JPH02256315A - Input/output isolation type semiconductor switch circuit - Google Patents

Abstract

PURPOSE:To obtain a circuit quickly turned on/off by connecting a capacitor always charged whose capacitance is sufficiently large to an input of a transistor(TR). CONSTITUTION:Since a DC power supply is always applied between power supply terminals 2, 2, a light emitting diode 8a is lighted and a capacitor 18 is charged by an output of a photovolatile element 8b. When an input is applied to an input terminal 17, a TR 16 is turned on and since a light emitting diode 13a is lighted, a photoconductive element 13b is turned on. Thus, a stray capacitance Cg of a TR 9 is charged by a charge in the capacitor 18 and when the level reaches a threshold level or over, the TR 9 is turned on. When the input signal is lost, since the TR 15 is turned off, the photoconductive element 13b is turned off via the light emitting diode 13a, and since the charge in the stray capacitor Cg of the TR 9 is discharged via a resistor 12, the TR 9 is turned off.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention provides an input/output isolated semiconductor switch in which input and output are insulated by photoforce, and a field effect transistor is used as an output transistor to open and close a load circuit. Regarding this.

[Conventional technology]

Input/output isolated type semiconductor switch circuits have been widely used for electrically insulating both circuits when connecting a load circuit to a load circuit. When using a P2 edge gate type field effect transistor, it is necessary to photodischarge the gate to open and close it, and a photocoupler that combines a light emitting element and a photovoltaic element is used for this photoelectric discharge. A discharge resistor is provided separately from the photocoupler. FIG. 2 shows a conventional example of this input/output isolated type semiconductor switch in the most general form. A semiconductor switch circuit J shown surrounded by a dashed line in the figure receives an input command made up of a DC power supply 3 and a switch 4 between 11 source terminals 2 and 2, and accordingly outputs an output terminal 5 to 5. It opens and closes a load 7 that is powered by another connected power source 6, and its input and output are insulated by a photocoupler 8. In this example, an n-channel field effect transistor is used as the output transistor 9.

It is connected between output terminals 5 and 5 in series with a series circuit of a power source 6 and a load 7. The photocoupler 8 is for photoelectric use of the gate capacitance Cg of the output transistor 9, and is composed of a light emitting diode 8a and a photovoltaic element 8b.When the switch 4 is turned on, the light emitting diode 8
When the light emitting diode 8a emits light by being supplied with power from the power source 3 through its series resistor lO, the photovoltaic element 8b is activated.

The gate of the output transistor 9 is photoelectronized by the electromotive force generated in the direction indicated by , and the output transistor 9 is closed. A predetermined number of photovoltaic elements 8b are connected in series so that their electromotive force exceeds the operating threshold of the output transistor 9. When the switch 4 is turned off, the light emitting diode 8a turns off and the light F, 11! Although the electromotive force of the force resistor 8b disappears, the gate capacitance Cg of the output transistor 9 is not discharged as it is, so a discharge resistor 12 is connected in parallel to the photovoltaic element 8b. Therefore, the gate of the output transistor 9 is discharged with a time constant equal to the product of its gate capacitance Cg and the resistance of the discharge resistor 12, and the output transistor 9 opens accordingly.

[Problem to be solved by the invention]

In the above-mentioned conventional technology, the stray capacitance Cg between the input terminals of the output transistor 9 is charged by the electromotive force of the photovoltaic element 8b generated by the input signal, and it takes a long time to reach the threshold value, and the output transistor 9 becomes conductive. There is a time when it is late. Furthermore, the larger the discharge resistance 12 is, the lighter the load on the electromotive force element 8b is, which is convenient for the closing operation of the output transistor 9. However, if the discharge resistance 12 is large, the discharge time of the stray capacitance between the input terminals of the output transistor 9: itCg is delayed. There is a problem that the opening operation is delayed. As described above, this conventional example has a problem in that it is difficult to speed up both the closing and opening operations.

SUMMARY OF THE INVENTION An object of the present invention is to provide an input/output isolated semiconductor switch circuit in which field effect transistors can quickly close and open.

[Means to solve the problem]

In order to solve the above-mentioned problems, the present invention provides an input/output isolated semiconductor switch circuit in which the input and output are insulated by a photocoupler and the load is switched on and off using a field effect transistor. an input circuit in which a circuit in which a light emitting element of a photocoupler and a collector/emitter of an input transistor are connected in series is connected in parallel with a light emitting element of a second photocoupler, and a base of the input transistor is connected to an input terminal; A capacitor is connected in parallel to a field effect transistor connected between a pair of output terminals on the load side and a photovoltaic element of the second photocoupler connected to the field effect transistor so as to be able to charge the gate of the field effect transistor. and a charging circuit connected to the conductive element of the first photocoupler in series, and a discharging resistor connected to the field effect transistor so as to be able to discharge the gate of the field effect transistor. be.

[Effect]

The capacitor is always charged by the output of the photovoltaic element of the second photocoupler, and the input signal charges the gate of the field effect transistor with the charge of the capacitor via the first photocoupler, so it can be charged at high speed. It works faster. Moreover, since sufficient charge is stored in the capacitor, the discharge resistance can be made relatively small, and the gate can be discharged quickly, so that the opening operation can be done quickly.

〔Example〕

FIG. 1 shows an embodiment of an input/output isolated type semiconductor switch according to the present invention, and the same parts as in FIG. 2 are given the same reference numerals as in FIG. 2. In FIG. 1, a pair of power supply terminals 2-
2, the light emitting diode 13a1 of the photocoupler 13, the resistor 14, the collector-emitter series circuit of the input transistor 15, and the series circuit of the light emitting diode 8a and the resistor 10 are connected in parallel. The base of transistor 15 is connected to input terminal 17 via resistor 16. A DC power supply (not shown) is connected to the outside between terminals 2-2.

The drain and source of an output transistor 9 consisting of a field effect transistor are connected between the terminals 5 and 5, and the photocoupler 13 is connected to a circuit in which the photovoltaic element 8b of the photocoupler 8 and the capacitor 18 are connected in parallel. photoconductive elements 13b are connected in series to the output transistor 9 so as to photoconduct the gate of the output transistor 9. Further, a discharge resistor 12 is connected to the output transistor 9 so as to discharge the charge on the gate of the output transistor 9. A series circuit of a power supply and a load (not shown) is connected between the output terminals 5 and 5.

Since DC power is always applied between the power terminals 2 and 2, the light emitting diode 8a emits light, and the capacitor 18 is charged by the voltage of the photovoltaic element 8b. Here, when an input is applied to the input terminal 17, the transistor 15 is turned on and the light emitting diode 13a emits light, so that the photoconductive element 13b is turned on. As a result, the stray capacitance Cg between the input terminals of the output transistor 9 is charged by the capacitor 18, and when the voltage exceeds the threshold value, the output transistor 9 is turned on. When the input signal disappears, transistor 15
is turned off, the photoconductive element 13b is turned off via the light emitting diode 13a, and the electric charge of the stray capacitance Cg between the input terminals of the output transistor 9 is discharged via the resistor 12, so the output transistor 9 is turned off.

Although a field effect transistor is used as the output transistor in the above, it is not necessary to use such an element and it can also be applied to an ignition circuit such as a thyristor, which is ignited by the discharge energy of a capacitor.

〔Effect of the invention〕

According to the present invention, in order to charge the stray capacitance between the input terminals of the field effect transistor, the capacitance is sufficiently large, and since a capacitor that is constantly photoelectrically charged is connected, it is possible to charge the field effect transistor at an extremely high speed, and to turn on the field effect transistor at a high speed. The effect is that it can be done. In addition, by using this capacitor, the discharge resistance of the stray capacitance between the input terminals of the gate of the field effect transistor can be reduced without increasing the number of photovoltaic elements, and the discharge of the gate can also be made faster. This has the effect that the operation of turning off the field effect transistor can be made faster, and the field effect transistor serving as the output transistor can be turned on and off extremely quickly.

[Brief explanation of drawings]

FIG. 1 is a wiring diagram showing an embodiment of an input/output P edge type semiconductor switch circuit according to the present invention, and FIG. 2 is a wiring diagram showing an example of a conventional input/output isolated type semiconductor switch circuit. 2-2: power supply terminal, 5-5: output terminal, 8: photocoupler, 8a: light emitting diode, 8b: photovoltaic element, 9: output transistor, 12: discharge resistor, 13: photocoupler,
13a: Light emitting diode, 13b: Photoconductive element, 1
5: Input transistor, 17 two-power terminal, 18: Capacitor. Figure 111Figure 2

Claims (1)

[Claims] 1) In an input/output isolated semiconductor switch circuit in which the input and output are insulated by a photocoupler and the load is switched on and off by a field effect transistor, the light emitting element of the first photocoupler and the collector of the input transistor are connected between a pair of power supply terminals. an input circuit in which a circuit in which emitters are connected in series is connected in parallel with a light emitting element of a second photocoupler, and a base of the input transistor is connected to an input terminal;
A capacitor is connected in parallel to the field effect transistor connected between the pair of output terminals on the load side and the photovoltaic element of the second photocoupler connected to the field effect transistor so that the gate of the field effect transistor can be photoelectrically connected. and a charging circuit connected to the photoconductive element of the first photocoupler in series, and a discharging resistor connected to the field effect transistor so as to be able to discharge the gate of the field effect transistor. Input/output isolated semiconductor switch.