JPH10257755A - Switching circuit for driving photocoupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching circuit for driving a photocoupler in which erroneous function of an output switching element can be prevented upon power supply voltage drop. SOLUTION: The switching circuit comprises a photocoupler 11 connected across a power supply having positive and negative (P, N) polarities to output positive and negative (1, 0) signals, an output switching element 21 being turned on/off, and a protective circuit 15 inserted between a zero potential and a signal line connecting the output terminal of the photocoupler 11 and the input terminal of the output switching element 21. The protective circuit 15 is normally opened so long as the power supply voltage of the photocoupler 11 is within a normal range and when it drops below normal voltage range, the protective circuit 15 is closed to holds the potential on the signal line on the output side of the photocoupler at a level close to zero potential thus preventing erroneous function of the output switching element 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a circuit for driving a switching element via a photocoupler.

[0002]

2. Description of the Related Art Circuit patterns in which an input signal is insulated from an output circuit by a photocoupler are widely used in various fields. For example, as shown in FIG. 5, an input signal s1 is insulated by a photo IC 91, and an output switching element (power MOS-
FET) 95 is turned on and off. In the figure, reference numeral 92 denotes a current booster circuit for relaying the output signal s2, which is a complementary connection of transistors 921 and 922.

A photocoupler 91 and a current booster circuit 92 are connected to a power supply circuit of both positive and negative (P, N) polarity, and output switching elements (power MOS-
FET) 95 is turned on / off by a positive / negative (1, 0) signal s3. The output switching element (power MOS-FET) 95 may be directly driven by the output signal s2 of the photocoupler 91 without providing the current booster circuit 92.

[0004]

However, if the power supply (P, N) voltage is interrupted or lost for some reason in the above switching circuit, the output switching element (power MOS-FET) 95 changes the input signal s1. If not, it may malfunction. That is, when the power supply (P, N) voltage is lost, the voltages P, N usually decrease according to a fixed time constant determined by the characteristics of the circuit of the power supply device as shown in FIG. 6 or FIG.

For example, as shown in FIG. 6, in one power supply device, the voltage on the negative (N) side is lost earlier due to a shorter time constant, and in another power supply device, as shown in FIG. Thus, the positive (P) voltage is lost early due to the short time constant. As shown in FIG. 6, when the voltage on the negative (N) side is lost first, the output s of the photocoupler 91 is reduced.
2 is 0 (N voltage) and there is no change in the input signal, the output of the photocoupler 91 is inverted as shown by the reference numeral 991 shown by a broken line in FIG. FET) 95 output 1
A malfunction may occur on the side (P voltage).

On the other hand, as shown in FIG. 7, when the voltage on the positive (P) side is lost first, the output s2 of the photocoupler 91 is at 1 (P voltage) and there is no change in the input signal. Nevertheless, the output signal of the photocoupler 91 is inverted as indicated by the reference numeral 992 shown by the broken line, and the output s4 of the output switching element (power MOS-FET) 95 malfunctions on the 0 side (N voltage). Sometimes. And the similar phenomenon caused by the narrow interval between the positive and negative power supply voltages is not only when the power supply is cut off or lost, but also
This is a phenomenon that can occur even when the power is turned on when the power is turned on.

The present invention has been made in view of such a conventional problem, and an object thereof is to provide a photocoupler-driven switching circuit capable of preventing a malfunction of an output switching element when a power supply voltage is reduced. Things.

[0008]

The present invention relates to a switching circuit for driving a switching element on and off by insulating an input signal with a photocoupler, and connected between positive and negative (P, N) bipolar power supplies. 0) A photocoupler that outputs a signal, an output switching element that performs on / off operation, and a signal line that connects between an output terminal of the photocoupler and an input terminal of the output switching element, and a signal line that is inserted between zero potential. A protection circuit, wherein the protection circuit is always open when the power supply voltage of the photocoupler is within a predetermined normal range, and the power supply voltage of the photocoupler is lower than a predetermined range. When the voltage drops, the circuit is closed and the potential of the output signal line of the photocoupler is held near zero potential to prevent malfunction of the output switching element. In the switching circuit of the photocoupler drive, characterized by.

What is particularly noteworthy in the present invention is that a protection circuit is inserted between the signal line connecting the output terminal of the photocoupler and the input terminal of the output switching element and the zero potential. . The protection circuit is normally open when the power supply voltage of the photocoupler is within a predetermined normal range, and is closed when the power supply voltage of the photocoupler falls below a predetermined range. Circuit.

As a result, it is possible to suppress malfunction of the switching circuit by the operation of the protection circuit. That is, for example, as shown in FIG. 2, in the case of a power supply device in which the voltage on the negative (N) side is lost first, if the output of the photocoupler is in a state where s2 is 0 (N voltage), As the voltage on the N) side decreases, the output s2 of the photocoupler decreases as shown by the curve 61 in FIG. In the circuit according to the present invention, positive and negative (P, N)
When the voltage between them falls below a predetermined value, the protection circuit is closed, and as a result, the input level of, for example, a current booster circuit on the output s2 side of the photocoupler is pulled toward the 0 potential, and the output s2 of the photocoupler becomes positive ( Even when the output switching element is inverted to the P) side, the output switching element is prevented from malfunctioning.

Further, as shown in FIG. 4, in the case of a power supply device in which the positive (P) side voltage is lost first, if the output of the photocoupler is in a state where s2 is 1 (P voltage), As the voltage on the positive (P) side decreases, the output s2 of the photocoupler decreases as indicated by the curve 62 in FIG. In the circuit according to the present invention, when the voltage between positive and negative (P, N) drops below a predetermined value, the protection circuit is closed, and as a result, the output s2 of the photocoupler is closed.
For example, even if the input level of the current booster circuit on the side is pulled closer to the 0 potential, the output switching element is prevented from malfunctioning even when the output of the photocoupler is inverted to the negative (N) side of s2.

As described above, in the switching circuit according to the present invention, when the power supply voltage of the photocoupler falls below a predetermined range, the potential of the signal line on the output side of the photocoupler is held close to zero potential. Malfunction of the output switching element can be prevented.

The protection circuit comprises a switching transistor and a diode, and a signal input terminal of the switching transistor is bias-connected to a positive or negative power supply side of a photocoupler. (FIG. 1 or FIG. 3)
reference). That is, as shown in FIG. 1 or FIG. 3, the switching transistor is always kept in the off state by biasing the positive or negative power supply side of the photocoupler, while the diode is connected to the normal state of the photocoupler. For example, the connection is made to the polarity shown in FIG. 1 or FIG.

The switching circuit operates between the photocoupler and the output switching element by an output signal of the photocoupler and applies a positive / negative operation signal to the output switching element. A relay switching circuit may be interposed. In this case, the protection circuit is provided between the signal line connecting the output terminal of the photocoupler and the input terminal of the relay switching circuit to zero potential. insert. Then, similarly to the above case, the signal of the photocoupler is erroneously inverted so that the relay circuit (output switching element) does not malfunction. The relay switching circuit includes a switching transistor circuit and the like which are complementarily connected between the positive and negative power supplies of the photocoupler (see FIGS. 1 and 3).

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 As shown in FIG. 1, this embodiment is a switching circuit 1 that insulates an input signal s1 with a photocoupler 11 and drives a switching element 21 on and off. The switching circuit 1 has positive and negative (P, N). Photocoupler 11 connected between bipolar power supplies and outputting positive and negative (1, 0) signals
And an output switching element 21 that is turned on and off, and a signal line 1 that connects between an output terminal 111 of the photocoupler 11 and an input terminal 211 of the output switching element 21.
It has a protection circuit 15 inserted between 19 and zero potential (GND).

The protection circuit 15 is always open when the power supply voltage (P, N) of the photocoupler 11 is within a predetermined normal range, and the power supply voltage (P, N) of the photocoupler 11 is maintained at a predetermined level. When the voltage falls below the range, the circuit is closed and the potential of the output signal line of the photocoupler 11 is held close to zero potential to prevent the output switching element 21 from malfunctioning.

The protection circuit 15 is connected in series with the switching transistor 151 so that the switching transistor 15 does not close (turn on) due to the normal operation (1 operation in this example) of the photocoupler 11 with the anode at the 0 potential side. And a diode 152 connected thereto. The signal input terminal of the switching transistor 151 is bias-connected to the negative power supply (N) side of the photocoupler 11, and the switching transistor 151 is always kept in the off state.

Further, between the photocoupler 11 and the output switching element 21, a relay switching circuit 31 operated by the output signal s2 of the photocoupler 11 and applying a positive / negative operation signal s3 to the output switching element 21.
The protection circuit 31 is inserted between a signal line connecting between the output terminal 111 of the photocoupler 11 and the input terminal of the relay switching circuit 31 and zero potential (GND). The relay switching circuit 31 is a current boost circuit having switching transistors 311 and 312 that are complementary connected between the positive and negative power supplies (P and N) of the photocoupler 11.

In the switching circuit 1 of the present embodiment, when the voltage of the power supply (P, N) maintains a normal value, the protection circuit 15 opens the transistor 151 (off).
In either case, the polarity of the diode 152 is reversed, and in any case, the protection circuit 15 holds the open (off) state between the signal line 119 and the zero potential (GND). That is, the output signal s2 of the photocoupler 11
Is in one signal (P-side potential), the transistor 1
When the output signal 51 is in an open (off) state and the output signal s2 of the photocoupler 11 is a 0 signal (N-side potential), the diode 152 has the opposite polarity, and the circuit 15 is open (off).
In state.

When the voltage of the power supply (P, N) is cut off (or rises) for some reason,
For example, even when the voltage drops as indicated by reference numerals P and N in FIG. 2, that is, when the transient characteristic of the power supply device loses the voltage on the negative (N) side first, the circuit 1 of the present embodiment has the above protection. Due to the operation of the circuit 15, as shown in FIG.
1 does not malfunction.

The output signal s2 of the photocoupler 11 is P
If the N-side voltage is lost while at the side potential, the protection circuit 1
5 is turned on, and the current booster circuit 2 is turned on.
The input potential of No. 5 is drawn to the 0 potential side. Thereby, the output switching element 21 is turned off to prevent malfunction. On the other hand, if the N-side voltage is lost while the output signal s2 of the photocoupler 11 is at the N-side potential, the protection circuit 15 does not operate. However, since the input of the current booster circuit 25 is N-side potential before that, The output switching element 21 is off and does not malfunction.

At this time, if the photocoupler 1 is
When the output signal s2 of 1 is inverted and the P-side potential is output, the transistor 151 of the protection circuit 15 is turned on to pull the input potential of the current booster circuit 25 to the 0 potential side, and the output switching element 21 malfunctions. (Turn off). As described above, according to the switching circuit 1 of the present embodiment, it is possible to prevent the output switching element 21 from malfunctioning in the power supply having the transient characteristics as shown in FIG.

Embodiment 2 In this embodiment, as shown in FIG. 3, the relay switching circuit 31 is not provided in the embodiment 1 and, as shown in FIG. A protection circuit 25 having an effective protection function against a power supply having characteristics
1 is an example of a switching circuit 10 including:

As shown in FIG. 3, the switching circuit 10 of the present embodiment has a signal line 118 connecting between the output terminal 111 of the photocoupler 11 and the input terminal 211 of the output switching element 21 and a zero potential (GND). And a protection circuit 25 inserted between them. The protection circuit 25 has a switching transistor 251 and a diode connected in series with the transistor 251 with the anode at the 0 potential side.

When the voltage of the power supply (P, N) maintains a normal value, the protection circuit 25 determines whether the transistor 251 is open (off) or whether the diode 252
Is reversed, and the circuit 25 maintains the open (off) state between the signal line 118 and the zero potential (GND) in any case. That is, the photocoupler 11
Is the 0 signal (N-side potential)
When the transistor 251 is in an open (off) state and the output signal s2 of the photocoupler 11 is at one signal (P-side potential), the diode 252 has the opposite polarity and the circuit 1
5 is in an open (off) state.

When the voltage of the power supply (P, N) decreases as shown by reference numerals P and N in FIG. 4, that is, when the transient characteristic of the power supply device is such that the voltage on the positive (P) side is lost first. In the circuit 10 of the present embodiment, the operation of the protection circuit 25 also causes the photocoupler 11 to operate as shown in FIG.
Does not malfunction. That is, when the output signal s2 of the photocoupler 11 is a 0 signal (N-side potential), the transistor 251 is off, and even when the power supply voltage changes as shown in FIG. There is no inversion operation.

When the output signal s2 of the photocoupler 11 is one signal (P-side potential), the transistor 251 is turned on and the diode 252 becomes forward polarity, and the power supply voltage changes as shown in FIG. In this case, the output signal s2 changes as indicated by the curve indicated by reference numeral 62 in FIG. 3, and even if the photocoupler 11 performs an inversion operation, the input potential of the output switching element 21 becomes 0 potential and turns off. Malfunction can be prevented. Others are the same as the first embodiment. As described above, according to the switching circuit 1 of the present embodiment, it is possible to prevent the output switching element 21 from malfunctioning in the power supply device having the transient characteristics as shown in FIG.

[0028]

According to the present invention, it is possible to obtain a photocoupler-driven switching circuit capable of preventing a malfunction of an output switching element when a power supply voltage decreases.

[Brief description of the drawings]

FIG. 1 is a connection diagram of a switching circuit according to a first embodiment.

FIG. 2 is a diagram illustrating an example of transition of a power supply voltage of a switching circuit and an output signal of a photocoupler according to the first embodiment.

FIG. 3 is a connection diagram of a switching circuit according to a second embodiment.

FIG. 4 is a diagram illustrating an example of transition of a power supply voltage of a switching circuit and an output signal of a photocoupler according to a second embodiment.

FIG. 5 is a connection diagram of a conventional switching circuit.

FIG. 6 is a diagram illustrating an example of transition of a power supply voltage of a conventional switching circuit and an output signal of a photocoupler.

FIG. 7 is a diagram showing another example of transition of the power supply voltage of the conventional switching circuit and the output signal of the photocoupler.

[Explanation of symbols]

 11. . . Photocoupler, 15, 25. . . Protection circuit,

Claims (4)

[Claims] 1. A switching circuit for driving a switching element on and off by insulating an input signal with a photocoupler, and connecting a positive and negative (P, N) bipolar power supply to output a positive and negative (1, 0) signal. And a protection circuit inserted between a signal line connecting the output terminal of the photocoupler and the input terminal of the output switching element and a zero potential. And the above protection circuit is
When the power supply voltage of the photocoupler is within a predetermined normal range, the photocoupler is normally open. When the power supply voltage of the photocoupler falls below the predetermined range, the photocoupler is closed and the output side of the photocoupler is closed. A switching circuit driven by a photocoupler, wherein a potential of a signal line is held near zero potential to prevent a malfunction of the output switching element. 2. The protection circuit according to claim 1, wherein the protection circuit includes a switching transistor, and a diode connected in series with a polarity such that the switching transistor does not close due to a normal operation of the photocoupler.
A switching circuit for driving a photocoupler, wherein a signal input terminal of the switching transistor is bias-connected to a positive or negative power supply side of the photocoupler, and the switching transistor or the diode is always kept in an off state. 3. The device according to claim 1, wherein the photocoupler and the output switching element are arranged between the photocoupler and the output switching element.
A relay switching circuit that operates according to the output signal of the photocoupler and applies a positive / negative operation signal to the output switching element is provided, and the protection circuit includes an output terminal of the photocoupler and an input terminal of the relay switching circuit. A switching circuit driven by a photocoupler, which is inserted between a signal line connecting between the two and a zero potential. 4. The photocoupler-driven switching circuit according to claim 3, wherein the relay switching circuit is a switching transistor circuit that is complementarily connected between the positive and negative power supplies of the photocoupler.