JPH02244907A - Photocoupling relay circuit - Google Patents

Abstract

PURPOSE:To attain rapid switching by photocoupling a photodiode array with a light emitting diode and supplying a gate current from the load side to an output MOSFET to which the photoelectromotive force of the photodiode array is impressed between the gate and source through a transistor(TR) to be driven by the optical current of the photodiode array. CONSTITUTION:The light emitting diode 1 is photocoupled with the photodiode array 2 and the positive and negative poles of the array 2 are respectively connected to the base of a TR 6 connected between the drain and gate of the output MOSFET 3 and the gate of a control TR 4 connected between the gate and source of the MOSFET 3. When an input signal is impressed to the diode 1, the TR 6 is turned to a conductive state by an optical current generated by the array 2, a gate current is allowed to flow from the negative side to the gate of the MOSFET 3 and the gate capacity can be rapidly charged. Thus, rapid switching can be attained.

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 input terminal 8a.

The photodiode array 2 receives the optical signal generated by the light emitting diode 1 connected between 8b and 8b, generates a photovoltaic force, and outputs this photovoltaic force via the resistor 5.
The drain and source of a control transistor 4 made of a depletion type JFET are connected to the gate and source of the output MOSFET 3, which applies the voltage to the gate and source of the ET 3, respectively. The gate and source are connected to both ends of the resistor 5, respectively.

In the relay circuit having such a configuration, when an input signal is applied to the light emitting diode array 2 and a photovoltaic force is generated in the photodiode array 2, a photocurrent is generated between the drain and source of the depression type control transistor 4 and through the resistor 5. flows, and a voltage is generated across the resistor 5. This voltage biases the control transistor 4 to a high resistance state, so the photoelectromotive force of the photodiode array 2 is applied between the gate and source of the output MOSFET 3, turning the output MOSFET 3 on. .

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 depression type control transistor 3 returns to the on state and the output MOSFET
Since the accumulated charge between the gate and source of the output MOS FET 3 is discharged, the output MOS FET 3 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, but since this uses a bipolar transistor on the output side, When controlling signal switching,
There is a problem that the signal is distorted due to the influence of the offset.

Therefore, for applications that control switching of minute signals, a MOS F E is used on the output side, as in the conventional circuit shown in Figure 2.
It is desirable to use T3. However, in the conventional example described above, the gate capacitance of MOSFET 3 is charged with the photocurrent of photodiode array 2, and the output MOSFET 3 is charged with the photocurrent of photodiode array 2.
Since the ET3 is turned on, the photocurrent is small relative to the gate capacitance, so there is a problem in that high-speed switching cannot be expected.

The present invention has been made in view of these points, and its object is to provide an optically coupled relay circuit that is capable of high-speed switching and has good output linearity.

[Means for Solving the Problems] In order to solve the above problems, the optically coupled relay circuit according to the present invention includes a light emitting diode 1 that generates an optical signal in response to an input signal, and a light emitting diode 1. a photodiode array 2 arranged to receive the optical signal;
The photovoltaic force of the photodiode array 2 is applied between the gate and source to switch between the conducting state and the non-conducting state between the drain and source of the output MOSFET 3, and the discharge of accumulated charge between the gate and source of the output MOSFET 3. Normally-on type control transistor 4 forming a path
Then, a photocurrent is applied to the photodiode array 2 via the control transistor 4 to bias the control transistor 4 to a high resistance state.

an impedance element 5 that generates a voltage, and an output MO
The collector and emitter of the SFET 3 are connected between the drain and gate, and the SFET 3 is connected in series with the transistor 6 which operates based on the photocurrent of the photodiode array 2, and the cathode is connected to the collector side of the transistor 6. It is characterized in that it is composed of a diode 7 whose anode is connected to the drain side of the output MOSFET 3.

[Function] In the present invention, the transistor 6 is connected between the drain and gate of the output MOS FET 3, and its base is connected to the photodiode array 2 which is optically coupled to the light emitting diode l. When an input signal is applied to the transistor 1, the photocurrent generated by the photodiode array 2 turns on the transistor 6, and a gate current flows from the load side to the gate of the output MOSFET 3, rapidly charging the gate capacitance. Therefore, M for output
OSFET 3 performs high-speed switching. Furthermore, when the control transistor 4 is biased to a high resistance state and the gate-source voltage of the output MOSFET 3 rises sufficiently, the photocurrent to the base of the transistor 6 decreases, so the transistor 6 becomes non-conductive. , the load current passes only through output MOSFET3,
This ensures output linearity. and,
In this state, the voltage of the photodiode array 2 is higher than the drain voltage of the output MOSFET 3, which prevents photocurrent from flowing into the output side.

[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 8a and 8b, and the light emitting diode 1 is optically coupled to a photodiode array 2. The positive terminal of diode array 2 is an output MOSFE
Connected between the drain and gate of T3 (base terminal of transistor, negative terminal is connected to gate of output MOSFET3)
They are respectively connected to the gates of the control transistors 4 connected between the sources. Control transistor 4 is normally on type, and its drain is connected to output MOSFET 3.
The gate and source of the output MOSFET 3 are connected to each other, and the gate is connected to the output MOSFET 3 through a resistor 5.
Connected to the source of SFET3. Output MOS
The drain of FET 3 is connected to one output terminal 9a, the source is connected to the other output terminal 9b, and the output terminals 9a, 9
A series circuit of a DC power supply 10 and a load 11 is connected between the terminals b. In addition, the drain of output MOSFET3 is
A diode 7 is connected, and the cathode of the diode 7 is connected to the gate of the output MOSFET 3 via the collector and emitter of the transistor 6.

Next, the operation of the above embodiment will be explained. First, input terminal 1
When a human power signal is applied between 3a and 8b, the light emitting diode 1 generates an optical signal, and upon receiving this optical signal, the photodiode array 2 generates a photocurrent, and this photocurrent turns the transistor 6 into a conductive state. Therefore, a load current flows through the load 11. This current flows from the diode 7 through the transistor 6 and the control transistor 4. In addition, this current is also shunted to the gate capacitance of the output MOSFET 3, and this gate capacitance is charged at high speed.
The gate-source voltage of S FET 3 rises rapidly. Therefore, the output MOSFET 3 is quickly turned on. On the other hand, in the photodiode array 2, a photovoltaic force is generated by receiving an optical signal. Output MOSF
When ET3 is turned on, this drain voltage becomes lower than the photovoltaic force, but during this time the diode 7 is connected in the opposite direction to the bias, so the inflow of photocurrent from the photodiode array 2 is prevented. This prevents this and makes the operation stable. Furthermore, the control transistor 4 is biased to a high resistance state by the photocurrent of the photodiode array 2, and the base current to the transistor 6 decreases.
Since the transistor 6 becomes non-conductive, the load current flows only through the output MOSFET 3, thereby ensuring output linearity.

[Effects of the Invention] As described above, in the present invention, a photodiode array is optically coupled to a light emitting diode that generates an optical signal in response to an input signal, and the photovoltaic force of the photodiode array is gate-sourced. Since the gate current is supplied from the load side to the output MOSFET applied between the two through the transistor operated by the photocurrent of the photodiode array, this has the effect of enabling high-speed switching. Since the control transistor connected in series with the transistor is turned off by the photocurrent of the diode array, the current flowing through the transistor is cut off in a steady state, and the output MO
Since the load current flows only through the SFET, even if a weak signal is controlled by switching, the offset is small and the signal linearity is not easily impaired.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional example. Engineering...Light emitting diode, 2...Photodiode array, 3...Output MOSFET, 4...Control transistor, 5...Impedance element, 6...Transistor, 7...Diode. Figure 1

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 optical signal from the light emitting diode, and a photovoltaic force of the photodiode array applied between the gate and source. an output MOSFET whose drain and source are switched between conducting and non-conducting states; a normally-on control transistor which forms a discharge path for accumulated charge between the gate and source of the output MOSFET; The collector and emitter of the output MOSFET are connected between an impedance element that generates a voltage that biases the control transistor to a high resistance state when the photocurrent of the photodiode array is passed through the control transistor, and the drain and gate of the output MOSFET. It consists of a transistor that operates based on the photocurrent of a photodiode array, and a diode connected in series to the transistor, with a cathode connected to the collector side of the transistor and an anode connected to the drain side of the output MOSFET. An optically coupled relay circuit featuring: