JPH03297219A - Optical coupling type relay circuit - Google Patents

Abstract

PURPOSE:To attain the turn-on of an output MOSFET at a high speed by connecting a phototransistor(PTR), a counter flow preventing diode, a control TR and a bias resistor between the drain and the gate of the output MOSFET. CONSTITUTION:When an input signal is energized between input terminals 10a, 10b, a light emitting diode 1 generates an optical signal. Since a phototransistor(PTR) 6 receives the light and is conductive, a current flows through a load 13, a bias resistor 9, a control TR 8, a diode 7 and a PTR 6. Since the current is far smaller than the current flowing when the output MOSFET 3 is turned on, a voltage generated across the resistor 9 is low. Thus, the TR 8 remains conductive as it is and the current flowing to the PTR 6 is not hindered. Since the current flowing through the PTR 6 charges up the gate capacitor of the MOSFET 3 at the high speed, the gate voltage is boosted rapidly and the MOSFET 3 is quickly turned on.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optically coupled relay circuit that uses an optical coupling method to insulate input and output.

[Prior Art] FIG. 2 is a circuit diagram showing a conventional optically coupled relay circuit. In this circuit, the phototransistor 6 which receives the optical signal generated by the light emitting diode 1 connected between the input terminals 10a and 10b becomes conductive, and the output MO3F
Since current flows from the load side to the gate of ET3 and the gate capacitance is rapidly charged, the output MO5FET3 is turned on at high speed.

On the other hand, the optical signal generated by the light emitting diode 1 is also applied to the photodiode array 2, and a photocurrent is generated in the photodiode array 2. This photocurrent flows between the drain and source of the depression type control transistor 4 and through the resistor 5, and the voltage generated across the resistor 5 biases the control transistor 4 into a high resistance state.

Therefore, the photovoltaic force of the photodiode array 2 is applied between the gate and source of the output MOSFET 3, and in order to keep the output MOSFET 3 in the on state, the relay operation is stabilized regardless of the voltage and current of the load power supply 12. It is said that

When the input signal to the light emitting diode 1 is cut off, the photovoltaic force of the photodiode array 2 disappears, and the voltage across the resistor 5 disappears, so the desorption type control transistor 4 returns to the on state, and the output MO5FET 3 The accumulated charge between the gate and source of the device is discharged. In addition, since the phototransistor 6 also returns to the off state, the output MO5
FET3 is turned off.

[Problems to be Solved by the Invention] Generally, semiconductor relays are required to have high switching speed. Conventionally, photocouplers, in which a light emitting diode and a phototransistor are optically coupled, have been widely used as optically coupled relays capable of high-speed switching.However, since this uses a bipolar transistor on the output side, it cannot handle minute signals. When switching is controlled, there is a problem that the signal is distorted due to the influence of offset.

Therefore, for applications that control switching of minute signals, MOSFET 3 is installed on the output side, as in the conventional circuit shown in Figure 2.
It is desirable to use In the conventional example described above, by combining the advantages of bipolar transistors and MOSFETs, it is possible to turn on at a higher distance than when using only MOSFETs on the output side, and it is also possible to control switching even with minute signals. In order to make the turn-off time determined only by the control transistor 4 and the resistor 5, a resistor 9 of several MΩ is required to limit the current flowing through the phototransistor 6, and the gate capacitance charging current at turn-on must also be reduced. There is a drawback. This is because if there is no resistor 9, more current flows through the phototransistor 6, but when it is off, the recovery of the phototransistor 6 is delayed, and the output MO5
This is because the gate charge of FET3 becomes difficult to discharge.

Therefore, a high turn-off speed must also be sacrificed.

The present invention has been made in view of these points, and its purpose is to turn on faster than before,
The object of the present invention is to provide an optically coupled relay circuit whose turn-off is the same as that of the conventional one and whose switching can be controlled even with very small signals.

[Means for Solving the Problems] In order to solve the above problems, in the present invention, as shown in FIG. A photodiode array 2 arranged to receive an optical signal of MOSFET3 and output MOS
In an optically coupled relay circuit including a control circuit 14 that forms a discharge path for accumulated charges between the gate and source of the FET 3, a phototransistor 6 arranged to receive an optical signal from the light emitting diode 1 is connected to the MO for output
A diode 7 for blocking reverse current and a normally-on control transistor 8 are connected between the drain and gate of the SFET 3.
A bias resistor 9 is connected between the gate and source of the control transistor 8.

[Function] In the present invention, a series circuit of a phototransistor 6 and a control transistor 8 whose conductive state and non-conductive state are switched depending on the presence or absence of a load current is connected between the drain and gate of the output MOSFET 3. At the same time, since the phototransistor 6 is optically coupled to the light emitting diode 1, when an input signal is applied to the light emitting diode 1, the phototransistor 6 becomes conductive, and a current flows from the load side to the gate of the output MO3FET3. Since the gate capacitance is rapidly charged, the output MO5FET3 is turned on quickly.

On the other hand, at turn-off, the control transistor 8 is biased to a high resistance state by the load current, so the phototransistor 6 is turned off at high speed, the current from the load side is cut off, and the light of the photodiode array 2 is As the electromotive force disappears, the accumulated charge between the gate and source of the output MO3FET 3 is discharged via the control circuit 14, and the output MO3FET 3 is turned off.

Furthermore, when the load becomes a minute voltage, diode 7
Since this prevents the photocurrent from flowing out of the photodiode array 2, the relay operates stably and can control switching even with minute signals.

[Embodiment] FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which a light emitting diode 1 is connected between input terminals 10a and 10b, and the light emitting diode 1 is optically coupled to a photodiode array 2 and a phototransistor 6. The positive electrode of the photodiode array 2 is connected to the gate of the output MOSFET 3, and the negative electrode is connected to the source of the output MOSFET 3 via the first bias resistor 5 for the first control transistor 4. ing. The gate of the output MO3FET 3 is connected to the drain of a normally-on first control transistor 4 made of a depletion type n-channel MOSFET, and the source of the first control transistor 4 is connected to the source of the output MO5FET 3. is connected to the gate, and the negative electrode of the photodiode array 2 is connected to the gate.

The drain of the output MO3FET 3 is connected to one output terminal 11a through the second bias resistor 9 for the second control transistor 8, and the source is connected to the other output terminal 11b. DC current tA12
A series circuit of a load I3 and a load I3 are connected. In addition, the drain of the output MO3FET3 has a depletion type P.
Normally-on type second channel MOSFET
The source of the control transistor 8 is connected to this second
The output terminal 11a is connected to the gate of the control transistor 8, and the positive electrode of the diode 7 is connected to the drain. Further, the collector of the phototransistor 6 is connected to the negative electrode of the diode 7, and the gate of the output MOSFET 3 is connected to the emitter of the phototransistor 6.

Next, the operation of the above embodiment will be explained. First, input terminal 1
When an input signal is applied between 0a and 10b, the light emitting diode 1 generates an optical signal, and upon receiving this optical signal, the phototransistor 6 becomes conductive.
A current flows through the bias resistor 9, the second control transistor 8, the diode 7, and the phototransistor 6. Since this current is much smaller than the current that flows when the output MO5FET 3 is turned on, the voltage generated at the second bias resistor 9 due to this current is low, and therefore the second control transistor 8 remains in a conductive state and does not impede the current flowing to the phototransistor 6.

The current flowing through the phototransistor 6 is the output MO
Since the gate capacitance of 3FET3 is charged at high speed, the output M
The gate voltage of O3FET3 rises rapidly. Therefore,
The output MOSFET 3 is quickly turned on.

On the other hand, when off, the second control transistor 8 is turned off due to the voltage generated in the second bias resistor 9 due to the load current flowing through the load 13, the second bias resistor 9, and the output MO5FET 3. Therefore, almost no current flows through the phototransistor 6, and therefore the phototransistor 6 is turned off at a high speed. Furthermore, since the bias voltage of the resistor 5 disappears due to the disappearance of the photovoltaic force of the photodiode array 2, the depletion type first control transistor 4 turns on, and the output MO3
FET3 turns off.

When the load becomes a minute voltage, the diode 7 prevents the photocurrent from the photodiode array 2 from flowing out, so the relay operates stably and can control switching even with minute signals.

In the above embodiment, the control circuit 14, which forms the discharge path for the accumulated charge between the gate and source of the output MO5FET 3, is composed of the control transistor 4 and the bias resistor 5, but the present invention is not limited to this. Of course.

[Effect of the invention] In the present invention, the drain of the output MOSFET
A series circuit of a control transistor that switches between a conductive state and a non-conductive state depending on the presence or absence of current in the phototransistor and load is connected between the gates, and the phototransistor is optically coupled to the light-emitting diode, so that input signals can be transmitted to the light-emitting diode. When the voltage is applied, the phototransistor becomes conductive, current is supplied from the load side to the gate of the output MOS FET, the gate capacitance is rapidly charged as in the conventional example above, and the output MOS FET is turned on. Although the battery turns on at a high speed, it is possible to turn on the battery at a higher speed because there is no resistance component that limits the charging current as in the conventional example. Furthermore, at turn-off, the charging current is limited by the control transistor which becomes non-conductive due to the load current, so turn-off remains the same as in the conventional case.Furthermore, a diode is connected between the phototransistor and the control transistor. This prevents photocurrent from flowing out from the photodiode array, allowing stable relay operation and enabling switching control even with minute signals.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional example. 1... Light emitting diode, 2... Photodiode array, 3... MO3FET for output, 6... Phototransistor, 7... Diode, 8... Control transistor, 9... Bias resistor , 14... control circuit.

Claims (1)

[Claims] (1) A light emitting diode that generates an optical signal in response to an input signal, a photodiode array arranged to receive the light signal from the light emitting diode, and a photovoltaic force of the photodiode array applied between the gate and source. The output MOSFET is switched between conductive state and non-conductive state between the drain and source, and the gate of the output MOSFET is
In the optically coupled relay circuit, the phototransistor arranged to receive the light signal of the light emitting diode is connected to the drain of the output MOSFET and the control circuit that forms a discharge path for the accumulated charge between the sources. 1. An optically coupled relay circuit characterized in that the gates are connected via a reverse current blocking diode and a normally-on type control transistor, and a bias resistor is connected between the gate and source of the control transistor.