JP3679524B2 - Transistor overcurrent protection circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an overcurrent protection circuit for a transistor, and more specifically, when a load such as a display lamp or a solenoid is driven using a DC power supply as a drive source, an overcurrent caused by a short circuit or the like of a transistor that opens / closes a current from the power supply to the load. The present invention relates to an overcurrent protection circuit for a transistor for protecting from a current.
[0002]
[Prior art]
Conventionally, a basic circuit for driving a lamp, a solenoid or the like by a transistor is shown in FIG.
This circuit (40) is for supplying electric power from a DC voltage source 10 of output voltage Vcc to a load 20 such as a solenoid, and a fuse 30 and an output transistor 42 (first transistor) are connected to the electric power supply line. Transistors) are sequentially inserted and connected in series.
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 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 circuit such as the output transistor 42.
[0003]
The overcurrent protection circuit 40 receives an input signal A from an external circuit in order to control the switching operation of the output transistor 42 in addition to the fuse 30 and the output transistor 42 composed of a power transistor of a MOS FET. A drive circuit 41 that generates a drive signal C to the gate of the output transistor 42, and a load 20 for suppressing the generation of a surge voltage due to a back electromotive force when the load 20 is an inductive load such as a solenoid. A flywheel diode 43 connected in parallel is also provided.
[0004]
This drive circuit 41 is composed of an NPN transistor whose base is connected to the voltage dividing point of the resistors R1 and R2 of the input signal A, whose emitter is grounded, and whose collector is connected to the fuse 30 via series resistors R3 and R4. The drive signal C is generated by the connection point voltage of R3 and R4. 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 causing the current B through the fuse 30 from the DC voltage source 10 to flow through 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.
[0005]
The output transistor 42 has a gate connected to the drive signal C line, a source connected to the fuse 30, and a drain connected to the load 20 and the cathode of a diode 43 connected in parallel thereto. The drive signal C and the input signal A are turned on / off in response to the drive signal C.
[0006]
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 overcurrent protection circuit 40 described above. As a result, when an overcurrent is detected, the output transistor 42 is immediately turned off regardless of the input signal A to try to prevent the overcurrent.
[0007]
For this purpose, the overcurrent detection circuit 51 includes a current detection resistor 52 and an overcurrent detection transistor 53 (second transistor).
The current detection resistor 52 is inserted and connected in series with the power supply line between the DC voltage source 10 and the source of the output transistor 42, and a voltage corresponding to the load current from the DC voltage source 10 to the load 20 is applied. It is generated at both ends of the resistor 52. Moreover, the resistance value is a voltage corresponding to the base-emitter voltage necessary for turning on the transistor when the load current is going to be excessive (that is, a voltage having a predetermined value corresponding to whether or not the overcurrent is present). Is set to occur.
[0008]
The overcurrent detection transistor 53 is a PNP transistor for switching, the emitter is connected to the power supply line, and the base is connected to the connection point between the current detection resistor 52 and the collector of the output transistor 42 via the protective resistor R5. And the collector is connected to the gate line of the output transistor 42. 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 is switched from the off state to the on state. As a result, the drive signal C is made substantially equal to the voltage Vcc, so that the output transistor 42 is forcibly turned off.
[0009]
The overcurrent protection circuit 60 shown in FIG. 5 is obtained by adding a spike current bypass capacitor 61 in parallel to 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 the current flowing into the capacitance that is parasitic or added to the load 20 and the current that cancels the charge / current of the diode 43. Depending on the degree, the overcurrent detection circuit 51 may malfunction.
The spike current bypass capacitor 61 is for diverting the spike current from the current detection resistor 52 to prevent malfunction of the overcurrent detection circuit 51, and its capacity is determined according to the amount of spike current.
[0010]
[Problems to be solved by the invention]
However, in such a conventional transistor overcurrent protection circuit, in the case of the circuit of FIG. 3 (fuse blown type), when an overcurrent flows due to an output short circuit accident or the like, the fuse is blown and the circuit is protected. Even if there is no damage to the pattern of the circuit element or the printed wiring board, the function of the circuit cannot be recovered unless the fuse is replaced. Also, it is difficult to determine how much the stress due to overcurrent has affected the life and performance of the output transistor. Even if the function is restored by replacing the fuse, the performance and reliability of the circuit / device It is impossible to conclude that it has recovered to the same level as before the occurrence of the short-circuit accident.
[0011]
On the other hand, in the case of the circuit of FIG. 4 (transistor drive type), since the fuse is not used, the function is restored immediately when 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. When the output transistor 42 is turned off, no current flows through the current detection resistor 52, and the overcurrent detection transistor 53 is turned off. The output transistor 42 is turned on again, but if the output short circuit state is not solved, the overcurrent detection transistor 53 is turned on again. As a result, the output transistor 42 oscillates at a high frequency corresponding to the switching speed of the transistor. Since energization / non-energization is frequently repeated, a considerably large average output current flows. In addition, the energy loss in the transistor is larger during such switching than in the off state or the on state. For this reason, if the output is short-circuited, the output transistor generates significant heat.
[0012]
In order to prevent damage to the output transistor due to this heat generation and a decrease in life, the output transistor itself must be sufficiently powered up, or a heat sink can be attached to the output transistor, or the heat sink can be replaced with a high-performance one. There are measures such as increasing the heat dissipation effect. However, the former has an excessive specification more than necessary and is difficult in terms of cost, while the latter is not only costly, but it cannot be expected to have a sufficient heat dissipation effect, for example, in devices that require operation in a high temperature environment. There are cases where stress due to fever is unavoidable.
[0013]
Another essential measure for solving such a problem is to suppress the heat generation of the output transistor itself. For example, a means for detecting the temperature of the output transistor using a temperature sensor or the like is provided, the temperature of the detected output transistor is monitored by this means, and a device that shuts off the power supply when the temperature exceeds a certain level is attached. Is possible.
However, this straightforward measure is a reliable measure, but the circuit / device becomes complicated and expensive, and can only be used in a limited field.
Therefore, in order to prevent damage to the output transistor due to heat generation when the output is short-circuited, shortening the service life, etc., it is necessary to suppress the heat generation of the output transistor due to overcurrent itself and to increase the practical value. There is a problem of realizing a simple and inexpensive circuit using a general and easily available element, not using an element or an expensive element / equipment.
[0014]
The present invention has been made to solve such a problem, and an object thereof is to realize a transistor overcurrent protection circuit that suppresses the heat generation of the output transistor itself when the output is short-circuited with a simple circuit configuration. And
[0015]
[Means for Solving the Problems]
In order to solve such a problem, in the present invention, a delay means embodied by a charge / discharge capacitor charged by an overcurrent detection circuit based on a transistor drive type circuit (FIG. 4). And a third transistor that switches based on the charging voltage of the capacitor, and this transistor is interposed to control the output transistor when overcurrent is detected.
[0016]
Thereby, the transistor switching timing at the time of overcurrent detection is delayed according to the charging / discharging time of the charging / discharging capacitor. Therefore, even if there is an output short circuit or the like and the oscillation state occurs, the frequency is low, so that heat generation due to switching is reduced. Furthermore, since the time during which the output current is cut off becomes longer and the energization time ratio is relatively reduced, the average current flowing through the output transistor is also reduced. As a result, the heat generation itself of the output transistor can be suppressed.
[0017]
Therefore, according to the present invention, a transistor overcurrent protection circuit that suppresses the heat generation of the output transistor itself when the output is short-circuited can be realized with a simple configuration related to the capacitor and its charge and discharge.
[0018]
In addition, when the output current cut-off time is increased in this way, the above-described advantages can be obtained. On the other hand, the spike-like overcurrent is clearly cut off only for a predetermined time. Therefore, in an area where output current interruption in normal use is not permitted or avoided apart from an emergency, the rate at which the spike current countermeasure circuit (FIG. 5) must be used as a prototype becomes high. . In this case, a spike current bypass capacitor is required in addition to the charge / discharge capacitor. However, capacitors are more subject to variations and aging than resistance elements. In addition, it is difficult to incorporate a circuit into an IC when it is made into an IC, which tends to hinder miniaturization. Therefore, by adopting a configuration capable of suppressing the response to spike-like overcurrent without using a spike current bypass capacitor (fourth embodiment below), inconvenience with respect to spike current can be reduced. .
[0019]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment for implementing the overcurrent protection circuit for a transistor of the present invention achieved by such a solution will be described.
[0020]
[First Embodiment]
The first embodiment of the present invention (as described in claim 1 at the beginning of the application)
A transistor overcurrent protection circuit comprising the following first transistor, current detection resistor, and second transistor is characterized by comprising the following capacitor, a third transistor, and an output current cutoff means: .
The first transistor is connected (inserted in series) between the load and the power supply, and switches between a conductive state and a cut-off state in response to an input signal (received from the outside or an internal preceding circuit). In this case, the output current from the power source to the load is switched.
The current detection resistor is connected (inserted in series) between the power source and the first transistor (so that the current passes therethrough).
The second transistor performs a switching operation based on a potential difference between both ends of the current detection resistor.
When the potential difference (at both ends of the current detection resistor) exceeds a predetermined value (corresponding to whether or not it is an overcurrent), the first transistor is turned off according to the switching of the second transistor. (And thus protected from overcurrent).
The charging / discharging capacitor is charged by a current through the second transistor.
The third transistor performs a switching operation based on the charging voltage of the capacitor.
The output current cut-off means performs switching control of the first transistor according to switching of the second transistor via the third transistor (indirectly).
[0021]
In such an overcurrent protection circuit for a transistor, when an overcurrent is detected, the first transistor is switched to a cut-off state in accordance with the switching of the second transistor. The transistor is not directly driven by the second transistor, but indirectly through a capacitor charged via the second transistor and a third transistor that switches based on the charging voltage of the capacitor. Driven by. Further, 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 is stopped. Then, the capacitor is discharged, and based on this, the third transistor is switched to return to the original state. Also at this time, the first transistor is indirectly driven through the discharging capacitor and the third transistor.
[0022]
Thereby, it is possible to easily design and set the cutoff time and conduction time of the output transistor when an overcurrent is detected easily by defining the charging state and discharging state of the capacitor. Then, 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 charging / discharging state of the capacitor can be specified by providing a resistor or the like that regulates the charging / discharging current around the capacitor and selecting its resistance value, etc., and the circuit configuration is also simple. You can do it with things. In addition, such a circuit is cheaper and can be miniaturized more easily than a circuit using a heat sink or a temperature sensor.
[0023]
[Second Embodiment]
A second embodiment of the present invention is an overcurrent protection circuit for a transistor according to the first embodiment described above (as described in claim 2 at the beginning of the application), wherein each transistor and output current cut-off means are as follows. It is characterized by that.
The first transistor is a MOS transistor.
(The second transistor performs a switching operation so as to be energized when the potential difference exceeds a predetermined value.)
(The third transistor performs a switching operation in accordance with a switching operation of the second transistor so as to be energized when the second transistor is energized.)
The output current cut-off means includes a fourth transistor that performs a switching operation according to a switching operation of the third transistor so as to be energized when the third transistor is energized. The fourth transistor controls the energization state between the gate and source of the first transistor.
[0024]
In this case, when any one of the second, third, and fourth transistors is turned on, the potentials of the gate and the source of the MOS transistor become substantially equal, so that the first transistor is turned off.
As a result, even if any of the second to fourth transistors, which are additional active elements, are damaged and become energized such as a short-circuited state, the first transistor is cut off. The circuit state is maintained on the safe side without causing any damage.
[0025]
[Third Embodiment]
The 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,
In response to the switching operation of the third transistor (because the potential difference has exceeded a predetermined value), the potential of the input signal line or its subsequent line is forcibly made non-conductive. It is characterized by being held at the potential level on the side.
[0026]
In this case, when the third transistor is activated in response 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 shared by the output cutoff control means, and it is not necessary to provide the fourth transistor or the like in the above-described embodiment. Can be reduced.
[0027]
[Fourth Embodiment]
A fourth embodiment of the present invention (as described in claim 4 at the beginning of the application) is an overcurrent protection circuit for the transistor of the first, second, and third embodiments described above, and the following charging And a discharging resistor.
The charging resistor (provided by connecting the second transistor and the capacitor) regulates the charging current of the capacitor when charging the capacitor by the current through the second transistor, The inrush current flowing through the second transistor is limited to prevent stress applied to the second transistor.
The discharging resistor regulates the discharging current of the capacitor upon discharging from the capacitor after charging. Thus, even after the second transistor is turned off, the third transistor remains on for a while.
[0028]
In this case, when the capacitor is charged, the charging voltage quickly rises above a predetermined voltage required to turn on the third transistor, and when the capacitor is discharged, the charging voltage slowly decreases to the predetermined voltage. Therefore, the transistor overcurrent protection circuit can be operated simply and reliably by simply 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, even if the spike current used to turn on the second transistor is not regulated, the third transistor is turned on for a short time. I can't do that.
As a result, the inconvenience of the spike current can be overcome to some extent without using a spike current bypass capacitor.
[0029]
【Example】
A specific configuration of the first embodiment of the transistor overcurrent protection circuit of the present invention will be described with reference to the circuit of FIG.
[0030]
This overcurrent protection circuit 100 is an improvement over the overcurrent protection circuit 50 shown in FIG. 4 in the conventional example. With respect to the connection line between the emitter of the overcurrent detection transistor 53 and the gate of the output transistor 42, The delay means (101, R101 to R103), the delay operation transistor 102 (third transistor), and the output cutoff control circuit 103 are sequentially inserted and connected.
[0031]
The delay means includes a charging resistor R101 and discharging resistors R102 and R103 connected in series between the emitter of the overcurrent detecting transistor 53 and the ground GND in order, one end connected to a connection point between the resistors R101 and R102, and the other end. It comprises a grounded charging / discharging capacitor 101. Here, the discharging resistors R102 and R103 are designed so that the discharging time constant of the charging / discharging capacitor 101 is several times the switching time of the output transistor 42 in order to discharge slowly and lengthen the output current interruption time. The resistance value is set to be relatively large.
[0032]
The delay operation transistor 102 is composed of an NPN transistor having a base connected to a connection point between the resistors R102 and R103, an emitter grounded, and a collector connected to the output cutoff control circuit 103 side. Switching operation is performed based on the partial pressure.
[0033]
The output cutoff control circuit 103 includes a cutoff control drive transistor 104 of a PNP transistor, a bias resistor R7 connected between the base and emitter of the transistor 104, and a resistor R6 connected to the base of the transistor 104. The resistor R6 To the collector of the transistor 102. Further, the collector of the cutoff control driving transistor 104 is connected to the gate of the output transistor 42. The resistance values of the resistors R6 and R7 are set so as to perform voltage setting and current limitation so as to turn on and off the cutoff control driving transistor 104 in response to the on / off of the delay operation transistor 102. As a result, the output cutoff control circuit 103 is driven by the delay operation transistor 102, and 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. ing.
[0034]
The use mode and operation of the overcurrent protection circuit for the transistor 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 there is no problem of overcurrent. Therefore, a case will be described in which the input signal A is high, the drive signal C is dropped to a significant voltage by the drive circuit 41, and the output transistor 42 is controlled to be in the ON state.
[0035]
In this case, in the normal state, since the output current D is within the appropriate range, the overcurrent detection transistor 53 is in the off state, and the charge / discharge capacitor 101 is not charged, so the delay operation transistor 102 is also in the off state. Correspondingly, the cutoff control drive transistor 104 is also in the OFF state, and these do not prevent the output current D from being switched according to the input signal A.
[0036]
On the other hand, when the output current D becomes abnormally large due to damage of the load 20 and overcurrent flows, the potential difference between both ends of the current detection resistor 52 exceeds a predetermined value, and the overcurrent detection transistor 53 is turned on. Furthermore, the current from the overcurrent detection transistor 53 is charged into the charging / discharging capacitor 101 via the charging resistor R101. This charging is performed more quickly than discharging by the discharging resistors R102 and R103. Then, when charging of the charge / discharge capacitor 101 proceeds to some extent, the delay operation transistor 102 is also turned on with a slight delay from the overcurrent detection transistor 53. As a result, the output cutoff control circuit 103 is activated, that is, the cutoff control drive transistor 104 is turned on, and the drive signal C is forced 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 overcurrent is prevented.
[0037]
At this time, the charging / discharging capacitor 101 is charged more quickly than the switching of the output transistor 42 from the on state to the off state. Therefore, when the output transistor 42 has been turned off, the charging / discharging capacitor 101 is used for delay operation. The battery 102 is charged to a state where a predetermined voltage necessary for turning on the transistor 102 is exceeded.
[0038]
When the output current D is cut off, the overcurrent detection transistor 53 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 slowly 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 charging / discharging capacitor 101 proceeds to that extent, 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 the state corresponding to the input signal A, and the output transistor 42 is turned on again.
[0039]
At this time, when the output current D becomes excessive again, the above operation is repeated, but the non-energization time is sufficiently longer than the energization time of the output current D. When the load 20 is repaired and no overcurrent flows, the above-described repetition is interrupted and the normal state is restored.
[0040]
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.
[0041]
This overcurrent protection circuit 200 is different from the above-described overcurrent protection circuit 100 in the following points. That is, in the drive circuit 241 that replaces the drive circuit 41, the resistor R1 is divided into series resistors R1a and R1b. Further, the collector of the delay operation transistor 102 is connected to the connection point of the resistors R1a and R1b, and the output cutoff control circuit 103 is eliminated.
[0042]
In the overcurrent protection circuit 200, when the output transistor 42 is controlled to be in an OFF state according to the input signal A, and when the output current D is within an appropriate range even if the output transistor 42 is controlled to be in an ON state, The overcurrent detection transistor 53 and the delay operation transistor 102 are kept off, and the output current D is normally switched in accordance with the input signal A.
[0043]
On the other hand, when the overcurrent is detected by the overcurrent detection circuit 51, the charging / discharging capacitor 101 is charged and the delay operation transistor 102 is switched in the same manner as described above. As a result, the connection point of the resistors R1a and R1b is almost grounded. 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 overcurrent is prevented.
[0044]
When the overcurrent detection transistor 53 is turned off, the charge / discharge capacitor 101 is discharged and the delay operation transistor 102 is switched in the same manner as in the above example. Then, the drive signal C returns to the 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 if the output cutoff control circuit 103 is omitted.
[0045]
【The invention's effect】
As is apparent from the above description, in the transistor overcurrent protection circuit of the present invention, the transistor switching timing at the time of overcurrent detection is delayed according to the charge / discharge time of the charge / discharge capacitor. Thus, there is an advantageous effect that the transistor overcurrent protection circuit that suppresses the heat generation of the output transistor itself when the output is short-circuited can be realized with a simple configuration related to the capacitor and its charge and discharge. In addition, there are effects such as low cost, miniaturization, and easy design of the output cutoff time.
[0046]
The second embodiment also has an effect that the circuit state is maintained on the safe side against damage of the second to fourth transistors.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of a first embodiment of an overcurrent protection circuit for a transistor of the present invention.
FIG. 2 is a circuit diagram of a second embodiment.
FIG. 3 is a conventional overcurrent protection circuit (fuse blown type).
FIG. 4 is a conventional overcurrent protection circuit (transistor drive type).
FIG. 5 is a conventional overcurrent protection circuit (spike current countermeasure type).
[Explanation of symbols]
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 Capacitor for spike current bypass
100 Overcurrent protection circuit
101 Charging / discharging capacitor
102 Delay actuating transistor (third transistor)
103 Output cutoff control circuit (output current cutoff means)
104 Shutdown control drive transistor (fourth transistor)
200 Overcurrent protection circuit
241 drive circuit
300 Overcurrent protection circuit
341 Drive circuit
R101 Resistor for charging
R102 Discharge resistance
R103 Discharge resistor
R301 Bias resistor
R302 Bias control resistor
R303 Current limiting resistor

Claims (4)

A first transistor connected between a load and a power supply and switching an output current from the power supply to the load in response to an input signal; and a current detection resistor 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 first transistor is cut off in accordance with the switching of the second transistor. In the overcurrent protection circuit of the transistor to be in a state, a charging / discharging capacitor charged by a current through the second transistor, a third transistor that performs a switching operation based on a charging voltage of the capacitor, and the third transistor The first transistor scans in response to switching of the second transistor through a transistor. An output current interrupting means for performing etching control, the second transistor and the charging resistor to perform rapid charging than the switching of the first transistor during charging of the capacitor provided interposed connected to said capacitor and A discharge resistor provided to be inserted and connected to the third transistor and the capacitor, and discharging at a time more than twice as long as the switching of the first transistor when discharging the capacitor. A transistor overcurrent protection circuit.   The first transistor is a MOS transistor, and the output current cut-off means includes a fourth transistor that performs a switching operation according to a switching operation of the third transistor so as to be energized when the third transistor is energized. 2. The overcurrent protection circuit for a transistor according to claim 1, wherein the fourth transistor controls an energization state between the gate and the source of the first transistor.   In accordance with the switching operation of the third transistor, the potential of the line of the input signal or the subsequent line is forcibly held at a potential level on the side where the first transistor becomes non-conductive. The overcurrent protection circuit of the transistor according to claim 1. The charging resistor limits the inrush current that flows to the collector of the second transistor when the capacitor is charged by the current through the second transistor, and prevents stress applied to the second transistor. The discharging resistor regulates a charging current , and the discharging resistor delays the on-time of the third transistor upon discharging from the capacitor after charging, and the average current flowing through the first transistor when the output is short-circuited is sufficient. overcurrent protection circuit of a transistor according to any of claims 1 to 3, characterized in that for regulating the discharge current of the capacitor to be smaller.

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