Power conversion device

Abstract

The present invention detects a switching circuit in which hard switching occurs in a power conversion circuit. In this power conversion device (100), a control device (50) has: a first determination unit (54) that determines a switching state in a power conversion circuit (11) on the basis of a ripple voltage included in a voltage (V15) at both ends of a regeneration capacitor (15) and a plurality of load currents (iU, iV, iW) output from a plurality of AC terminals (41); and a second determination unit (55) that, when it is determined by the first determination unit (54) that hard switching has occurred in the power conversion circuit (11), determines a switching circuit (10) in which hard switching occurs among the plurality of switching circuits (10) on the basis of the polarity of a plurality of load currents (iU, iV, iW) at a specific phase of the ripple voltage.

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 resonant inverter device (power conversion device).

[0003] In the resonant inverter device disclosed in Patent Document 1, a DC voltage from a DC voltage source is converted into an AC voltage by an inverter unit (power conversion circuit). This inverter unit has a configuration in which six main switching elements (three first switching elements and three second switching elements) are bridge-connected in three phases (U phase, V phase, and W phase) between a positive bus and a negative bus.

[0004] The resonant inverter device also includes two voltage-dividing capacitors connected in series between a positive bus and a negative bus. The two voltage-dividing capacitors form a voltage-dividing means for dividing the DC voltage of a DC voltage source, and a means for generating half the DC voltage of the DC voltage source at the junction between the two voltage-dividing capacitors. Furthermore, a resonant circuit is provided between the two voltage-dividing capacitors and the inverter unit to perform resonant operation when the main switching elements are switched. The resonant circuit includes a series circuit, each consisting of a resonant reactor (resonant inductor) and an auxiliary switch (switch), connected between the junction of the two voltage-dividing capacitors and the junction of the upper and lower arms of each phase, with a resonant capacitor connected in parallel to each series circuit. The inverter unit includes three inverter arms (switching circuits). In each of the three inverter arms, the upper-arm main switching element (first switching element) and the lower-arm main switching element (second switching element) are connected in series.

[0005] Each main switching element and each auxiliary switch is controlled to be turned on and off by a control unit.

[0006] In the power conversion device disclosed in Patent Document 1, for example, when an auxiliary switch connected to a resonance inductor fails, hard switching may occur in each of the first switching element and the second switching element connected to the failed auxiliary switch. The power conversion device disclosed in Patent Document 1 cannot detect in which of the multiple switching circuits of the power conversion circuit hard switching has occurred.

[0007] Japanese Patent Application Laid-Open No. 2000-32775

[0008] An object of the present disclosure is to provide a power conversion device capable of detecting a switching circuit in which hard switching has occurred in the power conversion circuit.

[0009] 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, at least one resonant inductor, a regenerative capacitor, and a control device. The power conversion circuit includes multiple first switching elements and multiple second switching elements. In the power conversion circuit, multiple switching circuits are connected in parallel, 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 and a second end, and the first end is 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. 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 at least one resonant inductor has a third end and a fourth end. The third end of the at least one resonant inductor is connected to the second end of the corresponding switch among the plurality of switches. The regenerative capacitor has a fifth end and a sixth end. The fifth end of the regenerative capacitor is connected to the second DC terminal, and the sixth end is connected to the fourth end of the at least one resonant inductor. The control device controls the on / off of each of the plurality of first switching elements, the plurality of second switching elements, and the plurality of switches. The control device has a first determination unit and a second determination unit.The first determination unit determines a switching state of the power conversion circuit based on a ripple voltage included in the voltage across the regenerative capacitor and a plurality of load currents output from the plurality of AC terminals, and when the first determination unit determines that hard switching is occurring in the power conversion circuit, the second determination unit determines which of the plurality of switching circuits is causing hard switching based on polarities of the plurality of load currents at a specific phase of the ripple voltage.

[0010] 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, at least one resonant inductor, a regenerative capacitor, and a control device. The power conversion circuit includes multiple first switching elements and multiple second switching elements. In the power conversion circuit, multiple switching circuits are connected in parallel, 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 and a second end, and the first end is 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. 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 at least one resonant inductor has a third end and a fourth end. The third end of the at least one resonant inductor is connected to the second end of the corresponding switch among the plurality of switches. The regenerative capacitor has a fifth end and a sixth end. The fifth end of the regenerative capacitor is connected to the second DC terminal, and the sixth end is connected to the fourth end of the at least one resonant inductor. The control device controls the on / off of each of the plurality of first switching elements, the plurality of second switching elements, and the plurality of switches. The control device has a first determination unit and a second determination unit.The first determination unit determines a switching state of the power conversion circuit based on a ripple voltage included in a voltage across the regenerative capacitor and a plurality of load currents output from the plurality of AC terminals. When the first determination unit determines that hard switching is occurring in the power conversion circuit, the second determination unit determines which of the plurality of switching circuits is experiencing hard switching based on control signals provided to the plurality of switches.

[0011] FIG. 1 is a circuit diagram of a system including a power conversion device according to a first embodiment. FIG. 2A is a waveform diagram illustrating the relationship between the load current and the voltage across a regenerative capacitor when the power conversion device is operating normally. FIG. 2B is a waveform diagram illustrating the relationship between the load current and the voltage across a regenerative capacitor when the power conversion device is operating abnormally. FIG. 3A is a waveform diagram illustrating the relationship between the load current and the voltage across a regenerative capacitor when a U-phase switch in the power conversion device fails. FIG. 3B is a waveform diagram illustrating the relationship between the load current and the voltage across a regenerative capacitor when a V-phase switch in the power conversion device fails. FIG. 3C is a waveform diagram illustrating the relationship between the load current and the voltage across a regenerative capacitor when a V-phase switch in the power conversion device fails. FIG. 4 is an explanatory diagram illustrating the operation of the power conversion device when hard switching occurs in a U-phase switching circuit. FIG. 5 is a waveform diagram illustrating the relationship between the load current and the voltage across a regenerative capacitor when a V-phase switch and a W-phase switch in the power conversion device fail. FIG. 6A is a waveform diagram illustrating the relationship between the load current and the voltage across the regenerative capacitor when the power conversion device is operating normally. FIG. 6B is a waveform diagram illustrating the relationship between the load current and the voltage across the regenerative capacitor when the U-phase, V-phase, and W-phase switches in the power conversion device are faulty. FIG. 7 is an explanatory diagram illustrating the operation of the control device in the power conversion device when performing basic operation when the load current is greater than 0 and the resonant capacitor is charging. FIG. 8 is another explanatory diagram illustrating the operation of the control device in the power conversion device when performing basic operation when the load current is greater than 0 and the resonant capacitor is charging. FIG. 9 is a diagram illustrating the time variation of the duty and the time variation of the load current corresponding to the voltage commands for each of the three phases in an AC load connected to multiple AC terminals of the power conversion device. FIG. 10 is an explanatory diagram illustrating the first current threshold and the second current threshold used by the control device in the power conversion device. FIG. 11 is an explanatory diagram illustrating the operation of the control device in the power conversion device when performing basic operation when the load current is greater than 0 and the resonant capacitor is discharging.Fig. 12 is an explanatory diagram of an operation when a control device in the above power conversion device performs a basic operation when a load current is less than 0 and a resonant capacitor is discharging. Fig. 13 is an explanatory diagram of an operation when a control device in the above power conversion device performs a basic operation when a load current is less than 0 and a resonant capacitor is charging. Fig. 14 is a circuit diagram of a system including a power conversion device according to embodiment 2. Fig. 15 is an explanatory diagram of an operation of the above power conversion device. Fig. 16 is an explanatory diagram of an operation of the above power conversion device. Fig. 17 is a circuit diagram of a system including a power conversion device according to embodiment 3. Fig. 18 is a circuit diagram of a system including a power conversion device according to embodiment 4. Fig. 19 is a circuit diagram of a system including a power conversion device according to modification 1 of embodiment 4. Fig. 20 is a circuit diagram of a system including a power conversion device according to modification 2 of embodiment 4. Fig. 21 is a circuit diagram of a system including a power conversion device according to modification 3 of embodiment 4. Fig. 22 is a circuit diagram of a system including a power conversion device according to modification 4 of embodiment 4. Fig. 23 is a circuit diagram of a system including a power conversion device according to embodiment 5.

[0012] First Embodiment A power conversion device 100 according to a first embodiment will be described below with reference to FIGS.

[0013] (1) Overall Configuration of the Power Conversion Device As shown in FIG. 1 , the power conversion device 100 includes a first DC terminal 31, a second DC terminal 32, and multiple (e.g., three) AC terminals 41. A DC power source E1 is connected between the first DC terminal 31 and the second DC terminal 32, and an AC load RA1 is connected to the multiple AC terminals 41. The AC load RA1 is, for example, a three-phase motor. The power conversion device 100 converts DC output from the DC power source E1 into AC power and outputs it to the AC load RA1. The DC power source E1 includes, for example, a solar cell or a fuel cell. The DC power source E1 may also include a DC-DC converter. In the power conversion device 100, when the multiple AC terminals 41 are three AC terminals 41, the AC power is, for example, three-phase AC power having a U phase, a V phase, and a W phase.

[0014] The power conversion device 100 includes a power conversion circuit 11, a plurality of (e.g., three) switches 8, a plurality of (e.g., three) resonant capacitors 9, a regenerative capacitor 15, a plurality of (e.g., three) resonant inductors L1, and a control device 50. The power conversion device 100 also includes a plurality of (e.g., three) protection circuits 17. Each of the plurality of switches 8 is, for example, a bidirectional switch.

[0015] The power conversion circuit 11 has a plurality of (e.g., three) first switching elements 1 and a plurality of (e.g., three) second switching elements 2. In the power conversion circuit 11, a plurality of (e.g., three) switching circuits 10, each having a plurality of first switching elements 1 and a plurality of second switching elements 2 connected in series in a one-to-one relationship, are connected in parallel to one another. In the power conversion circuit 11, the plurality of first switching elements 1 are connected to first DC terminals 31, and the plurality of second switching elements 2 are connected to second DC terminals 32. The plurality of AC terminals 41 correspond one-to-one to the plurality of switching circuits 10. Each of the plurality of AC terminals 41 is connected to a connection point 3 between the first switching element 1 and the second switching element 2 in a corresponding one of the plurality of switching circuits 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 and a second end 82. Each of the multiple switches 8 has a first end 81 connected to a connection point 3 between the first switching element 1 and the second switching element 2 in a corresponding switching circuit 10 among the multiple switching circuits 10. The multiple resonant capacitors 9 correspond one-to-one to the multiple switches 8. Each of the multiple resonant capacitors 9 is connected between the first end 81 and the second DC terminal 32 of a corresponding switch 8 among the multiple switches 8. Each of the multiple resonant inductors L1 has a third end and a fourth end. The fourth end of each resonant inductor L1 is connected to the regenerative capacitor 15. The third end of each of the multiple resonant inductors L1 is connected to the second end 82 of a corresponding switch 8 among the multiple switches 8. The regenerative capacitor 15 has a fifth end 153 and a sixth end 154. The fifth end 153 of the regenerative capacitor 15 is connected to the second DC terminal 32, and the sixth end 154 is connected to the fourth end of the resonant inductor L1. The control device 50 controls the plurality of first switching elements 1 , the plurality of second switching elements 2 and the plurality of switches 8 .

[0016] (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.

[0017] 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.

[0018] In the power conversion circuit 11, each of a plurality of (e.g., three) first switching elements 1 and a plurality of (e.g., three) second switching elements 2 has a control terminal, a first main terminal, and a second main terminal. The control terminals of the plurality of first switching elements 1 and the plurality of second switching elements 2 are connected to a control device 50. In each of the plurality of switching circuits 10 of the power conversion device 100, a first main terminal of the first switching element 1 is connected to a first DC terminal 31, a second main terminal of the first switching element 1 is connected to a first main terminal of the second switching element 2, and a second main terminal of the second switching element 2 is connected to a second DC terminal 32. In each of the plurality of switching circuits 10, the first switching element 1 is a high-side switching element (P-side switching element), and the second switching element 2 is a low-side switching element (N-side switching element). Each of the plurality of first switching elements 1 and the plurality of second switching elements 2 is, for example, an IGBT (Insulated Gate Bipolar Transistor). Therefore, the control terminal, the first main terminal and the second main terminal of each of the plurality of first switching elements 1 and the plurality of second switching elements 2 are the gate terminal, the collector terminal and the emitter terminal, respectively.

[0019] 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.

[0020] A connection point 3U between the first switching element 1U and the second switching element 2U is connected to, for example, the U-phase terminal of the AC load RA1 via an AC terminal 41U. A connection point 3V between the first switching element 1V and the second switching element 2V is connected to, for example, the V-phase 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, the W-phase terminal of the AC load RA1 via an AC terminal 41W.

[0021] 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.

[0022] Each of the multiple switches 8 has, for example, two IGBTs, a first IGBT 6 and a second IGBT 7, connected in anti-parallel. In each of the switches 8, the collector terminal of the first IGBT 6 is connected to the emitter terminal of the second IGBT 7, and the emitter terminal of the first IGBT 6 is connected to the collector terminal of the second IGBT 7. In each of the multiple switches 8, the emitter terminal of the first IGBT 6 is connected to the connection point 3 of the switching circuit 10 corresponding to the switch 8 having the first IGBT 6. In each of the multiple switches 8, the collector terminal of the second IGBT 7 is connected to the connection point 3 of the switching circuit 10 corresponding to the switch 8 having the second IGBT 7. The switch 8U is connected to the connection point 3U between the first switching element 1U and the second switching element 2U. The switch 8V is connected to the connection point 3V between the first switching element 1V and the second switching element 2V. The switch 8W is connected to a connection point 3W between the first switching element 1W and the second switching element 2W. Hereinafter, for convenience of explanation, the first IGBT 6 and the second IGBT 7 of the switch 8U may be referred to as the first IGBT 6U and the second IGBT 7U, the first IGBT 6 and the second IGBT 7 of the switch 8V may be referred to as the first IGBT 6V and the second IGBT 7V, and the first IGBT 6 and the second IGBT 7 of the switch 8W may be referred to as the first IGBT 6W and the second IGBT 7W, respectively.

[0023] The plurality of switches 8 are controlled by the control device 50. In other words, the first IGBT 6U, the second IGBT 7U, the first IGBT 6V, the second IGBT 7V, the first IGBT 6W, and the second IGBT 7W are controlled by the control device 50.

[0024] Each of the multiple resonant inductors L1 has a third end and a fourth end. The third end of each of the multiple resonant inductors L1 is connected to the second end 82 of a corresponding one of the multiple switches 8. The fourth end of each of the multiple resonant inductors L1 is connected to the sixth end 154 of the regenerative capacitor 15. The multiple resonant inductors L1 have the same inductance. That is, the inductances of the three resonant inductors L1 are the same. "The inductances of the three resonant inductors L1 are the same" does not necessarily mean that the inductances of two of the three resonant inductors L1 completely match the inductance of the remaining resonant inductor L1, but may mean that the inductances of the two resonant inductors L1 are within a range of 95% to 105% of the inductance of the remaining resonant inductor L1.

[0025] The regenerative capacitor 15 is connected between the fourth ends of the plurality of resonance inductors L1 and the second DC terminal 32. The regenerative capacitor 15 is, for example, a film capacitor.

[0026] Each of the multiple protection circuits 17 includes a third diode 13 and a fourth diode 14. In each of the multiple protection circuits 17, the third diode 13 is connected between the connection point between the resonant inductor L1 and the switch 8 and the first DC terminal 31. The anode of the third diode 13 is connected to the connection point between the resonant inductor L1 and the switch 8. The cathode of the third diode 13 is connected to the first DC terminal 31. The fourth diode 14 is connected between the connection point between the resonant inductor L1 and the switch 8 and the second DC terminal 32. The anode of the fourth diode 14 is connected to the second DC terminal 32. The cathode of the fourth diode 14 is connected to the connection point between the resonant inductor L1 and the switch 8. Therefore, in each of the multiple protection circuits 17, the fourth diode 14 is connected in series with the third diode 13.

[0027] The control device 50 controls the plurality of first switching elements 1 , the plurality of second switching elements 2 and the plurality of switches 8 .

[0028] The control device 50 outputs control signals SU1, SV1, and SW1 that control the on / off of the multiple first switching elements 1U, 1V, and 1W, respectively. Each of the control signals SU1, SV1, and SW1 is, for example, a PWM (Pulse Width Modulation) signal whose potential level changes between a first potential level (hereinafter also referred to as a low level) and a second potential level (hereinafter also referred to as a high level) that is higher than the first potential level. The first switching elements 1U, 1V, and 1W are turned on when the control signals SU1, SV1, and SW1 are at a high level, and turned off when the control signals SU1, SV1, and SW1 are at a low level. The control device 50 also outputs control signals SU2, SV2, and SW2 that control the on / off of the multiple second switching elements 2U, 2V, and 2W, respectively. Each of the control signals SU2, SV2, and SW2 is, for example, a PWM signal whose potential level changes between a first potential level (hereinafter also referred to as a low level) and a second potential level (hereinafter also referred to as a high level) that is higher than the first potential level. The second switching elements 2U, 2V, and 2W are turned on when the control signals SU2, SV2, and SW2 are at a high level, and turned off when they are at a low level.

[0029] The control device 50 uses a sawtooth carrier signal (see FIG. 7 ) to generate control signals SU1, SV1, SW1 corresponding to the plurality of first switching elements 1U, 1V, and 1W, respectively, and control signals SU2, SV2, and SW2 corresponding to the plurality of second switching elements 2U, 2V, and 2W, respectively. More specifically, the control device 50 generates the control signals SU1 and SU2 to be provided to the first switching element 1U and the second switching element 2U, respectively, based on at least the carrier signal and a U-phase voltage command. The control device 50 also generates the control signals SV1 and SV2 to be provided to the first switching element 1V and the second switching element 2V, respectively, based on at least the carrier signal and a V-phase voltage command. The control device 50 also generates the control signals SW1 and SW2 to be provided to the first switching element 1W and the second switching element 2W, respectively, based on at least the carrier signal and a W-phase voltage command. The U-phase voltage command, V-phase voltage command, and W-phase voltage command are, for example, sinusoidal signals whose phases differ by 120° from each other, and whose values ​​(voltage command values) change over time. The waveform of the carrier signal is not limited to a sawtooth waveform and may be, for example, a triangular wave or a sawtooth wave obtained by inverting the sawtooth wave shown in FIG. 7 . The U-phase voltage command, V-phase voltage command, and W-phase voltage command each have the same cycle length. The U-phase voltage command, V-phase voltage command, and W-phase voltage command each have a longer cycle length than the carrier signal.

[0030] The duty of the control signals SU1 and SU2 provided by the control device 50 to the first switching element 1U and the second switching element 2U, respectively, varies based on the U-phase voltage command. In FIG. 9, the duty of the control signal SU1 is shown as the U-phase duty. The control device 50 (see FIG. 1) compares the U-phase voltage command with a carrier signal to generate the control signal SU1 provided to the first switching element 1U. The control device 50 also inverts the control signal SU1 provided to the first switching element 1U to generate the control signal SU2 provided to the second switching element 2U. The control device 50 also sets a dead time Td (see FIG. 7) between the high-level period of the control signal SU1 and the high-level period of the control signal SU2 so that the on periods of the first switching element 1U and the second switching element 2U do not overlap.

[0031] The duties of the control signals SV1 and SV2 provided by the control device 50 to the first switching element 1V and the second switching element 2V, respectively, vary based on the V-phase voltage command. In FIG. 9, the duty of the control signal SV1 is shown as the V-phase duty. The control device 50 (see FIG. 1) compares the V-phase voltage command with a carrier signal to generate the control signal SV1 provided to the first switching element 1V. The control device 50 also inverts the control signal SV1 provided to the first switching element 1V to generate the control signal SV2 provided to the second switching element 2V. The control device 50 also sets a dead time period Td (see FIG. 7) between the high-level period of the control signal SV1 and the high-level period of the control signal SV2 so that the on periods of the first switching element 1V and the second switching element 2V do not overlap.

[0032] The duties of the control signals SW1 and SW2 provided by the control device 50 to the first switching element 1W and the second switching element 2W, respectively, vary based on the W-phase voltage command. In FIG. 9, the duty of the control signal SW1 is shown as the W-phase duty. The control device 50 (see FIG. 1) compares the W-phase voltage command with a carrier signal to generate the control signal SW1 provided to the first switching element 1W. The control device 50 also inverts the control signal SW1 provided to the first switching element 1W to generate the control signal SW2 provided to the second switching element 2W. The control device 50 also sets a dead time period Td (see FIG. 8) between the high-level period of the control signal SW1 and the high-level period of the control signal SW2 so that the on periods of the first switching element 1W and the second switching element 2W do not overlap.

[0033] The U-phase voltage command, V-phase voltage command, and W-phase voltage command are, for example, sinusoidal signals whose phases are different from each other by 120°, and whose values ​​change over time. Therefore, the duty of the control signal SU1 (U-phase duty), the duty of the control signal SV1 (V-phase duty), and the duty of the control signal SW1 (W-phase duty) change like sinusoidal waves whose phases are different from each other by 120°, as shown in Fig. 9 . Similarly, the duty of the control signal SU2, the duty of the control signal SV2, and the duty of the control signal SW2 change like sinusoidal waves whose phases are different from each other by 120°.

[0034] The control device 50 generates the control signals SU1, SU2, SV1, SV2, SW1, and SW2 based on the carrier signal, the voltage commands, and information about the state of the AC load RA1. For example, if the AC load RA1 is a three-phase motor, the information about the state of the AC load RA1 includes, for example, detection values ​​from a plurality of current sensors that detect output currents (hereinafter also referred to as load currents) iU, iV, and iW flowing through the U-phase, V-phase, and W-phase of the AC load RA1, respectively.

[0035] The plurality of switches 8, the plurality of resonant inductors 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.

[0036] In the power conversion device 100 , the control device 50 controls the plurality of switches 8 in addition to the plurality of first switching elements 1 and second switching elements 2 of the power conversion circuit 11 .

[0037] The control device 50 generates control signals SU6, SU7, SV6, SV7, SW6, and SW7 that control the on / off of the first IGBT6U, the second IGBT7U, the first IGBT6V, the second IGBT7V, the first IGBT6W, and the second IGBT7W, respectively, and outputs them to the gate terminals of the first IGBT6U, the second IGBT7U, the first IGBT6V, the second IGBT7V, the first IGBT6W, and the second IGBT7W, respectively.

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

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

[0040] When the first IGBT 6W is in the ON state and the second IGBT 7W is in the OFF state, the switch 8W allows a charging current to pass through the path of the regenerative capacitor 15, resonant inductor L1, switch 8W, and resonant capacitor 9W. The charging current is a current that charges the resonant capacitor 9W. When the first IGBT 6W is in the OFF state and the second IGBT 7W is in the ON state, the switch 8W allows a discharging current to pass through the path of the resonant capacitor 9W, switch 8W, resonant inductor L1, and regenerative capacitor 15. The discharging current is a current that discharges the charge of the resonant capacitor 9W.

[0041] The control device 50 includes a control unit 51 , a first acquisition unit 52 , a second acquisition unit 53 , a first determination unit 54 , and a second determination unit 55 .

[0042] The control unit 51 has a function of controlling the on / off of each of the plurality of first switching elements 1, the plurality of second switching elements 2, and the plurality of switches 8. In the control device 50, the control unit 51 has a function of generating each control signal SU1, SU2, SV1, SV2, SW1, SW2 based on the above-mentioned carrier signal, each voltage command, and information regarding the state of the AC load RA1. The information regarding the state of the AC load RA1 includes detection values ​​from multiple current sensors that detect load currents iU, iV, iW.

[0043] The first acquisition unit 52 has a function of acquiring a detection value of a voltage sensor 20 that detects a voltage V15 across the regenerative capacitor 15 .

[0044] The second acquisition unit 53 has a function of acquiring detection values ​​from the above-mentioned multiple current sensors. That is, the second acquisition unit 53 has a function of acquiring detection values ​​of multiple load currents iU, iV, iW output from the multiple AC terminals 41. Each of the multiple load currents iU, iV, iW is, for example, a sinusoidal AC current. The multiple load currents iU, iV, iW have a phase difference of 120° from each other, for example.

[0045] The first determination unit 54 determines the switching state of the power conversion circuit 11 based on the ripple voltage included in the voltage V15 across the regenerative capacitor 15 and the multiple load currents iU, iV, and iW output from the multiple AC terminals 41. The switching state of the power conversion circuit 11 includes at least one of the switching states of the multiple first switching elements 1 and the multiple second switching elements 2. With respect to the first determination unit 54, "determining the switching state of the power conversion circuit 11" means determining whether hard switching is occurring in the power conversion circuit 11. "Hard switching is occurring in the power conversion circuit 11" means that hard switching is occurring in at least one of the multiple first switching elements 1 and the multiple second switching elements 2. The operation of the first determination unit 54 will be described in more detail in the section "(3.2) Operation of the First Determination Unit and the Second Determination Unit."

[0046] When the first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11, the second determination unit 55 determines which of the multiple switching circuits 10 is experiencing hard switching. The second determination unit 55 determines which of the multiple switching circuits 10 is experiencing hard switching based on the polarities of the multiple load currents iU, iV, and iW at a specific phase of the ripple voltage included in the voltage V15 across the regenerative capacitor 15. The operation of the second determination unit 55 will be described in more detail in the section "(3.2) Operation of the First Determination Unit and the Second Determination Unit."

[0047] The executing entity of the control device 50 includes a computer system. The computer system has one or more computers. The computer system is mainly composed of a processor and memory as hardware. The processor executes a program recorded in the memory of the computer system, thereby realizing the function of the executing entity of the control device 50 in the present disclosure. The program may be pre-recorded in the memory of the computer system, provided via a telecommunications line, or recorded and provided on a non-transitory recording medium such as a memory card, optical disk, or hard disk drive (magnetic disk) readable by the computer system. The processor of the computer system is composed of one or more electronic circuits including a semiconductor integrated circuit (IC) or a large-scale integrated circuit (LSI). The multiple electronic circuits may be integrated into a single chip or distributed across multiple chips. The multiple chips may be integrated into a single device or distributed across multiple devices.

[0048] (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.

[0049] The control device 50 sets a dead time period Td between the high level period of the control signals SU1, SV1, SW1 to the first switching elements 1U, 1V, 1W and the high level period of the control signals SU2, SV2, SW2 to the second switching elements 2U, 2V, 2W for each of the multiple switching circuits 10.

[0050] Below, the basic operation of the control device 50 for realizing zero-voltage soft switching of each of the plurality of first switching elements 1 and the plurality of second switching elements 2 will be explained with reference to Figures 1 and 7 to 13, and then the operation of the first judgment unit 54 and the second judgment unit 55 will be explained with reference to Figures 2A, 2B, 3A, 3B, 3C, 4, 5, 6A, and 6B.

[0051] (3.1) Basic Operation In zero voltage soft switching of the first switching element 1, the voltage across the first switching element 1 must be set to zero immediately before the first switching element 1, which is the target of zero voltage soft switching, is turned on. In addition, in zero voltage soft switching of the second switching element 2, the voltage across the second switching element 2 must be set to zero immediately before the second switching element 2, which is the target of zero voltage soft switching, is turned on. Hereinafter, the switching element (first switching element 1 or second switching element 2) that is the target of zero voltage soft switching is also referred to as the target switching element.

[0052] The basic operation of the control device 50 differs depending on the polarity (positive / negative) of the load current flowing through the AC terminal 41 connected to the target switching element and the operation (charging operation / discharging operation) of the resonant capacitor 9 connected in series or parallel to the target switching element. The load currents iU, iV, and iW are positive when flowing from the AC terminal 41 to the AC load RA1 and negative when flowing from the AC load RA1 to the AC terminal 41. When the resonant capacitor 9 is charging, the voltage across the resonant capacitor 9 increases. When the resonant capacitor 9 is discharging, the voltage across the resonant capacitor 9 decreases. The voltage across each of the multiple second switching elements 2 is the same as the voltage across the resonant capacitor 9 connected in parallel to the second switching element 2. The basic operation of the control device 50 is executed by the control unit 51.

[0053] (3.1.1) Operation for Soft-Switching the First Switching Element When Load Current > 0 When the target of soft switching is the first switching element 1 (hereinafter also referred to as the target first switching element 1), and the polarity of the load current flowing through the AC terminal 41 connected to the target first switching element 1 is positive, the control device 50 turns on the first IGBT 6 corresponding to the target first switching element 1. As a result, the control device 50 causes resonance between the resonant inductor L1 and the resonant capacitor 9 connected to the target first switching element 1, charging the resonant capacitor 9 from the regenerative capacitor 15 and setting the voltage across the target first switching element 1 to zero. In this way, the power conversion device 100 can achieve zero-voltage soft switching of the target first switching element 1.

[0054] 7 illustrates control signals SU1 and SU2 provided from the control device 50 to the first switching element 1U and the second switching element 2U of the switching circuit 10U when the target first switching element is the first switching element 1U of the switching circuit 10U. Also illustrated in FIG. 7 are a control signal SU6 provided from the control device 50 to the first IGBT 6U of the switch 8U, a load current iU flowing through the U-phase of the AC load RA1, a current iL1 flowing through the resonant inductor L1, a voltage V1u across the first switching element 1U, and a voltage V2u across the second switching element 2U. Also illustrated in FIG. 7 are control signals SV1 and SV2 provided from the control device 50 to the first switching element 1V and the second switching element 2V of the switching circuit 10V when the target first switching element is the first switching element 1V of the switching circuit 10V. FIG. 7 also illustrates the control signal SV6 given from the control device 50 to the first IGBT 6V of the switch 8V, the load current iV flowing through the V phase of the AC load RA1, the current iL1 flowing through the resonant inductor L1, the voltage V1v across the first switching element 1V, and the voltage V2v across the second switching element 2V.

[0055] 7 also shows a dead time period Td set in the control device 50 to prevent the first switching element 1 and the second switching element 2, which are in phase, from being turned on simultaneously. Also, Fig. 7 shows an additional time Tau set in the control device 50 for the control signal SU6 of the first IGBT 6U of the switch 8U, and an additional time Tav set in the control device 50 for the control signal SV6 of the first IGBT 6V of the switch 8V. The additional time Tau and the additional time Tav will be described later.

[0056] FIG. 8 illustrates the control signals SW1 and SW2 provided from the control device 50 to the first switching element 1W and the second switching element 2W of the switching circuit 10W, respectively, when the target first switching element is the first switching element 1W of the switching circuit 10W. FIG. 8 also illustrates the control signal SW6 provided from the control device 50 to the first IGBT 6W of the switch 8W and the load current iW flowing through the W-phase of the AC load RA1. FIG. 8 also illustrates the current iL1 flowing through the resonant inductor L1. FIG. 8 also illustrates the voltage V1w across the first switching element 1W and the voltage V2w across the second switching element 2W. In FIG. 8, the voltage value of the DC power supply E1 is indicated as Vd.

[0057] 8 also shows a dead time period Td set in the control device 50 to prevent the first switching element 1W and the second switching element 2W from being turned on simultaneously. Also, Fig. 8 shows an additional time Taw set in the control device 50 for the control signal SW6 of the first IGBT 6W of the switch 8W. The additional time Taw will be described later.

[0058] As shown in FIG. 7 , the additional time Tau is set to advance the start time t1 of the high-level period of the control signal SU6 to earlier than the start time t2 of the dead time period Td, thereby making the high-level period of the control signal SU6 longer than the dead time period Td. The length of the additional time Tau is set based on the value of the load current iU. To start LC resonance at the start time t2 of the dead time period Td, it is desirable that the value of the current iL1 match the value of the load current iU at the start time t2 of the dead time period Td. This is because, while iL1 < iU, all of the current iL1 flows through the AC load RA1, preventing the resonant capacitor 9U from being charged. The end time of the high-level period of the control signal SU6 may be the same as or later than the end time t3 of the dead time period Td. FIG. 7 shows an example in which the end time of the high-level period of the control signal SU6 is set to the same as the end time t3 of the dead time period Td. The control device 50 sets the length of the high-level period of the control signal SU6 to Tau + Td. In the switching circuit 10U, the voltage V2u across the second switching element 2U becomes Vd at time t3, when the dead time period Td ends, and the voltage V1u across the first switching element 1U becomes zero at time t3, when the dead time period Td ends. In the example of FIG. 7 , the current iL1 flowing through the resonant inductor L1 begins at time t1, when the high-level period of the control signal SU6 begins, and becomes zero at time t4, when the additional time Tau has elapsed since time t3, when the dead time period Td ends. Since iL1≧iU from time t2, when the dead time period Td begins, the current iL1 in the shaded region of the current waveform in the fifth row from the top in FIG. 7 flows into the resonant capacitor 9U, generating LC resonance. After time t3 when the dead time period Td ends, the current iL1 is regenerated in the power conversion circuit 11 via the third diode 13 directly connected to the resonance inductor L1.

[0059] As described above, the control device 50 determines the additional time Tau based on the load current iU so that iL1 = iU at the time t2 when the dead time period Td starts, in order to start LC resonance at the time t2 when the dead time period Td starts and end the resonance half cycle at the time the dead time period Td ends. More specifically, the control device 50 determines the additional time Tau by calculating Tau = iU × (L / V15) using, for example, the detection result of the load current iU by a current sensor, its signal-processed value, or an estimated value of the load current iU, the pre-stored inductance L of the resonance inductor L1, and the detection result of the voltage V15 across the regenerative capacitor 15. The detection result of the load current iU or its signal-processed value is a detection value at the carrier cycle to which the additional time Tau is added, or at a timing closest to that carrier cycle. The estimated value of the load current iU is a value, for example, an estimated value of the load current iU at the carrier cycle to which the additional time Tau is added. The resonant half cycle is half the resonant cycle, which is the reciprocal of the resonant frequency of the resonant circuit including one resonant inductor L1 and one resonant capacitor 9. In the control device 50, the resonant half cycle is set to be equal to or shorter than the length of the dead time period Td, for example, to be the same as the length of the dead time period Td.

[0060] As shown in FIG. 7 , the additional time Tav is set to advance the start time t5 of the high-level period of the control signal SV6 to earlier than the start time t6 of the dead time period Td, thereby making the high-level period of the control signal SV6 longer than the dead time period Td. The length of the additional time Tav is set based on the value of the load current iV. To start LC resonance at the start time t6 of the dead time period Td, it is desirable that the value of the current iL1 match the value of the load current iV at the start time t6 of the dead time period Td. This is because, while iL1 < iV, all of the current iL1 flows through the AC load RA1, preventing the resonant capacitor 9V from being charged. The end time of the high-level period of the control signal SV6 may be the same as or later than the end time t7 of the dead time period Td. FIG. 7 shows an example in which the end time of the high-level period of the control signal SV6 is set to the same time as the end time t7 of the dead time period Td. The control device 50 sets the length of the high-level period of the control signal SV6 to Tav + Td. The voltage V1v across the first switching element 1V becomes zero at time t7, when the dead time period Td ends. In the example of FIG. 7 , the current iL1 flowing through the resonant inductor L1 begins at time t5, when the high-level period of the control signal SV6 begins, and becomes zero at time t8, when the additional time Tav has elapsed since time t7, when the dead time period Td ends. Since iL1≧iV holds true from time t6, when the dead time period Td begins, the current iL1 in the shaded region of the current waveform in the tenth row from the top in FIG. 7 flows into the resonant capacitor 9V, generating LC resonance. After time t7, when the dead time period Td ends, the current iL1 is regenerated to the power conversion circuit 11 via the third diode 13 directly connected to the resonant inductor L1.

[0061] As described above, in order to start LC resonance at time t6, when the dead time period Td starts, the control device 50 determines the additional time Tav based on the load current iV so that iL1 = iV at time t6, when the dead time period Td starts. More specifically, the control device 50 determines the additional time Tav by calculating Tav = iV × (L / V15) using, for example, the detection result of the load current iV by a current sensor, its signal-processed value, or an estimated value of the load current iV, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 across the regenerative capacitor 15. The detection result of the load current iV or its signal-processed value is a detection value at the carrier cycle to which the additional time Tav is added, or at a timing closest to that carrier cycle. The estimated value of the load current iV is a value, for example, an estimate of the load current iV at the carrier cycle to which the additional time Tav is added.

[0062] As shown in FIG. 8 , the additional time Taw is set to advance the start time t9 of the high-level period of the control signal SW6 to earlier than the start time t10 of the dead time period Td, thereby making the high-level period of the control signal SW6 longer than the dead time period Td. The length of the additional time Taw is set based on the value of the load current iW. To start LC resonance at the start time t10 of the dead time period Td, it is desirable that the value of the current iL1 match the value of the load current iW at the start time t10 of the dead time period Td. This is because, while iL1 < iW, all of the current iL1 flows through the AC load RA1, preventing the resonant capacitor 9W from being charged. The end time of the high-level period of the control signal SW6 may be the same as or later than the end time t11 of the dead time period Td. FIG. 8 illustrates an example in which the end time of the high-level period of the control signal SW6 is set to the same as the end time t11 of the dead time period Td. The control device 50 sets the length of the high-level period of the control signal SW6 to Taw+Td. The voltage V1w across the first switching element 1W becomes zero at time t11, when the dead time period Td ends. In the example of FIG. 8 , the current iL1 flowing through the resonant inductor L1 begins at time t9, when the high-level period of the control signal SW6 begins, and becomes zero at time t12, when the additional time Taw has elapsed since time t11, when the dead time period Td ends. Since iL1≧iW holds true from time t10, when the dead time period Td begins, the current iL1 in the shaded region of the current waveform in the fourth row from the top in FIG. 8 flows into the resonant capacitor 9W, generating LC resonance. After time t11, when the dead time period Td ends, the current iL1 is regenerated in the power conversion circuit 11 via the third diode 13, which is directly connected to the resonant inductor L1.

[0063] The control device 50 determines the additional time Taw based on the load current iW. More specifically, the control device 50 determines the additional time Taw by calculating Taw = iW × (L / V15) using the detection result of the load current iW by the current sensor, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 across the regenerative capacitor 15. The detection result of the load current iW or its signal processed value at this time uses a detection value at the carrier cycle to which the additional time Taw is added or at a timing closest to that carrier cycle. Furthermore, the estimated value of the load current iW at this time uses, for example, a value estimated from the load current iW at the carrier cycle to which the additional time Taw is added.

[0064] (3.1.2) Operation for Soft-Switching the Second Switching Element When Load Current > 0 When the soft-switching target is the second switching element 2 (hereinafter also referred to as the target second switching element 2), and when the polarity of the load current (load current iU, load current iV, or load current iW) flowing through the AC terminal 41 connected to the target second switching element 2 is positive, the control device 50 compares the current value of the load current with the first current threshold I1 (=Ith, see FIG. 10 ). When the current value of the load current is greater than the first current threshold I1, the control device 50 does not turn on the switch 8, and when the current value of the load current is less than the first current threshold I1, the control device 50 turns on the switch 8 during the dead time period Td. In the power conversion device 100, when the current value of the load current is greater than the first current threshold I1, the control device 50 does not turn on the switch 8 corresponding to the target second switching element 2, and allows the resonant capacitor 9 connected in parallel to the target second switching element 2 to discharge with the load current. This allows the power conversion device 100 to achieve zero voltage soft switching of the target second switching element 2.

[0065] 11 illustrates the control signals SU1, SU2, and SU7, the load current iU, the current i9U flowing from the resonant capacitor 9U, and the voltage V2u across the second switching element 2U when the target second switching element 2 is the second switching element 2U of the switching circuit 10U and the current value of the load current iU is greater than the first current threshold I1. Also illustrated in FIG. 11 is the dead time period Td and the additional time Tau that the control device 50 sets for the control signal SU7 to the second IGBT 7U of the switch 8U.

[0066] When the current value of the load current iU is greater than the first current threshold I1, the control device 50 does not provide a high-level period for the control signal SU7. In this case, in the power conversion device 100, the current i9U begins to flow from the resonant capacitor 9U at time t22, when the dead time period Td starts, and the current i9U decreases to zero before time t23, when the dead time period Td ends, and the voltage V2u across the second switching element 2U becomes zero before time t23, when the dead time period Td ends. As a result, in the power conversion device 100, when the control signal SU2 changes from low to high at time t23, when the dead time period Td ends, the second switching element 2U is subjected to zero-voltage soft switching.

[0067] When the current value of the load current iU is smaller than the first current threshold I1, the control device 50 provides a high-level period for the control signal SU7, as shown by the two-dot chain line in FIG. 11 . The start time of the high-level period of the control signal SU7 in this case is, for example, the same as time t22, when the dead time period Td starts. The end time of the high-level period of the control signal SU7 is the same as time t23, when the dead time period Td ends. As a result, in the power conversion device 100, the voltage V2u across the second switching element 2U becomes zero before time t23, when the dead time period Td ends. Therefore, in the power conversion device 100, when the control signal SU2 changes from low to high at time t23, when the dead time period Td ends, the second switching element 2U is subjected to zero-voltage soft switching. The start time of the high-level period of the control signal SU7 may be time t21, which is earlier than time t22, when the dead time period Td starts, by an additional time Tau. The end point of the high level period of the control signal SU7 may be time t24, which is later than time t23, when the dead time period Td ends, by the additional time Tau. Note that the time before and after the period during the high level period of the control signal SU7 that overlaps with the dead time period Td is not limited to the additional time Tau, and may be another set time.

[0068] (3.1.3) Operation for Soft-Switching the Second Switching Element When Load Current<0 When the polarity of the load current (load current iU, load current iV, or load current iW) flowing through the AC terminal 41 connected to the target second switching element 2 is negative, the control device 50 turns on the second IGBT 7 corresponding to the target second switching element 2. As a result, the control device 50 causes the resonant capacitor 9 and the resonant inductor L1 connected to the target second switching element 2 to resonate, discharging the resonant capacitor 9 and setting the voltage across the target second switching element 2 to zero. This allows the power conversion device 100 to achieve zero-voltage soft switching of the target second switching element 2.

[0069] Figure 12 illustrates the control signals SU1, SU2, and SU7, the load current iU, the current iL1 flowing through the resonant inductor L1, and the voltage V2u across the second switching element 2U when the target second switching element 2 is the second switching element 2U of the switching circuit 10U.

[0070] FIG. 12 also illustrates the dead time period Td set by the control device 50 to prevent the first switching element 1 and the second switching element 2, which are in phase, from being simultaneously turned on. FIG. 12 also illustrates the additional time Tau set by the control device 50 for the control signal SU7 of the second IGBT 7U of the switch 8U. The end point of the high-level period of the control signal SU7 may be the same as or later than the end point t33 of the dead time period Td. FIG. 12 illustrates an example in which the end point of the high-level period of the control signal SU7 is set to the same as the end point t33 of the dead time period Td. The control device 50 sets the length of the high-level period of the control signal SU7 to Tau + Td. In the switching circuit 10U, the voltage V2u across the second switching element 2U becomes zero at the end point t33 of the dead time period Td. 12 , the current iL1 flowing through the resonant inductor L1 starts at time t31, when the high-level period of the control signal SU7 starts, and becomes zero at time t34, when the additional time Tau has elapsed since time t33, when the dead time period Td ends. As for the current iL1, when the relationship iL1≦iU is established from time t32, when the dead time period Td starts, LC resonance occurs, and a resonant current (discharge current of the resonant capacitor 9U) flows from the resonant capacitor 9U toward the resonant inductor L1. After time t33, when the dead time period Td ends, the current iL1 is regenerated in the power conversion circuit 11 via the fourth diode 14, which is directly connected to the resonant inductor L1.

[0071] The control device 50 determines the additional time Tau based on the load current iU so that iL1 = iU at time t32, when the dead time period Td starts, so that the LC resonance starts at time t32, when the dead time period Td starts, and the resonant half cycle ends at time t33, when the dead time period Td ends. More specifically, the control device 50 determines the additional time Tau by calculating Tau = |iU| × (L / V15) using, for example, the detection result of the load current iU by a current sensor, its signal-processed value, or an estimated value of the load current iU, the inductance L of the resonance inductor L1 that is stored in advance, and the detection result of the voltage V15 across the regenerative capacitor 15. The detection result of the load current iU or its signal-processed value is a value detected in the carrier cycle to which the additional time Tau is added, or a value closest to that carrier cycle. The estimated value of the load current iU is a value estimated from the load current iU in the carrier cycle to which the additional time Tau is added.

[0072] (3.1.4) Operation for Soft-Switching the First Switching Element When Load Current < 0 When the polarity of the load current (load current iU, load current iV, or load current iW) flowing through the AC terminal 41 connected to the target first switching element 1 is negative, the control device 50 compares the current value of the load current with the second current threshold I2 (= −Ith, see FIG. 10 ). When the current value of the load current is smaller than the second current threshold I2, the control device 50 does not turn on the switch 8. When the current value of the load current is larger than the second current threshold I2, the control device 50 turns on the switch 8 during the dead time period Td. When the current value of the load current is smaller than the second current threshold I2, the power conversion device 100 can charge the resonant capacitor 9 connected in series to the target first switching element 1 with the load current without turning on the switch 8 corresponding to the target first switching element 1. This allows the power conversion device 100 to achieve zero-voltage soft switching of the target first switching element 1.

[0073] 13 illustrates the control signals SU1, SU2, and SU6, the load current iU, the current i9U flowing from the resonant capacitor 9U, the voltage V1u across the first switching element 1U, and the voltage V2u across the second switching element 2U when the target first switching element 1 is the first switching element 1U of the switching circuit 10U and the current value of the load current iU is greater than the second current threshold I2 (in other words, when the absolute value of the current value of the load current iU is smaller than the absolute value of the second current threshold I2). Also illustrated in FIG. 13 are the carrier signal and the dead time period Td.

[0074] When the current value of the load current iU is smaller than the second current threshold I2 (in other words, when the absolute value of the load current iU is greater than the absolute value of the second current threshold I2), the control device 50 does not provide a high-level period for the control signal SU6. In this case, in the power conversion device 100, the current i9U begins to flow through the resonant capacitor 9U at time t41, when the dead time period Td starts. As a result, in the power conversion device 100, the resonant capacitor 9U is charged, and the voltage V2u across the second switching element 2U increases. The current i9U becomes zero before time t42, when the dead time period Td ends, and the voltage V1u across the first switching element 1U becomes zero before time t42, when the dead time period Td ends. As a result, in the power conversion device 100, the first switching element 1U is subjected to zero-voltage soft switching when the control signal SU1 changes from low to high at time t42, when the dead time period Td ends.

[0075] When the current value of the load current iU is greater than the second current threshold I2 (in other words, when the absolute value of the load current iU is smaller than the absolute value of the second current threshold I2), the control device 50 sets a high-level period of the control signal SU6, as shown by the two-dot chain line in FIG. 13 . The start time of the high-level period of the control signal SU6 is the same as the start time t41 of the dead time period Td. The end time of the high-level period of the control signal SU6 is the same as the end time t42 of the dead time period Td. As a result, in the power conversion device 100, the voltage V1u across the first switching element 1U becomes zero before the end time t42 of the dead time period Td. Therefore, in the power conversion device 100, the first switching element 1U is subjected to zero-voltage soft switching when the control signal SU1 changes from low to high at the end time t42 of the dead time period Td.

[0076] (3.2) Operations of the First Determination Unit and the Second Determination Unit In the control device 50, the first determination unit 54 is operating even when the control unit 51 is performing the basic operations described above.

[0077] 2A and 2B illustrate the load current iU, load current iV, load current iW, and voltage V15 across the regenerative capacitor 15 as an example to explain operation when the U-phase switch 8U fails. In FIGS. 2A and 2B, the vertical axis is scaled to easily visualize the maximum value Vmax and minimum value Vmin of the ripple voltage included in the voltage V15 across the regenerative capacitor 15. The ripple voltage of the voltage V15 across the regenerative capacitor 15 in FIG. 2A is the ripple voltage when all three switches 8U, 8V, and 8W are operating normally. In FIG. 2A, the voltage V15 across the regenerative capacitor 15 is approximately Vd / 2. The ripple voltage of the voltage V15 across the regenerative capacitor 15 in FIG. 2B is the ripple voltage when switch 8U, one of the three switches 8U, 8V, and 8W, fails and is in an open state (off state). If the switch 8U fails, the resonant capacitor 9U connected to the switch 8U will no longer be charged or discharged, and the first switching element 1U and the second switching element 2U connected to the switch 8U will be switched hard. In the power conversion device 100, if the switch 8U fails, the resonant current of the U phase will decrease, and one cycle of the ripple voltage included in the voltage V15 across the regenerative capacitor 15 will be longer than one cycle of the ripple voltage included in the voltage V15 across the regenerative capacitor 15 when the switch 8U is not faulty.

[0078] FIG. 2B illustrates the operation when the U-phase switch 8U fails, but the operation is similar when the V-phase switch 8V fails, and the operation is similar when the W-phase switch 8W fails.

[0079] As described above, the first determination unit 54 determines the switching state of the power conversion circuit 11 based on the ripple voltage included in the voltage V15 across the regenerative capacitor 15 and the plurality of load currents iU, iV, iW output from the plurality of AC terminals 41. The switching state of the power conversion circuit 11 includes the switching states of the plurality of first switching elements 1 and the plurality of second switching elements 2.

[0080] The first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11 when a predetermined condition (hereinafter also referred to as a first predetermined condition) is satisfied. "Hard switching is occurring in the power conversion circuit 11" means that hard switching is occurring in at least one of the plurality of first switching elements 1 and the plurality of second switching elements 2. The first predetermined condition is a condition in which the number of intersections B1 of any two load currents iU, iV, and iW during a predetermined period Ts is greater than a predetermined value (e.g., 2). The predetermined period Ts is the period between a first occurrence time tg1 of a first peak P1 of the ripple voltage and a second occurrence time tg2 of a second peak P2 of the ripple voltage. In the first embodiment, as shown in FIGS. 2A and 2B , the first peak P1 is a maximum peak at which the ripple voltage reaches a maximum value Vmax, and the second peak P2 is a maximum peak at which the ripple voltage reaches a maximum value Vmax after the first peak P1 (in the first embodiment, next to the first peak P1). In other words, the first peak P1 is one of the multiple maximum peaks at which the ripple voltage reaches its maximum value Vmax, and the second peak P2 is the next maximum peak after the one maximum peak among the multiple maximum peaks. In this case, the length of the specified period Ts is the same as the length of one cycle of the ripple voltage. At the intersection B1, the two load currents have the same polarity and magnitude.

[0081] In FIG. 2A, the number of intersections B1 in the specified period Ts is two, whereas in FIG. 2B, the number of intersections B1 in the specified period Ts is seven, which is greater than two.

[0082] For example, the first determination unit 54 initializes the value of the variable to 0, increases the value of the variable by "1" each time the intersection B1 is detected, and decreases the value of the variable by "2" each time the maximum peak is detected. If the intersection B1 and the maximum peak are detected at the same time, +1 - 2 = -1 is assumed, and if the value of the variable becomes a negative value, the value of the variable is initialized to 0. As a result, if all three switches 8U, 8V, and 8W are operating normally, the value of the variable in the first determination unit 54 is 2 or less. On the other hand, if the switch 8U is faulty, for example, the value of the variable in the first determination unit 54 is 4, which is a value greater than 2.

[0083] The second peak P2 used in the first predetermined condition by the first determination unit 54 may be a single maximum value peak at which the ripple voltage reaches its maximum value Vmax after the first peak P1. For example, a single maximum value peak at which the ripple voltage reaches its maximum value Vmax two times after the first peak P1 may be used. In this case, the length of the specified period Ts is twice the length of one cycle of the ripple voltage, and the specified value used by the first determination unit 54 may be twice the value when the length of the specified period Ts is one cycle of the ripple voltage, i.e., 4 times 2. Furthermore, the second peak P2 used in the first predetermined condition by the first determination unit 54 may be a single maximum value peak at which the ripple voltage reaches its maximum value Vmax three times after the first peak P1. In this case, the length of the specified period Ts is three times the length of one cycle of the ripple voltage, and the specified value used by the first determination unit 54 may be three times the value when the length of the specified period Ts is one cycle of the ripple voltage, i.e., 6 times 2. In other words, the specified value used by the first determination unit 54 may be appropriately determined depending on the length of the specified period Ts. In other words, if the length of the specified period Ts is n cycles (n is a natural number) of the ripple voltage, the specified value may be set to n×2.

[0084] To determine whether the first predetermined condition is satisfied, the first determination unit 54 uses moving averages as signal-processed values ​​of the detection results of the load currents iU, iV, and iW and the voltage V15 across the regenerative capacitor 15. The periods for calculating the moving averages can be set arbitrarily as long as they avoid exactly one cycle of the fluctuation period of the load currents iU, iV, and iW and the voltage V15 across the regenerative capacitor 15. This reduces the influence of noise contained in the detection values ​​of the load currents iU, iV, and iW and the voltage V15 across the regenerative capacitor 15, thereby improving the accuracy of the determination. In other words, the first determination unit 54 can improve the accuracy of detecting the first peak P1, the second peak P2, and each intersection B1, thereby improving the accuracy of determining whether hard switching is occurring in the power conversion circuit 11.

[0085] The first determination unit 54 may determine that each of the first switching elements 1 and the second switching elements 2 is soft-switched when a predetermined condition (hereinafter also referred to as a second predetermined condition) different from the first predetermined condition is satisfied. "Each of the first switching elements 1 and the second switching elements 2 is soft-switched" means, in other words, "hard switching is not occurring in the power conversion circuit 11." The second predetermined condition is a condition that the number of intersections B1 of any two load currents among the plurality of load currents iU, iV, and iW during a specified period Ts is equal to or less than a specified value. The specified period Ts is the period between a first occurrence timing tg1 of a first peak P1 of the ripple voltage and a second occurrence timing tg2 of a second peak P2 of the ripple voltage.

[0086] In the power conversion device 100, when the first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11, the control device 50 stops the operation of the power conversion circuit 11, for example, after making a determination using the second determination unit 55. To stop the operation of the power conversion circuit 11, the control unit 51 sets each of the control signals SU1, SV1, SW1, SU2, SV2, and SW2 to a low level, or stops outputting the control signals SU1, SV1, SW1, SU2, SV2, and SW2, for example.

[0087] When the first judgment unit 54 judges that hard switching is occurring in the power conversion circuit 11, the second judgment unit 55 judges which of the multiple switching circuits 10 is experiencing hard switching based on the polarities of the multiple load currents iU, iV, and iW at a specific phase of the ripple voltage.

[0088] In the power conversion device 100, the relationship between the polarities of the multiple load currents iU, iV, and iW in a specific phase of the ripple voltage varies depending on which of the three switches 8 has failed, as shown in Figures 3A, 3B, and 3C. Each of Figures 3A, 3B, and 3C can be viewed in the same way as Figure 2B.

[0089] If only the U-phase switch 8U among the three switches 8U, 8V, and 8W fails, the polarity of the load current iU will be positive, the polarity of the load current iV will be positive, and the polarity of the load current iW will be negative in a specific phase, as shown in FIG. 3A . In other words, if only the U-phase switch 8U fails, of the three load currents iU, iV, and iW, only the polarity of the load current iW will be negative. If the switch 8U fails, the resonant capacitor 9U connected to the switch 8U will no longer be charged or discharged, and the switching of the first switching element 1U and the second switching element 2U connected to the switch 8U will be hard switched. In this case, during the dead time immediately before the first switching element 1W is turned off, as shown in FIG. 4 , in a specific phase, the load current iU flows to discharge the resonant capacitor 9U, the load current iV flows to discharge the resonant capacitor 9V, and the load current iW flows to charge the resonant capacitor 9W.

[0090] If only the V-phase switch 8V of the three switches 8U, 8V, and 8W fails, the polarity of the load current iU will be negative, the polarity of the load current iV will be positive, and the polarity of the load current iW will be positive in a specific phase, as shown in Figure 3B. In other words, if only the V-phase switch 8V fails, of the three load currents iU, iV, and iW, only the load current iU will be negative in polarity. If the switch 8V fails, charging and discharging of the resonant capacitor 9V connected to the switch 8V will no longer be performed, and the switching of the first switching element 1V and the second switching element 2V connected to the switch 8V will become hard switching.

[0091] If only the W-phase switch 8W of the three switches 8U, 8V, and 8W fails, the polarity of the load current iU will be positive, the polarity of the load current iV will be negative, and the polarity of the load current iW will be positive in a specific phase, as shown in Figure 3C. In other words, if only the W-phase switch 8W fails, of the three load currents iU, iV, and iW, only the load current iV will be negative. If the switch 8W fails, charging and discharging of the resonant capacitor 9W connected to the switch 8W will no longer be performed, and the switching of the first switching element 1W and the second switching element 2W connected to the switch 8W will be hard switching.

[0092] The specific phase is, for example, the first occurrence timing tg1 of the first peak P1 of the ripple voltage. The first peak P1 is the single maximum peak at which the ripple voltage reaches its maximum value Vmax. In other words, the specific phase is the phase of one of the multiple maximum peaks of the ripple voltage of the voltage V15 across the regenerative capacitor 15. When only one of the multiple load currents iU, iV, and iW has negative polarity, the second determination unit 55 determines that hard switching has occurred in the switching circuit 10 corresponding to the load current that is 120° behind the phase of the one load current among the multiple switching circuits 10.

[0093] More specifically, when only the polarity of the load current iW among the three load currents iU, iV, and iW is negative, the second judgment unit 55 judges that hard switching is occurring in the first switching element 1U and the second switching element 2U of the switching circuit 10U corresponding to the load current iU, which is 120° out of phase with the load current iW.

[0094] Furthermore, when only the polarity of the load current iU among the three load currents iU, iV, and iW is negative, the second judgment unit 55 judges that hard switching is occurring in the first switching element 1V and the second switching element 2V of the switching circuit 10V corresponding to the load current iV, which is 120° out of phase with the load current iU.

[0095] Furthermore, when only the polarity of the load current iV among the three load currents iU, iV, and iW is negative, the second judgment unit 55 judges that hard switching is occurring in the first switching element 1W and the second switching element 2W of the switching circuit 10W corresponding to the load current iW, which is 120° out of phase with the load current iV.

[0096] In the power conversion device 100, if two of the three switches 8U, 8V, and 8W, namely, the switches 8V and 8W, fail, the charging and discharging of the regenerative capacitor 15 is determined only by the U-phase load current iU. Therefore, if two of the three switches 8U, 8V, and 8W, namely, the switches 8V and 8W, fail, and as shown in FIG. 5 , at the first occurrence timing tg1 in the ripple voltage of the voltage V15 across the regenerative capacitor 15, the load current iU becomes zero, only the polarity of the load current iV becomes negative, and the polarity of the load current iW becomes positive, the second determination unit 55 can determine that hard switching is occurring in the switching circuit 10V.

[0097] Furthermore, when only two of the multiple load currents iU, iV, and iW have negative polarities, the second judgment unit 55 judges, based on the load current having the larger absolute value among the two load currents, that hard switching has occurred in the switching circuit 10 among the multiple switching circuits 10 corresponding to the load current whose phase is delayed by 120° from the one load current.

[0098] 5 , when two of the three switches 8U, 8V, and 8W fail and the first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11, the load current iU may not become zero and the polarity of the load current iU may become negative at the first generation timing tg1. When only two of the load currents iU, iV, and iW have negative polarities, the second determination unit 55 determines that hard switching is occurring in the first switching element 1W and the second switching element 2W of the switching circuit 10W corresponding to the load current iW that is 120° out of the phase of the load current iV, based on the load current iV having the larger absolute value among the two load currents iU and iV.

[0099] In the power conversion device 100, the first determination unit 54 determines that hard switching is occurring in each of the multiple switching circuits 10 when the voltage V15 across the regenerative capacitor 15 is constant (see FIG. 6B ). The phrase "the voltage V15 across the regenerative capacitor 15 is constant" means, for example, that the voltage V15 across the regenerative capacitor 15 during one cycle of the load current is within a range of 95% to 105% of the average value between the maximum value Vmax and the minimum value Vmin of the ripple voltage when the three switches 8U, 8V, and 8W are not failing. The ripple voltage of the voltage V15 across the regenerative capacitor 15 in FIG. 6A is the ripple voltage when all three switches 8U, 8V, and 8W are operating normally.

[0100] (4) Summary In the power conversion device 100 according to the first embodiment, the control device 50 has a first determination unit 54 and a second determination unit 55. The first determination unit 54 determines the switching state in the power conversion circuit 11 based on the ripple voltage included in the voltage V15 across the regenerative capacitor 15 and the multiple load currents iU, iV, and iW output from the multiple AC terminals 41. When the first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11, the second determination unit 55 determines which of the multiple switching circuits 10 is experiencing hard switching, based on the polarities of the multiple load currents iU, iV, and iW in a specific phase of the ripple voltage.

[0101] According to the power conversion device 100 of the first embodiment, it is possible to detect the switching circuit 10 in which hard switching occurs in the power conversion circuit 11 .

[0102] Furthermore, the power conversion device 100 according to embodiment 1 can reduce costs by reducing the number of components compared to a case where six voltage sensors are provided to detect the voltages across each of the three first switching elements 1 and three second switching elements 2 of the power conversion circuit 11, and can also reduce the number of input ports required for the computer system (processor) that constitutes the control device 50.

[0103] Furthermore, in the power conversion device 100 according to the first embodiment, the first determination unit 54 determines that hard switching has occurred in the power conversion circuit 11 when a predetermined condition (first predetermined condition) is satisfied. The predetermined condition (first predetermined condition) is that the number of intersections B1 of any two of the load currents iU, iV, and iW during a predetermined period Ts is greater than a predetermined value. The predetermined period Ts is the period between a first occurrence timing tg1 of a first peak P1 of the ripple voltage and a second occurrence timing tg2 of a second peak P2 of the ripple voltage. Therefore, the power conversion device 100 according to the first embodiment can detect that hard switching has occurred in the power conversion circuit 11 when hard switching has occurred in at least one of the first switching elements 1 and the second switching elements 2.

[0104] Second Embodiment A power conversion device 100A according to a second embodiment will be described below with reference to FIGS.

[0105] (1) Overall Configuration of Power Conversion Device As shown in Fig. 14 , the overall configuration of the power conversion device 100A according to the second embodiment is substantially the same as that of the power conversion device 100 according to the first embodiment (see Fig. 1 ), but differs from the power conversion device 100 according to the first embodiment in that a second determination unit 55A is provided instead of the second determination unit 55 of the control device 50 in the power conversion device 100. Regarding the power conversion device 100A according to the second embodiment, components similar to those of the power conversion device 100 according to the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0106] The second judgment unit 55A judges which of the multiple switching circuits 10 is experiencing hard switching based on the control signals SU6, SU7, SV6, SV7, SW6, and SW7 given to each of the multiple switches 8 when the first judgment unit 54 judges that hard switching is occurring in the power conversion circuit 11.

[0107] (2) Operation of the Power Conversion Device The operation of the power conversion device 100A according to the second embodiment is substantially the same as the operation of the power conversion device 100 according to the first embodiment, and only the operation of the second determination unit 55A differs from the operation of the second determination unit 55. Note that, regarding the operation of the power conversion device 100A, the description of the same operations as those of the power conversion device 100 will be omitted as appropriate.

[0108] Fig. 15 shows the control signals SU1, SU2, SV1, SV2, SW1, SW2, SU6, SU7, SV6, SV7, SW6, and SW7 when none of the three switches 8U, 8V, and 8W has failed, and Fig. 16 shows the control signals SU1, SU2, SV1, SV2, SW1, SW2, SU6, SU7, SV6, SV7, SW6, and SW7 when only the U-phase switch 8U has failed. Although Figs. 15 and 16 are not described in the first embodiment, they are similarly applied to the first embodiment.

[0109] If only the U-phase switch 8U fails, the polarity of the load current iW becomes negative immediately before the W-phase first switching element 1W is turned on, and the resonant capacitor 9W is charged as shown in Fig. 4. Therefore, the high-level period of the control signal SW6 (see Fig. 16) when only the U-phase switch 8U fails is shorter than the high-level period of the control signal SW6 (see Fig. 15) when the U-phase switch 8U is not failed. If only the U-phase switch 8U fails, the high-level period of the control signal SW6 (see Fig. 16) may become zero.

[0110] Furthermore, if only the V-phase switch 8V fails, the high level period of the control signal SU6 becomes shorter than when the switch 8V does not fail.

[0111] Furthermore, if only the W-phase switch 8W fails, the period during which the control signal SV6 is at high level will be shorter than if the switch 8W is not at fault.

[0112] The second judgment unit 55A judges which of the multiple switching circuits 10 is experiencing hard switching based on the control signals given to each of the multiple switches 8 when the first judgment unit 54 judges that hard switching is occurring in the power conversion circuit 11.

[0113] When the high level period of the control signal SW6 becomes shorter than the normal high level period, the second determination unit 55A determines which of the switching circuits 10U, 10V, and 10W is experiencing hard switching.

[0114] Furthermore, when the high level period of the control signal SU6 becomes shorter than the normal high level period, the second determination unit 55A determines which of the multiple switching circuits 10U, 10V, and 10W is the switching circuit 10V in which hard switching is occurring.

[0115] Furthermore, when the high level period of the control signal SV6 becomes shorter than the normal high level period, the second determination unit 55A determines which of the multiple switching circuits 10U, 10V, and 10W is the switching circuit 10W in which hard switching is occurring.

[0116] (3) Summary In the power conversion device 100A according to the second embodiment, the control device 50 includes a first determination unit 54 and a second determination unit 55A. The first determination unit 54 determines the switching state in the power conversion circuit 11 based on the ripple voltage included in the voltage V15 across the regenerative capacitor 15 and the multiple load currents iU, iV, and iW output from the multiple AC terminals 41. The second determination unit 55A determines which of the multiple switching circuits 10 is experiencing hard switching based on control signals provided to the multiple switches 8 when the first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11.

[0117] According to the power conversion device 100A of the second embodiment, it is possible to detect the switching circuit 10 in which hard switching occurs in the power conversion circuit 11.

[0118] (Embodiment 3) A power conversion device 100B according to embodiment 3 will be described with reference to Fig. 17. Regarding the power conversion device 100B according to embodiment 3, 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.

[0119] The power conversion device 100B of embodiment 3 differs from the power conversion device 100 of embodiment 1 in that it further includes a regenerative capacitor 16 (hereinafter also referred to as the second regenerative capacitor 16) connected between the sixth terminal 154 of the regenerative capacitor 15 (hereinafter also referred to as the first regenerative capacitor 15) and the first DC terminal 31.

[0120] The second regenerative capacitor 16 is connected in series to the first regenerative capacitor 15. Therefore, in the power conversion device 100B, 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. 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.

[0121] In the power conversion device 100B according to the third embodiment, the voltage V15 across the first regenerative capacitor 15 (the potential at the sixth terminal 154 of the first regenerative capacitor 15) is a value obtained by dividing the voltage value Vd of the DC power supply E1 between the second regenerative capacitor 16 and the first regenerative capacitor 15. Therefore, the voltage V15 across the first regenerative capacitor 15 is approximately Vd / 2 except when it is transiently changing, but it includes ripple voltages due to the resonant currents of the U phase, V phase, and W phase.

[0122] The operation of the control device 50 of the power conversion device 100B according to the third embodiment is similar to the operation of the control device 50 of the power conversion device 100 according to the first embodiment. Therefore, like the power conversion device 100 according to the first embodiment, the power conversion device 100B according to the third embodiment can detect a switching circuit 10 in which hard switching is occurring in the power conversion circuit 11.

[0123] (Fourth embodiment) A power conversion device 100C according to a fourth embodiment will be described with reference to Fig. 18. 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 Fig. 1) are denoted by the same reference numerals, and description thereof will be omitted.

[0124] (1) Configuration The power conversion device 100C differs from the power conversion device 100 according to the first embodiment in that it includes only one resonant inductor L1. In the power conversion device 100C, the resonant inductor L1 is common to the multiple resonant circuits. In the power conversion device 100C, the third end of the resonant inductor L1 is connected to a common connection point 25. The second ends 82 of the multiple switches 8 are commonly connected to the common connection point 25.

[0125] In the power conversion device 100C, the third end of the resonance inductor L1 is connected to a common connection point 25. The common connection point 25 is connected in common to second ends 82 of the plurality of switches 8.

[0126] The power conversion device 100C also differs from the power conversion device 100 according to the first embodiment in that it includes only one protection circuit 17 .

[0127] In the power conversion device 100C, the third diode 13 in the protection circuit 17 is connected between the common connection point 25 and the first DC terminal 31. In the third diode 13, the anode of the third diode 13 is connected to the common connection point 25. In addition, in the third diode 13, the cathode of the third diode 13 is connected to the first DC terminal 31. The fourth diode 14 in the protection circuit 17 is connected between the common connection point 25 and the second DC terminal 32. In the fourth diode 14, the anode of the fourth diode 14 is connected to the second DC terminal 32. In the fourth diode 14, the cathode of the fourth diode 14 is connected to the common connection point 25. Therefore, the fourth diode 14 is connected in series with the third diode 13.

[0128] (2) Operation of the Power Conversion Device In the power conversion device 100C, similarly to the power conversion device 100, the control device 50 controls a plurality of (e.g., three) first switching elements 1, a plurality of (e.g., three) second switching elements 2, and a plurality of (e.g., three) switches 8. The control device 50 (control unit 51) performs basic operations and shift control operations.

[0129] (2.1) Basic Operation The basic operation of the control device 50 is the same as the operation of the control device 50 in the power conversion device 100 according to embodiment 1. The basic operation is an operation performed when no resonant current flows through two or more of the multiple switches 8 simultaneously through the resonant inductor L1.

[0130] (2.2) Shift Control Operation The shift control operation is an operation of the control device 50 when the control device 50 determines that resonant currents flow simultaneously through two or more of the switches 8 .

[0131] When the control device 50 determines that resonant currents passing through two switches 8 out of the plurality of switches 8 simultaneously flow through the resonant inductor L1, the control device 50 performs shift control to shift the high-level period of the control signal to one of the two switches 8 so that the resonant currents passing through the two switches 8 do not simultaneously flow through the resonant inductor L1. "When it is determined that resonant currents passing through two switches 8 out of the plurality of switches 8 simultaneously flow" means that it has been estimated in advance that the resonant currents passing through the two switches 8 simultaneously flow through the resonant inductor L1.

[0132] (2.2.1) Determination of whether two-phase resonant currents flow simultaneously In the power conversion device 100C, the phases of the voltage commands for three phases (U phase, V phase, and W phase) differ 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 control signals for the two phases approach each other (see areas A1 and A2 in FIG. 9 ). In area A1 in FIG. 9 , the duties of the U-phase control signal and the V-phase control signal are approximately 0.75. In area A2 in FIG. 9 , the duties of the U-phase control signal and the V-phase control signal are approximately 0.25. The polarity of the resonant current is the same as the polarity of current iL1; in area A1, the polarity of the resonant current is positive, and in area A2, the polarity of the resonant current is negative. In the case of region A1, for example, during one cycle of the carrier signal, the time difference between time t1 (see FIG. 7) at which the high-level period of the control signal SU6 supplied to the first IGBT 6U starts and time t5 (see FIG. 7) at which the high-level period of the control signal SV6 supplied to the first IGBT 6V starts 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 100C, 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.

[0133] Assuming that the capacitance of each of the multiple resonant capacitors 9U, 9V, and 9W is Cr, 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×Cr) of the resonant capacitors 9U and 9V is connected in series to the resonant inductor L1. Therefore, in the power conversion device 100C, if two-phase currents flow simultaneously through the resonant inductor L1, the resonant frequency of the resonant circuit including the resonant inductor L1 will change compared to when a single-phase current flows through the resonant inductor L1, and zero-voltage soft switching may not be achieved.

[0134] (2.2.2) In the case of charging operation of the resonant capacitor The boundary conditions for when the U-phase resonant current and the V-phase resonant current do not overlap (do not flow simultaneously) and when they overlap (flow simultaneously) will be described with reference to Figure 7.

[0135] In the power conversion device 100C (see FIG. 18 ), if the time difference ΔTuv between time t3 when the high-level period of the control signal SU1 starts and time t7 when the high-level period of the control signal SV1 starts is equal to or greater than (Tau + Tav + Td), the U-phase resonant current and the V-phase resonant current do not overlap, and if the time difference ΔTuv is less than (Tau + Tav + Td), the U-phase resonant current and the V-phase resonant current overlap. The control device 50 sets a threshold for the time difference ΔTuv to, for example, (Tau + Tav + Td), and if the time difference ΔTuv is less than the threshold, it estimates that resonant currents corresponding to two phases, switching circuit 10U and switching circuit 10V, of the multiple switching circuits 10, will flow simultaneously through the resonant inductor L1. The above threshold setting is merely an example, and other values ​​may also be considered. For example, it may be possible to set the threshold value to a value even greater than (Tau + Tav + Td) in consideration of errors in the additional times Tau and Tav. Furthermore, the method for calculating the time difference ΔTuv used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example, and any other calculation method may be used as long as it can calculate a time difference equivalent to the time difference ΔTuv. For example, the time difference ΔTuv used to determine whether two-phase resonant currents flow simultaneously may be the time difference between time t2, at which the high-level period of the control signal SU2 ends, and time t6, at which the high-level period of the control signal SV2 ends.

[0136] Furthermore, in the power conversion device 100C, if the time difference between time t3, when the high-level period of the control signal SU1 starts, and time t11, when the high-level period of the control signal SW1 starts, is equal to or greater than (Tau + Taw + Td), the U-phase resonant current and the W-phase resonant current do not overlap. If the time difference is less than (Tau + Taw + Td), the U-phase resonant current and the W-phase resonant current overlap. The control device 50 sets a threshold for this time difference to, for example, (Tau + Taw + Td). If the time difference is less than the threshold, the control device 50 estimates that resonant currents corresponding to two phases, the switching circuit 10U and the switching circuit 10W, of the multiple switching circuits 10 simultaneously flow through the resonant inductor L1. The above threshold setting is merely an example, and other values ​​may also be considered. For example, the threshold may be set to a value greater than (Tau + Taw + Td) in consideration of errors in the additional time Tau and the additional time Taw. Furthermore, the method of calculating the time difference used to determine whether the two-phase resonant currents flow simultaneously is not limited to the above example, and any other calculation method may be used as long as it can calculate a time difference equivalent to the time difference. For example, the time difference used to determine whether the two-phase resonant currents flow simultaneously may be the time difference between time t2, at which the high-level period of the control signal SU2 ends, and time t10, at which the high-level period of the control signal SW2 ends.

[0137] Furthermore, in the power conversion device 100C, if the time difference between time t7 at which the high-level period of the control signal SV1 provided to the first switching element 1V of the switching circuit 10V starts and time t11 at which the high-level period of the control signal SW1 provided to the first switching element 1W of the switching circuit 10W starts is (Tav + Taw + Td) or more, the V-phase resonant current and the W-phase resonant current do not overlap, and if the time difference is less than (Tav + Taw + Td), the V-phase resonant current and the W-phase resonant current overlap. The control device 50 sets a threshold for this time difference to, for example, (Tav + Taw + Td), and if the time difference is less than the threshold, it estimates that resonant currents corresponding to two phases, the switching circuit 10V and the switching circuit 10W, of the multiple switching circuits 10 will flow simultaneously through the resonant inductor L1. The above threshold setting is merely an example, and other values ​​may also be considered. For example, it is possible to set the threshold value to a value greater than (Tav + Taw + Td) in consideration of errors in the additional times Tav and Taw. Furthermore, the method for calculating the time difference used to determine whether two-phase resonant currents flow simultaneously is not limited to the above example, and any other calculation method may be used as long as it can calculate a time difference equivalent to the time difference. For example, the time difference used to determine whether two-phase resonant currents flow simultaneously may be the time difference between time t6, at which the high-level period of the control signal SV2 ends, and time t10, at which the high-level period of the control signal SW2 ends.

[0138] (2.2.3) In the case of discharging operation of the resonant capacitor In the case of discharging operation of the resonant capacitor 9, the control device 50 can determine whether two-phase resonant currents flow simultaneously using the same time difference and threshold value as in the case of charging operation of the resonant capacitor 9.

[0139] For example, the control device 50 estimates that the U-phase resonant current and the V-phase resonant current overlap if the time difference between the start point of the high-level period of the control signal SU2 and the start point of the high-level period of the control signal SV2 is less than a threshold value (e.g., Tau + Tav + Td).

[0140] In addition, the control device 50 estimates that the U-phase resonant current and the W-phase resonant current overlap if the time difference between the start point of the high-level period of the control signal SU2 and the start point of the high-level period of the control signal SW2 is less than a threshold value (e.g., Tau + Taw + Td).

[0141] In addition, the control device 50 estimates that the V-phase resonant current and the W-phase resonant current overlap if the time difference between the start of the high-level period of the control signal SV2 and the start of the high-level period of the control signal SW2 is less than a threshold value (e.g., Tav + Taw + Td).

[0142] (2.2.4) Shift Control When It is Determined that Two-Phase Resonant Currents Flow Simultaneously The control device 50 performs shift control to shift the high-level period of the control signal to one of the two switches 8, for example, so that the resonant currents passing through the two switches 8 respectively do not flow simultaneously through the resonant inductor L1.

[0143] When performing shift control, the control device 50 shifts the high-level period of the control signal to one of the two switches 8 so as not to change the length of the high-level period of the control signal supplied to each of the first switching element 1 and the second switching element 2 of one switching circuit 10 corresponding to one of the two switches 8. For example, when shifting the high-level period of the control signal SU6 or SU7 supplied to switch 8U, the control device 50 shifts the high-level periods of the control signal SU1 and the control signal SU2, but does not change the duties of the control signal SU1 and the control signal SU2 in one period of the carrier signal. Furthermore, when shifting the high-level period of the control signal SV6 or SV7 supplied to switch 8V, the control device 50 shifts the high-level periods of the control signal SV1 and the control signal SV2, but does not change the duties of the control signal SV1 and the control signal SV2 in one period of the carrier signal. Furthermore, when the control device 50 shifts the high-level period of the control signal SW6 or SW7 provided to the switch 8W, it shifts the high-level period of each of the control signal SW1 and the control signal SW2, but does not change the duty of each of the control signal SW1 and the control signal SW2 in one cycle of the carrier signal.

[0144] In the power conversion device 100C, when the control device 50 executes shift control to soft-switch the first switching element 1, for example, the voltages V2u and V2v across the second switching elements 2U and 2V rise to Vd at the point in time when the control signals SU1 and SV1 change from a low-level period to a high-level period (the end point of the dead time period Td corresponding to the U phase and the V phase, respectively). In other words, when the control device 50 executes shift control, charging of the resonant capacitors 9U and 9V ends at the end of the dead time period Td corresponding to the U phase and the V phase, respectively. Therefore, in the power conversion device 100C, when the control device 50 executes shift control, the switching of the first switching elements 1U and 1V becomes zero-voltage soft switching.

[0145] Although the above example shows an example of shift control when the control device 50 determines in advance that a U-phase resonant current and a V-phase resonant current will simultaneously flow through the resonant inductor L1, the present invention is not limited to this. For example, when the control device 50 determines in advance that a V-phase resonant current and a W-phase resonant current will simultaneously flow through the resonant inductor L1, the control device 50 also performs shift control in a case where it has determined in advance that a W-phase resonant current and a U-phase resonant current will simultaneously flow through the resonant inductor L1, thereby enabling zero-voltage soft switching.

[0146] Furthermore, in the power conversion device 100C, when the control device 50 executes shift control to soft-switch the second switching element 2, for example, the voltages V1u and V1v across the first switching elements 1U and 1V rise to Vd at the point in time when the control signals SU2 and SV2 change from a low-level period to a high-level period (the end point of the dead time period Td corresponding to the U phase and the V phase, respectively). In other words, when the control device 50 executes shift control, the discharge of the resonant capacitors 9U and 9V ends at the end of the dead time period Td corresponding to the U phase and the V phase, respectively. Therefore, in the power conversion device 100C, when the control device 50 executes shift control, the switching of the second switching elements 2U and 2V becomes zero-voltage soft switching.

[0147] Although the above example shows an example of shift control when the control device 50 determines in advance that a U-phase resonant current and a V-phase resonant current will simultaneously flow through the resonant inductor L1, the present invention is not limited to this. For example, when the control device 50 determines in advance that a V-phase resonant current and a W-phase resonant current will simultaneously flow through the resonant inductor L1, the control device 50 also performs shift control in a case where it has determined in advance that a W-phase resonant current and a U-phase resonant current will simultaneously flow through the resonant inductor L1, thereby enabling zero-voltage soft switching.

[0148] (2.3) Operation of First Determination Unit and Second Determination Unit The operation of the first determination unit 54 is the same as the operation of the first determination unit 54 in the control device 50 of the power conversion device 100 according to embodiment 1. The operation of the second determination unit 55 is the same as the operation of the second determination unit 55 in the control device 50 of the power conversion device 100 according to embodiment 1. Therefore, when the first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11, the second determination unit 55 determines which of the multiple switching circuits 10 is causing hard switching, based on the polarities of the multiple load currents iU, iV, and iW in a specific phase of the ripple voltage.

[0149] (3) Summary The power conversion device 100C according to the fourth embodiment includes a first determination unit 54 that determines the switching state of the power conversion circuit 11 based on the ripple voltage included in the voltage V15 across the regenerative capacitor 15 and the multiple load currents iU, iV, and iW output from the multiple AC terminals 41. Therefore, the power conversion device 100C according to the fourth embodiment is capable of detecting the switching state of the power conversion circuit 11. Furthermore, the power conversion device 100C includes a second determination unit 55 that, when the first determination unit 54 determines that hard switching is occurring in the power conversion circuit 11, determines which of the multiple switching circuits 10 is experiencing hard switching based on the polarities of the multiple load currents iU, iV, and iW in a specific phase of the ripple voltage. Therefore, the power conversion device 100C according to the fourth embodiment is capable of detecting which of the multiple switching circuits 10 is experiencing hard switching.

[0150] Furthermore, the power conversion device 100C according to the fourth embodiment has one resonant inductor L1, and the second ends 82 of the multiple switches 8 are commonly connected to the single resonant inductor L1. This allows the power conversion device 100C according to the fourth embodiment to have a reduced number of components and be more compact.

[0151] Furthermore, in the power conversion device 100C according to the fourth embodiment, when the control device 50 determines that resonant currents passing through two of the multiple switches 8 respectively flow through one resonant inductor L1 simultaneously, the control device 50 performs control to shift the high-level periods of the control signals to the two switches 8 respectively so that the resonant currents passing through the two switches 8 respectively do not flow through one resonant inductor L1 simultaneously. This makes it possible for the power conversion device 100C according to the fourth embodiment to more reliably achieve soft switching.

[0152] (4) Modifications of the Fourth Embodiment (4.1) Modification 1 A power conversion device 100C according to Modification 1 will be described with reference to Fig. 19. Regarding the power conversion device 100C according to Modification 1, components similar to those of the power conversion device 100C according to the fourth embodiment (see Fig. 18) are denoted by the same reference numerals, and description thereof will be omitted.

[0153] In a power conversion device 100C according to the first modification, the first IGBT 6 and the second IGBT 7 are connected in anti-series in each of the plurality of switches 8. In a power conversion device 100B according to the first modification, the collector terminal of the first IGBT 6 and the collector terminal of the second IGBT 7 are connected in each of the plurality of switches 8, the emitter terminal of the first IGBT 6 is connected to the connection point 3 of a corresponding one of the plurality of switching circuits 10, and the emitter terminal of the second IGBT 7 is connected to the common connection point 25. Each of the plurality of switches 8 further includes a diode 61 connected in anti-parallel to the first IGBT 6 and a diode 71 connected in anti-parallel to the second IGBT 7.

[0154] In the power conversion device 100C according to the first modification, each of the first IGBT 6 and the second IGBT 7 may be replaced with a MOSFET or a bipolar transistor. In this case, the diode 61 and the diode 71 in FIG. 19 may be substituted with a parasitic diode of the replaced element, or an element built into the chip of the replaced element. Furthermore, in the power conversion device 100C according to the first modification, the diode 61 and the diode 71 are not limited to being externally connected to the first IGBT 6 and the second IGBT 7, but may be an element built into the chip.

[0155] The operation of the control device 50 is the same as that of the control device 50 of the fourth embodiment, for example.

[0156] (4.2) Modification 2 A power conversion device 100C according to Modification 2 will be described with reference to Fig. 20. Regarding the power conversion device 100C according to Modification 2, components that are the same as those of the power conversion device 100C according to the fourth embodiment (see Fig. 18) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0157] In a power conversion device 100C according to the second modification, the first IGBT 6 and the second IGBT 7 are connected in anti-series in each of the multiple switches 8. In the power conversion device 100C according to the second modification, the emitter terminal of the first IGBT 6 and the emitter terminal of the second IGBT 7 are connected in each of the multiple switches 8, the collector terminal of the first IGBT 6 is connected to a common connection point 25, and the collector terminal of the second IGBT 7 is connected to a connection point 3 of a corresponding one of the multiple switching circuits 10. Each of the multiple switches 8 further includes a diode 61 connected in anti-parallel to the first IGBT 6 and a diode 71 connected in anti-parallel to the second IGBT 7.

[0158] In the power conversion device 100C according to the second modification, each of the first IGBT 6 and the second IGBT 7 may be replaced with a MOSFET or a bipolar transistor. In this case, the diode 61 and the diode 71 in FIG. 20 may be substituted with a parasitic diode of the replaced MOSFET or bipolar transistor, or a diode element built into a single chip including a MOSFET or a bipolar transistor. Furthermore, in the power conversion device 100C according to the second modification, the diode 61 and the diode 71 are not limited to being externally attached to the first IGBT 6 and the second IGBT 7, but may also be elements built into a single chip.

[0159] The operation of the control device 50 is the same as that of the control device 50 of the fourth embodiment, for example.

[0160] (4.3) Modification 3 A power conversion device 100C according to Modification 3 will be described with reference to Fig. 21. Regarding the power conversion device 100C according to Modification 3, components that are the same as those of the power conversion device 100C according to the fourth embodiment (see Fig. 18) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0161] In a power conversion device 100C according to the third modification, a first MOSFET 6A and a second MOSFET 7A are connected in anti-series in each of the multiple switches 8. In the power conversion device 100C according to the third modification, the drain terminal of the first MOSFET 6A and the drain terminal of the second MOSFET 7A are connected in anti-parallel in each of the multiple switches 8. Each of the multiple switches 8 further includes a diode 61 connected in anti-parallel to the first MOSFET 6A and a diode 71 connected in anti-parallel to the second MOSFET 7A. In each of the multiple switches 8, the source terminal of the second MOSFET 7A is connected to a common connection point 25. In each of the multiple switches 8, the source terminal of the first MOSFET 6A is connected to a connection point 3 of a switching circuit 10 corresponding to the switch 8 having the first MOSFET 6A. Control signals SU6 and SU7 are provided from a control device 50 to the first MOSFET 6A and the second MOSFET 7A of a switch 8U. The first MOSFET 6A and the second MOSFET 7A of the switch 8V are supplied with control signals SV6 and SV7 from the control device 50. The first MOSFET 6A and the second MOSFET 7A of the switch 8W are supplied with control signals SW6 and SW7 from the control device 50.

[0162] The operation of the control device 50 is similar to that of the control device 50 of the fourth embodiment, for example.

[0163] (4.4) Modification 4 A power conversion device 100C according to Modification 4 will be described with reference to Fig. 22. Regarding the power conversion device 100C according to Modification 4, components that are the same as those of the power conversion device 100C according to the fourth embodiment (see Fig. 18) are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0164] In a power conversion device 100C according to the fourth modification, a diode 63 is connected in series to the first MOSFET 6A, and a diode 73 is connected in series to the second MOSFET 7A in each of the multiple switches 8. In a power conversion device 100B according to the fourth modification, a series circuit of the first MOSFET 6A and the diode 63 and a series circuit of the second MOSFET 7A and the diode 73 are connected in anti-parallel.

[0165] The operation of the control device 50 is similar to that of the control device 50 of the fourth embodiment, for example.

[0166] Fifth Embodiment A power conversion device 100D according to a fifth embodiment will be described with reference to Fig. 23. Regarding the power conversion device 100D according to the fifth embodiment, components similar to those of the power conversion device 100C according to the fourth embodiment (see Fig. 18) are denoted by the same reference numerals, and description thereof will be omitted.

[0167] The power conversion device 100D of embodiment 5 differs from the power conversion device 100C of embodiment 4 in that it further includes a regenerative capacitor 16 (hereinafter also referred to as the second regenerative capacitor 16) connected between the sixth terminal 154 of the regenerative capacitor 15 (hereinafter also referred to as the first regenerative capacitor 15) and the first DC terminal 31.

[0168] The second regenerative capacitor 16 is connected in series to the first regenerative capacitor 15. Therefore, in the power conversion device 100D, 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. 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.

[0169] In the power conversion device 100D according to the fifth embodiment, the voltage V15 across the first regenerative capacitor 15 (the potential at the sixth terminal 154 of the first regenerative capacitor 15) is equal to the voltage Vd of the DC power supply E1 divided by the voltages of the second regenerative capacitor 16 and the first regenerative capacitor 15. Therefore, the voltage V15 across the first regenerative capacitor 15 is approximately Vd / 2, but includes a ripple voltage. This ripple voltage occurs in conjunction with the operation of discharging the charge of the first regenerative capacitor 15 to charge the resonant capacitor 9 and the operation of discharging the resonant capacitor 9 to charge the first regenerative capacitor 15.

[0170] The operation of the control device 50 of the power conversion device 100D according to the fifth embodiment is similar to the operation of the control device 50 of the power conversion device 100C according to the fourth embodiment. Therefore, like the power conversion device 100C according to the fourth embodiment, the power conversion device 100D according to the fifth embodiment is capable of detecting the switching state of the power conversion circuit 11. Also, like the power conversion device 100C according to the fourth embodiment, the power conversion device 100D according to the fifth embodiment is capable of detecting a switching circuit 10 in which hard switching is occurring in the power conversion circuit 11.

[0171] (Other Modifications) The above-described first to fifth embodiments are merely examples of various embodiments of the present disclosure. The above-described first to fifth embodiments can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved.

[0172] For example, in the control device 50 of the power conversion device 100C according to the fourth embodiment, the operation of "determining that two-phase resonant currents are flowing simultaneously" is not limited to the operation of "determining that two-phase resonant currents are flowing simultaneously" when the time difference described in the fourth embodiment is less than the threshold value.

[0173] For example, the control device 50 may determine that two-phase resonant currents are flowing simultaneously when any one of the current difference between the U-phase load current iU and the V-phase load current iV, the current difference between the V-phase load current iV and the W-phase load current iW, and the current difference between the W-phase load current iW and the U-phase load current iU is less than a current difference threshold.

[0174] Furthermore, the power conversion device 100A according to the second embodiment may be configured to include only one resonance inductor L1 instead of three, similar to the power conversion device 100C according to the fourth embodiment.

[0175] In addition, the control device 50 may determine that "two-phase resonant currents flow simultaneously" when the electrical angle calculated from sensor information output from a sensor device (e.g., an encoder or resolver) for detecting the rotation speed of the motor, or the estimated electrical angle, is within a first rotation angle range (e.g., 55 degrees or more and 65 degrees or less), a second rotation angle range (e.g., 115 degrees or more and 125 degrees or less), a third rotation angle range (e.g., 175 degrees or more and 185 degrees or less), a fourth rotation angle range (e.g., 235 degrees or more and 245 degrees or less), a fifth rotation angle range (295 degrees or more and 305 degrees or less), or a sixth rotation angle range (e.g., 355 degrees or more and 365 degrees or less).

[0176] Furthermore, each of the plurality of first switching elements 1 and the plurality of second switching elements 2 is not limited to an IGBT and may be a MOSFET. In this case, each of the plurality of first diodes 4 may be substituted with a parasitic diode of a 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 a 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 (Gate Injection Transistor).

[0177] In addition, in the power conversion devices 100, 100A, 100B, 100C, and 100D, 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.

[0178] Furthermore, the length of the dead time period Td is not limited to being set to be the same as the resonance half cycle, and may be set to a length different from the resonance half cycle.

[0179] The dead time period Td may be set by a dead time generation circuit included in a gate driver IC (Integrated Circuit) or the like that is provided separately from the control device 50. Alternatively, the control device 50 may include a gate driver IC, and the dead time period Td may be set by a dead time generation circuit included in the gate driver IC.

[0180] The first peak P1 used in the first judgment unit 54 may be one minimum value peak at which the ripple voltage contained in the voltage V15 across the regenerative capacitor 15 reaches a minimum value Vmin, and the second peak P2 used in the first judgment unit 54 may be another minimum value peak at which the ripple voltage reaches a minimum value Vmin after the first peak P1.

[0181] Furthermore, the specific phase determined by the second determination unit 55 may be a phase of a single minimum peak at which the ripple voltage included in the voltage V15 across the regenerative capacitor 15 reaches a minimum value Vmin. In this case, when the polarity of only one of the multiple load currents iU, iV, and iW is positive, the second determination unit 55 determines that hard switching has occurred in the switching circuit 10 corresponding to the load current whose phase lags behind that of the one load current by 120° among the multiple switching circuits 10.

[0182] Alternatively, the first peak P1 used in the first determination unit 54 may be a maximum peak at which the ripple voltage included in the voltage V15 across the regenerative capacitor 15 reaches a maximum value Vmax, and the second peak P2 may be a minimum peak at which the ripple voltage reaches a minimum value Vmin after the first peak P1. In this case, the specified period Ts is, for example, half the cycle of the ripple voltage, and the specified value is 1.

[0183] Furthermore, the first peak P1 used in the first judgment unit 54 may be a single minimum peak at which the ripple voltage reaches its minimum value Vmin, and the second peak P2 may be a single maximum peak at which the ripple voltage reaches its maximum value Vmax after the first peak P1.

[0184] Furthermore, the power conversion devices 100, 100A, 100B, 100C, and 100D are not limited to being configured to output three-phase AC, but may be configured to output polyphase AC of three or more phases. The specified value used by the first determination unit 54 may be appropriately determined depending on the number of switching circuits 10 in the power conversion circuit 11, which is determined by the number of phases of the polyphase AC.

[0185] (Aspects) The following aspects are disclosed in this specification.

[0186] A power conversion device (100; 100B; 100C; 100D) 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), at least one resonant inductor (L1), a regenerative capacitor (15), and a control device (50). The power conversion circuit (11) has a plurality of first switching elements (1) and a plurality of second switching elements (2). In the power conversion circuit (11), a plurality of switching circuits (10) are connected in parallel to each other, each of which has a plurality of first switching elements (1) and a plurality of second switching elements (2) connected in series in a one-to-one relationship. In the power conversion circuit (11), the plurality of first switching elements (1) are connected to the first DC terminal (31), and the plurality of second switching elements (2) are connected to the second DC terminal (32). The multiple AC terminals (41) correspond one-to-one to the multiple switching circuits (10). Each of the multiple AC terminals (41) is connected to a connection point (3) between a first switching element (1) and a second switching element (2) in a corresponding switching circuit (10) among the multiple switching circuits (10). The multiple switches (8) correspond one-to-one to the multiple switching circuits (10). Each of the multiple switches (8) has a first terminal (81) and a second terminal (82) connected to the connection point (3) between the first switching element (1) and the second switching element (2) in a corresponding switching circuit (10) among the multiple switching circuits (10). The multiple resonant capacitors (9) correspond one-to-one to the multiple switches (8). Each of the multiple resonant capacitors (9) is connected between the first terminal (81) of the corresponding switch (8) among the multiple switches (8) and a second DC terminal (32). At least one resonant inductor (L1) has a third end and a fourth end. The third end of the at least one resonant inductor (L1) is connected to the second end (82) of a corresponding one of the plurality of switches (8). The regenerative capacitor (15) has a fifth end (153) and a sixth end (154).The regenerative capacitor (15) has a fifth terminal (153) connected to the second DC terminal (32) and a sixth terminal (154) connected to a fourth terminal of at least one resonant inductor (L1). The control device (50) controls the on / off of each of the plurality of first switching elements (1), the plurality of second switching elements (2), and the plurality of switches (8). The control device (50) has a first determination unit (54) and a second determination unit (55). The first determination unit (54) determines the switching state of the power conversion circuit (11) based on a ripple voltage included in a voltage (V15) across the regenerative capacitor (15) and a plurality of load currents (iU, iV, iW) output from a plurality of AC terminals (41). When the first determination unit (54) determines that hard switching is occurring in the power conversion circuit (11), the second determination unit (55) determines which of the multiple switching circuits (10) is experiencing hard switching based on the polarities of multiple load currents (iU, iV, iW) at a specific phase (first occurrence timing tg1) of the ripple voltage.

[0187] According to this aspect, it is possible to detect a switching circuit (10) in which hard switching occurs in a power conversion circuit (11).

[0188] A power conversion device (100A) according to a second aspect includes a first DC terminal (31) and 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), at least one resonant inductor (L1), a regenerative capacitor (15), and a control device (50). The power conversion circuit (11) has a plurality of first switching elements (1) and a plurality of second switching elements (2). In the power conversion circuit (11), a plurality of switching circuits (10) are connected in parallel to each other, each of which has a plurality of first switching elements (1) and a plurality of second switching elements (2) connected in series in a one-to-one relationship. In the power conversion circuit (11), the plurality of first switching elements (1) are connected to the first DC terminal (31), and the plurality of second switching elements (2) are connected to the second DC terminal (32). The multiple AC terminals (41) correspond one-to-one to the multiple switching circuits (10). Each of the multiple AC terminals (41) is connected to a connection point (3) between a first switching element (1) and a second switching element (2) in a corresponding switching circuit (10) among the multiple switching circuits (10). The multiple switches (8) correspond one-to-one to the multiple switching circuits (10). Each of the multiple switches (8) has a first terminal (81) and a second terminal (82) connected to the connection point (3) between the first switching element (1) and the second switching element (2) in a corresponding switching circuit (10) among the multiple switching circuits (10). The multiple resonant capacitors (9) correspond one-to-one to the multiple switches (8). Each of the multiple resonant capacitors (9) is connected between the first terminal (81) of the corresponding switch (8) among the multiple switches (8) and a second DC terminal (32). At least one resonant inductor (L1) has a third end and a fourth end. The third end of the at least one resonant inductor (L1) is connected to the second end (82) of a corresponding one of the plurality of switches (8). The regenerative capacitor (15) has a fifth end (153) and a sixth end (154).The regenerative capacitor (15) has a fifth terminal (153) connected to the second DC terminal (32) and a sixth terminal (154) connected to a fourth terminal of at least one resonant inductor (L1). The control device (50) controls the on / off of each of the plurality of first switching elements (1), the plurality of second switching elements (2), and the plurality of switches (8). The control device (50) has a first determination unit (54) and a second determination unit (55A). The first determination unit (54) determines the switching state of the power conversion circuit (11) based on a ripple voltage included in a voltage (V15) across the regenerative capacitor (15) and a plurality of load currents (iU, iV, iW) output from a plurality of AC terminals (41). The second determination unit (55A) determines which of the multiple switching circuits (10) is experiencing hard switching based on the control signals given to each of the multiple switches (8) when the first determination unit (54) determines that hard switching is occurring in the power conversion circuit (11).

[0189] According to this aspect, it is possible to detect a switching circuit (10) in which hard switching occurs in a power conversion circuit (11).

[0190] In the power conversion device (100; 100B; 100C; 100D) according to the third aspect, in the first aspect, the specific phase (first occurrence timing tg1) is the phase of one of a plurality of maximum peaks of the ripple voltage. When only one of the plurality of load currents (iU, iV, iW) has negative polarity, a second determination unit (55) determines that hard switching has occurred in one of the plurality of switching circuits (10) corresponding to a load current whose phase lags behind that of the one load current by 120°.

[0191] In the power conversion device (100; 100B; 100C; 100D) according to the fourth aspect, in the first aspect, when only two of the plurality of load currents (iU, iV, iW) have negative polarity, the second determination unit (55) determines, based on the load current having the larger absolute value among the two load currents, that hard switching has occurred in a switching circuit (10) among the plurality of switching circuits (10) that corresponds to a load current whose phase lags behind that of the one load current by 120°.

[0192] According to this aspect, it becomes possible to detect one of the two switching circuits (10) in which hard switching has occurred in the power conversion circuit (11).

[0193] In a power conversion device (100; 100A; 100B; 100C; 100D) according to a fifth aspect, in any one of the first to fourth aspects, a first determination unit (54) determines that hard switching is occurring in each of a plurality of switching circuits (10) when a voltage (V15) across the regenerative capacitor (15) is constant.

[0194] According to this aspect, when hard switching occurs in all the switching circuits (10) in the power conversion circuit (11), it becomes possible to detect all the switching circuits (10) in which hard switching has occurred.

[0195] In a power conversion device (100; 100A; 100B; 100C; 100D) according to a sixth aspect, in any one of the first to fifth aspects, a first determination unit (54) determines that hard switching is occurring in the power conversion circuit (11) when a predetermined condition is satisfied. The predetermined condition is that the number of intersections (B1) of any two load currents among a plurality of load currents (iU, iV, iW) in a specified period (Ts) is greater than a specified value. The specified period (Ts) is the period between a first occurrence timing (tg1) of a first peak (P1) of the ripple voltage and a second occurrence timing (tg2) of a second peak (P2) of the ripple voltage.

[0196] According to this aspect, when hard switching occurs in at least one of the plurality of first switching elements (1) and the plurality of second switching elements (2), it is possible to detect that hard switching has occurred in the power conversion circuit (11).

[0197] In the power conversion device (100; 100A; 100B; 100C; 100D) according to the seventh aspect, in the sixth aspect, the first peak (P1) is one maximum value peak at which the ripple voltage reaches its maximum value (Vmax), and the second peak (P2) is another maximum value peak at which the ripple voltage reaches its maximum value (Vmax) after the first peak (P1).

[0198] In the power conversion device (100C; 100D) according to the eighth aspect, in any one of the first to seventh aspects, at least one resonant inductor (L1) is a single resonant inductor (L1), and the second ends (82) of the multiple switches (8) are commonly connected to the single resonant inductor (L1).

[0199] According to this aspect, the number of resonance inductors (L1) can be reduced to one, making it possible to achieve miniaturization.

[0200] 1 First switching element 2 Second switching element 3 Connection point 8 Switch 81 First terminal 82 Second terminal 9 Resonant capacitor 10 Switching circuit 11 Power conversion circuit 15 Regenerative capacitor 153 Fifth terminal 154 Sixth terminal 31 First DC terminal 32 Second DC terminal 41 AC terminal 50 Control device 54 First determination unit 55, 55A Second determination unit 100, 100A, 100B, 100C, 100D Power conversion device B1 Intersection iU, iV, iW Output current (load current) L1 Resonant inductor P1 First peak P2 Second peak RA1 AC load SU1, SU2, SU6, SU7 Control signal SV1, SV2, SV6, SV7 Control signal SW1, SW2, SW6, SW7 Control signal Ts: Specified period tg1: First generation timing tg2: Second generation timing V15: Voltage across both ends

Claims

1. a first DC terminal and a second DC terminal; a power conversion circuit including a plurality of first switching elements and a plurality of second switching elements, wherein a plurality of switching circuits, each having the plurality of first switching elements and the plurality of second switching elements connected in series in a one-to-one relationship, are connected in parallel with each other, the plurality of first switching elements are connected to the first DC terminal, and the plurality of second switching elements are connected to the second DC terminal; a plurality of AC terminals corresponding one-to-one to the plurality of switching circuits, each AC terminal being connected to a connection point between the first switching element and the second switching element in the corresponding switching circuit; a plurality of switches corresponding one-to-one to the plurality of switching circuits, each having a first end and a second end, the first end of which is connected to the connection point between the first switching element and the second switching element in the corresponding switching circuit; a plurality of resonance capacitors corresponding one-to-one to the plurality of switches, each of which is connected between the first end and the second DC terminal of the corresponding switch; at least one resonant inductor having a third end and a fourth end, the third end being connected to the second end of a corresponding one of the plurality of switches; a regenerative capacitor having a fifth end and a sixth end, the fifth end being connected to the second DC terminal and the sixth end being connected to the fourth end of the at least one resonant inductor; a control device that controls the on / off of each of the first switching elements, the second switching elements, and the switches, The control device a first determination unit that determines a switching state of the power conversion circuit based on a ripple voltage included in a voltage across the regenerative capacitor and a plurality of load currents output from the plurality of AC terminals; a second determination unit that, when it is determined by the first determination unit that hard switching is occurring in the power conversion circuit, determines which of the plurality of switching circuits is causing hard switching based on polarities of the plurality of load currents in a specific phase of the ripple voltage. Power conversion device.

2. a first DC terminal and a second DC terminal; a power conversion circuit including a plurality of first switching elements and a plurality of second switching elements, wherein a plurality of switching circuits, each having the plurality of first switching elements and the plurality of second switching elements connected in series in a one-to-one relationship, are connected in parallel with each other, the plurality of first switching elements are connected to the first DC terminal, and the plurality of second switching elements are connected to the second DC terminal; a plurality of AC terminals corresponding one-to-one to the plurality of switching circuits, each AC terminal being connected to a connection point between the first switching element and the second switching element in the corresponding switching circuit; a plurality of switches corresponding one-to-one to the plurality of switching circuits, each having a first end and a second end, the first end of which is connected to the connection point between the first switching element and the second switching element in the corresponding switching circuit; a plurality of resonance capacitors corresponding one-to-one to the plurality of switches, each of which is connected between the first end and the second DC terminal of the corresponding switch; at least one resonant inductor having a third end and a fourth end, the third end being connected to the second end of a corresponding one of the plurality of switches; a regenerative capacitor having a fifth end and a sixth end, the fifth end being connected to the second DC terminal and the sixth end being connected to the fourth end of the at least one resonant inductor; a control device that controls the on / off of each of the first switching elements, the second switching elements, and the switches, The control device a first determination unit that determines a switching state of the power conversion circuit based on a ripple voltage included in a voltage across the regenerative capacitor and a plurality of load currents output from the plurality of AC terminals; a second determination unit that determines which of the plurality of switching circuits is causing hard switching based on control signals provided to the plurality of switches when the first determination unit determines that hard switching is occurring in the power conversion circuit, Power conversion device.

3. the specific phase is the phase of one maximum peak among a plurality of maximum peaks of the ripple voltage, the second determination unit determines, when the polarity of only one of the plurality of load currents is negative, that hard switching has occurred in a switching circuit of the plurality of switching circuits corresponding to a load current whose phase lags behind that of the one load current by 120°; The power conversion device according to claim 1 .

4. When only two of the plurality of load currents have negative polarities, the second determination unit determines, based on one of the two load currents having a larger absolute value, that hard switching has occurred in one of the plurality of switching circuits, a switching circuit corresponding to a load current whose phase lags behind that of the one of the plurality of load currents by 120°. The power conversion device according to claim 1 .

5. The first determination unit determining that hard switching is occurring in each of the plurality of switching circuits when the voltage across the regenerative capacitor is constant; The power conversion device according to any one of claims 1 to 4.

6. The first determination unit When a predetermined condition is satisfied, it is determined that hard switching is occurring in the power conversion circuit; the predetermined condition is a condition that the number of intersections between any two load currents among the plurality of load currents in a specified period is greater than a specified value; the specified period is a period between a first occurrence timing of a first peak of the ripple voltage and a second occurrence timing of a second peak of the ripple voltage. The power conversion device according to any one of claims 1 to 4.

7. the first peak is one maximum value peak at which the ripple voltage reaches a maximum value, and the second peak is another maximum value peak at which the ripple voltage reaches a maximum value after the first peak; The power conversion device according to claim 6.

8. the at least one resonant inductor is one resonant inductor, the second terminals of the plurality of switches are commonly connected to the one resonant inductor; The power conversion device according to any one of claims 1 to 4.