JP2015037226A - Gate drive circuit for switching element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem of the possibility of applying a rated potential or higher to a gate of a switching element, and the problem of a greater loss due to a gradual change in the waveform of a control signal during a change in the switching element from on to off.SOLUTION: A gate voltage drive circuit for a switching element of the present invention, which is a gate drive circuit for driving a gate of a switching transistor as a switching element, includes: a control voltage generation circuit for generating a voltage capable of polarity reversal; a negative potential applying capacitor; a capacitor application circuit for applying a constant voltage across the negative potential applying capacitor for a predetermined period; and a Vgs limiting circuit connected between a gate electrode and a source electrode of the switching transistor.

Description

  The present invention relates to a gate drive circuit for a switching element used in a power control circuit.

  A power control circuit using a switching element such as an inverter for driving a motor or a power conditioner of a photovoltaic power generation apparatus is currently widely used because of its high power efficiency. Furthermore, in order to prevent global warming and to solve the energy shortage after the Great East Japan Earthquake, further enhancement of the efficiency of the power control circuit is strongly desired.

  As a key to realizing power saving, wide gap semiconductors such as silicon carbide (hereinafter referred to as “SiC”) and gallium nitride (hereinafter referred to as “GaN”) are expected to be used in power control circuits. SiC semiconductors, which are considered to be put to practical use faster than GaN, have a breakdown voltage about 10 times that of silicon semiconductors and a thermal conductivity about 3 times that of silicon semiconductors. Therefore, if a switching element is realized using SiC, a low on-resistance exceeding the limit value of the silicon semiconductor can be realized. Furthermore, since the power loss can be reduced by the low on-resistance and the high-temperature operation is possible, the device can be expected to be downsized. However, it is hard to say that SiC semiconductors are still in practical use due to manufacturing difficulties. Currently, mass production from device manufacturers is finally starting.

  Among the devices using SiC, the development of diodes and junction gate type field effect transistors (hereinafter referred to as “JFETs”) is ahead, and the metal-oxide-semiconductor field effect expected as the favorite of switching devices. Development of type transistors (hereinafter referred to as “MOSFETs”) is delayed. Among them, the development of a SiC-static induction transistor (hereinafter referred to as “SIT”), which is a normally-on type element, is ahead. SiC-SIT is suitable for low on-resistance because the impurity diffusion coefficient in SiC can be extremely low. In general, normally-on type switching elements are said to be unsuitable for application to power control circuits because it is difficult to ensure safety. This is because a normally-on type switching element is in a conductive state when the input signal is zero (the gate-source voltage is 0 V), and a normally-on type element is used to form a bridge circuit. This is because the positive and negative terminals of the power supply having no input signal to the switching element are short-circuited.

  Therefore, in order to apply the normally-on type element to the power control circuit, a drive circuit for safely driving the normally-on type switching element is required. Even a switching element such as a MOSFET using Si or an insulated gate bipolar transistor (hereinafter referred to as “IGBT”) requires a drive circuit, and so far many drive circuit developments and proposals have been made. Yes. However, there are few proposals of drive circuits that can be used for normally-on type switching elements.

  Non-Patent Document 1 discloses a drive circuit that is assumed to be applied to a normally-on type SiC-JFET switching element. This is a circuit that generates a negative power source from a positive single power source using a polar transformer and applies it between the gate and the source, and can shut off a normally-on type switching element.

  FIG. 1 is a circuit diagram of a gate driving circuit described in Non-Patent Document 1. FIG. 2 shows a circuit in which the MOS transistor M1 in which the ASIC portion is provided between the transformer and the ground is used for circuit simulation. FIG. 3 shows a simulation result of the control signal potential Vcnt applied to the gate of M1 and the voltage Vgs applied as an output between the gate and source of the switching element M0. When Vcnt is sufficiently large and M1 conducts, Vgs becomes a negative voltage and M0 is cut off. On the other hand, during the period when Vcnt becomes 0 and M1 is cut off, M0 conducts only during a certain period depending on the circuit constant.

  Next, the operation of this gate drive circuit will be described in detail. First, when M1 is short-circuited at time t0, Vin is applied to the primary winding of the transformer, and a voltage Vs having the opposite polarity is generated in the secondary winding of the transformer. At this time, a reverse voltage is applied to the parasitic diode existing between the gate and the source of M0, and the current Is does not flow and energy is accumulated in the transformer. Vgs is negative, and when M0 is normally on, the threshold voltage Vth is negative. However, if Vs is a sufficiently low negative value, M0 is cut off.

  In this case, when M1 is short-circuited at time t0, a large current instantaneously flows in the secondary winding of the transformer, and a negative voltage with a large absolute value may be instantaneously applied to the gate of M0. There is a problem that the allowable rated voltage may be exceeded.

  Next, when M1 is cut off at time t1, Is flows in the direction shown in FIG. 2 by the energy stored in the transformer. At this time, Vgs becomes a forward voltage of the parasitic diode existing between the gate and the source of M0, M0 becomes conductive, and-(VD1 + | VZ1 |) is charged to C1.

  Is decreases with time and becomes 0 at time t2. At this time, since the voltages at both ends of Vgs and C1 are different from each other, a reverse current flows through the secondary winding of the transformer from the next moment, and Vgs decreases. Since this charge movement flows depending on the gate-source capacitance of the switching transistor M0 to be controlled and the value of the secondary inductor of the transformer, Vgs decreases with a slope as shown in FIG.

  M0 remains conductive until Vgs falls below the threshold voltage of M0, and M0 shuts off when Vgs falls below the threshold voltage. Ideally, the switching element does not consume power when it is completely cut off or conducting. This is because the current flowing through the switching element is zero when cut off, and the voltage across the switch is zero when conducting.

  However, while the switch element transitions from the conductive state to the cut-off state, there is a time during which the flowing current and the voltage across the switch are non-zero, and power is consumed during that period. This loss is called “switching loss”. In the above-described conventional gate drive circuit, Vgs changes gradually, so that the time for the switching element to change from conduction to cutoff becomes long. For this reason, there also exists a problem that the switching loss of a switching element will become large. In addition, since the time during which the switching element to be driven is turned on depends on the circuit constant, there is a problem that the design becomes difficult if the maximum value of the duty ratio is determined by selecting the circuit constant.

R. Kelley, M.S. Mazzola, “SiC JFET gate driver design for use in DC / DCconverters” Proc. Of IEEE Applied Power Electronics Conference and Exposition, 2006

  As described above, in the gate drive circuit described in Non-Patent Document 1, there is a possibility that a potential higher than the rating may be applied to the gate of the switching element, and a control signal when the switching element changes from conduction to cutoff. However, there is a problem that the loss is large.

  A gate voltage driving circuit for a switching element according to the present invention is a gate driving circuit for driving a gate of a switching transistor as a switching element, a control voltage generating circuit for generating a voltage that reverses positive and negative, a capacitor for applying a negative potential, A capacitor applying circuit for applying a constant voltage between the electrodes of the negative potential applying capacitor for a predetermined period, and a Vgs limiting circuit connected between the gate electrode and the source electrode of the switching transistor, To do.

  According to the present invention, a voltage higher than the rated voltage is not applied to the gate potential of the switching transistor, and the loss of the loss is small because the waveform of the control signal when the switching transistor changes from conduction to cutoff is quickly changed. There is.

FIG. 4 is a circuit diagram of a conventional gate drive circuit described in Non-Patent Document 1. Is a circuit for simulating a gate drive circuit according to the prior art based on the gate drive circuit described in Non-Patent Document 1. These are graphs showing Vcnt and Vgs simulation waveforms of the gate drive circuit according to the prior art shown in FIG. These are the circuit diagrams for demonstrating embodiment of the gate drive circuit for switching elements by this invention. These are the circuit diagrams for demonstrating Example 1 of the gate drive circuit for switching elements by this invention. These are the figures which showed the simulation waveform for demonstrating Example 1 of the gate drive circuit for switching elements by this invention. These are the figures which showed the Vgs simulation waveform for demonstrating Example 1 of the gate drive circuit for switching elements by this invention, and are shown in contrast with the simulation waveform by a prior art. These are the circuit diagrams for demonstrating Example 2 of the gate drive circuit for switching elements by this invention. These are the figures which showed the simulation waveform for demonstrating Example 2 of the gate drive circuit for switching elements by this invention. These are the circuit diagrams for demonstrating Example 3 of the gate drive circuit for switching elements by this invention.

  Hereinafter, embodiments of the present invention will be described in detail.

  A schematic circuit diagram of a gate drive circuit for a switching element according to the present invention is shown in FIG. The control voltage generation circuit 1 connected to the constant voltage source 6 outputs a voltage Vs whose polarity is inverted between the nodes N1 and N2 by a control signal generated internally or given from the outside. The node N1 is connected to the output terminal OUTG via the current limiting circuit 5. On the other hand, the node N2 is connected to the output terminal OUTS through a parallel circuit of the negative potential applying capacitor 2 and the capacitor applying circuit 3. A Vgs limiting circuit 4 is connected between OUTG and OUTS. When actually used as a switching element drive circuit, OUTG is connected to the gate G of the switching transistor 7 which is a switching element, and OUTS is connected to the source S of the switching transistor 7. Here, each of the current limiting circuit 5, the Vgs limiting circuit 4, and the capacitor applying circuit 3 has a predetermined current-voltage characteristic, and exhibits a substantially constant voltage when forward and reverse currents exceeding a predetermined value flow. Such a circuit having voltage limiting characteristics. In particular, the capacitor application circuit 3 is a rectifying circuit that exhibits a constant voltage at a predetermined current value or more in the forward direction, but does not flow a current in the reverse direction. Further, the current limiting circuit 5 is not necessarily required depending on the allowable current and the cycle of the switching operation, and the node N1 may be directly connected to the output terminal OUTG. It is also possible to substitute a resistance element or a parasitic resistance connected in series to the gate of the switching transistor for stable operation.

  When Vs changes from negative to positive, the potential of N1 becomes larger than the potential of N2, and the current Is flows from N1 to N2 via the current limiting circuit 5, the Vgs limiting circuit 4, and the capacitor applying circuit 3. When Is flows in this direction, a predetermined voltage is generated at both ends of the Vgs limiting circuit 4, and this voltage is applied between the gate and source of the switching transistor 7. If the gate-source voltage Vgs is larger than the threshold voltage Vth of the switching transistor 7, the switching transistor 7 becomes conductive. At the same time, Is flows through the capacitor application circuit 3, whereby a predetermined voltage is generated at both ends of the capacitor application circuit 3. The potential is applied to both ends of the negative potential application capacitor 2, and charges corresponding to the capacitance of the capacitor are stored.

  On the other hand, when Vs changes from positive to negative, due to the rectification of the capacitor application circuit 3, no current flows through the capacitor application circuit 3, and the negative potential application capacitor 2 to the Vgs limiting circuit 4, current limiting circuit. A discharge current flows through 5. A voltage corresponding to a reverse current is generated at both ends of the Vgs limiting circuit 4, and the voltage is applied between the gate and source of the switching transistor 7. In this case, since the source potential is higher, Vgs becomes a negative value, and the switching transistor 7 can be shut off by setting it lower than the threshold voltage Vth of the switching transistor 7. When the switching transistor 7 is normally on, Vth has a negative value and must be set sufficiently low.

  Here, as the switching transistor, a junction gate type field effect transistor (hereinafter referred to as “J-FET”), a metal semiconductor junction type field effect transistor (hereinafter referred to as “MESFET”), an electrostatic induction transistor. (Hereinafter referred to as “SIT”), various transistors such as MOSFETs can be used.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 5 shows a circuit diagram of a gate drive circuit for a switching element according to this embodiment. In this embodiment, the control voltage generation circuit is composed of a depolarizing transformer T1 and a MOSFET M1 that functions as a control switch connected to the primary side. The capacitor application circuit is a circuit in which a normal diode D1 and a Zener diode Z1 are connected in series. The Vgs limiting circuit is a Zener diode Z3. Further, the current limiting circuit is a parallel connection circuit of a Zener diode Z2 and a resistor R1. Here, the resistance value of R1 is a relatively large resistance such that the current flowing in the normal bias state is smaller than the current flowing in Z2.

  The operation of this embodiment will be described with reference to the simulation waveform of FIG. At the rising time T0 of the potential Vcnt of the control signal given from the outside, the MOSFET M1 becomes conductive, the voltage Vin of the constant power source is applied to the primary winding of the transformer T1, and the primary current Ip flows. At the same time, an exciting current Is is generated in the secondary winding, and a voltage Vs is generated. It flows in the forward direction of the Zener diode Z2, the Zener diode Z3, the reverse direction of the Zener diode Z1, and the forward direction of the diode D1.

  At this time, since the Zener diode Z3 conducts in the forward direction, the gate-source potential Vgs of the connected switching transistor M0 becomes the forward voltage of Z3. If M0 is a normally-on transistor, the threshold voltage Vth is negative, so that the transistor is sufficiently conductive. Further, the sum VD1 + | VZ1 | of the forward voltage of the diode D1 and the Zener voltage of the Zener diode Z1 is applied between the terminals of the negative potential applying capacitor C1, and charges corresponding to the capacitance of C1 are charged. At this time, the terminal on the transformer side of C1 is negative, and the terminal on the switching transistor M0 side is positive.

  Here, depending on the control signal switching time (time from rising to falling), Is may decrease thereafter and become zero. In that case, since the direction of the Zener diode Z2 is opposite from the gate of M0 to the transformer, Vgs gradually decreases due to a current corresponding to the value of the resistor R1 at a potential difference equal to or lower than the Zener voltage. By setting the resistance value of R1 large according to the switching time of the control signal, it is possible to prevent a decrease in Vgs and maintain a conductive state.

  Next, at time T1, when Vcnt falls and MOSFET M1 is cut off, Ip becomes zero. At the same time, since an exciting current in the reverse direction is generated in the secondary winding, Is has a negative value. However, since the diode D1 is reverse-biased, no current flows through D1 and the Zener diode Z1. Instead, the charge stored in the negative potential applying capacitor C1 flows in the opposite direction of the Zener diode Z3 and Zener diode Z2, and the Zener voltages of Z3 and Z2 are generated at both ends. At this time, since the Zener diode Z3 conducts in the reverse direction, the gate-source potential Vgs of the connected switching transistor M0 is limited to the negative Zener voltage of Z3 and does not exceed the rating. Although Vth is negative, if the Zener voltage is set lower than Vth, M0 is sufficiently cut off. When the capacitance of C1 is sufficiently large compared to the charge flowing in the discharge current, the voltage across the two ends is small, and the voltage VD1 + | VZ1 | is maintained approximately equal to the sum of the forward voltage of D1 and the Zener voltage of Z1. .

  When the energy stored in the transformer is consumed, the absolute value of Is decreases and eventually becomes zero at time t2. After that, the gate-source potential Vgs of the switching transistor M0 changes to become the voltage-(VD1 + | VZ1 |) at both ends of C1, and the speed is determined by the gate-source capacitance Cgs of M0 and the resistor R1. By setting Cgs smaller than C1, it changes in a short time. By setting − (VD1 + | VZ1 |) sufficiently lower than the threshold voltage Vth of M0, M0 is sufficiently cut off.

  FIG. 7 shows the Vgs waveform of this example compared with the prior art. The upper part of FIG. 7 is a Vgs waveform according to the present example, and the lower part is a Vgs waveform of the prior art. For comparison, the rise time of Vgs is aligned. As the control signal is switched, the Vgs waveform of this embodiment rises sharply and falls. On the other hand, the fall of Vgs in the prior art occurs at a time when Is becomes zero, and falls with a slope depending on the circuit constant. According to this comparison, the proposed circuit has a slope about 25 times that of the prior art circuit near Vth. Therefore, according to the present invention, it can be said that the switching loss due to the transient time is small.

  As described above, according to the present embodiment, the gate-source voltage rating of the switching transistor is not exceeded, and the transient time in each of the conductive and cut-off states is reduced, so that the loss is low and stable. The switching element can be controlled.

  Hereinafter, another embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 8 shows a circuit diagram of a gate drive circuit for a switching element according to this embodiment. In this embodiment, the current limiting circuit 5 is not provided as compared with the first embodiment.

  The operation of this embodiment will be described with reference to the simulation waveform of FIG. For comparison, it is shown together with the case where a Zener diode Z2 is provided. At the time T0 when the potential Vcnt of the control signal applied from the outside rises, an excitation current Is is generated in the secondary winding, and the forward direction of the Zener diode Z3, the reverse direction of the Zener diode Z1, and the forward direction of the diode D1 Flowing. Is is a large value as compared with the case where the Zener diode Z2 is connected, and the amount of decrease over time is large. However, if the switching time of the control signal is shortened, the conduction of the switching transistor is maintained during this period.

  As in the first embodiment, the Zener diode Z3 conducts in the forward direction. Therefore, the gate-source potential Vgs of the connected switching transistor M0 becomes the forward voltage of Z3, and M0 is sufficiently conducted. Further, the sum VD1 + | VZ1 | of the forward voltage of the diode D1 and the Zener voltage of the Zener diode Z2 is applied between the terminals of the negative potential applying capacitor C1, and charges corresponding to the capacitance of C1 are charged.

  Next, at time T1, when Vcnt falls and MOSFET M1 is cut off, a reverse exciting current is generated in the secondary winding, and the charge stored in negative potential applying capacitor C1 is transferred to zener diode Z3. The Z3 Zener voltage is generated at both ends. At this time, since the Zener diode Z3 conducts in the reverse direction, the gate-source potential Vgs of the connected switching transistor M0 is limited to the negative Zener voltage of Z3 and does not exceed the rating. Although Vth is negative, if the Zener voltage is set lower than Vth, M0 is sufficiently cut off. As in the first embodiment, sufficient switching transistor control is possible.

  Examples in which the transformer of the present invention is not used will be described below.

  FIG. 10 is a circuit diagram of the gate drive circuit for the switching element according to this embodiment. In this embodiment, the control voltage generation circuit is configured by a capacitor and a switching element controlled by φ1 and φ2 which are opposite phase signals without using a transformer.

  Vin is a DC voltage of a constant voltage source, and is input to a control voltage generation circuit indicated by a dotted rectangle. The control voltage generation circuit includes a second negative potential applying capacitor C2 and six switch elements. Switching elements SW1 and SW2 are switching elements for applying Vin and ground potential to the control voltage generation circuit, respectively. SW3 to SW6 are a capacitor C2 and a capacitor applying circuit, that is, a Zener diode Z3, and a Vgs limiting circuit, that is, a diode D1 and a Zener diode This is a switching element for inverting the connection of Z1 with the series connection circuit.

  Φ1 and φ2 are pulse signals, and φ1 and φ2 are complementary signals. That is, φ1 and φ2 are signals with inverted potentials. The four switch elements SW1, SW2, SW3, and SW6 are controlled by φ1, and the two switch elements SW4 and SW5 are controlled by φ2. That is, SW4, SW5 are open when SW1, SW2, SW3, SW6 are closed, and SW4, SW5 are closed when SW1, SW2, SW3, SW6 are open.

  When SW1, SW2, SW3, SW6 are closed and SW4, SW5 are open, Vin is applied between nodes N1 and N2, Zener diode Z3 is forward, Zener diode Z1 is reverse, and diode D1 is A forward voltage is applied, and a current corresponding to the voltage flows. At the same time, a voltage VD1 + | VZ1 | equal to the sum of the forward voltage of D1 and the Zener voltage of Z1 is applied to the capacitor C1, and the forward voltage of Z1 is applied to Vgs of the switching transistor, and the switching transistor becomes conductive. .

  Next, when SW1, SW2, SW3, SW6 are open and SW4, SW5 are closed, the external potential connection is cut off, the voltage of Vin stored in the internal capacitor C2 is reversed, and the nodes N1 and N2 In between, -Vin is applied, and the Zener diode Z3 is applied with a reverse voltage, the Zener diode Z1 is applied with a forward voltage, and the diode D1 is applied with a reverse voltage. Since the reverse current does not flow through D1, the charge stored in the capacitor C1 is discharged, and the Zener voltage of Z3 is applied to Vgs of the switching transistor, and if it is lower than Vth, the switching transistor is cut off. By sufficiently increasing the capacitance of C2, if the switching time is short, a sufficient potential difference can be continuously applied to N1 and N2, and stable operation is possible.

  In the present embodiment, it is not necessary to use a transformer, and the size can be reduced and the manufacturing cost can be reduced as compared with the first and second embodiments.

  The switching element gate drive circuit according to the present invention can stably control a normally-on type switching transistor. It can also be used to control normally-off switching transistors without being limited to normally-on types, and can be applied not only to JFETs but also to MOSFETs, SITs, and IGBTs. It can also be applied to transistors of GaN, Si and other compound semiconductor materials.

DESCRIPTION OF SYMBOLS 1 Control voltage generation circuit 2 Negative potential application capacitor 3 Capacitor application circuit 4 Vgs limit circuit 5 Current limit circuit 6 Constant voltage source 7 Switching transistor

Claims (13)

A gate drive circuit for driving the gate of a switching transistor as a switching element,
A control voltage generation circuit that generates a voltage that is inverted between positive and negative;
A negative potential applying capacitor;
A capacitor application circuit for applying a constant voltage between the electrodes of the negative potential application capacitor for a predetermined period;
A Vgs limiting circuit connected between the gate electrode and the source electrode of the switching transistor;
A gate voltage driving circuit for a switching element, comprising:   The gate voltage driving circuit for a switching element according to claim 1, wherein the Vgs limiting circuit includes a Zener diode.   3. The switching element gate voltage drive circuit according to claim 1, wherein the capacitor application circuit includes a diode and a Zener diode connected in series.   4. The switching element gate voltage driving circuit according to claim 1, further comprising a current limiting circuit for limiting a current flowing through the Vgs limiting circuit.   The switching element gate voltage driving circuit according to claim 4, wherein the current limiting circuit includes a Zener diode.   6. The switching element gate voltage driving circuit according to claim 5, wherein the current limiting circuit includes a resistor connected in parallel with the Zener diode.   7. The switching element gate voltage drive circuit according to claim 1, wherein the control voltage generation circuit includes a transformer.   8. The switching element gate voltage drive circuit according to claim 7, wherein the transformer is depolarized.   7. The switching element gate voltage drive circuit according to claim 1, wherein the control voltage generation circuit includes a second negative potential application capacitor.   10. The switching element gate voltage drive circuit according to claim 9, wherein the control voltage generation circuit further comprises a plurality of switching elements controlled by opposite phase control signals φ1 and φ2. A gate drive circuit for driving the gate of a switching transistor as a switching element,
A depolarizing transformer whose primary side is controlled to conduct and cut off current by a control signal;
A negative potential application capacitor having one end connected to one terminal on the secondary side of the depolarizing transformer;
A diode having a cathode connected to a node to which the depolarizing transformer and the negative potential applying capacitor are connected;
A capacitor applying Zener diode connected to the anode of the diode and the other end of the negative potential applying capacitor;
A Vgs limiting Zener diode having a cathode connected to a node to which the negative potential applying capacitor and the capacitor applying Zener diode are connected;
A current limiting Zener diode connected to the anode of the Vgs limiting Zener diode and the other terminal of the depolarizing transformer;
A resistor connected in parallel across the zener diode for current limiting;
Terminals connecting the gate and source of the switching transistor to both ends of the Vgs limiting Zener diode;
A gate voltage driving circuit for a switching element, comprising: A gate drive circuit for driving the gate of a switching transistor as a switching element,
A depolarizing transformer whose primary side is controlled to conduct and cut off current by a control signal;
A negative potential application capacitor having one end connected to one terminal on the secondary side of the depolarizing transformer;
A diode having a cathode connected to a node to which the depolarizing transformer and the negative potential applying capacitor are connected;
A capacitor applying Zener diode connected to the anode of the diode and the other end of the negative potential applying capacitor;
A node connected to the negative potential applying capacitor and the capacitor applying Zener diode, and a Vgs limiting Zener diode connected to the other terminal on the secondary side of the depolarizing transformer;
Terminals connecting the gate and source of the switching transistor to both ends of the Vgs limiting Zener diode;
A gate voltage driving circuit for a switching element, comprising: A gate drive circuit for driving the gate of a switching transistor as a switching element,
A first switch having one end connected to a constant voltage power supply;
A second switch having one end connected to the ground potential;
A third switch having one end connected to the other end of the first switch;
A fourth switch having one end connected to the other end of the second switch;
A fifth switch having one end connected to the other end of the first switch;
A sixth switch having one end connected to the other end of the second switch;
A negative potential applying capacitor having one end connected to the other end of the fifth switch and the other end of the sixth switch;
A diode having a cathode connected to one end of the negative potential applying capacitor;
The other end of the negative potential application capacitor, and a capacitor application Zener diode connected to the anode of the diode;
A node to which the negative potential application capacitor and the capacitor application zener diode are connected; a Vgs limiting zener diode having an anode connected to the other end of the third switch and the other end of the fourth switch; ,
A second negative potential applying capacitor connected to the other end of the first switch and the other end of the second switch;
Terminals connecting the gate and source of the switching transistor to both ends of the Vgs limiting Zener diode;
A gate voltage drive circuit for a switching element, comprising: