JPH06204410A - Protection circuit for insulated-gate control transistor - Google Patents

Abstract

PURPOSE:To reduce a current capacitance required by a diode which protects insulated-gate control transistors, including insulated-gate bipolar transistors and field effect transistors, from an overvoltage by a dynamic clamping method, and to reduce the size of the diode. CONSTITUTION:A protective diode 30 having an avalanche breakdown voltage which is lower than a withstand voltage of an insulated-gate control transistor 10 is connected between a gate and a main terminal of the transistor 10. For example, a depletion type bipolar transistor 41 is connected, as a nonlinear element 40 which has a current saturation characteristic, between the gate and a drive circuit 20. When a voltage Vc applied to the transistor 10 exceeds the withstand voltage of the transistor 10, the diode 30 causes breakdown. An avalanche current Id, which flows into the diode 30 while the transistor 10 is temporarily turned on and an overvoltage is absorbed, is limited to a saturation current value of the non-linear element 40 by means of the non-linear element 40, so that the avalanche current is reduced to some fraction of one when compared with an avalanche current fl-owing through a conventional protection circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an insulated gate control transistor having an insulated gate such as an insulated gate bipolar transistor or a field effect transistor, the on / off state of which is controlled by a gate drive by a drive circuit, between the pair of main terminals. The present invention relates to a protection circuit for protecting against applied overvoltage.

[0002]

2. Description of the Related Art The above-mentioned insulated gate control transistor has a high withstand voltage of several hundred to 1,000 V and a large current capacity of several tens to several hundred A and is used for high speed control of various loads under a circuit voltage of several hundred V. Depending on the type of load, a large overvoltage may be applied especially in the off state, and if avalanche breakdown occurs and a large avalanche current flows, damage or destruction may occur due to local heating. The protection circuit is to prevent such trouble, but instead of using a circuit element that absorbs overvoltage as in the past, recently, when the overvoltage is applied, the insulated gate control transistor is rather turned on temporarily. A so-called dynamic clamp method is often adopted to prevent the avalanche breakdown. The present invention relates to this type of protection circuit, and a conventional example thereof will be briefly described with reference to FIG.

An insulated gate control transistor 10 shown on the right side of FIG. 5 (a) is an insulated gate bipolar transistor, and its gate is driven by a drive circuit 20 shown on the left side through a gate resistor 10. The drive circuit 20 is composed of a pair of bipolar type complementary transistors 21 and 22 in the example shown in the figure.
It is an inverter type circuit in which both transistors 21 and 22 are alternately turned on and off according to the state of the input signal Si of FIG. When the input signal Si in Fig. 5 (b) is high and the transistor 22 of the drive circuit 20 is on, the insulated gate control transistor 10 turns off as shown in Fig. 5 (c). In order to prevent avalanche breakdown when Vc becomes excessive, in the illustrated example, a protective diode 30 is connected between the collector side and the gate of the insulated gate control transistor 10.

The avalanche breakdown voltage of the diode 30 is set lower than the withstand voltage value between the pair of main terminals of the insulated gate control transistor 10 by about several tens of volts, and depends on the current flowing through the gate resistor 10a when the diode 30 breaks down. The gate voltage is raised by the amount of the voltage drop and the insulated gate control transistor 10 is temporarily turned on as shown in FIG. 5 (c) to prevent damage or destruction due to avalanche breakdown between its main terminals. To prevent the diode 30 from conducting in the forward direction when a gate drive voltage is applied to turn on the insulated gate control transistor 10, another diode 31 is usually connected in anti-series with it as shown in the figure. Is.

[0005]

As described above, the dynamic clamp method in which the insulated gate control transistor is protected by the diode connected between the main terminal to which the overvoltage is applied and the gate has the advantage that the overvoltage is absorbed by the object to be protected. However, because of its operating principle, the protection diode is avalanche-breakdown, so it is necessary to give it a current capacity that can sufficiently withstand the avalanche current, and its size is considerably large because it is necessary to have a high avalanche breakdown voltage of several hundred volts. There is a growing problem.

For example, the gate resistance 10a in FIG. 5 (a) is 5Ω.
When the gate operation threshold of the insulated gate control transistor 10 is 3V, the diode 30 has a minimum of 3V / 5Ω = O.6.
Since it is necessary to have a current capacity of A, and in order to reliably generate a voltage drop of 3 V or more in the gate resistance 10a in the vicinity of the avalanche starting voltage, it is necessary to allow a considerable margin for this minimum current capacity. The actual size or area of diode 30 should be quite large. If the resistance value of the gate resistor 10a is increased, the current capacity required for the diode 30 can be reduced, but since the ON operation speed of the insulated gate control transistor 10 is reduced, the resistance value is usually more than the above several Ω. It is not allowed to raise to.

The object of the present invention is to solve the above problems,
It is to reduce the current capacity required for a diode used in a protection circuit and to reduce the size thereof while taking advantage of the above-mentioned advantages of the dynamic clamp method.

[0008]

According to the first invention of the present application, a pair of transistors is provided as a protection circuit for protecting the above-mentioned insulated gate control transistor whose gate is driven by a drive circuit from an overvoltage applied to its main terminal. The above object is achieved by providing a diode having a breakdown voltage lower than the withstand voltage between the main terminals and connected between the gate and the main terminal, and a current-saturating nonlinear element connected between the gate and the drive circuit. To do. It should be noted that it is preferable to use a small-sized transistor that is always conducting as the non-linear element. In particular, it is advantageous to use a depletion-type or enhancement-type field-effect transistor in which the gate is biased in the always-on state. It is preferable that the internal pn junction is connected in a direction in which it conducts when the transistor is turned on.

In the second aspect of the present invention, the diode is combined with a control circuit for controlling the operation of the drive circuit, and the impedance of the circuit element for turning off the transistor in the drive circuit is controlled by the control circuit to start the turn-off operation of the transistor. The objective described above is achieved by sometimes lowering and then raising after a short time. It is preferable that the control circuit in the second aspect of the invention controls the off operation circuit element in the drive circuit by using, for example, a one-shot circuit that generates a control signal having a constant pulse width, and the pulse width generated in this is protected. Of course, it depends on the load condition of the transistor, but it is usually about 1 μS. A protection circuit according to the second aspect of the present invention is such that a resistance is connected in parallel to the OFF operation circuit element, and a constant voltage element such as a Zener diode is connected to the gate of the insulated gate control transistor to protect it. This is advantageous in stabilizing the operation of.

In the third aspect of the present invention, the above-mentioned protection diode is combined with a voltage circuit connected to the drive circuit, and at the start of the turn-off operation of the transistor, the voltage circuit is applied to the gate of the transistor via the off operation circuit element in the drive circuit. The above-mentioned problem is solved by applying a voltage that promotes turn-off and placing the voltage circuit in a high impedance state shortly thereafter. For example, a charging / discharging circuit is used for the voltage circuit, which is always charged, and when the transistor is turned off, this is discharged to promote the off operation, and then the high impedance state corresponding to the charging resistance is protected during the off period. It is good practice to limit the avalanche current when the diode breaks down. Also in the third invention, it is advantageous to connect the gate of the insulated gate control transistor to the constant voltage element for protection thereof in order to stabilize the operation of the protection circuit.

In the first and second inventions, a gate resistor having a relatively low resistance value is connected between the gate of the insulated gate control transistor and the drive circuit as in the conventional case, if necessary in the first and second inventions. It is advantageous to surely absorb the overvoltage applied to the transistor by the temporary ON operation. Further, in any of the inventions, it is advantageous to connect another diode to the protection diode in anti-series in the same manner as in the prior art in order to ensure the operation during the ON period of the transistor, and further to connect the two diodes in anti-series connection. It is particularly advantageous to use an open-base bipolar transistor instead to simplify the circuit structure.

[0012]

In any of the first to third inventions of the present application, a protection diode having a breakdown voltage lower than the withstand voltage value between the pair of main terminals of the insulated gate control transistor to be protected is provided between the gate and the main terminal. Although the same is connected between them, in the first invention, a current-saturating non-linear element is connected between the gate and the drive circuit to limit the current that can flow therethrough to a saturation current value or less. The circuit is connected to the drive circuit to control the impedance of the circuit element for the OFF operation so as to be lowered at the time of turning off the transistor and to be increased after a short time. In the third invention, the voltage circuit is connected to the drive circuit to accelerate the turn-off. By applying a voltage and setting it in a high impedance state after a short time, both inventions accelerate or accelerate the turn-off operation of the transistor to be protected. The protection diode avalanche breakdown due to the overvoltage applied when the transistor is off, and the avalanche current flowing through the diode while the transistor is temporarily on and absorbing the overvoltage causes the saturation current value and high impedance to reduce the avalanche current to a fraction of the conventional value. It is limited to below.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. 1 to 3 show embodiments of the first to third inventions of the present invention, and FIG. 4 shows an embodiment using a bipolar transistor having an open base structure as a protection diode. Although the insulated gate control transistor is an insulated gate bipolar transistor in the embodiments described below, the present invention is of course applicable to a field effect transistor. Further, the drive circuit is assumed to be composed of bipolar transistors, but it may be composed of field effect transistors.

FIG. 1 (a) is a circuit diagram showing a state in which the protection circuit of the first invention is incorporated. Insulated gate control transistor 10
Receives a power supply voltage V of several hundreds V via one of its main terminals indicated by Vc through a resonance type load 1, and drives the load 1 while the other main terminal indicated by E is grounded. Used for. The drive circuit 20 shown on the left side of the figure is an inverter type circuit composed of complementary bipolar transistors 21 and 22 as in the case of FIG. 5 described above, which receives an input signal Si from the interconnection point of both transistors 21 and 22. The output drives the insulated gate control transistor 10.

In this embodiment, the protection circuit for the transistor 10 which is an insulated gate bipolar transistor is a diode 30 connected between the collector side main terminal and the gate.
And a non-linear element connected between the gate and the drive circuit 20.
It consists of 40 and. As in the conventional case, as the diode 30, a diode having an avalanche voltage lower than the withstand voltage value between the pair of main terminals of the transistor 10 by several tens of volts is used, and the diode 31 is connected in reverse series to the diode 30 to turn on the transistor 10. It is sometimes desirable to prevent diode 30 from conducting in the forward direction. In order to protect the gate of the transistor 10, a constant voltage element 32, for example, a Zener voltage of 20 is provided between it and the main terminal on the emitter side.
It is recommended to connect a ~ 30V bidirectional Zener diode. These diodes 30 to 32 are transistors
It is advantageous to build it in the same chip as 10.

A current saturating element is used as the non-linear element 40, and it is advantageous to use, for example, the depletion type field effect transistor 41 shown in FIG. 1 (b) so that it can be incorporated in the same chip as the transistor 10. Yes, or Figure 1
The power supply 43 may be connected to the gate of the enhancement-type field effect transistor 42 of (c) so that it is always kept in the ON state. FIG. 1 (d) shows an example of the characteristics of the voltage v and the current i of the non-linear element 40. Compared with the linear characteristic L, the current i has a limited saturation characteristic S as shown in the figure. When the transistors 41 and 42 are used, the current i is constantly saturated within a range where the voltage v is equal to or higher than the threshold Vt of the gate of the transistor 10 as a small element having a small current capacity. Since the transistors 41 and 42 have a pn junction indicated by a parasitic diode d in parallel with the channel, it is preferable to connect the non-linear element 40 in a direction in which the transistor 41 and 42 conduct in the forward direction when the transistor 10 is turned on.

The operation of the protection circuit configured as described above will be described with reference to FIG. The waveform of voltage Vc and current I of transistor 10 is shown in Fig. 1 (e), and the diode is shown in Fig. 1 (f).
The waveform of the avalanche current Id flowing in 20 and the waveform of the gate voltage Vg of the transistor 10 are shown in FIG. 1 (g). The left side of the figure is the ON state of the transistor 10 and the right side is the OFF state thereof. When turned on, the transistor of the drive circuit 20 in Fig. 1 (a)
21 is turned on, the gate voltage Vg corresponding to the power supply voltage Vd of the drive circuit 20 is applied to the gate of the transistor 10 through the non-linear element 40, and the gate of the transistor 10 is connected between the main terminals of the transistor 10 as shown in FIG. As shown on the left side, a large current I flows and a very low ON voltage Vn is applied.

When the transistor 22 of the drive circuit 20 is turned on, the gate voltage Vg disappears, so that the transistor 10 is turned off and its current I in FIG. 1 (e) becomes a very small so-called tail current It within a short time. It disappears after flowing, but if the load 1 in Fig. 1 (a) is a resonant circuit, for example,
The voltage Vc applied between the main terminals of 10 rises sinusoidally as shown on the right side of FIG. 1 (e) with this turn-off operation, and the maximum value may exceed the withstand voltage value of the transistor 10. However, since the breakdown voltage Va of the protection diode 20 is lower than the withstand voltage value of the transistor 10, the voltage Vc
When it reaches that level, it breaks down and its avalanche current Id in FIG. 1 (f) flows through the non-linear element 40 and the transistor 22 of the drive circuit 20 as can be seen from FIG. 1 (a).

In the non-linear element 40 having the current saturation property as shown in FIG. 1D, the gate voltage exceeding the operation threshold value Vt of the transistor 10 as shown in FIG. 1G is caused by a slight flow of the avalanche current Id. Since Vg is generated, the transistor 10 is temporarily turned on as shown by the current In in FIG. 1 (a) to absorb the overvoltage. As described above, according to the first aspect of the invention, the voltage Vc applied to the transistor 10 is clamped to the breakdown voltage Va by the diode 30, and when the diode 30 receives an overvoltage.
It is possible to limit the avalanche current Id flowing to the saturation current value by the non-linear element 40 to reduce it to a fraction of the conventional avalanche current Id1 shown in FIG. 1 (f).

FIG. 2A shows a circuit incorporating the protection circuit according to the second invention. Protected transistor 10 as shown
The connection of the diodes 30, 31 and the constant voltage element 32 to the gate of the transistor is the same as in FIG. 1 (a), but in the second invention, the control circuit 50 is connected to the input side of the drive circuit 20 to turn off the transistor 10. In this example, which is a circuit element for use, the operation of the transistor 22 is controlled according to the input signal Si. In the example of the input signal Si of FIG. 2 (b), the control circuit 50 responds to the rising edge of the control signal with a predetermined pulse width τ as shown in FIG. 2 (b).
For example, a one-shot circuit that generates Sn may be used, and the pulse width τ varies depending on the type of load of the transistor 10, but is usually set to about 1 μS. Further, in the second invention, the drive circuit 20 and the transistor
Between the 10 gates, a gate resistance of about several Ω 10
It is preferable to connect a, and it is desirable to connect a resistor 23 having a relatively high resistance value of, for example, 1 to 10 kΩ in parallel to the transistor 22 as shown in the figure.

When the input signal Si and the control signal Sn are low, the transistor 21 of the drive circuit 20 is on and the transistor 22 is off, so that the transistor 10 is on. input signal
When Si goes high and the control signal Sn goes high accordingly, the on / off states of the transistors 21 and 22 are reversed, so that the transistor 10 turns off, and then the control signal Sn goes low after a time corresponding to the pulse width τ. When this happens, the transistor 22 turns off, but the transistor 10 maintains its off state, and in this example the resistor 23 serves to ensure that this off state is maintained. As you can see,
If the drive circuit 20 is composed of field effect transistors, the resistance 23
Is not particularly necessary.

When an overvoltage is applied while the transistor 10 is in the off state, the diode 30 breaks down and the avalanche current Id shown in FIG. 2D flows, and the transistor 10 is turned on temporarily to absorb the overvoltage. Since the transistor 22 and the resistor 23 as the off-operation circuit element are in a high resistance state, the avalanche current Id is reduced to a fraction of the conventional value. As can be seen from Fig. 1 (e), the voltage Vc applied to the transistor 10
Exceeds the withstand voltage value after some time has elapsed from the start of turn-off, so by setting the pulse width τ of the control signal Sn shorter than this time, the impedance value of the off operation circuit element can be adjusted by the time overvoltage is applied. It can be raised.

As described above, according to the second aspect of the present invention, the control circuit 50 lowers the impedance value of the off-operation element of the drive circuit 20 to promote the turn-off operation of the transistor 10, and then the impedance value of the off-operation element by the time when the overvoltage is applied. Can be increased to reduce the avalanche current Id flowing in the diode 30. According to the second aspect of the invention, the constant voltage element 32 has the high impedance value so that the transistor 10
Is especially useful in protecting the gate when the gate voltage of the gate rises too high.

FIG. 3 (a) shows a protection circuit according to the third invention. Diode 30, 31 for the gate of transistor 10
The connection between the constant voltage element 32 and the constant voltage element 32 is the same as in the first invention as shown in the figure, but in the third invention, the voltage circuit 60, in the example shown in the figure, is connected to the drive circuit 20. The voltage circuit 60 includes a low-voltage power source 62 of about 5V with respect to the capacitor 61 as shown in the figure.
A series circuit of charging resistors 63 of 0.1 to 1 kΩ is connected in parallel, and is connected in series to the off operation circuit element 22 side of the drive circuit 20,
And the power supply 62 to the gate of the transistor 10 in the off state.
Is connected in the direction of applying a negative voltage from.

When the input signal Si of FIG. 3 (b) is low and therefore the transistor 10 is in the on state, the transistor 22 of the drive circuit 20 is of course off and the capacitor in the voltage circuit 60 is
61 is charged to the voltage of the power supply 62. When the input signal Si becomes high, the transistor 22 of the drive circuit 20 turns on and the transistor 10 turns off.At this time, since the voltage circuit 60 is connected and the charging voltage of the capacitor 61 is added, the charge is transferred from the gate of the transistor 10. The pull-out gate current Ig of FIG. 3 (c) is strengthened, and this turn-off operation is promoted. To obtain the turn-off promoting effect on the transistor 10, it is preferable to set the capacitance of the capacitor 61 in the voltage circuit 60 to 0.1 to 2 times the capacitance of the gate of the transistor 10.

During the turn-off time of the transistor 10, which is generally about 1 μS, the capacitor 61 is discharged, and when the transistor 10 is turned off thereafter, the voltage circuit 60 is applied to its gate.
Negative voltage of the power supply 62 in the inside, and high resistance charging resistance
63 is connected to it. This off transistor 10
When the diode 30 breaks down due to overvoltage, its avalanche current Id flows as shown in Fig. 3 (d).
Although 10 temporarily turns on and absorbs the overvoltage, the avalanche current Id can be limited to a fraction of the conventional value by the high resistance of the charging resistor 63 in this third invention as well. The capacitor 61 is charged after the avalanche current Id flows, and maintains this charged state until the next turn-off operation is started.

As described above, in the third invention, the transistor 10
After the turn-on is accelerated by the voltage of the voltage circuit 60, the voltage circuit 60 is brought into a high impedance state to reduce the avalanche current Id flowing through the diode 30 when an overvoltage is applied. When a charge / discharge circuit is used for the voltage circuit 60, the capacitor 61 is charged by the power source 62, the turn-off of the transistor 10 is promoted by the charge voltage, and then the voltage circuit 60 is charged by the charging resistor 63 when the transistor 10 is off. It is possible to limit the avalanche current Id when the protection diode 30 breaks down due to overvoltage in a high impedance state due to. Further, by applying a negative gate voltage from the power supply 62 to the transistor 10, the off state can be stably maintained. Also in the third invention, it is advantageous to connect the constant voltage element 32 for protecting the gate of the transistor 10 in order to stabilize the operation of the protection circuit.

An open base bipolar transistor suitable for two diodes 30 and 31 connected in anti-series in any of the above embodiments is shown in FIG. Figure 4
As shown in the cross-sectional view of (a), this pnp-type transistor
70 is a strong p-type semiconductor substrate 71, in which an n-type layer 72 is diffused at a high impurity concentration on the surface of the figure, and then an n-type epitaxial layer 73 is grown as a base to a predetermined thickness.
The terminals are led out from the electrode film 75 connected to the semiconductor substrate 71 by diffusing 74 with a high impurity concentration. As a result, a pnp transistor 70 whose base is not opened or connected is obtained as shown in FIG. 4 (b).
It can be used as two diodes 30 and 31 connected in anti-series as shown in the equivalent circuit of (c).

Since the open base bipolar transistor 70 for the diodes 30 and 31 has a vertical structure, it is advantageous to build it in the chip of the insulated gate control transistor 10 having the same vertical structure. The avalanche breakdown voltage to be given to the diode 30 can be accurately set by the thickness of the n-type epitaxial layer 73 and the impurity concentration of the p-type layer 74 shown in FIG. 4 (a). Further, since the transistor 70 having a so-called wide base structure in which the base epitaxial layer 73 is thick has a low current amplification factor, there is no possibility of conduction or latch-up even when used as an open base.

[0030]

As described above, in any of the first to third inventions of the present invention, a protection diode having a breakdown voltage lower than the withstand voltage value between the main terminals of the insulated gate control transistor to be protected is mainly used as the gate. In the first aspect of the present invention, a current saturating non-linear element is connected between the gate and the drive circuit to limit the current flowing to the saturation current value or less.
In the second aspect of the invention, the control circuit is connected to the drive circuit to control the impedance of the off operation circuit element so as to be lowered at the time of turning off the transistor and to be increased after a short time. Since the high impedance state is applied after applying the acceleration voltage during operation, the protection diode avalanche breakdown due to the overvoltage applied to the transistor to be protected, and the protection diode flows to the protection diode while it is temporarily turned on and absorbing the overvoltage. It is possible to limit the avalanche current to a fraction of the conventional value or less, reduce the current capacity required for the protection diode, and miniaturize it.

Furthermore, (a) it is easy to make the protection diode smaller and to incorporate it in the same chip as the insulated gate control transistor, and (b) the operation reliability of the protection diode is improved over a long period of time by reducing the avalanche current. Let
(c) It is possible to obtain the effect that the gate driving force by the drive circuit can be strengthened and the operating speed at turn-on and turn-off of the insulated gate control transistor can be increased without being restricted by the avalanche current of the protection diode.

[Brief description of drawings]

FIG. 1 shows an embodiment of the first invention of the present case, FIG. 1A is a circuit diagram including a protection circuit, and FIGS. 1B and 1C are partial circuit diagrams showing different configuration examples of nonlinear elements. , (D) is a characteristic diagram of a nonlinear element, (e) is a waveform diagram of voltage and current of an insulated gate control transistor, (f) is a waveform diagram of its gate voltage, and (g) is the same. FIG. 6 is a waveform diagram of an avalanche current of a protection diode.

2 shows an embodiment of the second invention, FIG. 2 (a) is a circuit diagram including a protection circuit, FIG. 2 (b) is a waveform diagram of input signals of a driving circuit and a control circuit, and FIG. A waveform diagram of the control signal by the control circuit, and FIG. 6D is a waveform diagram of the avalanche current of the protection diode.

3A and 3B show an embodiment of the third invention, wherein FIG. 3A is a circuit diagram including a protection circuit, FIG. 3B is a waveform diagram of an input signal of a drive circuit, and FIG. 3C is an insulated gate control. Waveform diagram of the gate current of the transistor, Figure (d) is a waveform diagram of the avalanche current of the protection diode.

FIG. 4 shows an embodiment in which a bipolar transistor having an open base structure is used as a protection diode or the like used in the first to third inventions. FIG. 4 (a) is its sectional structure view and FIG. 4 (b) is its circuit diagram. , (C) is an equivalent circuit diagram thereof.

FIG. 5 is a circuit diagram including a protection diode, FIG. 5 (b) is a waveform diagram of an input signal to a driving circuit, and FIG. 5 (c) is an on / off state of an insulated gate control transistor in the related art. FIG.

[Explanation of symbols]

1 Insulated gate control transistor load 10 Insulated gate control transistor 10a Gate resistance 20 Drive circuit 30 Protective diode 40 Non-linear element 41 Depletion type field effect transistor for non-linear element 42 Enhancement type field-effect transistor for non-linear element 50 Control Circuit 60 Voltage circuit 70 Open base bipolar transistor I Current flowing through insulated gate control transistor Id Avalanche current of protection diode Si drive circuit input signal Sn Control signal by control circuit Va Avalanche breakdown voltage of protection diode Vc Voltage applied to insulated gate control transistor Vg Insulated gate control transistor gate voltage

Claims (3)

[Claims] 1. A circuit for protecting an insulated gate control type transistor whose gate is driven by a drive circuit from an overvoltage applied to its main terminal, the circuit having a breakdown voltage lower than a withstand voltage between a pair of main terminals of the transistor. A protection circuit for an insulated gate control transistor, comprising: a diode connected between the gate and the main terminal; and a current-saturating non-linear element connected between the gate and the drive circuit. 2. A circuit for protecting an insulated gate control type transistor whose gate is driven by a drive circuit from an overvoltage applied to its main terminal, the circuit having a breakdown voltage lower than a withstand voltage between a pair of main terminals of the transistor. A diode connected between the gate and the main terminal and a control circuit for controlling the operation of the drive circuit are provided, and the impedance of the off operation circuit element in the drive circuit is lowered by the control circuit at the start of the turn-off operation of the transistor. A protection circuit for an insulated gate control transistor, characterized in that it is increased after a short time. 3. A circuit for protecting an insulated gate control type transistor whose gate is driven by a drive circuit from an overvoltage applied to its main terminal, the circuit having a breakdown voltage lower than a withstand voltage between a pair of main terminals of the transistor. A diode connected between the gate and the main terminal and a voltage circuit connected to the drive circuit are provided, and when the turn-off operation of the transistor is started, the voltage circuit turns off the gate through the off operation circuit element in the drive circuit. A protection circuit for an insulated gate control transistor, characterized in that a voltage circuit that accelerates the voltage is applied and the voltage circuit is placed in a high impedance state after a short time.