JPS61296819A - Composite switch element circuit - Google Patents

Abstract

PURPOSE:To attain protection against excess current by supervising the drain- source voltage of the 2nd FET 2 and decreasing the on-signal voltage given to the 1st FET and the 2nd FET when it is discriminated as an abnormal voltage so as to limit the current of a composite switch element. CONSTITUTION:When an on-signal is applied between signal terminals G and S to increase the collector current, the source-drain voltage of the 2nd FET 2 is increased proportional to it. When the source-drain voltage of the 2nd FET 2 exceeds the voltage of a constant voltage diode 5, an additional TR 6 is turned on to decrease the gate-source voltage of the 1st FET. As a result, the 1st FET 1 is about to turn off, but the current of the 2nd FET 2 is decreased and then the source-drain voltage is decreased to turn off the additional TR 6 thereby making the collector current constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a composite switch element circuit constructed by combining a bipolar transistor and an FET (field effect transistor).

[Conventional technology]

As shown in FIG. 4, a circuit of such a composite switch element includes a bipolar transistor 3 which becomes the main switch element.
The source and drain of the first FET 1 are connected between the base and collector of the bipolar transistor 3, and the drain of the second FET 2 is connected to the emitter side of the bipolar transistor 3.
It is already known that I and the gate of the second FET 2 are commonly connected, and a constant voltage diode 4 is connected between the base of the bipolar transistor 3 and the source of the 20th FET 2.

In the composite switch element, when an ON signal is applied to the gate-source terminal, the first FETI and the second FET 2 are brought into conduction, and a forward base current flows through the bipolar transistor 3, and the collector current flows through the bipolar transistor 3.

The sources also become conductive.

Next, when an off signal is applied to the gate source terminal, the first FE
TI and the second FET 2 turn off, and the collector current of the bipolar transistor 3, which had been flowing in the direction of the core emitter, has nowhere to go and flows from the collector to the base.
The current is commutated to the path of the constant voltage diode 4.

As a result, an extremely large reverse base current flows through the bipolar transistor 3, so that the bipolar transistor 3 can be turned off in a short period of time.

[Problem that the invention seeks to solve]

FIG. 5 shows the output characteristics of the composite switching element shown in FIG. 4 as described above, and this switching element has a characteristic that allows a current to flow extremely large compared to the rated current.

The reason is that the switch element is the collector.

A low source-to-source on-voltage is desired, and in order to meet this requirement, the first FETI is designed to have a large current capacity margin. As a result, an extremely large forward base current can be supplied to the bipolar transistor 3.

Figure 6 shows the equivalent of a DC short circuit accident in the impark device or chopper device of the circuit of the composite switch element. In the event of this type of short circuit accident, the terminal voltage of the switch element becomes equal to the DC power supply voltage. At this time, the larger the current flowing through the switch element, the more easily the element is destroyed.

In this way, the composite switch element shown in FIG.
Although it has the great advantage of being able to switch large currents at high speed using voltage signals, it has the disadvantage of being susceptible to overcurrent damage.

SUMMARY OF THE INVENTION An object of the present invention is to provide a composite switch element circuit which eliminates the disadvantages of the conventional example and provides protection against overcurrent.

[Means for solving problems]

In order to achieve the above-mentioned object, the present invention has a first transistor between the base and collector of the bipolar transistor 3, as shown in FIG.
The source and drain of the 10th FETI are connected, the drain of the second FET 2 is connected to the emitter side of the bipolar transistor 3, the gates of the 10th FETI and the second FET 2 are commonly connected, and the base of the bipolar transistor 3 is connected to the drain of the second FET 2. In a composite switch element in which a constant voltage diode 4 is connected between the sources of the second FET 2, the voltage between the drain and the source of the second FET 2 is monitored, and if it is determined that this is an abnormal voltage, the first FETI is The gist is that a protection circuit 10 is added that reduces the on-signal voltage applied to the second FET 2 and limits the current of the composite switch element.

[Effect]

When the FET is in the on state, it can be considered that the resistance between the source and drain becomes low. Therefore, since the voltage between the source and drain of the second FET 2 has a proportional relationship with the current of the bipolar transistor 3, the current value can be accurately determined from this voltage. Therefore, according to the present invention, when the voltage between the source and drain of the second FET 2 exceeds the reference voltage value, by lowering the on-signal level of the first and second FETs, the bipolar transistor 3 If the on-signal level of the first and second FETs is lowered when the reference voltage value is exceeded, a current exceeding that determined by the reference voltage value does not flow through the bipolar transistor 3.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 is a circuit diagram showing one embodiment of the circuit of the present invention, in which an additional transistor 6 is added as a protection circuit 10 of the circuit shown in FIG.
and a constant voltage diode 5 were used.

That is, a constant voltage diode 5 is connected between the train of the second FET 2 and the base of the additional transistor 6, connected between the emitter of the additional transistor 6 and the source of the second FET 2, and connected between the collector of the additional transistor 6. The first
connected to the gates of the first and second FETIs,
A resistor 7 is connected between the gates of the second FET2.

Added on/off signals between sources.

Next, to explain the operation, it is assumed that an on signal (for example, 10V) is applied between signal terminals G and S.

In this state, when the collector current is increased, the voltage between the source and drain of the 20th FET 2 increases in proportion to this.

When the collector current increases and the voltage between the source and drain of the second FET 2 exceeds the voltage of the constant voltage diode 5, the additional transistor 6 (bipolar transistor, FET, can be used in common) is turned on. 1 F
Reduce the voltage between the gate and source of ETI.

As a result, the first FETI tries to turn off, but if the current in the second FET 2 decreases and the voltage between the source and drain decreases, the additional transistor 6 turns off. It will remain at a constant value. The resistor 7 is inserted to prevent the signal source from overcurrent due to conduction of the additional transistor 6.

FIG. 3 shows the output characteristics of the circuit of the present invention.

〔Effect of the invention〕

As described above, the composite switch element circuit of the present invention is
The source and drain of the first FET are connected between the base and collector of the bipolar transistor, the drain of the second FET is connected to the emitter side of the bipolar transistor, and the gates of the first FET and the second FET are connected. commonly connected, the base of the bipolar transistor and the second FET
In a composite switch element circuit in which a constant voltage diode is connected between the sources of
This facilitates overcurrent protection.

[Brief explanation of drawings]

Fig. 1 is a principle circuit diagram of the composite switch element circuit of the present invention, Fig. 2 is a circuit diagram showing one embodiment of the same as above, Fig. 3 is a graph showing output characteristics, and Fig. 4 is a conventional composite switch element circuit. A circuit diagram of the element, FIG. 5 is a graph showing its output characteristics, and FIG. 6 is an equivalent circuit diagram at the time of a DC short circuit. 1...First FET 2...Second FET3
... Bipolar transistor 4 ... Constant voltage diode 5 ... Constant voltage diode 6 ... Transistor 7 ... Resistor 10 ... Overcurrent protection circuit Fig. 1 Fig. 3 Fig. 4

Claims (1)

[Claims] The source and drain of the first FET are connected between the base and collector of the bipolar transistor, the drain of the second FET is connected to the emitter side of the bipolar transistor, and the gates of the first FET and the second FET are connected. commonly connected, the base of the bipolar transistor and the second FET
In a composite switch element circuit in which a constant voltage diode is connected between the sources of the second FET, the voltage between the drain and source of the second FET is monitored, and upon determining that this is an abnormal voltage, the voltage between the first FET and the second FET is switched off. 1. A composite switch element circuit characterized in that a protection circuit is added to limit the current of the composite switch element by reducing the on-signal voltage applied to the composite switch element.