JPH10107605A - Overcurrent protective circuit of transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control the heat generation of an output transistor at the short circuit of output by delaying the timing of transistor switching at the overcurrent detection in accordance with the time of charge and discharge of a charging and discharging transistor. SOLUTION: The timing of transistor switching at the overcurrent detection is made to delay in accordance with the charging and discharging time of a charging and discharging condenser 101. A delaying means comprises charging resistance R 101 and discharging resistance R 102 and R 103 which are serially connected between the emitter of a transistor 53 for overcurrent detection and a GND, and the capacitor 101. The resistance R 102 and R 103 discharge slowly to extend an output current cutoff time. Therefore, their resistance values are set to be relatively large so that the discharge time constant of the capacitor 101 may become several times more than the switching time of an output transistor 42. A delay operational transistor 102 performs a switching operation, based on the divided voltage of the capacitor 101.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transistor overcurrent protection circuit and, more particularly, to a transistor for opening and closing a current from a power supply to a load when driving a load such as a display lamp or a solenoid using a DC power supply as a drive source. Overcurrent protection circuit for protecting the transistor from overcurrent due to a load short circuit or the like.

[0002]

2. Description of the Related Art Conventionally, a circuit shown in FIG. 3 is known as a basic circuit for driving a lamp, a solenoid or the like by a transistor. This circuit (40) has an output voltage V
For supplying power from a DC voltage source 10 of cc to a load 20 such as a solenoid, a fuse 30 and an output transistor 42 (first transistor) are inserted and connected in series to the power supply line. It was done. When the output transistor 42 is turned on / off to open / close the power supply line, the output current D from the DC voltage source 10 to the load 20 is turned on / off, and when an overcurrent flows when the output transistor 42 is turned on. When the fuse 30 is blown, the power supply line is forcibly cut off to protect the output transistor 42 and other circuits.

The overcurrent protection circuit 40 includes an input signal A from an external circuit for controlling a switching operation of the output transistor 42 in addition to the fuse 30 and an output transistor 42 formed of a power transistor of a MOS type FET. And a drive circuit 41 for generating a drive signal C to the gate of the output transistor 42 in response to the load, and a load for suppressing the generation of a surge voltage due to the back electromotive force at the time of interruption when the load 20 is an inductive load such as a solenoid. The flywheel diode 43 connected in parallel to 20 also
Is provided.

The driving circuit 41 has a base whose input signal A
Is composed of an NPN transistor connected to the voltage dividing point of the resistors R1 and R2, the emitter is grounded, and the collector is connected to the fuse 30 via the series resistors R3 and R4. Is generated. Then, when the input signal A becomes significant (high) and the transistor 41a is turned on, the drive signal C drops from the voltage Vcc to a significant voltage by flowing the current B through the fuse 30 from the DC voltage source 10 to the resistors R3 and R4. On the other hand, when the transistor 41a is turned off, the drive signal C is returned from the significant voltage to the voltage Vcc.

The output transistor 42 has a gate connected to the drive signal C line and a source connected to the fuse 30.
And the drain is connected to the load 20 and the cathode of the diode 43 connected in parallel to the load 20. Then, it is turned on / off in accordance with the drive signal C, and hence the input signal A.

The overcurrent protection circuit 50 shown in FIG. 4 is provided with an overcurrent detection circuit 51 in place of the fuse 30 in the above-described overcurrent protection circuit 40. As a result, when an overcurrent is detected, the output transistor 42 is immediately turned off regardless of the input signal A, thereby preventing the overcurrent.

For this purpose, the overcurrent detection circuit 51 includes a current detection resistor 52 and an overcurrent detection transistor 53 (second
Transistor). Current detection resistor 52
Is connected in series with a power supply line between the DC voltage source 10 and the source of the output transistor 42, and a voltage corresponding to a load current from the DC voltage source 10 to the load 20 is applied across the resistor 52. Is to occur. Moreover, when the load current becomes excessively large, the resistance value is a voltage corresponding to the base-emitter voltage required to turn on the transistor (that is, a voltage of a predetermined value corresponding to whether or not an overcurrent occurs). Is set to occur.

The overcurrent detecting transistor 53 is
A switching PNP transistor is employed, the emitter is connected to the power supply line, the base is connected to the connection point between the current detection resistor 52 and the collector of the output transistor 42 via the protection resistor R5, and the collector is connected to the output transistor 42. It is connected to the gate line. When an overcurrent flows through the output transistor 42 and the current detection resistor 52 and the potential difference between both ends of the current detection resistor 52 exceeds the predetermined voltage, the overcurrent detection transistor 53 switches from the off state to the on state. , The drive signal C is made substantially equal to the voltage Vcc, so that the output transistor 42 is forcibly turned off.

The overcurrent protection circuit 60 shown in FIG. 5 is obtained by adding a spike current bypass capacitor 61 in parallel with the current detection resistor 52 in the above-described overcurrent protection circuit 50. Immediately after the output transistor 42 is turned on, a spike current appears in the output current D due to a current flowing into the parasitic or added capacitance of the load 20 or a current that cancels out the charge and current of the diode 43.
Depending on the degree, the overcurrent detection circuit 51 may malfunction. The spike current bypass capacitor 61 is
This is to prevent such a spike current from erroneously operating the overcurrent detection circuit 51 by bypassing the current detection resistor 52,
The capacitance is determined according to the amount of spike current.

[0010]

However, in such a conventional transistor overcurrent protection circuit, in the case of the circuit shown in FIG. 3 (fuse blowing type), the fuse blows when an overcurrent flows due to an output short circuit accident or the like. Although the circuit is protected, even if there is no damage to the circuit element or the pattern of the printed wiring board, if the fuse is not replaced,
The function of the circuit cannot be restored. In addition, it is difficult to determine how much the stress due to the overcurrent has affected the life and performance of the output transistor.
Even if the function is restored by replacing the fuse, it cannot be concluded that the performance and reliability of the circuit / device have been restored to the same level as before the occurrence of the short circuit accident.

On the other hand, the circuit of FIG. 4 (transistor driven type)
In this case, since the fuse is not used, the function is restored as soon as the output short-circuit state is released. However, in the output short-circuit state, when the overcurrent detection transistor 53 is turned on, the output transistor 42 is turned off, and the output transistor 4 is turned off.
2 turns off, the current stops flowing through the current detection resistor 52, the overcurrent detection transistor 53 is turned off, and the output transistor 42 is turned on again. If the output short-circuit state is not resolved, the overcurrent detection is performed again. As a result, the transistor 53 is repeatedly turned on. As a result, the transistor 53 is oscillated at a high frequency corresponding to the switching speed of the transistor, and energization / de-energization is frequently repeated, so that a considerably large average output current flows. In addition, during such switching, the energy loss in the transistor is larger than in the off state or the on state. Therefore, if the output transistor is left short-circuited, the output transistor will generate considerable heat.

In order to prevent the output transistor from being damaged or shortened in life due to the heat generation, sufficient power up of the output transistor itself is required, or a heatsink is attached to the output transistor or a high-performance heatsink is used. There are measures such as increasing the heat radiation effect by replacing it with something. However, the former has excessive specifications more than necessary and is difficult in terms of cost, and the latter not only raises the cost, but also cannot expect a sufficient heat radiation effect in, for example, equipment required to operate in a high temperature environment. In some cases, stress due to fever is inevitable.

Another essential measure for solving such a problem is to suppress the heat generation of the output transistor itself. For example, a device that detects the temperature of the output transistor using a temperature sensor or the like is provided, and a device that monitors the temperature of the output transistor detected by the device and shuts off the power supply when the temperature reaches a certain level or more is attached. However, it is possible. However, such a straightforward measure is a reliable measure, but can be used only in a limited field because circuits and devices are complicated and expensive. Therefore, in order to reliably prevent the output transistor from being damaged or shortened in life due to the heat generated when the output is short-circuited, etc., it is necessary to suppress the heat itself of the output transistor caused by the overcurrent, and to increase the practical value. It is an issue to realize a circuit with a simple and inexpensive configuration by using general and easily available elements instead of using elements and expensive elements and devices.

The present invention has been made to solve such a problem, and realizes a transistor overcurrent protection circuit that suppresses heat generation of an output transistor itself when the output is short-circuited, with a simple circuit configuration. The purpose is to:

[0015]

In order to solve such a problem, according to the present invention, a transistor drive type circuit (FIG. 4) is used as a prototype, and charging and discharging are performed by an overcurrent detection circuit. Delay means embodied in a capacitor for
A third transistor that switches based on the charging voltage of the capacitor is provided, and the output transistor is controlled when an overcurrent is detected by interposing the third transistor.

Thus, the charging / discharging of the charging / discharging capacitor is performed.
The timing of transistor switching at the time of overcurrent detection is delayed according to the discharge time. Therefore, even if an oscillation occurs due to an output short circuit or the like, heat generation due to switching is reduced because the frequency is low. Moreover,
Since the time during which the output current is cut off is prolonged and the proportion of the energization time is relatively reduced, the average current flowing through the output transistor is also reduced. As a result, heat generation of the output transistor itself can be suppressed.

Therefore, according to the present invention, a transistor overcurrent protection circuit for suppressing the heat generation of the output transistor itself when the output is short-circuited or the like can be realized with a simple configuration related to the capacitor and its charging and discharging.

When the output current cut-off time is lengthened in this way, the above-mentioned advantages are obtained, but the spike-like overcurrent is also cut off clearly for a predetermined time. Therefore, in a field in which the output current interruption in a normal use state is not allowed or avoided except in an emergency, the rate at which the spike current countermeasure type circuit (FIG. 5) must be used as a prototype increases. . In this case, a spike current bypass capacitor is required in addition to the charge / discharge capacitor. However, a capacitor has larger variation and aging than a resistor or the like. In addition, it is difficult to incorporate the circuit into an IC when the circuit is formed into an IC, which tends to hinder miniaturization. Therefore, by adopting a configuration capable of suppressing a response to a spike-like overcurrent without using a spike current bypass capacitor (fourth embodiment below), inconvenience with respect to the spike current can be reduced. .

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A description will be given of an embodiment for implementing a transistor overcurrent protection circuit of the present invention achieved by such a solution.

[First Embodiment] The first embodiment of the present invention (as described in claim 1 at the time of filing the application) comprises the following first transistor, current detection resistor, and second transistor. In the transistor overcurrent protection circuit provided with the following, the following capacitor, a third transistor, and output current cutoff means are provided. The first transistor is connected (inserted in series) between a load and a power supply, and switches between a conduction state and a cutoff state in response to an input signal (received from outside or from an internal preceding circuit). And) switching an output current from the power supply to the load. The current detection resistor is connected (inserted in series) between the power supply and the first transistor (so that the current passes). The second transistor performs a switching operation based on a potential difference between both ends of the current detection resistor. When the potential difference (both ends of the current detection resistor) exceeds a predetermined value (corresponding to whether or not an overcurrent occurs), the second
The first transistor is turned off (and protected from overcurrent) in response to the switching of the transistor. The charge / discharge capacitor is charged by a current passing through the second transistor. The third transistor performs a switching operation based on a charging voltage of the capacitor. The output current cutoff means performs switching control of the first transistor according to switching of the second transistor via the third transistor (indirectly).

In such a transistor overcurrent protection circuit, when an overcurrent is detected, the first transistor is switched to the cut-off state in accordance with the switching of the second transistor. Instead of the first transistor being driven directly by the second transistor, a capacitor that is charged via the second transistor and a third transistor that switches based on the charged voltage of the capacitor are interposed. , Driven indirectly. When the current is cut off, the first transistor is switched to the conductive state in accordance with the switching of the second transistor to the original state. At this time, charging of the capacitor by the second transistor stops. The capacitor is discharged from the capacitor, and the third transistor is switched based on the discharge to return to the original state.
Also at this time, the first transistor is indirectly driven via the discharging capacitor and the third transistor.

Thus, by specifying the charge state and the discharge state of the capacitor, the cutoff time and conduction time of the output transistor when an overcurrent is detected can be easily designed and set. By increasing the ratio of the cutoff time, the average current flowing through the first transistor when an accident such as an output short circuit occurs can be reduced, and heat generation there can be suppressed. Furthermore, the charge state / discharge state of the capacitor can be embodied by providing a resistor or the like that regulates the charge / discharge current around the capacitor and selecting the resistance value, and the circuit configuration is simple. Can be done with things. Moreover, such a circuit is cheaper and easier to miniaturize than a circuit using a heat sink or a temperature sensor.

[Second Embodiment] The second embodiment of the present invention (as described in claim 2 at the beginning of the application) is the transistor overcurrent protection circuit of the first embodiment described above. Each of the transistors and the output current cutoff means is as follows. The first transistor is a MOS transistor. (The second transistor performs a switching operation so as to be turned on when the potential difference exceeds a predetermined value.) (The third transistor is turned on when the second transistor is turned on. The output current cutoff means performs a switching operation in accordance with a switching operation of the third transistor so as to conduct when the third transistor is energized. A fourth transistor is provided. The fourth transistor controls the state of conduction between the gate and the source of the first transistor.

In this case, when any one of the second, third, and fourth transistors is turned on, the potential of the gate and the source of the MOS transistor become substantially equal, so that the first transistor is turned off. Thus, even when one of the second to fourth transistors, which is an additional active element, is damaged and becomes an energized state such as a short circuit state, the first
Are turned off, so that the circuit state is maintained on the safe side where there is no possibility of causing a chain damage or the like.

[Third Embodiment] A third embodiment of the present invention (as described in claim 3 at the beginning of the application) is the transistor overcurrent protection circuit of the first embodiment described above. Then, in response to the switching operation of the third transistor (due to the potential difference exceeding a predetermined value), the first transistor is forced to turn off the potential of the line of the input signal or the line following the input signal. Is maintained at a potential level on the side where

In this case, when the third transistor operates according to the overcurrent, the line side of the input signal is held at a predetermined potential level, and the first transistor is turned off. As a result, the drive circuit that receives the input signal and generates the drive signal for the first transistor is also used as the output cutoff control means, eliminating the need for providing the fourth transistor and the like in the above-described embodiment. Can be reduced.

[Fourth Embodiment] A fourth embodiment of the present invention (as described in claim 4 at the outset of the application) is the transistor of the first, second and third embodiments described above. An overcurrent protection circuit including the following charging resistor and discharging resistor. The charging resistor (provided by connecting the second transistor and the capacitor) is provided, by regulating a charging current of the capacitor when charging the capacitor with a current passing through the second transistor. The inrush current flowing through the second transistor is limited to prevent stress applied to the second transistor. The discharge resistor regulates a discharge current of the capacitor when discharging from the capacitor after charging. This keeps the third transistor on for a while after the second transistor is turned off.

In this case, when the capacitor is charged, its charging voltage rapidly rises above a predetermined voltage required for turning on the third transistor, and when the capacitor is discharged, the charging voltage slowly drops to the predetermined voltage. . Therefore, it is possible to simply and reliably operate the transistor overcurrent protection circuit simply by connecting the charging resistor and the discharging resistor to the charging / discharging capacitor. In addition, since the charging speed of the capacitor is regulated by the charging resistor, if there is no regulation, even if the spike current has turned on the second transistor, the spike current for a short time turns on the third transistor. You will not be able to do it. As a result, the disadvantage of the spike current can be overcome to some extent without using the spike current bypass capacitor.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a transistor overcurrent protection circuit according to the present invention will be described in detail with reference to the circuit shown in FIG.

The overcurrent protection circuit 100 is an improvement of the overcurrent protection circuit 50 shown in FIG.
A delay means (101, R101 to R103), a delay operation transistor 102 (third transistor), and an output cutoff control circuit are connected to a connection line between the emitter of the overcurrent detection transistor 53 and the gate of the output transistor 42. 103 are sequentially inserted and connected.

The delay means includes an overcurrent detecting transistor 5
A charging resistor R101 and discharging resistors R102 and R103, which are connected in series between the emitter of No. 3 and the ground GND;
One end is connected to a connection point of the resistors R101 and R102, and the other end is connected to a charging / discharging capacitor 101 grounded. Here, the discharging resistors R102 and R103 have a discharging time constant of the charging / discharging capacitor 101 of the output transistor 42 in order to perform the discharging slowly and prolong the output current interruption time.
The resistance value is set to be relatively large so as to be several times or more the switching time.

The delay operation transistor 102 is composed of an NPN transistor having a base connected to the connection point of the resistors R102 and R103, an emitter grounded, and a collector connected to the output cutoff control circuit 103 side. The switching operation is performed based on the divided voltage of the charging voltage.

The output cutoff control circuit 103 comprises a cutoff control drive transistor 104 of a PNP transistor, a bias resistor R7 connected between the base and the emitter of the transistor 104, and a resistor R6 connected to the base of the transistor 104. , And the collector of the transistor 102 via the resistor R6. The collector of the cutoff control drive transistor 104 is connected to the gate of the output transistor 42. Each resistor R6, R7 is
The respective resistance values are set so as to perform voltage setting and current limitation for turning on / off the cutoff control drive transistor 104 in response to turning on / off of the delay operation transistor 102. Thereby, the output cutoff control circuit 103
Is driven by the delay operation transistor 102,
When it is turned on, the output transistor 42 is turned off by making the gate voltage of the output transistor 42, that is, the drive signal C substantially equal to the source voltage.

The usage and operation of the transistor overcurrent protection circuit of the first embodiment will be described. When the input signal A is low, the output transistor 42 is controlled to be in the off state, so that there is no problem of overcurrent. Therefore,
When the input signal A is high and the drive signal C has dropped to a significant voltage by the drive circuit 41, the output transistor 4
A case in which 2 is controlled to the ON state will be described.

In this case, in the normal state, the output current D is within the proper range, so that the overcurrent detecting transistor 53 is off, and the charging / discharging capacitor 101 is not charged, so that the delay operating transistor 102 is also off. State, and correspondingly, the cutoff control drive transistor 104
Are also in the off state, and they do not prevent the output current D from being switched according to the input signal A.

On the other hand, when the output current D becomes abnormally large due to the damage of the load 20 and an overcurrent flows, the potential difference between both ends of the current detecting resistor 52 exceeds a predetermined value, and the overcurrent detecting transistor 53 is turned on. State, and the current from the overcurrent detection transistor 53 is charged by the charging resistor R1.
01, the capacitor 101 is charged.
This charging is performed more quickly than the discharging by the discharging resistors R102 and R103. And the charging and discharging capacitor 1
01 to a certain extent, the delay operation transistor 10 is slightly delayed from the overcurrent detection transistor 53.
2 is also turned on. As a result, the output cutoff control circuit 103 operates, that is, the cutoff control drive transistor 104 is turned on, and the drive signal C is forcibly set to the power supply Vcc side, so that the output transistor 42 is turned off from the on state. Switch to state. As a result, the output current D is cut off and the overcurrent is prevented.

At this time, the charging of the charging / discharging capacitor 101 is performed more quickly than the switching of the output transistor 42 from the on state to the off state, so that when the output transistor 42 is completely turned off, the charging / discharging capacitor 101 becomes: The battery is charged to a state exceeding a predetermined voltage required for turning on the delay operation transistor 102.

When the output current D is cut off, the transistor 53 for detecting overcurrent is turned off. As a result, charging of the charging / discharging capacitor 101 is stopped, and discharging from the charging / discharging capacitor 101 is performed gently via the discharging resistors R102 and R103. This state continues until the charging voltage of the charging / discharging capacitor 101 drops to the predetermined voltage. When the discharging of the charge / discharge capacitor 101 proceeds to that point, the delay operation transistor 102 is turned off, and the output cutoff control circuit 103 also stops operating. Then, the drive signal C returns to a state corresponding to the input signal A, and the output transistor 42 is turned on again.

At this time, when the output current D becomes excessive again, the above-mentioned operation is repeated, but the non-energization time is sufficiently longer than the output current D energization time. When the load 20 is repaired and the overcurrent stops flowing, the above-described repetition is interrupted and the normal state is restored.

The specific configuration of the second embodiment of the transistor overcurrent protection circuit of the present invention will be described with reference to the circuit of FIG.

This overcurrent protection circuit 200 is different from the above-mentioned overcurrent protection circuit 100 in the following points. That is, in the drive circuit 241 instead of the drive circuit 41, the resistor R1 is divided into the series resistors R1a and R1b. Further, the collector of the delay operation transistor 102 is connected to the resistors R1a and R1b.
, The output cutoff control circuit 103 is eliminated.

The overcurrent protection circuit 200 is configured to receive the input signal A
When the output transistor 42 is controlled to be in an off state in accordance with the following equation, and when the output current D is within an appropriate range even when the output transistor 42 is controlled to be in an on state, the overcurrent detection transistor 53 and the delay operation transistor 102 maintains the off state, and the output current D
Switching normally.

On the other hand, when the overcurrent is detected by the overcurrent detection circuit 51, the charging of the charging / discharging capacitor 101 and the delay operation of the transistor 10 are performed in the same manner as described above.
2 is performed. Then, the resistors R1a, R
The connection point 1b is almost in the ground state. As a result, the input signal A becomes low for the transistor 41a, the drive signal C becomes invalid, and the output transistor 42 switches from the on state to the off state. As a result, the output current D is cut off and the overcurrent is prevented.

When the overcurrent detecting transistor 53 is turned off, the discharging of the charging / discharging capacitor 101 and the delay operation of the transistor 1 are performed in the same manner as in the above-described example.
02 is performed. Then, the drive signal C returns to a state corresponding to the input signal A, and the output transistor 42 is turned on again. Thus, the overcurrent protection circuit 200 exhibits the same function as the overcurrent protection circuit 100 even when the output cutoff control circuit 103 is omitted.

[0045]

As is apparent from the above description, in the transistor overcurrent protection circuit of the present invention, the timing of transistor switching at the time of overcurrent detection is determined according to the charge / discharge time of the charge / discharge capacitor. The delay is advantageous because a transistor overcurrent protection circuit that suppresses the heat generation of the output transistor itself when the output is short-circuited or the like can be realized with a simple configuration related to the capacitor and its charging and discharging. Has a significant effect. Further, there are effects such as low cost, downsizing, and easy design of output cutoff time.

In the second embodiment, the second to second embodiments
There is also an effect that the circuit state is maintained on the safe side against the damage of the fourth transistor.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of a transistor overcurrent protection circuit of the present invention.

FIG. 2 is a circuit diagram of a second embodiment.

FIG. 3 is a conventional overcurrent protection circuit (fuse blowing type).

FIG. 4 shows a conventional overcurrent protection circuit (transistor drive type).

FIG. 5 shows a conventional overcurrent protection circuit (spike current countermeasure type).

[Explanation of symbols]

Reference Signs List 10 DC voltage source 20 Load 30 Fuse 40 Overcurrent protection circuit 41 Drive circuit 42 Output transistor (P-channel MOS type FET;
(First transistor) 43 diode 50 overcurrent protection circuit 51 overcurrent detection circuit 52 current detection resistor 53 overcurrent detection transistor (second transistor) 60 overcurrent protection circuit 61 spike current bypass capacitor 100 overcurrent protection circuit 101 filling Discharge capacitor 102 Delayed operation transistor (third transistor) 103 Output cutoff control circuit (output current cutoff means) 104 Cutoff control drive transistor (fourth transistor) 200 Overcurrent protection circuit 241 drive circuit 300 Overcurrent protection circuit 341 drive circuit R101 Charging resistor R102 Discharging resistor R103 Discharging resistor R301 Bias resistor R302 Bias control resistor R303 Current limiting resistor

Claims (4)

[Claims] A first transistor connected between a load and a power supply for switching an output current from the power supply to the load in response to an input signal; and a first transistor connected between the power supply and the first transistor. And a second transistor that performs a switching operation based on a potential difference between both ends of the current detection resistor. When the potential difference exceeds a predetermined value, the second transistor is switched in accordance with the switching of the second transistor. In a transistor overcurrent protection circuit in which one transistor is turned off, a charge / discharge capacitor charged by a current through the second transistor, and a third transistor that performs a switching operation based on a charge voltage of the capacitor. , The first transistor according to the switching of the second transistor via the third transistor. Overcurrent protection circuit of the transistor, characterized in that an output current interrupting means for performing switching control of the register. 2. The semiconductor device according to claim 1, wherein the first transistor is a MOS transistor, and the output current cutoff means performs a switching operation in accordance with a switching operation of the third transistor so as to conduct when the third transistor is energized. 2. The semiconductor device according to claim 1, further comprising a fourth transistor, wherein the fourth transistor controls a current-carrying state between a gate and a source of the first transistor.
An overcurrent protection circuit for the transistor as described. 3. A potential of a line of the input signal or a line following the input signal is forcibly held at a potential level on a side where the first transistor is turned off in accordance with a switching operation of the third transistor. The transistor overcurrent protection circuit according to claim 1, wherein: 4. The method according to claim 1, wherein the charging of the capacitor is performed in order to limit an inrush current flowing to the collector of the second transistor when charging the capacitor with a current passing through the second transistor and to prevent a stress applied to the second transistor. A charging resistor for regulating current, and the capacitor so that the on-time of the third transistor is delayed when discharging from the capacitor after charging, so that the average current flowing through the first transistor when the output is short-circuited is sufficiently small. 4. A transistor overcurrent protection circuit according to claim 1, further comprising: a discharge resistor for regulating a discharge current of the transistor.