JPS61284114A - Composite switch element - Google Patents

Abstract

PURPOSE:To prevent erroneous on-operation just after turn-off by providing a discharge circuit between a base and a collector of a bipolar transistor (TR) in an element composing of a series connection of the bipolar TR and an FET. CONSTITUTION:A drain of the FET is connected in series with the emitter of the bipolar TR 1. A discharge composite of a capacitor 7, a diode 8 and a resistor 9 is connected between the collector and base of the TR 1 of the composite switch element circuit. In the circuit constitution, the FET 2 starts being turned off at first and then the commutation of the emitter current TR 1 is finished in a Zener diode 3. The voltage across the capacitor 7 is discharged up to nearly zero V by the resistor 9 at this point of time and when the TR 1 starts turning off after that time, the capacitor 7 starts flowing a current. It is possible to reduce the rate of change in the current flowing to an inductance 6 by using the current flowing to the capacitor 7 so as to reduce the voltage generated in the inductance 6 thereby preventing erroneous on-operation.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is a composite type that combines two or more types of semiconductor elements such as bipolar transistors and FETs (field effect transistors) to improve performance compared to when used alone. Related to switch elements.

[Conventional technology]

This is generally called an emitter open switch, and as shown in FIG. 3, a composite type switch element is known in which a bipolar transistor 1 and an FET 2 are connected in series to the emitter side of the bipolar transistor 1.

In this circuit, the base of bipolar transistor 1 and F
A series circuit of a current limiting resistor 4 and a DC power supply 5 is provided between the sources of the ET 2, and a Zener diode 3 is connected in parallel to the series circuit.

The bipolar transistor 1 of this composite element is cut off by cutting off the FET 2 connected in series to the emitter side of the bipolar transistor 1.

When FET2 is cut off, the load current flowing through the collector-emitter of bipolar transistor 1 is entirely commutated between the collector and base of bipolar transistor 1.

This current has the same effect as the reverse bias current in the normal use of bipolar transistor 1, and the behavior is the same as when an extremely large reverse bias base current is supplied to bipolar transistor 1, and bipolar transistor 1 has an extremely short You can turn off in time.

[Problem that the invention seeks to solve]

However, in the composite switch element as shown in FIG. 3, as shown in FIG. In particular, it adversely affects turn-off operation.

FIG. 5 is an explanatory diagram of the operation of the circuit shown in FIG. Complete the commutation.

At time t3, bipolar transistor 1 begins to turn off, and the collector current (iC in the figure) begins to decrease sharply. During this period, the collector current flows from the collector to the base of the bipolar transistor 1 to the Zener diode 3 to the inductance 6, so
The currents in Zener diode 3 and inductance 6 also decrease sharply. (Period from t3 to t4) Due to the change in current during the period from t3 to t4, the inductance 6 generates a voltage expressed by the following formula.

V e = Il diz /dt 12:6 inductance value diz /dt: Rate of change of current in inductance 6 during period t 3 to t 4 If the voltage of inductance 6 is extremely large, the current accompanying this voltage flows through the path from the inductance 6 to the emitter and base of the built-in diode bipolar transistor 1 of the FET 2 and the Zener diode 3.

As a result, during the period from t3 to t4, the value between the drain and source of FET2 is low. This is the same as FET2 being conductive, and bipolar transistor 1
The base potential of becomes low, and the DC power supply 5 to current limiting resistor 4
However, during the period from t3 to t4, the current flows from the emitter to the base of the bipolar transistor 1 due to the voltage generated by the inductance 6. Therefore, a current flows through the path from the DC power supply 5 to the current limiting resistor 4 to the Zener diode 3.

There are no problems with the operation up to this point. At time t4, F
Since the voltage between the drain and source of ET2 cannot rise instantaneously, the following problem develops.

No matter how FET it is, it cannot instantly restore its voltage blocking ability. During the period t4 when the voltage between the drain and source of FET2 rises, the current in the inductance 6 has already become zero, so the bipolar Transistor l is again supplied with forward base current. As a result, the composite switch element once turns off and then turns off again.
It will turn on by mistake and cause a big loss.

SUMMARY OF THE INVENTION An object of the present invention is to provide a composite switching element which eliminates the disadvantages of the conventional example, prevents erroneous on-operation immediately after turn-off, and provides good switching operation.

[Means for solving problems]

In order to achieve the above object, the present invention provides a composite switching element in which a FET is connected in series with a bipolar transistor, and the bipolar transistor is turned on and off by turning on and off the FET.

The gist is that a discharge circuit using a capacitor, diode, and resistor is provided between the collectors.

[Effect]

According to the present invention, focusing on the fact that the cause of erroneous on operation immediately after turn-off of a composite switch element is the voltage generated by inductance due to the wiring of a Zener diode, etc., in order to reduce this voltage, the voltage flows through this path. The rate of change of current can be reduced.

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

FIG. 1 is a circuit diagram showing one embodiment of the composite switch element of the present invention, in which the same components as in the circuit of FIG. 3 showing the conventional example are given the same reference numerals.

In other words, F is placed on the emitter side of bipolar transistor 1.
The drains of ET2 are connected in series, a series circuit of a current limiting resistor 4 and a DC power source 5 is provided between the base of the bipolar transistor 1 and the source of FET2, and a Zener diode 3 is connected in parallel to this series circuit. This is similar to the conventional circuit shown in FIG.

The present invention further provides such a composite switch element circuit by connecting one terminal of a capacitor 7 to the collector of the bipolar transistor 1, connecting the cathode of a diode 8 to the base of the bipolar transistor 1, and connecting the other terminal of the capacitor 7 to the base of the bipolar transistor 1. Connect the anode terminal of diode 8, and then connect the common connection point of capacitor 7 and diode 8 to FET 2.
A resistor 9 was connected between the source terminals of. The circuit composed of these capacitor 7, diode 8, and resistor 9 is as follows:
It performs a discharging operation as a kind of snubber circuit.

Next, to explain the operation, FIG. 2 shows its waveform. At the time t2, the voltage of the capacitor 7 is discharged to almost zero V by the resistor 9, and at the time t2, the voltage of the capacitor 7 is discharged to almost zero V.
When the bipolar transistor 1 starts to turn off after 2, the capacitor 7 starts to flow current as the voltage between its collector and base increases. The current flowing through the capacitor 7 makes it possible to reduce the rate of change of the current flowing through the inductance 6, thereby reducing the voltage generated by the inductance 6 and preventing erroneous ON operation. In addition, in the same figure, the dotted line indicates the case where no conventional discharge means is provided.

〔Effect of the invention〕

As described above, the composite switch element of the present invention has an FET connected in series to a bipolar transistor, and the FE
In a composite switching element in which a bipolar transistor is turned on and off by turning on and off 7', an erroneous on operation immediately after turn-off is prevented, and as a result, a good switching operation can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the composite switch element of the present invention, FIG. 2 is an operating waveform diagram of the circuit shown in FIG. 1, FIG. 3 is a circuit diagram of a conventional composite switch element, and FIG. This figure is a circuit diagram showing problems in the circuit of FIG. 3, and FIG. 5 is an operating waveform diagram of the circuit of FIG. 3. 1... Bipolar transistor 2... FET 3... Zener diode 4... Current limiting resistor 5... DC power supply 6...
Inductance 7... Capacitor 8... Diode 9... Resistance Applicant Fuji Electric Co., Ltd. Figure 1 Figure 2 Figure 3 C Figure 4 Figure 5

Claims (1)

[Claims] In a composite switching element in which an FET is connected in series with a bipolar transistor, and the bipolar transistor is turned on and off by turning on and off the FET,
A composite switching element characterized in that a discharge circuit including a capacitor, a diode, and a resistor is provided between the base and collector of the bipolar transistor.