and the second switching element 2 do not overlap. The variable time Tp1 is determined based on the current values ​​of the load currents iU, iV, and iW flowing through the AC terminals 41, which correspond to each of the multiple switching circuits 10, the inductance of the resonance inductor L1, and the potential V15 at the fourth end 154 of the regenerative capacitor 15. The signal generating circuit 52 generates, for each of the multiple switches 8, a second control signal having a high-level period corresponding to the second dead time period Td2 for the corresponding switching circuit 10 among the multiple switching circuits 10. The control system 50 switches between a first control operation in which the high-level periods of the second control signals of the multiple switches 8 do not overlap, and a second control operation in which the high-level periods of the second control signals of two of the multiple switches 8 overlap, based on the potential V15 at the fourth end 154 of the regenerative capacitor 15 and the polarities of the multiple load currents iU, iV, and iW.

[0132] According to the above configuration, it is possible to improve the power conversion efficiency without directly controlling the plurality of switches 8 for zero voltage soft switching in the control device 51. More specifically, according to the above configuration, it is not necessary to generate a second control signal for directly controlling the plurality of switches 8 for zero voltage soft switching in the control device 51. Therefore, it is possible to realize zero voltage soft switching of each of the plurality of first switching elements 1 and the plurality of second switching elements 2 without directly controlling the plurality of switches 8 in the control device 51. Furthermore, according to the above configuration, the control system 50 switches between a first control operation that does not allow the high-level periods of the second control signals of the multiple switches 8 to overlap, and a second control operation that causes the high-level periods of the second control signals of two of the multiple switches 8 to overlap, based on the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 and the polarities of the multiple load currents iU, iV, and iW. This makes it possible to suppress fluctuations in the potential V15 of the fourth terminal 154 of the regenerative capacitor 15, thereby improving power conversion efficiency and reducing noise compared to when nothing is done regarding fluctuations in the potential V15 of the fourth terminal 154 of the regenerative capacitor 15.

[0133] Furthermore, according to the above configuration, there is no need for the control device 51 to generate and output multiple second control signals SU6, SU7, SV6, SV7, SW6, and SW7, so it is possible to simplify the control device 51, and for example, it is possible to suppress an increase in the number of control ports of the microcomputer included in the control device 51 and suppress an increase in the size of the control device 51.

[0134] Furthermore, in the power conversion device 100 according to the first embodiment, the control system 50 performs the first control operation when the first condition or the third condition is satisfied, and performs the second control operation when the second condition or the fourth condition is satisfied, thereby making it possible to suppress fluctuations in the potential V15 of the fourth end 154 of the regenerative capacitor 15 regardless of the polarities of the multiple load currents iU, iV, iW output from the multiple AC terminals 41.

[0135] Furthermore, in the power conversion device 100 according to the first embodiment, the control system 50 performs the first control operation when the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is greater than the upper threshold Vth2 and the product of the multiple load currents iU, iV, and iW is positive.

[0136] According to the above configuration, it is possible to suppress fluctuations in the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 when the product of the multiple load currents iU, iV, iW is positive.

[0137] Furthermore, in the power conversion device 100 according to the first embodiment, the control system 50 performs the second control operation when the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is smaller than the lower threshold Vth1 and the product of the multiple load currents iU, iV, and iW is positive.

[0138] According to the above configuration, it is possible to suppress fluctuations in the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 when the product of the multiple load currents iU, iV, iW is positive.

[0139] In addition, in the power conversion device 100 according to the first embodiment, the control system 50 performs the second control operation when the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is greater than the upper threshold Vth2 and the product of the multiple load currents iU, iV, and iW is negative.

[0140] According to the above configuration, it is possible to suppress fluctuations in the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 when the product of the multiple load currents iU, iV, iW is negative.

[0141] Furthermore, in the power conversion device 100 according to the first embodiment, the control system 50 performs the first control operation when the potential V15 at the fourth terminal 154 of the regenerative capacitor 15 is smaller than the lower threshold Vth1 and the product of the multiple load currents iU, iV, and iW is negative.

[0142] According to the above configuration, it is possible to suppress fluctuations in the potential V15 of the fourth terminal 154 of the regenerative capacitor 15 when the product of the multiple load currents iU, iV, iW is negative.

[0143] Furthermore, in the first control operation, the control system 50 in the power conversion device 100 according to the first embodiment shifts the high-level periods of two second control signals sent to two of the multiple switches 8 to make the high-level periods overlap each other. This enables the power conversion device 100 to increase the absolute value of the current iL1 flowing through the resonance inductor L1, and to increase the adjustment amount (range of change) of the potential V15 at the fourth end 154 of the regenerative capacitor 15.

[0144] Furthermore, in the second control operation, the control system 50 in the power conversion device 100 according to the first embodiment shifts the high-level periods of two second control signals to two of the multiple switches 8 to eliminate overlap between the high-level periods. This enables the power conversion device 100 to reduce the absolute value of the current iL1 flowing through the resonant inductor L1.

[0145] (Embodiment 2) A power conversion device 100A according to embodiment 2 will be described with reference to Figures 20 to 23. Regarding the power conversion device 100A according to embodiment 2, components similar to those of the power conversion device 100 according to embodiment 1 (see Figures 1 and 2) are denoted by the same reference numerals and descriptions thereof will be omitted.

[0146] (1) Configuration As shown in FIG. 20, the power conversion device 100A differs from the power conversion device 100 in that it includes a signal generation circuit 52A instead of the signal generation circuit 52 of the power conversion device 100.

[0147] In the power conversion device 100A, a signal generating circuit 52A controls a plurality of switches 8.

[0148] The signal generating circuit 52A generates second control signals SU6, SU7, SV6, SV7, SW6, and SW7 that control the on / off of the third switching element 6U, the fourth switching element 7U, the third switching element 6V, the fourth switching element 7V, the third switching element 6W, and the fourth switching element 7W, respectively, and outputs these signals to the gate terminals of the third switching element 6U, the fourth switching element 7U, the third switching element 6V, the fourth switching element 7V, the third switching element 6W, and the fourth switching element 7W, respectively.

[0149] The signal generation circuit 52A generates, for each of the multiple switches 8, a second control signal having a high-level period corresponding to the second dead-time period Td2 for the corresponding switching circuit 10 among the multiple switching circuits 10. In the second embodiment, the signal generation circuit 52A synchronizes the start of the high-level period of the second control signal to each of the multiple switches 8 with the start of the second dead-time period Td2, and synchronizes the end of the high-level period of the second control signal to each of the multiple switches 8 with the end of the second dead-time period Td2. Therefore, in the second embodiment, the length of the high-level period of the second control signal to each of the multiple switches 8 is the same as the length of the second dead-time period Td2. The signal generation circuit 52A includes multiple logic circuits 521 to 526 and multiple gate drive circuits 531 to 536, similar to the signal generation circuit 52 of the first embodiment (see FIG. 2 ). Since the signal generating circuit 52A synchronizes the end point of the high level period of the second control signal to each of the multiple switches 8 with the end point of the second dead time period Td2, the configuration of the multiple logic circuits 521 to 526 is different from the configuration of the multiple logic circuits 521 to 526 in the signal generating circuit 52.

[0150] (2) Operation (2.1) Basic Operation Fig. 21 shows the first control signals SU1 and SU2, the second control signal SU6, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V1u across the first switching element 1U when the target switching element is the first switching element 1U of the switching circuit 10U. Also, in Fig. 21, the voltage value of the DC power supply E1 is shown as Vd.

[0151] When the target switching element is the first switching element 1U, in the switching circuit 10U, the voltage V2u across the second switching element 2U becomes Vd at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends, and the voltage V1u across the first switching element 1U becomes zero at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends. Therefore, when the first control signal SU1 changes from low level to high level at time t13, the first switching element 1U is subjected to zero-voltage soft switching. 21 , the current iL1 flowing through the resonant inductor L1 begins at time t11, when the high-level period of the second control signal SU6 begins, reaches the same value as the load current iU at time t12, when the variable time Tp1 has elapsed, reaches the same value as the load current iU at time t13, when the second dead time period Td2 ends, and reaches zero at time t14, when the time Tad2, which is the same length as the variable time Tp1, has elapsed since time t13. In the signal generating circuit 52A, the second control signal SU6 changes from low to high at time t11, when the first control signal SU2 changes from high to low. The current iL1 flowing between time t12 and time t13 is a resonant current (a charging current for the resonant capacitor 9U) that flows from the regenerative capacitor 15 to the resonant capacitor 9U via the resonant inductor L1.

[0152] During the first period T01, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are all in an OFF state, and the third switching element 6U is in an ON state. During the first period T01, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, the third switching element 6U, and the AC terminal 41U, as in Fig. 8. Also, during the first period T01, the load current iU flows through the path passing through the second diode 5 connected in anti-parallel to the second switching element 2U, the AC terminal 41U, and the AC load RA1, as in Fig. 8.

[0153] During the second period T02, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are each in an OFF state, and the third switching element 6U is in an ON state. During the second period T02, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, and the third switching element 6U, as in Fig. 9, and is divided into a current flowing to the AC terminal 41U and a current flowing to the resonant capacitor 9U (resonant current). During the second period T02, the resonant capacitor 9U is charged by the current flowing through the resonant capacitor 9U.

[0154] During the third period T03, the first switching element 1U is in an ON state, and the second switching element 2U, the third switching element 6U, and the fourth switching element 7U are each in an OFF state. In FIG. 22, the current path of the current iL1 flowing through the resonant inductor L1 during the third period T03 in FIG. 21 is shown by a bold solid line, and the current path of the load current iU flowing during the third period T03 is shown by a bold dashed line. During the third period T03, the first switching element 1U is in an ON state, and the second switching element 2U, the third switching element 6U, and the fourth switching element 7U are each in an OFF state. During the third period T03, the current iL1 flows through the regenerative capacitor 15, the resonant inductor L1, and the first clamp diode 13. This reduces the current iL1 to zero. Furthermore, during the third period T03, the load current iU flows through a path passing through the first DC terminal 31, the first switching element 1U, the AC terminal 41U and the AC load RA1.

[0155] When the control device 51 sets the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is negative, it adds a variable time Tp1 to the first dead time period Td1 by shortening the high-level period of the first control signal to the first switching element 1.

[0156] For example, when the control device 51 sets the second dead time period Td2 for the switching circuit 10U, if the polarity of the load current iU is negative, as shown in FIG. 23, the control device 51 adds the variable time Tp1 to the first dead time period Td1 by shortening the high-level period of the first control signal SU1 to the first switching element 1U by the same amount as the variable time Tp1.

[0157] 23 illustrates the first control signals SU1 and SU2, the second control signal SU7, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V2u across the second switching element 2U when the target switching element is the second switching element 2U of the switching circuit 10U. Also, in FIG. 23, the voltage value of the DC power supply E1 is illustrated as Vd.

[0158] When the target switching element is the second switching element 2U, in the switching circuit 10U, the voltage V1u across the first switching element 1U becomes Vd at time t43, which is the end of the second dead time period Td2 immediately before the high-level period of the first control signal SU2, and the voltage V2u across the second switching element 2U becomes zero at time t43, which is the end of the second dead time period Td2. Therefore, when the first control signal SU2 changes from low to high at time t43, the second switching element 2U is subjected to zero-voltage soft switching. In the example of FIG. 23 , the current iL1 flowing through the resonant inductor L1 begins at time t41, which is the start of the high-level period of the second control signal SU7, and becomes the same value as the load current iU at time t42, when the variable time Tp1 has elapsed. The current iL1 also becomes the same value as the load current iU at time t43, when the second dead time period Td2 ends. Finally, the current iL1 becomes zero at time t44, when the variable time Tp1 has elapsed since time t43. In the signal generating circuit 52A, the second control signal SU7 changes from low to high when the first control signal SU1 changes from high to low. The current iL1 flowing between time t42 and time t43 is the resonant current (the discharge current of the resonant capacitor 9U) flowing from the resonant capacitor 9U to the resonant inductor L1.

[0159] 23, the first period T01 is the period from time t41 to time t42. During the first period T01, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the first period T01, the current iL1 flows through a path that passes through the AC terminal 41U, the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. Also, during the first period T01, the load current iU flows through a path that passes through the AC load RA1, the AC terminal 41U, and the first diode 4 connected in anti-parallel to the first switching element 1U.

[0160] In FIG. 23 , the second period T02 is the period from time t42 to time t43. During the second period T02, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the second period T02, the current iL1 flowing through the resonant inductor L1 is a combined current of a current flowing from the AC terminal 41U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15, and a current (resonant current) flowing from the resonant capacitor 9U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. During the second period T02, the resonant capacitor 9 is discharged.

[0161] In FIG. 23, the third period T03 is the period from time t43 to time t44. During the third period T03, the second switching element 2U is in the ON state, and the first switching element 1U, the third switching element 6U, and the fourth switching element 7U are each in the OFF state. During the third period T03, the current iL1 flows through the second clamp diode 14, the resonant inductor L1, and the regenerative capacitor 15. This reduces the absolute value of the current iL1 to zero. Furthermore, during the third period T03, the load current iU flows through the AC load RA1, the AC terminal 41U, the second switching element 2U, and the second DC terminal 32.

[0162] Although an example of setting the second dead time period Td2 for the switching circuit 10U has been described above, the same applies to the switching circuit 10V and the switching circuit 10W.

[0163] (2.2) First Control Operation and Second Control Operation The first control operation and second control operation are similar to the first control operation and second control operation of the first embodiment, and therefore a description thereof will be omitted.

[0164] (3) Advantages As with the power conversion device 100 according to the first embodiment, the power conversion device 100A according to the second embodiment can improve power conversion efficiency without directly controlling the multiple switches 8 for zero voltage soft switching in the control device 51.

[0165] A power conversion device 100B according to a third embodiment will be described with reference to Figures 24 to 26. Regarding the power conversion device 100B according to the third embodiment, components that are the same as those of the power conversion device 100 according to the first embodiment (see Figure 1) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0166] (1) Configuration As shown in FIG. 24, the power conversion device 100B differs from the power conversion device 100 in that it includes a control device 51B instead of the control device 51 of the power conversion device 100.

[0167] In the power conversion device 100B, the control device 51B, like the control device 51, generates first control signals SU1, SU2, SV1, SV2, SV2, SW1, and SW2 to control multiple first switching elements 1 and multiple second switching elements 2.

[0168] When setting the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is positive, the control device 51B advances the end point of the high-level period of the first control signal to the second switching element 2 and delays the start point of the high-level period of the first control signal to the first switching element 1, thereby lengthening the first dead time period Td1 by a variable time Tp1. If the time by which the end point of the high-level period of the first control signal to the second switching element 2 is advanced is Ta21 (see FIG. 25 ) and the time by which the start point of the high-level period of the first control signal to the first switching element 1 is delayed is Ta11 (see FIG. 25 ), then the variable time Tp1 is Ta11 + Ta21, i.e., Ta11 = Ta21 = 0.5 × Tp1.

[0169] When setting the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is negative, the control device 51B advances the end point of the high-level period of the first control signal to the first switching element 1 and delays the start point of the high-level period of the first control signal to the second switching element 2, thereby lengthening the first dead time period Td1 by a variable time Tp1. If the time by which the end point of the high-level period of the first control signal to the first switching element 1 is advanced is Ta12 (see FIG. 26 ) and the time by which the start point of the high-level period of the first control signal to the second switching element 2 is delayed is Ta22 (see FIG. 26 ), then the variable time Tp1 is Ta12 + Ta22, where Ta12 = Ta22 = 0.5 × Tp1.

[0170] As in embodiment 1, the signal generating circuit 52 generates a second control signal for each of the multiple switches 8 using a first control signal to the first switching element 1 and a first control signal to the second switching element 2 of a corresponding switching circuit 10 among the multiple switching circuits 10.

[0171] (2) Operation (2.1) Basic Operation Fig. 25 illustrates the first control signals SU1 and SU2, the second control signal SU6, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V1u across the first switching element 1U when the target switching element is the first switching element 1U of the switching circuit 10U. Also, in Fig. 25, the voltage value of the DC power supply E1 is illustrated as Vd.

[0172] When the target switching element is the first switching element 1U, in the switching circuit 10U, the voltage V2u across the second switching element 2U becomes Vd at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends, and the voltage V1u across the first switching element 1U becomes zero at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends. Therefore, when the first control signal SU1 changes from low level to high level at time t13, the first switching element 1U is subjected to zero-voltage soft switching. 25 , the current iL1 flowing through the resonant inductor L1 starts at time t11, when the high-level period of the second control signal SU6 begins, reaches the same value as the load current iU at time t12, when a time period equal to the variable time Tp1 has elapsed, reaches the same value as the load current iU at time t13, when the second dead time period Td2 ends, and reaches zero at time t14, when the third period T03 has elapsed since time t13. In the signal generating circuit 52, the second control signal SU6 changes from low to high at time t11, when the first control signal SU2 changes from high to low. The current iL1 flowing between time t12 and time t13 is a resonant current (a charging current for the resonant capacitor 9U) that flows from the regenerative capacitor 15 through the resonant inductor L1 to the resonant capacitor 9U.

[0173] During the first period T01, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are all in an OFF state, and the third switching element 6U is in an ON state. During the first period T01, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, the third switching element 6U, and the AC terminal 41U, as in Fig. 8. Also, during the first period T01, the load current iU flows through the path passing through the second diode 5 connected in anti-parallel to the second switching element 2U, the AC terminal 41U, and the AC load RA1, as in Fig. 8.

[0174] During the second period T02, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are each in an OFF state, and the third switching element 6U is in an ON state. During the second period T02, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, and the third switching element 6U, as in Fig. 9, and is divided into a current flowing to the AC terminal 41U and a current flowing to the resonant capacitor 9U (resonant current). During the second period T02, the resonant capacitor 9U is charged by the current flowing through the resonant capacitor 9U.

[0175] During the third period T03, the first switching element 1U and the third switching element 6U are each in an ON state, and the second switching element 2U and the fourth switching element 7U are each in an OFF state. During the third period T03, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, the third switching element 6U, and the AC terminal 41U, as in FIG. 10. This reduces the current iL1 to zero. Also, during the third period T03, the load current iU flows through the path passing through the first DC terminal 31, the first switching element 1U, the AC terminal 41U, and the AC load RA1, as in FIG. 10.

[0176] 26 illustrates the first control signals SU1 and SU2, the second control signal SU7, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V2u across the second switching element 2U when the target switching element is the second switching element 2U of the switching circuit 10U. Also, in FIG. 26, the voltage value of the DC power supply E1 is illustrated as Vd.

[0177] When the target switching element is the second switching element 2U, in the switching circuit 10U, the voltage V1u across the first switching element 1U becomes Vd at time t43 when the second dead time period Td2 immediately before the high-level period of the first control signal SU2 ends, and the voltage V2u across the second switching element 2U becomes zero at time t43 when the second dead time period Td2 ends. Therefore, when the first control signal SU2 changes from low level to high level at time t43, the second switching element 2U is subjected to zero-voltage soft switching.

[0178] 26 , the current iL1 flowing through the resonant inductor L1 starts at time t41, when the high-level period of the second control signal SU7 begins, reaches the same value as the load current iU at time t42, when a time equal to the variable time Tp1 has elapsed, reaches the same value as the load current iU at time t43, when the second dead time period Td2 ends, and reaches zero at time t44, when a time equal to the variable time Tp1 has elapsed from time t43. In the signal generating circuit 52, the second control signal SU7 changes from low to high when the first control signal SU1 changes from high to low. The current iL1 flowing between time t42 and time t43 is the resonant current (the discharge current of the resonant capacitor 9U) flowing from the resonant capacitor 9U to the resonant inductor L1.

[0179] 26, the first period T01 is the period from time t41 to time t42. During the first period T01, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the first period T01, the current iL1 flows through a path that passes through the AC terminal 41U, the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. Furthermore, during the first period T01, the load current iU flows through a path that passes through the AC load RA1, the AC terminal 41U, and the first diode 4 connected in anti-parallel to the first switching element 1U.

[0180] In FIG. 26 , the second period T02 is the period from time t42 to time t43. During the second period T02, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the second period T02, the current iL1 flowing through the resonant inductor L1 is a combined current of a current flowing from the AC terminal 41U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15, and a current (resonant current) flowing from the resonant capacitor 9U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. During the second period T02, the resonant capacitor 9 is discharged.

[0181] In FIG. 26, the third period T03 is the period from time t43 to time t44. During the third period T03, the second switching element 2U and the fourth switching element 7U are each in an ON state, and the first switching element 1U and the third switching element 6U are each in an OFF state. During the third period T03, the current iL1 flows through the AC terminal 41U, the connection point 3U, the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. This reduces the absolute value of the current iL1 to zero. Furthermore, during the third period T03, the load current iU flows through the AC load RA1, the AC terminal 41U, the second switching element 2U, and the second DC terminal 32.

[0182] Although an example of setting the second dead time period Td2 for the switching circuit 10U has been described above, the same applies to the switching circuit 10V and the switching circuit 10W.

[0183] (2.2) First Control Operation and Second Control Operation The first control operation and second control operation are similar to the first control operation and second control operation of the first embodiment, and therefore a description thereof will be omitted.

[0184] (3) Advantages As with the power conversion device 100 according to the first embodiment, the power conversion device 100B according to the third embodiment can improve power conversion efficiency without directly controlling the multiple switches 8 for zero voltage soft switching in the control device 51B.

[0185] Furthermore, in the power conversion device 100B, when the control device 51B sets the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is positive, the control device 51B advances the end point of the high-level period of the first control signal to the second switching element 2 and delays the start point of the high-level period of the first control signal to the first switching element 1, thereby adding a variable time Tp1 to the first dead time period Td1, and if the polarity of the load current is negative, the control device 51B advances the end point of the high-level period of the first control signal to the first switching element 1 and delays the start point of the high-level period of the first control signal to the second switching element 2, thereby adding a variable time Tp1 to the first dead time period Td1.

[0186] According to the above configuration, it is possible to realize zero-voltage soft switching while reducing dead-time loss and dead-time error.

[0187] A power conversion device 100C according to a fourth embodiment will be described with reference to Figures 27 to 29. Regarding the power conversion device 100C according to the fourth embodiment, components similar to those of the power conversion device 100 according to the first embodiment (see Figure 1) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0188] (1) Configuration As shown in FIG. 27, the power conversion device 100C differs from the power conversion device 100 in that it includes a control device 51C instead of the control device 51 of the power conversion device 100.

[0189] In the power conversion device 100C, the control device 51C, like the control device 51, generates first control signals SU1, SU2, SV1, SV2, SV2, SW1, and SW2 to control multiple first switching elements 1 and multiple second switching elements 2.

[0190] When the control device 51C sets the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is positive, it adds a variable time Tp1 to the first dead time period Td1 by shortening the high-level period of the first control signal to the first switching element 1.

[0191] When the control device 51C sets the second dead time period Td2 for each of the multiple switching circuits 10, if the polarity of the load current is negative, it adds a variable time Tp1 to the first dead time period Td1 by shortening the high-level period of the first control signal to the second switching element 2.

[0192] As in embodiment 1, the signal generating circuit 52 generates a second control signal for each of the multiple switches 8 using a first control signal to the first switching element 1 and a first control signal to the second switching element 2 of a corresponding switching circuit 10 among the multiple switching circuits 10.

[0193] (2) Operation (2.1) Basic Operation Fig. 28 illustrates the first control signals SU1 and SU2, the second control signal SU6, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V1u across the first switching element 1U when the target switching element is the first switching element 1U of the switching circuit 10U. Also, in Fig. 28, the voltage value of the DC power supply E1 is illustrated as Vd.

[0194] When the target switching element is the first switching element 1U, in the switching circuit 10U, the voltage V2u across the second switching element 2U becomes Vd at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends, and the voltage V1u across the first switching element 1U becomes zero at time t13 when the second dead time period Td2 immediately before the high-level period of the first control signal SU1 ends. Therefore, when the first control signal SU1 changes from low level to high level at time t13, the first switching element 1U is subjected to zero-voltage soft switching. 28 , the current iL1 flowing through the resonant inductor L1 starts at time t11, when the high-level period of the second control signal SU6 begins, reaches the same value as the load current iU at time t12, when the first period T01 has elapsed since time t11, reaches the same value as the load current iU at time t13, when the second dead time period Td2 ends, and becomes zero at time t14, when the third period has elapsed since time t13. In the signal generating circuit 52, the second control signal SU6 changes from low to high at time t11, when the first control signal SU2 changes from high to low. The current iL1 flowing between time t12 and time t13 is a resonant current (a charging current for the resonant capacitor 9U) that flows from the regenerative capacitor 15 through the resonant inductor L1 to the resonant capacitor 9U.

[0195] During the first period T01, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are all in an OFF state, and the third switching element 6U is in an ON state. During the first period T01, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, the third switching element 6U, and the AC terminal 41U, as in Fig. 8. Also, during the first period T01, the load current iU flows through the path passing through the second diode 5 connected in anti-parallel to the second switching element 2U, the AC terminal 41U, and the AC load RA1, as in Fig. 8.

[0196] During the second period T02, the first switching element 1U, the second switching element 2U, and the fourth switching element 7U are each in an OFF state, and the third switching element 6U is in an ON state. During the second period T02, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, and the third switching element 6U, as in Fig. 9, and is divided into a current flowing to the AC terminal 41U and a current flowing to the resonant capacitor 9U (resonant current). During the second period T02, the resonant capacitor 9U is charged by the current flowing through the resonant capacitor 9U.

[0197] During the third period T03, the first switching element 1U and the third switching element 6U are each in an ON state, and the second switching element 2U and the fourth switching element 7U are each in an OFF state. During the third period T03, the current iL1 flows through the path passing through the regenerative capacitor 15, the resonant inductor L1, the diode 71, the third switching element 6U, and the AC terminal 41U, as in FIG. 10. This reduces the current iL1 to zero. Also, during the third period T03, the load current iU flows through the path passing through the first DC terminal 31, the first switching element 1U, the AC terminal 41U, and the AC load RA1, as in FIG. 10.

[0198] 29 illustrates the first control signals SU1 and SU2, the second control signal SU7, the current iL1 flowing through the resonant inductor L1, the load current iU, and the voltage V2u across the second switching element 2U when the target switching element is the second switching element 2U of the switching circuit 10U. Also, in FIG. 29, the voltage value of the DC power supply E1 is illustrated as Vd.

[0199] When the target switching element is the second switching element 2U, in the switching circuit 10U, the voltage V1u across the first switching element 1U becomes Vd at time t43 when the second dead time period Td2 immediately before the high-level period of the first control signal SU2 ends, and the voltage V2u across the second switching element 2U becomes zero at time t43 when the second dead time period Td2 ends. Therefore, when the first control signal SU2 changes from low level to high level at time t43, the second switching element 2U is subjected to zero-voltage soft switching. 29 , the current iL1 flowing through the resonant inductor L1 starts at time t41, when the high-level period of the second control signal SU7 begins, reaches the same value as the load current iU at time t42, when a time equal to the variable time Tp1 has elapsed, reaches the same value as the load current iU at time t43, when the second dead time period Td2 ends, and reaches zero at time t44, when a time Tad2 equal to the variable time Tp1 has elapsed from time t43. In the signal generating circuit 52, the second control signal SU7 changes from low to high when the first control signal SU1 changes from high to low. The current iL1 flowing between time t42 and time t43 is the resonant current (the discharge current of the resonant capacitor 9U) flowing from the resonant capacitor 9U to the resonant inductor L1.

[0200] 29, the first period T01 is the period from time t41 to time t42. During the first period T01, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the first period T01, the current iL1 flows through a path that passes through the AC terminal 41U, the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. Also, during the first period T01, the load current iU flows through a path that passes through the AC load RA1, the AC terminal 41U, and the first diode 4 connected in anti-parallel to the first switching element 1U.

[0201] In FIG. 29 , the second period T02 is the period from time t42 to time t43. During the second period T02, the first switching element 1U, the second switching element 2U, and the third switching element 6U are all in an OFF state, and the fourth switching element 7U is in an ON state. During the second period T02, the current iL1 flowing through the resonant inductor L1 is a combined current of a current flowing from the AC terminal 41U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15, and a current (resonant current) flowing from the resonant capacitor 9U through the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. During the second period T02, the resonant capacitor 9 is discharged.

[0202] In FIG. 29 , the third period T03 is the period from time t43 to time t44. During the third period T03, the second switching element 2U and the fourth switching element 7U are each in an ON state, and the first switching element 1U and the third switching element 6U are each in an OFF state. During the third period T03, the current iL1 flows through the AC terminal 41U, the diode 61, the fourth switching element 7U, the resonant inductor L1, and the regenerative capacitor 15. This reduces the absolute value of the current iL1 to zero. Furthermore, during the third period T03, the load current iU flows through the AC load RA1, the AC terminal 41U, the second switching element 2U, and the second DC terminal 32.

[0203] The above describes an example in which the first dead time period Td1 is extended to the second dead time period Td2 for the switching circuit 10U, but the same applies to the switching circuits 10V and 10W.

[0204] (2.2) First Control Operation and Second Control Operation The first control operation and second control operation are similar to the first control operation and second control operation of the first embodiment, and therefore a description thereof will be omitted.

[0205] (3) Advantages As with the power conversion device 100 according to the first embodiment, the power conversion device 100C according to the fourth embodiment can improve power conversion efficiency without directly controlling the multiple switches 8 for zero voltage soft switching in the control device 51C.

[0206] Fifth Embodiment A power conversion device 100 according to a fifth embodiment will be described with reference to Fig. 30. Regarding the power conversion device 100 according to the fifth embodiment, components similar to those of the power conversion device 100 according to the first embodiment (see Fig. 1) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0207] In the power conversion device 100 according to the fifth embodiment, the third switching element 6 and the fourth switching element 7 are connected in anti-series in each of the multiple switches 8. In the power conversion device 100 according to the fifth embodiment, in each of the multiple switches 8, the second main terminal (emitter terminal) of the third switching element 6 and the second main terminal (emitter terminal) of the fourth switching element 7 are connected, the first main terminal (collector terminal) of the third switching element 6 is connected to the connection point 3 of a corresponding one of the multiple switching circuits 10, and the first terminal (collector terminal) of the fourth switching element 7 is connected to the resonance inductor L1. Each of the multiple switches 8 further includes a diode 61 connected in anti-parallel to the third switching element 6 and a diode 71 connected in anti-parallel to the fourth switching element 7.

[0208] The power conversion device 100 according to the fifth embodiment has the same advantages as the power conversion device 100 according to the first embodiment.

[0209] In the power conversion device 100 according to the fifth embodiment, each of the third switching element 6 and the fourth switching element 7 may be replaced with a MOSFET or a bipolar transistor. In this case, the diode 61 and the diode 71 in Fig. 30 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 100 according to the fifth embodiment, the diode 61 and the diode 71 are not limited to being externally attached to the third switching element 6 and the fourth switching element 7, respectively, and may be elements built into the chip.

[0210] Sixth Embodiment A power conversion device 100 according to a sixth embodiment will be described with reference to Fig. 31. Regarding the power conversion device 100 according to the sixth embodiment, components that are the same as those of the power conversion device 100 according to the first embodiment (see Fig. 1) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0211] In the power conversion device 100 according to the sixth embodiment, in each of the multiple switches 8, the third switching element 6 and the fourth switching element 7 are each a MOSFET, and the third switching element 6 and the fourth switching element 7 are connected in anti-series. In the power conversion device 100 according to the sixth embodiment, in each of the multiple switches 8, a first main terminal (drain terminal) of the third switching element 6 is connected to a first main terminal (drain terminal) of the fourth switching element 7. Each of the multiple switches 8 further includes a diode 61 connected in anti-parallel to the third switching element 6 and a diode 71 connected in anti-parallel to the fourth switching element 7. In each of the multiple switches 8, a second main terminal (source terminal) of the fourth switching element 7 is connected to a resonance inductor L1. In each of the multiple switches 8, a second main terminal (source terminal) of the third switching element 6 is connected to a connection point 3 of a switching circuit 10 corresponding to the switch 8 having the third switching element 6.

[0212] The power conversion device 100 according to the sixth embodiment has the same advantages as the power conversion device 100 according to the first embodiment.

[0213] Seventh Embodiment A power conversion device 100 according to a seventh embodiment will be described with reference to Fig. 32. Regarding the power conversion device 100 according to the seventh embodiment, components similar to those of the power conversion device 100 according to the first embodiment (see Fig. 1) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0214] In the power conversion device 100 according to the seventh embodiment, in each of the plurality of switches 8, the third switching element 6 and the fourth switching element 7 are each a MOSFET, a diode 63 is connected in series to the third switching element 6, and a diode 73 is connected in series to the fourth switching element 7. In the power conversion device 100 according to the seventh embodiment, in each of the plurality of switches 8, a series circuit of the third switching element 6 and the diode 63 and a series circuit of the fourth switching element 7 and the diode 73 are connected in anti-parallel.

[0215] The power conversion device 100 according to the seventh embodiment has the same advantages as the power conversion device 100 according to the first embodiment.

[0216] Eighth Embodiment A power conversion device 100D according to an eighth embodiment will be described with reference to Fig. 33. With respect to the power conversion device 100D according to the eighth embodiment, components similar to those of the power conversion device 100 according to the first embodiment (see Fig. 1) are denoted by the same reference numerals, and description thereof will be omitted.

[0217] In the power conversion device 100D 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 81 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 82 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.

[0218] The power conversion device 100D according to the eighth embodiment includes a signal generation circuit 52D instead of the signal generation circuit 52 (see FIGS. 1 and 2) of the power conversion device 100 according to the first embodiment.

[0219] The MOSFETs 80 of the multiple switches 8 are controlled by a signal generation circuit 52D. The signal generation circuit 52D outputs a second control signal SU8 that controls the on / off state of the MOSFET 80 of the switch 8U, a second control signal SV8 that controls the on / off state of the MOSFET 80 of the switch 8V, and a second control signal SW8 that controls the on / off state of the MOSFET 80 of the switch 8W. The signal generation circuit 52D generates the second control signal SU8 using the first control signals SU1 and SU2. The signal generation circuit 52D also generates the second control signal SV8 using the first control signals SV1 and SV2. The signal generation circuit 52D generates the second control signal SW8 using the first control signals SW1 and SW2.

[0220] 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 100D according to the eighth embodiment, 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 100D according to the eighth embodiment, 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.

[0221] The power conversion device 100D according to the eighth embodiment has the same advantages as the power conversion device 100 according to the first embodiment.

[0222] In the power conversion device 100D according to the eighth embodiment, each of the plurality of MOSFETs 80 may be replaced with an IGBT. Furthermore, in the power conversion device 100D 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.

[0223] Ninth Embodiment A power conversion device 100 according to a ninth embodiment will be described with reference to Fig. 34. In the power conversion device 100 according to the ninth embodiment, components similar to those of the power conversion device 100 according to the first embodiment (see Fig. 1) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0224] In the power conversion device 100 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 100 according to the ninth embodiment, a second 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 second control signal SU7 is applied between the second gate terminal and the second source terminal of the dual-gate GaN-based GIT constituting the switch 8V. A second 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 second control signal SV7 is applied between the second gate terminal and the second source terminal of the dual-gate GaN-based GIT constituting the switch 8W. A second control signal SW6 is applied between the first gate terminal and the first source terminal of the dual-gate GaN-based GIT, and a second control signal SW7 is applied between the second gate terminal and the second source terminal of the dual-gate GaN-based GIT constituting the switch 8W.

[0225] The power conversion device 100 according to the ninth embodiment has the same advantages as the power conversion device 100 according to the first embodiment.

[0226] (Tenth Embodiment) A power conversion device 100E according to a tenth embodiment will be described with reference to Fig. 35. With respect to the power conversion device 100E according to the tenth embodiment, components similar to those of the power conversion device 100 according to the first embodiment (see Fig. 1) are denoted by the same reference numerals, and description thereof will be omitted. Note that in Fig. 35, the capacitor C10 of Fig. 1 is not shown.

[0227] (1) Configuration The power conversion device 100E differs from the power conversion device 100 in that it further includes a regenerative capacitor 16 (hereinafter also referred to as the second regenerative capacitor 16) connected between the fourth end 154 of the regenerative capacitor 15 (hereinafter also referred to as the first regenerative capacitor 15) and the first DC terminal 31.

[0228] The second regenerative capacitor 16 is connected in series to the first regenerative capacitor 15. Therefore, in the power conversion device 100E, a series circuit of the second regenerative capacitor 16 and the first regenerative capacitor 15 is connected between the first DC terminal 31 and the second DC terminal 32. In the power conversion device 100E, a first end of the resonance inductor L1 is connected to the connection point between the first regenerative capacitor 15 and the second regenerative capacitor 16. The capacitance of the second regenerative capacitor 16 is the same as the capacitance of the first regenerative capacitor 15. The phrase "the capacitance of the second regenerative capacitor 16 is the same as the capacitance of the first regenerative capacitor 15" does not necessarily mean that the capacitance of the second regenerative capacitor 16 exactly matches the capacitance of the first regenerative capacitor 15, but may mean that the capacitance of the second regenerative capacitor 16 is within a range of 95% to 105% of the capacitance of the first regenerative capacitor 15.

[0229] In the power conversion device 100E according to the tenth embodiment, the potential V15 at the fourth terminal 154 of the first regeneration capacitor 15 is equal to the value obtained by dividing Vd, which is the voltage value of the DC power supply E1, between the second regeneration capacitor 16 and the first regeneration capacitor 15. Therefore, the potential V15 at the fourth terminal 154 of the first regeneration capacitor 15 is approximately Vd / 2.

[0230] (2) Advantages The operation of the control device 51 and the signal generating circuit 52 of the power conversion device 100E according to the tenth embodiment is similar to the operation of the control device 51 and the signal generating circuit 52 of the power conversion device 100 according to the first embodiment. Therefore, similar to the power conversion device 100 according to the first embodiment, the power conversion device 100E according to the tenth embodiment can improve the power conversion efficiency without directly controlling the plurality of switches 8 for zero voltage soft switching in the control device 51.

[0231] (Embodiment 11) A power conversion device 100F according to embodiment 11 will be described with reference to Fig. 36. With respect to the power conversion device 100F according to embodiment 11, components similar to those of the power conversion device 100 according to embodiment 1 (see Fig. 1) are denoted by the same reference numerals, and description thereof will be omitted.

[0232] (1) Configuration The power conversion device 100F differs from the power conversion device 100 in that the third end 153 of the regenerative capacitor 15 is connected to the first DC terminal 31.

[0233] (2) Operation The operation of the control device 51 and the signal generating circuit 52 of the power conversion device 100F according to the eleventh embodiment is similar to the operation of the control device 51 and the signal generating circuit 52 of the power conversion device 100 according to the first embodiment.

[0234] (3) Advantages As with the power conversion device 100 according to the first embodiment, the power conversion device 100F according to the eleventh embodiment makes it possible to improve the power conversion efficiency without directly controlling the multiple switches 8 for zero voltage soft switching in the control device 51.

[0235] (Other 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.

[0236] For example, 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.

[0237] Furthermore, the signal generating circuits 52, 52A, 52D may be configured to generate the second control signal for each of the plurality of switches 8 using one of the first control signal to the first switching element 1 and the first control signal to the second switching element 2 of the corresponding switching circuit 10 among the plurality of switching circuits 10. In any case, it is preferable that the signal generating circuits 52, 52A, 52D do not include a microcontroller.

[0238] In the power conversion device 100 according to the first embodiment, the diode 61 and the diode 71 are not limited to being externally attached to the third switching element 6 and the fourth switching element 7, respectively, but may also be elements built into one chip.

[0239] In addition, in the power conversion devices 100A to 100C according to embodiments 2 to 4, the configuration of the switch 8 is the same as the configuration of the switch 8 in the power conversion device 100 according to embodiment 1, but it may also be the same as the configuration of the switch 8 in any of embodiments 5 to 11.

[0240] Furthermore, the power conversion devices 100, 100B to 100F may not be provided with the first clamp diode 13 and the second clamp diode 14.

[0241] Furthermore, in the power conversion devices 100, 100A to 100F, 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.

[0242] The length of the resonance half cycle is set to be the same as the length of the first dead time period Td1, but may be different from the length of the first dead time period Td1. In either case, however, it is desirable that the end point of the resonance half cycle coincides with the end point of the first dead time period Td1.

[0243] Furthermore, the power conversion devices 100, 100A to 100F are not limited to a configuration that outputs three-phase AC, but may be configured to output polyphase AC with three or more phases.

[0244] (Aspects) The following aspects are disclosed in this specification.

[0245] A power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) 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 system (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), each including 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 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) connected to a common connection point (25). The plurality of resonance capacitors (9) correspond one-to-one to the plurality of switches (8). Each of the multiple resonant capacitors (9) is connected between the first end (81) of the corresponding switch (8) and the 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). In the regenerative capacitor (15), the fourth end (154) is connected to the second end of the resonant inductor (L1).The control system (50) includes a control device (51; 51B; 51C) and a signal generating circuit (52; 52A; 52D). The control device (51; 51B; 51C) provides a first control signal, the potential of which changes between a high level and a low level, to each of a plurality of first switching elements (1) and a plurality of second switching elements (2). The signal generating circuit (52; 52A; 52D) provides a second control signal, the potential of which changes between a high level and a low level, to each of a plurality of switches (8). The control device (51; 51B; 51C) sets a second dead time period (Td2) for each of the plurality of switching circuits (10) by adding a variable time (Tp1) to a predetermined first dead time period (Td1) between a high level period of the first control signal to the first switching element (1) and a high level period of the first control signal to the second switching element (2) so that the on periods of the first switching element (1) and the second switching element (2) do not overlap. The variable time (Tp1) is determined according to the current value of a load current corresponding to each of the plurality of switching circuits (10) among the plurality of load currents flowing through the plurality of AC terminals (41), the inductance of the resonance inductor (L1), and the potential (V15) of the fourth end (154) of the regenerative capacitor (15). The signal generating circuit (52; 52A; 52D) generates, for each of the plurality of switches (8), a second control signal having a high-level period corresponding to the second dead time period (Td2) for the corresponding switching circuit (10) among the plurality of switching circuits (10). The control system (50) switches between a first control operation that does not overlap the high-level periods of the second control signals of the multiple switches (8) and a second control operation that overlaps the high-level periods of the second control signals of two of the multiple switches (8) based on the potential (V15) of the fourth terminal (154) of the regenerative capacitor (15) and the polarities of the multiple load currents (iU, iV, iW).

[0246] According to this aspect, it is possible to improve the power conversion efficiency without directly controlling the plurality of switches (8) for zero voltage soft switching in the control device (51; 51B; 51C).

[0247] In the power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to the second aspect, in the first aspect, the control system (50) performs a first control operation when the potential (V15) of the fourth end (154) of the regenerative capacitor (15) is greater than an upper threshold (Vth2) that is greater than half the voltage value applied between the first DC terminal (31) and the second DC terminal (32), and the product of the multiple load currents (iU, iV, iW) is positive.

[0248] According to this aspect, when the product of the multiple load currents (iU, iV, iW) is positive, it is possible to suppress fluctuations in the potential (V15) of the fourth terminal (154) of the regenerative capacitor (15).

[0249] In the power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to the third aspect, in the first or second aspect, the control system (50) performs the second control operation when the potential (V15) of the fourth end (154) of the regenerative capacitor (15) is smaller than a lower limit threshold (Vth1) that is smaller than half the voltage value applied between the first DC terminal (31) and the second DC terminal (32), and the product of the multiple load currents (iU, iV, iW) is positive.

[0250] According to this aspect, when the product of the multiple load currents (iU, iV, iW) is positive, it is possible to suppress fluctuations in the potential (V15) of the fourth terminal (154) of the regenerative capacitor (15).

[0251] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a fourth aspect, in any one of the first to third aspects, the control system (50) performs a second control operation when the potential (V15) of the fourth end (154) of the regenerative capacitor (15) is greater than an upper threshold (Vth2) that is greater than half the voltage value applied between the first DC terminal (31) and the second DC terminal (32), and the product of the multiple load currents (iU, iV, iW) is negative.

[0252] According to this aspect, when the product of the multiple load currents (iU, iV, iW) is negative, it is possible to suppress fluctuations in the potential (V15) of the fourth terminal (154) of the regenerative capacitor (15).

[0253] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a fifth aspect, in any one of the first to fourth aspects, the control system (50) performs a first control operation when the potential (V15) of the fourth end (154) of the regenerative capacitor (15) is smaller than a lower limit threshold (Vth1) that is smaller than half of the voltage value applied between the first DC terminal (31) and the second DC terminal (32), and the product of multiple load currents (iU, iV, iW) is negative.

[0254] According to this aspect, when the product of the plurality of load currents (iU, iV, iW) is negative, it is possible to suppress fluctuations in the voltage value of the regenerative capacitor (15).

[0255] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a sixth aspect, in any one of the first to fifth aspects, when the control system (50) determines that resonant currents passing through two or more of the multiple switches (8) simultaneously flow through the resonant inductor (L1), the control system (50) determines the variable time (Tp1) to be the sum of two or more additional times (Tad) corresponding one-to-one to the two or more switches (8) and the difference between a first resonant half cycle when the resonant currents do not simultaneously flow and a second resonant half cycle when the resonant currents simultaneously flow. Each of the two or more additional times (Tad) is determined using the current value of the load current flowing through the corresponding switch (8) of the two or more switches (8), the inductance of the resonant inductor (L1), and the potential (V15) at a fourth end (154) of the regenerative capacitor (15). The first resonant half cycle is half the length of the resonant cycle of a first resonant circuit including the resonant inductor (L1) and only one of the plurality of resonant capacitors (9). The second resonant half cycle is half the length of the resonant cycle of a second resonant circuit including the resonant inductor (L1) and two or more resonant capacitors (9) corresponding to two or more switches (8) among the plurality of resonant capacitors (9).

[0256] In the power conversion device (100; 100A) according to the seventh aspect, in any one of the first to sixth aspects, when the control device (51) sets the second dead time period (Td2) for each of the plurality of switching circuits (10), if the polarity of the load current is positive, it adds a variable time (Tp1) to the first dead time period (Td1) by shortening the high-level period of the first control signal to the second switching element (2), and if the polarity of the load current is negative, it adds the variable time (Tp1) to the first dead time period (Td1) by shortening the high-level period of the first control signal to the first switching element (1).

[0257] According to this aspect, it is possible to achieve zero voltage soft switching while further reducing the dead time loss and the dead time error.

[0258] In a power conversion device (100B) according to an eighth aspect, in any one of the first to sixth aspects, when setting a second dead time period (Td2) for each of the plurality of switching circuits (10), if the polarity of the load current is positive, the control device (51B) advances the end point of the high-level period of the first control signal to the second switching element (2) and delays the start point of the high-level period of the first control signal to the first switching element (1), thereby adding a variable time (Tp1) to the first dead time period (Td1), and if the polarity of the load current is negative, the control device (51B) advances the end point of the high-level period of the first control signal to the first switching element (1) and delays the start point of the high-level period of the first control signal to the second switching element (2), thereby adding a variable time (Tp1) to the first dead time period (Td1).

[0259] According to this aspect, it is possible to achieve zero voltage soft switching while further reducing the dead time loss and the dead time error.

[0260] In a power conversion device (100C) according to a ninth aspect, in any one of the first to sixth aspects, when the control device (51C) sets the second dead time period (Td2) for each of the plurality of switching circuits (10), if the polarity of the load current is positive, a variable time (Tp1) is added to the first dead time period (Td1) by shortening the high-level period of the first control signal to the first switching element (1), and if the polarity of the load current is negative, the variable time (Tp1) is added to the first dead time period (Td1) by shortening the high-level period of the first control signal to the second switching element (2).

[0261] According to this embodiment, it is possible to realize zero-voltage soft switching.

[0262] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a tenth aspect, in any one of the first to ninth aspects, the signal generation circuit (52; 52A; 52D) generates a second control signal for each of the plurality of switches (8) using at least one of a first control signal to a first switching element (1) and a first control signal to a second switching element (2) of a corresponding switching circuit (10) among a plurality of switching circuits (10).

[0263] According to this aspect, it is possible to simplify the signal generating circuit (52; 52A; 52D).

[0264] In the power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) of the eleventh aspect, in the tenth aspect, the signal generating circuit (52; 52A; 52D) includes a plurality of logic circuits (521 to 526) and does not include a microcontroller.

[0265] According to this aspect, it is possible to simplify the signal generating circuit (52; 52A; 52D).

[0266] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a twelfth aspect, in any one of the first to eleventh aspects, the control system (50) shifts high-level periods of two second control signals to two of the plurality of switches (8) to make the high-level periods overlap each other in the second control operation.

[0267] According to this aspect, the absolute value of the current (iL1) flowing through the resonant inductor (L1) can be increased, and the adjustment amount (variation range) of the potential (V15) of the fourth terminal (154) of the regenerative capacitor (15) can be increased.

[0268] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a thirteenth aspect, in any one of the first to eleventh aspects, the control system (50) shifts high-level periods of two second control signals to two switches (8) out of the plurality of switches (8) in the second control operation, so that parts of the high-level periods overlap each other.

[0269] According to this aspect, the absolute value of the current (iL1) flowing through the resonant inductor (L1) can be increased, and the adjustment amount (variation range) of the potential (V15) of the fourth terminal (154) of the regenerative capacitor (15) can be increased.

[0270] In a power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a fourteenth aspect, in any one of the first to thirteenth aspects, the control system (50) shifts high-level periods of two second control signals to two switches (8) out of the plurality of switches (8) in the first control operation to eliminate overlap between the high-level periods.

[0271] According to this aspect, the absolute value of the current (iL1) flowing through the resonance inductor (L1) can be reduced.

[0272] A power conversion device (100; 100A; 100B; 100C; 100D; 100E; 100F) according to a fifteenth aspect is based on any one of the first to fourteenth aspects. The control system (50) includes, in the first control operation and the second control operation, control that does not shift high-level periods of two second control signals to two switches (8) among the plurality of switches (8).

[0273] REFERENCE SIGNS LIST 1 First switching element 2 Second switching element 3 Connection point 8 Switch 9 Resonant capacitor (first resonant capacitor) 10 Switching circuit 11 Power conversion circuit 15 Regenerative capacitor 153 Third terminal 154 Fourth terminal 31 First DC terminal 32 Second DC terminal 41 AC terminal 50 Control system 51, 51B, 51C Control device 52, 52A, 52D Signal generating circuit 100, 100A, 100B, 100C, 100D, 100E, 100F Power conversion device iU, iV, iW Load current L1 Resonant inductor RA1 AC load SU1, SU2, SV1, SV2, SW1, SW2 First control signal SU6, SU7, SU8, SV6, SV7, SV8, SW6, SW7, SW8 Second control signal Td1 First dead time period Td2 Second dead time period Tp1 Variable time V15 Potential Vd Voltage value Vth1 Lower limit threshold Vth2 Upper limit threshold

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 being connected in series in a one-to-one relationship, and a plurality of switching circuits being connected in parallel to each other, 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 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 having 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 connected to a common connection point; a plurality of resonant capacitors corresponding one-to-one to the plurality of switches, each 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 the fourth end being connected to the second end of the resonant inductor; and a control system, wherein the control system includes: a control device that provides a first control signal, the potential of which changes between a high level and a low level, to each of the plurality of first switching elements and the plurality of second switching elements; and a signal generation circuit that provides a second control signal, the potential of which changes between a high level and a low level, to each of the plurality of switches, wherein the control device sets, for each of the plurality of switching circuits, a second dead time period that is obtained by adding a variable time to a preset first dead time period between a high level period of the first control signal to the first switching element and a high level period of the first control signal to the second switching element so that on periods of the first switching element and the second switching element do not overlap,the variable time is determined according to a current value of a load current corresponding to each of the plurality of switching circuits among a plurality of load currents flowing through each of the plurality of AC terminals, an inductance of the resonant inductor, and a potential of the fourth end of the regenerative capacitor; the signal generating circuit generates, for each of the plurality of switches, the second control signal having a high level period corresponding to the second dead time period for a corresponding switching circuit among the plurality of switching circuits; and the control system switches between a first control operation in which high level periods of the second control signals of the plurality of switches do not overlap and a second control operation in which high level periods of the second control signals of two of the plurality of switches overlap, based on the potential of the fourth end of the regenerative capacitor and the polarities of the plurality of load currents.

2. The power conversion device according to claim 1, wherein the control system performs the first control operation when the potential at the fourth end of the regenerative capacitor is greater than an upper threshold value that is greater than half of the voltage value applied between the first DC terminal and the second DC terminal, and when the product of the multiple load currents is positive.

3. The power conversion device according to claim 1 or 2, wherein the control system performs the second control operation when the potential at the fourth end of the regenerative capacitor is smaller than a lower limit threshold that is smaller than half of the voltage value applied between the first DC terminal and the second DC terminal, and when the product of the multiple load currents is positive.

4. The power conversion device according to any one of claims 1 to 3, wherein the control system performs the second control operation when the potential at the fourth end of the regenerative capacitor is greater than an upper threshold value that is greater than half of the voltage value applied between the first DC terminal and the second DC terminal, and when the product of the multiple load currents is negative.

5. The power conversion device according to any one of claims 1 to 4, wherein the control system performs the first control operation when the potential at the fourth end of the regenerative capacitor is smaller than a lower limit threshold that is smaller than half of the voltage value applied between the first DC terminal and the second DC terminal, and when the product of the multiple load currents is negative.

6. The power conversion device according to any one of claims 1 to 5, wherein, when it is determined that a resonant current flows simultaneously through two or more switches of the plurality of switches in the resonant inductor, the control system determines the variable time to be the sum of two or more additional times corresponding one-to-one to the two or more switches and a difference between a first resonant half cycle in which the resonant currents do not flow simultaneously and a second resonant half cycle in which the resonant currents flow simultaneously, each of the two or more additional times is determined using the current value of the load current flowing through a corresponding switch of the two or more switches, the inductance of the resonant inductor, and the potential of the fourth end of the regenerative capacitor, the first resonant half cycle having a length half the resonant cycle of a first resonant circuit including the resonant inductor and only one resonant capacitor of the plurality of resonant capacitors, and the second resonant half cycle having a length half the resonant cycle of a second resonant circuit including the resonant inductor and two or more resonant capacitors of the plurality of resonant capacitors corresponding to each of the two or more switches.

7. The power conversion device according to any one of claims 1 to 6, wherein when setting the second dead time period for each of the plurality of switching circuits, the control device adds the variable time to the first dead time period by shortening a high-level period of the first control signal to the second switching element when the polarity of the load current is positive, and adds the variable time to the first dead time period by shortening a high-level period of the first control signal to the first switching element when the polarity of the load current is negative.

8. The power conversion device according to any one of claims 1 to 6, wherein when setting the second dead time period for each of the plurality of switching circuits, the control device adds the variable time to the first dead time period by advancing the end point of a high level period of the first control signal to the second switching element and delaying the start point of the high level period of the first control signal to the first switching element when the polarity of the load current is positive, and adds the variable time to the first dead time period by advancing the end point of a high level period of the first control signal to the first switching element and delaying the start point of the high level period of the first control signal to the second switching element when the polarity of the load current is negative.

9. The power conversion device according to any one of claims 1 to 6, wherein when setting the second dead time period for each of the plurality of switching circuits, the control device adds the variable time to the first dead time period by shortening a high-level period of the first control signal to the first switching element when the polarity of the load current is positive, and adds the variable time to the first dead time period by shortening a high-level period of the first control signal to the second switching element when the polarity of the load current is negative.

10. A power conversion device according to any one of claims 1 to 9, wherein the signal generation circuit generates the second control signal for each of the plurality of switches using at least one of the first control signal to the first switching element of a corresponding one of the plurality of switching circuits and the first control signal to the second switching element.

11. The power conversion device according to claim 10, wherein the signal generating circuit includes a plurality of logic circuits and does not include a microcontroller.

12. The power conversion device according to any one of claims 1 to 11, wherein, in the second control operation, the control system shifts high-level periods of two second control signals to two of the plurality of switches so that the high-level periods overlap each other.

13. The power conversion device according to any one of claims 1 to 11, wherein, in the second control operation, the control system shifts high-level periods of two second control signals to two of the plurality of switches so that parts of the high-level periods overlap each other.

14. The power conversion device according to any one of claims 1 to 13, wherein in the first control operation, the control system shifts high-level periods of two second control signals to two of the plurality of switches to eliminate overlap between the high-level periods.

15. The power conversion device according to any one of claims 1 to 14, wherein the control system includes control that does not shift high-level periods of two second control signals to two switches of the plurality of switches in the first control operation and the second control operation.

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