Switching Circuit

Abstract

To reduce a load added to a switching element in which avalanche breakdown occurs when avalanche breakdown occurs in one of a plurality of switching elements which are connected in parallel.SOLUTION: When one side breakdown in which either one of a first switching element and a second switching element undergoes avalanche breakdown in a turn-off period (Ttoff) which transitions from an ON-period (Ton) in which both the first switching element and the second switching element are turned on to an OFF-period (Toff) in which both the first switching element and the second switching element are turned off, specific potential application operation for applying specific potential higher than a gate threshold is performed on a gate of a non-breakdown element which does not undergo avalanche breakdown in the first switching element and the second switching element.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The technology disclosed in this specification relates to a switching circuit.

[0002] Patent Document 1 discloses a switching circuit having multiple switching elements connected in parallel. The gate potential of each switching element is controlled by a gate control circuit. In this switching circuit, the impedance of the wiring from the gate control circuit to the gate of each switching element is adjusted so that the gate current of each switching element is equal. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-156304 A Summary of the Invention [Problem to be solved by the invention]

[0004] When multiple switching elements connected in parallel are turned off together, a surge voltage may be applied to each switching element. Since there is variation in the avalanche voltage of each switching element, the application of a surge voltage generated at the time of turn-off may cause avalanche breakdown in one of the multiple switching elements connected in parallel. When avalanche breakdown occurs in one switching element in this way, a high load is applied to that switching element. This specification proposes a technology for reducing the load applied to the switching element in which avalanche breakdown occurs when avalanche breakdown occurs in one of multiple switching elements connected in parallel. [Means for solving the problem]

[0005] The switching circuit disclosed in this specification includes a first wiring, a second wiring, a first switching element connected between the first wiring and the second wiring, a second switching element connected in parallel to the first switching element between the first wiring and the second wiring, and a gate control circuit connected to a gate of the first switching element and a gate of the second switching element. The gate control circuit controls a gate potential of the first switching element and a gate potential of the second switching element so that an on-period in which both the first switching element and the second switching element are on and an off-period in which both the first switching element and the second switching element are off are alternately repeated. When one-sided breakdown occurs in which either one of the first switching element or the second switching element undergoes avalanche breakdown during a turn-off period transitioning from the on-period to the off-period, the gate control circuit executes a specific potential application operation of applying a specific potential higher than a gate threshold to a gate of a non-breakdown element that does not undergo avalanche breakdown among the first switching element and the second switching element.

[0006] In this specification, avalanche breakdown refers to a state in which the gate potential of a switching element is equal to or lower than the gate threshold, the voltage between the main terminals of the switching element (e.g., the drain-source voltage) is in a clamped state, and a current flows between the main terminals of the switching element.

[0007] In this switching circuit, when one-sided breakdown occurs (i.e., when either the first switching element or the second switching element is in avalanche breakdown), a specific potential higher than the gate threshold is applied to the gate of the non-breakdown element that is not in avalanche breakdown. Therefore, the main current flows through the non-breakdown element. In this way, by passing the main current through the non-breakdown element connected in parallel to the switching element in avalanche breakdown, the current flowing through the switching element in avalanche breakdown (i.e., the avalanche current) can be reduced. This makes it possible to reduce the load on the switching element in avalanche breakdown. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a circuit diagram of an inverter circuit. [Diagram 2] FIG. 2 is a circuit diagram of a switching circuit according to the first embodiment. [Diagram 3] Graph showing normal operation. [Figure 4] 11 is a graph showing one-sided breakdown when a specific potential application operation is not performed. [Diagram 5] 4 is a flowchart showing a process executed by a control IC. [Figure 6] 4 is a graph showing a specific potential application operation in the first embodiment. [Figure 7] FIG. 4 is a control block diagram showing feedback control of potential Vm. [Figure 8] FIG. 11 is a circuit diagram of a switching circuit according to a second embodiment. [Figure 9] 13 is a graph showing a specific potential application operation in Example 2. [Figure 10] FIG. 11 is a circuit diagram of a switching circuit according to a third embodiment. [Figure 11] FIG. 11 is a circuit diagram of a switching circuit according to a fourth embodiment. [Figure 12] 4 is a flowchart showing a process executed by a control IC. [Figure 13] 13 is a graph showing a specific potential application operation in Example 4. [Figure 14] 4 is a flowchart showing a process executed by a control IC. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] (Configuration 1) In one example of a switching circuit disclosed in the present specification, the specific potential may be lower than a gate potential of the non-breakdown element during the on-period.

[0010] According to this configuration, it is possible to limit the current flowing through the non-breakdown element during the specific potential application operation.

[0011] The switching circuit of the above configuration 1 may have a power supply that charges the gate of the non-breakdown element. In the specific potential application operation, the output voltage of the power supply may be lowered compared to the ON period.

[0012] When the gate control circuit includes the power supply, the gate control circuit may feedback control the output voltage of the power supply based on a main current flowing through the non-breakdown element in the specific potential application operation.

[0013] This configuration allows accurate control of the current flowing through the breakdown elements and the non-breakdown elements.

[0014] In the switching circuit of the above configuration 1, the gate control circuit may control the specific potential to a value lower than the gate potential of the non-breakdown element in the on-period by alternately repeating charging and discharging of the gate of the non-breakdown element in the specific potential application operation.

[0015] In the switching circuit of the above configuration 1, the gate control circuit may stop applying the specific potential when a main current (I1) flowing through a breakdown element that is undergoing avalanche breakdown, of the first switching element and the second switching element, drops to zero.

[0016] According to this configuration, it is possible to suppress the main current from flowing through the non-breakdown element after the avalanche breakdown of the breakdown element has ended.

[0017] In an example switching circuit disclosed in the present specification, the gate control circuit may determine whether or not to execute the specific potential application operation during the turn-off period based on the voltage between the first wiring and the second wiring, the main current of the first switching element, and the main current of the second switching element.

[0018] According to this configuration, it is possible to appropriately determine whether or not one-sided breakdown has occurred and execute the specific potential application operation.

[0019] (Configuration 2) In an example switching circuit disclosed in the present specification, the gate control circuit may determine, based on a total current (I3) flowing through the first switching element and the second switching element during the on period, whether or not to execute the specific potential application operation during the subsequent turn-off period.

[0020] According to this configuration, it is possible to determine whether or not one-sided breakdown will occur at a point before the turn-off period, and to appropriately execute the specific potential application operation during the turn-off period.

[0021] In the switching circuit of the above configuration 2, the gate control circuit may increase the specific potential in the subsequent specific potential application operation as the total current in the on-period increases. In the switching circuit of the above configuration 2, the gate control circuit may increase the application time of the specific potential in the subsequent specific potential application operation as the total current in the on-period increases. In the switching circuit of the above configuration 2, the gate control circuit may shorten the interval from the timing at which the gate potential of the non-breakdown element is lowered to the timing at which the specific potential application operation is started in the subsequent turn-off period as the total current in the on-period increases.

[0022] This configuration allows the load on the breakdown element to be reduced more effectively.

[0023] In the switching circuit of the above configuration 2, the gate control circuit may determine whether or not to execute the specific potential application operation in the subsequent turn-off period based on the voltage between the first wiring and the second wiring during the off period. In this case, the gate control circuit may extend the application time of the specific potential in the subsequent specific potential application operation as the voltage between the first wiring and the second wiring during the off period increases.

[0024] This configuration allows the load on the breakdown element to be reduced more effectively.

[0025] In the switching circuit disclosed herein, the gate charge current of the non-breakdown element at the start of application of the specific potential may be greater than the gate charge current of the non-breakdown element at the transition from the off period to the on period. Also, in the switching circuit disclosed herein, the gate discharge current of the non-breakdown element at the end of application of the specific potential may be greater than the gate discharge current of the non-breakdown element in the turn-off period when the one-sided breakdown does not occur. These configurations allow the gate potential to be controlled at a higher speed in the specific potential application operation.

[0026] In the switching circuit according to the embodiment disclosed herein, the gate control circuit may have a storage unit that stores which of the avalanche voltages of the first switching element and the second switching element is higher. When the one-sided breakdown occurs, the gate control circuit may control one of the first switching element and the second switching element, which has a higher avalanche voltage, as the non-breakdown element.

[0027] According to this configuration, it is possible to know in advance whether an avalanche breakdown will occur in the first switching element or the second switching element, which facilitates control. EXAMPLES

[0028] FIG. 1 shows an inverter circuit 10 mounted on a vehicle. The vehicle is also mounted with a battery 12 and a motor 14. The inverter circuit 10 converts DC power supplied from the battery 12 into AC power and supplies the AC power to the motor 14. When the motor 14 is driven, the drive wheels of the vehicle rotate and the vehicle runs. The inverter circuit 10 has a high potential wiring 20, a low potential wiring 22, and three output wirings 24a to 24c. The positive electrode of the battery 12 is connected to the high potential wiring 20. The negative electrode of the battery 12 is connected to the low potential wiring 22. The three output wirings 24a to 24c are connected to the motor 14. The inverter circuit 10 has six switching circuits 30. Three series circuits are configured, in which two switching circuits 30 are connected in series. Each series circuit is connected between the high potential wiring 20 and the low potential wiring 22. In each series circuit, a corresponding one output wiring 24 is connected to a connection point of two switching circuits 30 connected in series. The switching of each switching circuit 30 switches the current path between a conductive state and a cut-off state. That is, the switching circuit 30 connected to the high potential wiring 20 switches between a conductive state and a cut-off state between the high potential wiring 20 and the output wiring 24. The switching circuit 30 connected to the low potential wiring 22 switches between a conductive state and a cut-off state between the output wiring 24 and the low potential wiring 22. The switching of each switching circuit 30 outputs three-phase AC power to the output wirings 24a to 24c.

[0029] FIG. 2 shows a circuit diagram of each switching circuit 30. The switching circuit 30 has switching elements 31 and 32. In this embodiment, the switching elements 31 and 32 are n-channel type field effect transistors (FETs). For example, the switching elements 31 and 32 may be n-channel type metal oxide semiconductor field effect transistors (MOSFETs). However, in other embodiments, the switching elements 31 and 32 may be p-channel type FETs or other gate-type transistors. The switching elements 31 and 32 are connected in parallel between a wiring 36 and a wiring 38. That is, the drain of the switching element 31 and the drain of the switching element 32 are connected to the wiring 36, and the source of the switching element 31 and the source of the switching element 32 are connected to the wiring 38. The wiring 36 is a wiring that connects the switching circuit 30 to a wiring on the high potential side, and the wiring 38 is a wiring that connects the switching circuit 30 to a wiring on the low potential side. For example, when the switching circuit 30 is a switching circuit connected to the high-potential wiring 20, the wiring 36 is the wiring 25 shown in FIG. 1 (i.e., the wiring that connects the high-potential wiring 20 and the switching circuit 30), and the wiring 38 is the wiring 26 shown in FIG. 1 (i.e., the wiring that connects the switching circuit 30 and the output wiring 24). When the switching circuit 30 is a switching circuit connected to the low-potential wiring 22, the wiring 36 is the wiring 26 shown in FIG. 1 (i.e., the wiring that connects the output wiring 24 and the switching circuit 30), and the wiring 38 is the wiring 27 shown in FIG. 1 (i.e., the wiring that connects the switching circuit 30 and the low-potential wiring 22). When the switching element 31 is turned on, a current flows from the wiring 36 to the wiring 38 via the switching element 31. When the switching element 32 is turned on, a current flows from the wiring 36 to the wiring 38 via the switching element 32.

[0030] A diode 33 is connected to the switching element 31. The anode of the diode 33 is connected to the source of the switching element 31, and the cathode of the diode 33 is connected to the drain of the switching element 31. A diode 34 is connected to the switching element 32. The anode of the diode 34 is connected to the source of the switching element 32, and the cathode of the diode 34 is connected to the drain of the switching element 32.

[0031] The switching circuit 30 has a gate control circuit 40. The gate control circuit 40 controls the potential of the gate G1 of the switching element 31 (hereinafter referred to as gate potential Vg1) and the potential of the gate G2 of the switching element 32 (hereinafter referred to as gate potential Vg2). The gate control circuit 40 has a first control circuit 50, a second control circuit 60, and a control IC 70. In the following description, the potential of each part will be described assuming that the potential of the wiring 38 is 0V (i.e., ground).

[0032] The first control circuit 50 has a power supply wiring 51, a gate power supply 52, a gate-on switch 53, a gate-on resistor 54, a gate-off resistor 55, and a gate-off switch 56. The gate power supply 52 applies a potential VCC1 to the power supply wiring 51. The gate-on switch 53 and the gate-on resistor 54 are connected in series between the power supply wiring 51 and the gate G1. The gate-on switch 53 switches in response to a signal Sig1 input from the control IC 70. As shown in FIG. 3, the signal Sig1 is a signal that changes between a potential HIGH and a potential LOW. The gate-on switch 53 is turned on when the signal Sig1 is HIGH, and is turned off when the signal Sig1 is LOW. When the gate-on switch 53 is turned on, a current flows from the power supply wiring 51 to the gate G1 through the gate-on switch 53 and the gate-on resistor 54, and the gate G1 is charged. The gate-off resistor 55 and the gate-off switch 56 are connected in series between the gate G1 and the ground. The gate-off switch 56 switches in response to a signal Sig1 input from the control IC 70. The gate-off switch 56 is turned off when the signal Sig1 is HIGH, and turned on when the signal Sig1 is LOW. When the gate-off switch 56 is turned on, a current flows from the gate G1 to the ground via the gate-off resistor 55 and the gate-off switch 56, and the gate G1 is discharged. Therefore, when the signal Sig1 is HIGH, the gate-on switch 53 is turned on and the gate-off switch 56 is turned off, so that a potential VCC1 is applied to the gate G1. When the signal Sig1 is LOW, the gate-on switch 53 is turned off and the gate-off switch 56 is turned on, so that 0 V is applied to the gate G1.

[0033] The second control circuit 60 has a power supply wiring 61, a gate power supply 62, a gate-on switch 63, a gate-on resistor 64, a gate-off resistor 65, and a gate-off switch 66. The gate power supply 62 applies a potential VCC2 to the power supply wiring 61. The gate-on switch 63 and the gate-on resistor 64 are connected in series between the power supply wiring 61 and the gate G2. The gate-on switch 63 switches in response to a signal Sig2 input from the control IC 70. As shown in FIG. 3, the signal Sig2 is a signal that changes between a potential HIGH and a potential LOW. The gate-on switch 63 is turned on when the signal Sig2 is HIGH, and is turned off when the signal Sig2 is LOW. When the gate-on switch 63 is turned on, a current flows from the power supply wiring 61 to the gate G2 via the gate-on switch 63 and the gate-on resistor 64, and the gate G2 is charged. The gate-off resistor 65 and the gate-off switch 66 are connected in series between the gate G2 and ground. The gate-off switch 66 switches in response to a signal Sig2 input from the control IC 70. The gate-off switch 66 is turned off when the signal Sig2 is HIGH, and turned on when the signal Sig2 is LOW. When the gate-off switch 66 is turned on, a current flows from the gate G2 to the ground via the gate-off resistor 65 and the gate-off switch 66, and the gate G2 is discharged. Therefore, when the signal Sig2 is HIGH, the gate-on switch 63 is turned on and the gate-off switch 66 is turned off, so that a potential VCC2 is applied to the gate G2. When the signal Sig2 is LOW, the gate-on switch 63 is turned off and the gate-off switch 66 is turned on, so that 0 V is applied to the gate G2.

[0034] As described above, the control IC 70 inputs the signal Sig1 to the first control circuit 50 and inputs the signal Sig2 to the second control circuit 60. The control IC 70 also receives the signal Sig0 from the outside. The control IC 70 generates the signals Sig1 and Sig2 based on the signal Sig0. The switching circuit 30 also includes current detection circuits 41, 42, and 43 and a voltage detection circuit 44. The current detection circuit 41 detects the current I1 flowing through the switching element 31. The current detection circuit 42 detects the current I2 flowing through the switching element 32. The current detection circuit 43 detects the current I3 flowing through the wiring 38. The current I3 is equal to the sum of the currents I1 and I2. The current I3 is equal to the current (so-called phase current) flowing through the output wirings 24a to 24c shown in FIG. 1. Therefore, the current I3 may be detected by the output wirings 24a to 24c. The voltage detection circuit 44 detects the voltage Vds between the wiring 36 and the wiring 38 (i.e., the drain-source voltage of the switching elements 31 and 32). The value of the current I3 detected by the current detection circuit 43 and the value of the voltage Vds detected by the voltage detection circuit 44 are input to the control IC 70. In addition, the control IC 90 receives a judgment value obtained by comparing the current I1 detected by the current detection circuit 41 with the threshold Ith1 and a judgment value obtained by comparing the current I2 detected by the current detection circuit 42 with the threshold Ith2. Since the threshold Ith1 is set to a low value, the judgment value of the current I1 indicates whether or not the current I1 is flowing. Since the threshold Ith2 is set to a low value, the judgment value of the current I2 indicates whether or not the current I2 is flowing. The control IC 70 controls the signals Sig1 and Sig2 based on the judgment value of the current I1, the judgment value of the current I2, the current I3, and the voltage Vds that are input. Furthermore, the control IC 70 controls the gate power supplies 52 and 62 to change the potentials VCC1 and VCC2.

[0035] FIG. 3 shows the change of each value during the operation of the switching circuit 30. FIG. 3 shows a state in which avalanche breakdown does not occur. As shown in FIG. 3, the signal Sig0 input to the control IC 70 changes between the potential HIGH and the potential LOW. In a state in which avalanche breakdown does not occur, the control IC 70 generates the signals Sig1 and Sig2 so that the signals Sig1 and Sig2 have the same waveform as the signal Sig0. In addition, in a state in which avalanche breakdown does not occur, the control IC 70 controls the potentials VCC1 and VCC2 to the potential Von at all times. In addition, the potential Vth1 shown in FIG. 3 is the gate threshold of the switching element 31 (i.e., the minimum gate potential Vg1 required to turn on the switching element 31). In addition, the potential Vth2 shown in FIG. 3 is the gate threshold of the switching element 32 (i.e., the minimum gate potential Vg2 required to turn on the switching element 32). The gate threshold Vth1 is approximately equal to the gate threshold Vth2. As shown in FIG. 3, the gate threshold values ​​Vth1 and Vth2 are lower than the potential Von and higher than 0V.

[0036] When the signal Sig0 is in a HIGH state, both the signals Sig1 and Sig2 are HIGH. In this state, the gate control circuit 40 controls the gate potential Vg1 to the potential VCC1 (i.e., the potential Von) and controls the gate potential Vg2 to the potential VCC2 (i.e., the potential Von). Therefore, both the switching elements 31 and 32 are turned on, and the currents I1 and I2 flow approximately equally through the switching elements 31 and 32, respectively. Hereinafter, the period during which both the switching elements 31 and 32 are turned on is referred to as the on period Ton. After that, when the signal Sig0 falls from HIGH to LOW, both the signals Sig1 and Sig2 fall from HIGH to LOW. Then, the gate control circuit 40 lowers the gate potential Vg1 to 0V and the gate potential Vg2 to 0V. Therefore, the switching elements 31 and 32 are turned off. Therefore, the voltage Vds rises and the currents I1 and I2 fall to zero. In the following, the period during which both the switching elements 31 and 32 are off is referred to as the off period Toff. Also, the period during which the on period Ton transitions to the off period Toff is referred to as the turn-off period Ttoff. After that, when the signal Sig0 rises from LOW to HIGH, both the signals Sig1 and Sig2 rise from LOW to HIGH. Then, the gate control circuit 40 raises the gate potential Vg1 to the potential Von and raises the gate potential Vg2 to the potential Von. Therefore, the switching elements 31 and 32 are turned on. As a result, the voltage Vds decreases and the currents I1 and I2 increase. That is, the on period Ton occurs again. In the following, the period during which the off period Toff transitions to the on period Ton is referred to as the turn-on period Tton. As shown in FIG. 3, the gate control circuit 40 controls the switching elements 31 and 32 so that the on period Ton and the off period Toff alternately repeat.

[0037] As shown in FIG. 3, in the turn-off period Ttoff, the voltage Vds may suddenly rise and generate a surge voltage Vs. The voltage Va1 shown in FIG. 3 is the avalanche voltage of the switching element 31, and the voltage Va2 shown in FIG. 3 is the avalanche voltage of the switching element 32. In FIG. 3, the avalanche voltage Va1 is lower than the avalanche voltage Va2, but the avalanche voltage Va1 may be higher than the avalanche voltage Va2. When the voltage applied between the drain and source of the switching element exceeds the avalanche voltage, avalanche breakdown occurs inside the switching element. When avalanche breakdown occurs, a current flows through the switching element even if the gate potential of the switching element is an off potential (i.e., a potential lower than the gate threshold). In FIG. 3, the surge voltage Vs is lower than the avalanche voltages Va1 and Va2, so no avalanche breakdown occurs.

[0038] When the surge voltage Vs reaches the avalanche voltage Va1 or Va2, avalanche breakdown may occur in one of the switching elements 31 and 32. In the following, the occurrence of avalanche breakdown in one of the switching elements 31 and 32 is referred to as one-sided breakdown. The operation at the time of one-sided breakdown will be described below. Note that the following will be described using an example in which the switching element 31 undergoes avalanche breakdown (i.e., the avalanche voltage Va1 is lower than the avalanche voltage Va2). When one-sided breakdown occurs, the switching circuit 30 of the first embodiment performs a specific potential application operation to reduce the load of the switching element in which the avalanche breakdown has occurred. First, as a comparative example, the change in each value at the time of one-sided breakdown when the specific potential application operation is not performed will be described with reference to FIG. 4.

[0039] In FIG. 4, at the start timing t1 of the turn-off period Ttoff, the gate potentials Vg1 and Vg2 drop, and the voltage Vds rises sharply. At timing t2 during the turn-off period Ttoff, the surge voltage causes the voltage Vds to reach the avalanche voltage Va1. Therefore, at timing t2, avalanche breakdown occurs in the switching element 31. When the avalanche breakdown occurs, the current I1 flows through the switching element 31, even though the gate potential Vg1 is controlled to 0V. Furthermore, while the avalanche breakdown occurs in the switching element 31, the voltage Vds is clamped to the avalanche voltage Va1. Furthermore, while the avalanche breakdown occurs in the switching element 31, almost no current I2 flows through the switching element 32. Therefore, while the avalanche breakdown occurs, a current is concentrated in the switching element 31, and a high load is applied to the switching element 31. Thereafter, the current I1 drops to almost zero and the voltage Vds drops to a value lower than the avalanche voltage Va1, whereby the avalanche breakdown of the switching element 31 ends and the switching element 31 turns off. In Fig. 4, the current I1 of the switching element 31 becomes very high at the timing t2 when the avalanche breakdown occurs. Since the voltage Vds is high at the timing t2, the switching element 31 generates a lot of heat and a high load is applied to the switching element 31. In the following, a switching element in which an avalanche breakdown has occurred is referred to as a breakdown element, and a switching element in which an avalanche breakdown has not occurred is referred to as a non-breakdown element.

[0040] Next, the specific potential application operation performed by the switching circuit 30 of the first embodiment will be described. FIG. 5 shows the process performed by the control IC 70 during the turn-off period Ttoff. The control IC 70 repeatedly performs the process of FIG. 5 during the turn-off period Ttoff. In step S2, the control IC 70 judges whether the current I3 flows or not. In step S4, the control IC 70 judges whether the voltage Vds is clamped or not (i.e., whether the time rate of change dVds / dt of the voltage Vds is equal to or less than a reference value or not). In step S6, the control IC 70 judges whether only one of the currents I1 and I2 flows or not. If the control IC 70 judges NO in any of steps S2, S4, and S6, one-sided breakdown has not occurred, so the control IC 70 continues normal operation in step S8. If the control IC 70 judges YES in all of steps S2, S4, and S6, one-sided breakdown has occurred, so the control IC 70 performs the specific potential application operation in step S10. In step S10, the control IC 70 performs a specific potential application operation on the non-breakdown elements. That is, the control IC 70 performs a specific potential application operation on the switching element 32 when the current I1 is flowing and the current I2 is not flowing, and performs a specific potential application operation on the switching element 31 when the current I2 is flowing and the current I1 is not flowing. The control IC 70 repeats steps S10 and S12 and continues the specific potential application operation until the condition of step S12 is satisfied. When the condition of step S12 is satisfied, the control IC 70 returns to normal operation in step S14.

[0041] If avalanche breakdown does not occur in either of the switching elements 31 and 32 during the turn-off period Ttoff (i.e., if one-sided breakdown does not occur), the control IC 70 judges NO in any of steps S2 to S6 and continues normal operation. Therefore, if one-sided breakdown does not occur, the control IC 70 operates as shown in Fig. 3 above. Fig. 6 shows the operation of the switching circuit 30 when one-sided breakdown occurs.

[0042] In FIG. 6, similarly to FIG. 4, the voltage Vds rises sharply at the start timing t1 of the turn-off period Ttoff, and the voltage Vds reaches the avalanche voltage Va1 at timing t2 during the turn-off period Ttoff. Since avalanche breakdown has not yet occurred in the period between timing t1 and timing t2, the control IC 70 judges NO in any of steps S2 to S6 in FIG. 5 and continues normal operation. When avalanche breakdown occurs in the switching element 31 at timing t2, the voltage Vds is clamped. Also, when avalanche breakdown occurs at timing t2, a current flows biasedly through the switching element 31, and the current I2 drops to zero. Note that the period during which the current I2 drops to zero is extremely short, so FIG. 6 does not show that the current I2 drops to zero at timing t2. Immediately after timing t2, the control IC 70 judges YES in steps S2 to S6 and executes step S10 (i.e., a specific potential application operation).

[0043] When the specific potential application operation is started at timing t2, the control IC 70 controls the gate power supply 62 for the switching element 32 (i.e., the non-breakdown element) to lower the potential VCC2 from the potential Von to the potential Vm. The potential Vm is lower than the potential Von and slightly higher than the gate threshold Vth2. At the same time, the control IC 70 raises the signal Sig2 from LOW to HIGH. Therefore, the gate potential Vg2 of the switching element 32 rises from 0V to the potential Vm. Since the potential Vm is higher than the gate threshold Vth2, the switching element 32 turns on and a current I2 flows. However, since the potential Vm is lower than the potential Von, the on-resistance when the switching element 32 turns on is relatively high. Therefore, immediately after timing t2, the current I1 flows through the switching element 31 and the current I2 flows through the switching element 32. That is, the current branches and flows through the switching element 31 and the switching element 32, and it is possible to prevent the current from flowing unevenly through the switching element 31. Therefore, the current I1 immediately after the timing t2 in Figure 6 is smaller than the current I1 immediately after the timing t2 in Figure 4. This reduces the load on the switching element 31 (that is, the breakdown element).

[0044] The control IC 70 repeats steps S10 and S12 in FIG. 5 during the execution of the specific potential application operation. As shown in FIG. 6, the current I1 of the switching element 31, which is a breakdown element, decreases after timing t2, and decreases to zero at timing t3. The control IC 70 continues the specific potential application operation by repeating steps S10 and S12 during the period between timing t2 and timing t3. When the current I1 decreases to zero at timing t3, the control IC 70 judges YES at step S12 and returns to normal operation at step S14. Therefore, at timing t3, the control IC 70 raises the potential VCC2 from the potential Vm to the potential Von, and lowers the signal Sig2 from HIGH to LOW. Therefore, at timing t3, the gate potential Vg2 decreases from the potential Vm to 0V, and the switching element 32 is turned off. Therefore, after timing t3, the current flows biasedly through the switching element 31. However, because the current I3 flowing after timing t3 is low, even if the current flows unevenly through the switching element 31, the load on the switching element 31 is small. After that, when the current I1 drops to zero and the voltage Vds becomes lower than the avalanche voltage Va1, the turn-off of the switching elements 31 and 32 is completed.

[0045] As described above, in the switching circuit 30 of the first embodiment, when one-sided breakdown occurs, a potential Vm lower than the potential Von and higher than the gate threshold Vth is applied to the gate of the non-breakdown element, and the non-breakdown element turns on. This reduces the load on the breakdown element. Also, in the switching circuit 30 of the first embodiment, the specific potential application operation is terminated at the stage where the current of the breakdown element drops to zero. This prevents the non-breakdown element from being kept on longer than necessary, and allows the switching circuit 30 to properly complete turning off.

[0046] During the specific potential application operation of Example 1, the potential Vm (i.e., the gate potential during the specific potential application operation) may be feedback-controlled based on the current flowing through the non-breakdown element. For example, when the breakdown element is a switching element 31, as shown in Fig. 7, half of the current I3 at the start timing t2 of the specific potential application operation may be set as a target current It, and the potential Vm may be controlled so that the current I2 coincides with the target current It.

[0047] In the above embodiment 1, the case where the switching element 31 undergoes avalanche breakdown (i.e., the avalanche voltage Va1 is lower than the avalanche voltage Va2) has been described as an example. However, the specific potential application operation can also be performed when the switching element 32 undergoes avalanche breakdown (i.e., the avalanche voltage Va2 is lower than the avalanche voltage Va1). In this case, since the non-breakdown element is the switching element 31, the specific potential application operation can be performed on the switching element 31. Also, in other embodiments described later, the specific potential application operation when the switching element 31 undergoes avalanche breakdown will be described as an example, but the specific potential application operation may also be performed when the switching element 32 undergoes avalanche breakdown. EXAMPLES

[0048] A switching circuit 30a according to the second embodiment shown in FIG.

[0049] The high frequency drive circuit 57 is provided on a wiring that inputs a signal Sig1 from the control IC 70 to the first control circuit 50. When the high frequency drive circuit 57 is not operating, the signal output from the control IC 70 is input as it is to the first control circuit 50 as the signal Sig1. When the high frequency drive circuit 57 is operating, the high frequency drive circuit 57 inputs a signal that oscillates at a high frequency (for example, about 10 MHz) between HIGH and LOW as the signal Sig1 to the first control circuit 50. The high frequency drive circuit 67 is provided on a wiring that inputs a signal Sig2 from the control IC 70 to the second control circuit 60. When the high frequency drive circuit 67 is not operating, the signal output from the control IC 70 is input as it is to the second control circuit 60 as the signal Sig2. When the high frequency drive circuit 67 is operating, the high frequency drive circuit 67 inputs a signal that oscillates at a high frequency (for example, about 10 MHz) between HIGH and LOW as the signal Sig2 to the second control circuit 60. In the switching circuit 30a of the second embodiment, the potential VCC1 is fixed to the potential Von, and the potential VCC2 is fixed to the potential Von. The other configurations of the switching circuit 30a of the second embodiment are the same as those of the switching circuit 30 of the first embodiment.

[0050] In the switching circuit 30a of the second embodiment, the control IC 70 executes a specific potential application operation when one-sided breakdown occurs, in the same manner as in the switching circuit 30 of the first embodiment.

[0051] FIG. 9 shows the change of each value during the operation of the switching circuit 30a of the second embodiment. During normal operation, the control IC 70 stops the high frequency drive circuits 57 and 67. Therefore, during normal operation, the signals Sig1 and Sig2 match the signal Sig0. In FIG. 9, as in FIG. 6, an avalanche breakdown occurs in the switching element 31 at timing t2. The control IC 70 detects the one-sided breakdown at timing t2 and executes a specific potential application operation. In the specific potential application operation, the control IC 70 operates the high frequency drive circuit 67 on the non-breakdown element side. Since the high frequency drive circuit 67 operates, the signal Sig2 oscillates at high frequency between HIGH and LOW immediately after timing t2. As a result, the second control circuit 60 repeatedly charges and discharges the gate G2 at high frequency. Therefore, the gate potential Vg2 is controlled to a potential lower than the potential Von and higher than the gate threshold value Vth2. Therefore, as in the first embodiment (i.e., FIG. 6), the switching element 32 is turned on in a state where the on-resistance is relatively high, and the current I2 flows. Therefore, immediately after the timing t2, the current branches and flows to the switching element 31 and the switching element 32, and the load applied to the switching element 31 (i.e., the breakdown element) is reduced. When the current I1 drops to zero during the execution of the specific potential application operation, the control IC 70 ends the specific potential application operation and returns to normal operation. That is, the control IC 70 stops the high frequency drive circuit 67 at the timing t3 in FIG. 9. Therefore, after the timing t3, the gate potential Vg2 drops to 0V, and the switching element 32 turns off. Therefore, after the timing t3, the current I1 flows to the switching element 31. Thereafter, when the current I1 drops to zero and the voltage Vds becomes lower than the avalanche voltage Va1, the turn-off of the switching elements 31 and 32 is completed.

[0052] As described above, in the switching circuit 30a of the second embodiment, when one-sided breakdown occurs, a potential lower than the potential Von and higher than the gate threshold Vth is applied to the gate of the non-breakdown element, and the non-breakdown element turns on. This reduces the load on the breakdown element. In addition, in the switching circuit 30a of the second embodiment, the specific potential application operation is terminated when the current of the breakdown element drops to zero. This prevents the non-breakdown element from being kept on longer than necessary, and allows the switching circuit 30 to properly complete turning off. EXAMPLES

[0053] The switching circuit 30b of the third embodiment shown in FIG. 10 has a configuration in which a gate-on switch 53f, a gate-on resistor 54f, a gate-off resistor 55f, a gate-off switch 56f, a gate-on switch 63f, a gate-on resistor 64f, a gate-off resistor 65f, and a gate-off switch 66f are added to the switching circuit 30 of the first embodiment. The gate-on switch 53f and the gate-on resistor 54f are connected in series between the power supply wiring 51 and the gate G1. The resistance value of the gate-on resistor 54f is lower than the resistance value of the gate-on resistor 54. The gate-off resistor 55f and the gate-off switch 56f are connected in series between the gate G1 and the ground. The resistance value of the gate-off resistor 55f is lower than the resistance value of the gate-off resistor 55. The gate-on switch 63f and the gate-on resistor 64f are connected in series between the power supply wiring 61 and the gate G2. The resistance value of the gate-on resistor 64f is lower than the resistance value of the gate-on resistor 64. The gate-off resistor 65f and the gate-off switch 66f are connected in series between the gate G2 and the ground. The resistance value of the gate-off resistor 65f is lower than the resistance value of the gate-off resistor 65. The gate-on switch 53f, the gate-off switch 56f, the gate-on switch 63f, and the gate-off switch 66f are controlled by the control IC 70.

[0054] In the switching circuit 30b of the third embodiment, the control IC 70 can charge the gate G1 by turning on the gate-on switch 53, and can also charge the gate G1 by turning on the gate-on switch 53f. Since the resistance value of the gate-on resistor 54f is lower than the resistance value of the gate-on resistor 54, when the gate-on switch 53f is turned on, the charging current that charges the gate G1 becomes higher than when the gate-on switch 53 is turned on, and the gate potential Vg1 can be increased more quickly.

[0055] In the switching circuit 30b of the third embodiment, the control IC 70 can discharge the gate G1 by turning on the gate-off switch 56, and can also discharge the gate G1 by turning on the gate-off switch 56f. Since the resistance value of the gate-off resistor 55f is lower than the resistance value of the gate-off resistor 55, when the gate-off switch 56f is turned on, the discharge current that discharges the gate G1 becomes higher than when the gate-off switch 56 is turned on, and the gate potential Vg1 can be reduced more quickly.

[0056] In the switching circuit 30b of the third embodiment, the control IC 70 can charge the gate G2 by turning on the gate-on switch 63, and can also charge the gate G2 by turning on the gate-on switch 63f. Since the resistance value of the gate-on resistor 64f is lower than the resistance value of the gate-on resistor 64, when the gate-on switch 63f is turned on, the charging current that charges the gate G2 becomes higher than when the gate-on switch 63 is turned on, and the gate potential Vg2 can be increased more quickly.

[0057] In the switching circuit 30b of the third embodiment, the control IC 70 can discharge the gate G2 by turning on the gate-off switch 66, and can also discharge the gate G2 by turning on the gate-off switch 66f. Since the resistance value of the gate-off resistor 65f is lower than the resistance value of the gate-off resistor 65, when the gate-off switch 66f is turned on, the discharge current that discharges the gate G2 becomes higher than when the gate-off switch 66 is turned on, and the gate potential Vg2 can be reduced more quickly.

[0058] In the third embodiment, the control IC 70 uses the gate-on switch 63f and the gate-off switch 66f to control the gate potential Vg2 when performing a specific potential application operation on the switching element 32, and uses the gate-on switch 63 and the gate-off switch 66 to control the gate potential Vg2 during normal operation. Therefore, the charge / discharge current of the gate G2 becomes larger during the specific potential application operation than during normal operation. That is, the gate charge current of the gate G2 at the start timing t2 of the specific potential application operation becomes larger than the gate charge current of the gate G2 during the turn-on period Tton. Also, the gate discharge current of the gate G2 at the end timing t3 of the specific potential application operation becomes larger than the gate discharge current of the gate G2 during the turn-off period Ttoff when one-sided breakdown does not occur. Therefore, the gate potential Vg2 changes faster during the specific potential application operation than during normal operation. According to this configuration, the switching element 32 can be switched quickly during the specific potential application operation when avalanche breakdown occurs in the switching element 31, and the switching element 31 can be appropriately protected. Furthermore, in normal operation, the switching element 32 can be switched at a relatively slow speed, and the loss occurring in the switching element 32 and the second control circuit 60 can be reduced.

[0059] In the third embodiment, the control IC 70 controls the gate potential Vg1 using the gate-on switch 53f and the gate-off switch 56f when performing a specific potential application operation on the switching element 31, and controls the gate potential Vg1 using the gate-on switch 53 and the gate-off switch 56 during normal operation. Therefore, the charge / discharge current of the gate G1 becomes larger during the specific potential application operation than during normal operation. Therefore, the gate potential Vg1 changes faster during the specific potential application operation than during normal operation. With this configuration, the switching element 31 can be switched quickly in the specific potential application operation when avalanche breakdown occurs in the switching element 32, and the switching element 32 can be appropriately protected. Moreover, the switching element 31 can be switched relatively slowly during normal operation, and the loss occurring in the switching element 31 and the first control circuit 50 can be reduced.

[0060] In the third embodiment, the resistance value of the gate charge / discharge circuit used during the specific potential application operation is made lower than the resistance value of the gate charge / discharge circuit used during normal operation, thereby making the charge / discharge current during the specific potential application operation higher than the gate charge / discharge current during normal operation. However, when the gate charge / discharge current is controlled by a constant current circuit, the charge / discharge current during the specific potential application operation may be made higher than the gate charge / discharge current during normal operation by changing the set value of the gate charge / discharge current in the constant current circuit. In addition, the charge / discharge current during the specific potential application operation may be made higher than the gate charge / discharge current during normal operation by other configurations. In addition, the configuration in which the charge / discharge current during the specific potential application operation is made higher than the gate charge / discharge current during normal operation may be applied to the above-mentioned second embodiment or to the below-mentioned fourth embodiment. EXAMPLES

[0061] In the switching circuits 30, 30a, and 30b of the above-mentioned first to third embodiments, a one-sided breakdown is detected during the turn-off period Ttoff, and a specific potential application operation is performed. For this reason, the control IC 70 needs to perform the one-sided breakdown detection and the specific potential application operation at high speed. In contrast, the switching circuit 30c of the third embodiment shown in FIG. 11 judges whether or not a one-sided breakdown occurs based on the state of the switching elements 31 and 32 before the turn-off period Ttoff. If the switching circuit 30c judges that a one-sided breakdown will occur, it performs the specific potential application operation in the subsequent turn-off period Ttoff.

[0062] As shown in FIG. 11, the switching circuit 30c of the third embodiment has a storage device 72. The storage device 72 stores avalanche voltage data 72a and map data 72b for a specific potential application operation. The avalanche voltage data 72a is data indicating which of the avalanche voltage Va1 of the switching element 31 and the avalanche voltage Va2 of the switching element 32 is lower. When the switching circuit 30c is manufactured, the avalanche voltages Va1 and Va2 are measured, and the avalanche voltage data 72a is written to the storage device 72 based on the measurement results. The map data 72b for a specific potential application operation is data that specifies a method of controlling the gate potential of a non-breakdown element during a specific potential application operation.

[0063] FIG. 12 shows the process executed by the control IC 70 during normal operation in the fourth embodiment. The control IC 70 repeatedly executes the process of FIG. 12 during normal operation. Step S42 is executed during the off period Toff (for example, timing ta in FIG. 13). In step S42, the control IC 70 judges whether the voltage Vds is higher than the reference value Vdsth. Also, step S44 is executed during the on period Ton (for example, timing tb in FIG. 13). In step S44, the control IC 70 judges whether the current I3 (i.e., the total current flowing through the switching elements 31 and 32) is higher than the reference value I3th. If NO is judged in either step S42 or S44, the control IC 70 continues normal operation in step S46. In this case, the control IC 70 does not execute the specific potential application operation in the subsequent turn-off period Ttoff. If the determination is YES at both steps S42 and S44, the control IC 70 executes a specific potential application operation in the subsequent turn-off period Ttoff (ie, step S48).

[0064] If the voltage Vds is high during the off period Toff and the current I3 is high during the on period Ton, a high surge voltage is generated during the subsequent turn-off period Ttoff, which makes it easy for one-sided breakdown to occur. Therefore, based on the voltage Vds during the off period Toff and the current I3 during the on period Ton, it is possible to predict whether one-sided breakdown will occur during the subsequent turn-off period Ttoff. Therefore, as shown in FIG. 12, if the determination is YES in both steps S42 and S44, a specific potential application operation is performed during the subsequent turn-off period Ttoff, thereby protecting the breakdown element during the turn-off period Ttoff.

[0065] When the control IC 70 determines to execute the specific potential application operation in step S48, it executes the process shown in Fig. 14. Steps S50 and S52 are executed before the start of the turn-off period Ttoff.

[0066] In step S50, the control IC 70 identifies a breakdown element (i.e., a switching element in which avalanche breakdown occurs in the subsequent turn-off period Ttoff) and a non-breakdown element based on the avalanche voltage data 72a. For example, in the case of Fig. 13, the avalanche voltage data 72a indicates that the avalanche voltage Va1 is lower than the avalanche voltage Va2. Therefore, the control IC 70 identifies the switching element 31 as a breakdown element and the switching element 32 as a non-breakdown element.

[0067] In step S52, the control IC 70 determines a waveform to be applied as the gate potential of the non-breakdown element during the turn-off period Ttoff (i.e., gate potential Vg2 in the case of FIG. 13) based on the map data 72b for specific potential application operation. Here, the control IC 70 determines the potential Vp, application time Tp, and delay time Td shown in FIG. 13. The map data 72b for specific potential application operation is map data configured to determine an appropriate potential Vp, application time Tp, and delay time Td from the voltage Vds detected in step S42 and the current I3 detected in step S44.

[0068] The potential Vp is the potential applied to the gate of the non-breakdown element during the specific potential application operation (i.e., the level of the specific potential). The specific potential application operation map data 72b is configured to calculate a higher potential Vp as the current I3 increases. The larger the current I3, the higher the surge voltage generated, and the more likely it is that a large avalanche current will flow through the breakdown element. Therefore, by increasing the potential Vp as the current I3 increases, the load on the breakdown element can be reduced appropriately.

[0069] The application time Tp is the length of time during which the specific potential application operation is performed. In other words, the application time Tp is the pulse width of the waveform of the potential Vp. The specific potential application operation map data 72b is configured so that the larger the current I3 is, the longer the application time Tp is calculated, and the higher the voltage Vds is, the longer the application time Tp is calculated. Since the larger the current I3 and the higher the voltage Vds are, the longer the avalanche current flows through the breakdown element, the load on the breakdown element can be suitably reduced by making the application time Tp longer as the current I3 is larger and the voltage Vds is higher.

[0070] The delay time Td is the time interval from the start timing t1 of the turn-off period Ttoff (i.e., the timing when the gate potentials Vg1, Vg2 are lowered from HIGH) to the start timing t2 of the specific potential application operation. The specific potential application operation map data 72b is configured to calculate a shorter delay time Td as the current I3 is larger. Since the avalanche current flows at an earlier timing as the current I3 is larger, the load applied to the breakdown element can be suitably reduced by shortening the delay time Td as the current I3 is larger.

[0071] After determining the waveform of the potential to be applied to the gate of the non-breakdown element in step S52, the control IC 70 executes step S54 simultaneously with the start timing t1 of the turn-off period Ttoff. That is, the control IC 70 controls the gate potential of the non-breakdown element (i.e., the gate potential Vg2 in FIG. 13) so that the gate potential has the waveform determined in step S52 at the start timing t1 of the turn-off period Ttoff. Therefore, when one-sided breakdown occurs during the turn-off period Ttoff, the current flows in a distributed manner in the switching elements 31 and 32, and the load applied to the breakdown element (i.e., the switching element 31 in FIG. 13) is reduced.

[0072] As described above, the switching circuit 30c of the fourth embodiment determines whether or not one-sided breakdown occurs based on the state of the switching elements 31 and 32 before the turn-off period Ttoff, and if it is determined that one-sided breakdown occurs, executes the specific potential application operation in the subsequent turn-off period Ttoff. Therefore, according to this configuration, the response speed required of the switching circuit 30c is lower than when avalanche breakdown is detected during the turn-off period. In other words, according to this configuration, even a switching circuit with a low response speed can execute the specific potential application operation.

[0073] 13, the potential Vp is output by lowering the potential VCC2 output by the gate power supply 62 during the specific potential application operation, as in the first embodiment. However, the potential Vp may be output by oscillating the signal Sig2 at a high frequency, as in the second embodiment.

[0074] In the fourth embodiment, the control IC 70 adjusts the potential Vp, the application time Tp, and the delay time Td based on the voltage Vds and the current I3. However, the potential Vp, the application time Tp, and the delay time Td may be fixed values ​​without adjustment. Even with such a configuration, it is possible to reduce the load on the breakdown element.

[0075] In the fourth embodiment, the control IC 70 determines whether to execute the specific potential application operation based on the voltage Vds and the current I3. However, the control IC 70 may determine whether to execute the specific potential application operation in the subsequent turn-off period Ttoff based only on the current I3 during the on-period Ton.

[0076] In the fourth embodiment, the storage device 72 stores avalanche voltage data 72a (i.e., data indicating which of the avalanche voltage Va1 of the switching element 31 and the avalanche voltage Va2 of the switching element 32 is lower). However, in the manufacturing process of the switching circuit 30c, the switching elements 31 and 32 may be selected and mounted so that the avalanche voltage Va1 is lower than the avalanche voltage Va2 (or vice versa). In this case, the storage device 72 does not need to store the avalanche voltage data 72a.

[0077] The following are features of the switching circuit described in this specification: (Item 1) A switching circuit comprising: A first wiring (36); A second wiring (38); a first switching element (31) connected between the first wiring and the second wiring; a second switching element (32) connected in parallel to the first switching element between the first wiring and the second wiring; a gate control circuit (40) connected to a gate of the first switching element and a gate of the second switching element; having the gate control circuit controls a gate potential of the first switching element and a gate potential of the second switching element so that an on-period (Ton) in which both the first switching element and the second switching element are on and an off-period (Toff) in which both the first switching element and the second switching element are off are alternately repeated; the gate control circuit executes a specific potential application operation of applying a specific potential higher than a gate threshold to a gate of a non-breakdown element that has not undergone avalanche breakdown, of the first switching element and the second switching element, when one-side breakdown occurs in which either one of the first switching element and the second switching element undergoes avalanche breakdown during a turn-off period (Ttoff) transitioning from the on period to the off period; Switching circuit. (Item 2) 2. The switching circuit according to item 1, wherein the specific potential is lower than a gate potential (Von) of the non-breakdown element during the on-period. (Item 3) a power supply (62) for charging the gate of the non-breakdown element; In the specific potential application operation, the output voltage (VCC2) of the power supply is lowered compared to the ON period. Item 2. A switching circuit according to item 2. (Item 4) 4. The switching circuit according to item 3, wherein the gate control circuit feedback controls the output voltage of the power supply based on a main current (I2) flowing through the non-breakdown element during the specific potential application operation. (Item 5) 3. The switching circuit according to item 2, wherein the gate control circuit controls the specific potential to a value lower than the gate potential of the non-breakdown element during the on-period by alternately repeating charging and discharging of the gate of the non-breakdown element in the specific potential application operation. (Item 6) 6. The switching circuit according to any one of items 1 to 5, wherein the gate control circuit stops the application of the specific potential when a main current (I1) flowing through one of the first switching element and the second switching element, which is undergoing avalanche breakdown, drops to zero. (Item 7) 7. The switching circuit according to any one of items 1 to 6, wherein the gate control circuit determines whether or not to execute the specific potential application operation based on a voltage (Vds) between the first wiring and the second wiring, a main current (I1) of the first switching element, and a main current (I2) of the second switching element during the turn-off period. (Item 8) 7. The switching circuit according to any one of claims 1 to 6, wherein the gate control circuit determines whether or not to execute the specific potential application operation in the subsequent turn-off period based on a total current (I3) flowing through the first switching element and the second switching element during the on period. (Item 9) 9. The switching circuit according to item 8, wherein the gate control circuit increases the specific potential (Vp) in the subsequent specific potential application operation as the total current in the on-period increases. (Item 10) 10. The switching circuit according to item 8 or 9, wherein the gate control circuit increases the application time (Tp) of the specific potential in the subsequent specific potential application operation as the total current in the on-period increases. (Item 11) 11. The switching circuit according to any one of items 8 to 10, wherein the gate control circuit shortens an interval (Td) between a timing at which the gate potential of the non-breakdown element is lowered and a timing at which the specific potential application operation is started in the subsequent turn-off period, as the total current in the on-period is larger. (Item 12) 12. The switching circuit according to any one of claims 8 to 11, wherein the gate control circuit determines whether or not to execute the specific potential application operation in the subsequent turn-off period based on a voltage between the first wiring and the second wiring in the off period. (Item 13) Item 13. The switching circuit according to item 12, wherein the gate control circuit extends an application time of the specific potential in the subsequent specific potential application operation as the voltage between the first wiring and the second wiring during the off period increases. (Item 14) 14. The switching circuit according to any one of items 1 to 13, wherein a gate charging current of the non-breakdown element at the start of application of the specific potential is greater than a gate charging current of the non-breakdown element at the transition from the off period to the on period. (Item 15) 15. The switching circuit according to any one of items 1 to 14, wherein a gate discharge current of the non-breakdown element when the application of the specific potential is stopped is larger than a gate discharge current of the non-breakdown element during the turn-off period when the one-sided breakdown does not occur. (Item 16) the gate control circuit has a storage unit (72) that stores which of the avalanche voltage of the first switching element and the avalanche voltage of the second switching element is higher, the gate control circuit controls, when the one-sided breakdown occurs, one of the first switching element and the second switching element, which has a higher avalanche voltage, as the non-breakdown element. 16. A switching circuit according to any one of items 1 to 15.

[0078] Although the embodiments have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or drawings achieves multiple objectives simultaneously, and achieving one of the objectives itself has technical utility. [Explanation of symbols]

[0079] 30: switching circuit, 31: switching element, 32: switching element, 50: first control circuit, 60: second control circuit, 70: control IC

Claims

1. A switching circuit comprising: A first wiring (36); A second wiring (38); a first switching element (31) connected between the first wiring and the second wiring; a second switching element (32) connected in parallel to the first switching element between the first wiring and the second wiring; a gate control circuit (40) connected to a gate of the first switching element and a gate of the second switching element; having the gate control circuit controls a gate potential of the first switching element and a gate potential of the second switching element so that an on-period (Ton) in which both the first switching element and the second switching element are on and an off-period (Toff) in which both the first switching element and the second switching element are off are alternately repeated; the gate control circuit executes a specific potential application operation of applying a specific potential higher than a gate threshold to a gate of a non-breakdown element that has not undergone avalanche breakdown, of the first switching element and the second switching element, when one-side breakdown occurs in which either one of the first switching element and the second switching element undergoes avalanche breakdown during a turn-off period (Ttoff) transitioning from the on period to the off period; Switching circuit.

2. 2. The switching circuit according to claim 1, wherein the specific potential is lower than a gate potential (Von) of the non-breakdown element during the on-period.

3. a power supply (62) for charging the gate of the non-breakdown element; In the specific potential application operation, the output voltage (VCC2) of the power supply is lowered compared to the ON period.

3. The switching circuit of claim 2.

4. 4. The switching circuit according to claim 3, wherein the gate control circuit feedback controls the output voltage of the power supply based on a main current (I2) flowing through the non-breakdown element during the specific potential application operation.

5. 3. The switching circuit according to claim 2, wherein the gate control circuit, in the specific potential application operation, controls the specific potential to a value lower than the gate potential of the non-breakdown element during the on-period by alternately repeating charging and discharging of the gate of the non-breakdown element.

6. 2. The switching circuit according to claim 1, wherein the gate control circuit stops application of the specific potential when a main current (I1) flowing through a breakdown element that is in avalanche breakdown of one of the first switching element and the second switching element drops to zero.

7. The switching circuit according to any one of claims 1 to 6, wherein the gate control circuit determines whether or not to execute the specific potential application operation based on a voltage (Vds) between the first wiring and the second wiring, a main current (I1) of the first switching element, and a main current (I2) of the second switching element during the turn-off period.

8. The switching circuit according to any one of claims 1 to 6, wherein the gate control circuit determines whether or not to execute the specific potential application operation in the subsequent turn-off period based on a total current (I3) flowing through the first switching element and the second switching element during the on period.

9. 9. The switching circuit according to claim 8, wherein the gate control circuit increases the specific potential (Vp) in the subsequent specific potential application operation as the total current in the on-period increases.

10. 9. The switching circuit according to claim 8, wherein the gate control circuit extends an application time (Tp) of the specific potential in the subsequent specific potential application operation as the total current in the on-period increases.

11. 9. The switching circuit according to claim 8, wherein the gate control circuit shortens an interval (Td) between a timing at which the gate potential of the non-breakdown element is lowered and a timing at which the specific potential application operation is started in the subsequent turn-off period, as the total current in the on-period is larger.

12. 9. The switching circuit according to claim 8, wherein the gate control circuit determines whether or not to execute the specific potential application operation in the subsequent turn-off period based on a voltage between the first wiring and the second wiring during the off period.

13. 13. The switching circuit according to claim 12, wherein the gate control circuit extends an application time of the specific potential in the subsequent specific potential application operation as the voltage between the first wiring and the second wiring during the off period increases.

14. 2. The switching circuit according to claim 1, wherein a gate charging current of said non-breakdown element at the start of application of said specific potential is greater than a gate charging current of said non-breakdown element at the transition from said off period to said on period.

15. 2. The switching circuit according to claim 1, wherein a gate discharge current of the non-breakdown element when application of the specific potential is stopped is greater than a gate discharge current of the non-breakdown element during the turn-off period when the one-sided breakdown does not occur.

16. the gate control circuit has a storage unit (72) for storing which of the avalanche voltage of the first switching element and the avalanche voltage of the second switching element is higher; the gate control circuit controls, when the one-sided breakdown occurs, one of the first switching element and the second switching element, which has a higher avalanche voltage, as the non-breakdown element.

2. The switching circuit of claim 1.

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