JPH0746901B2 - Low voltage detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention detects the output voltage from a switching regulator or the like when it drops or becomes zero, and shuts off the output of the switching regulator or the like. The present invention relates to the low voltage detection circuit.

[Conventional technology and problems]

Conventionally, for example, in a switching regulator or the like,
Although a desired output voltage is obtained by pulse-controlling the input DC voltage, if this output voltage drops or becomes zero due to, for example, a short circuit or damage, this is detected by a low voltage detection circuit. Is detected by shutting off the output to the control circuit that performs the pulse control, and stopping the output of the switching regulator by stopping the oscillation of the control circuit to protect the switching regulator. There is.

As such a low voltage detection circuit, for example, the one having the configuration shown in FIG. 3 is known. That is, in FIG.
The low voltage detection circuit 1 includes a transistor 3 to which an output voltage from a switching regulator or the like (not shown) is input to its base via an input terminal 2, a + side is a collector of the transistor 3, and a − side is an emitter of the transistor 3. An electrolytic capacitor 5 which is connected and to which a constant voltage Vc is applied to the + side via a resistor 4, and the + side of this electrolytic capacitor 5 is connected to the gate via a Zener diode 6 having a predetermined Zener voltage and a resistor, and It is composed of an SCR7 into which the constant voltage V _C is input to the anode via a resistor, and an output terminal 8 connected to the anode side of the SCR7.

According to the low-voltage detection circuit 1 configured as above, when the output voltage of the switching regulator or the like input to the input terminal 2 is equal to or higher than the reference voltage, the H-level signal is input to the base of the transistor 3. Therefore, since the emitter and collector of the transistor 3 are turned on, the constant voltage V _C input to the + side of the electrolytic capacitor 5 flows to the ground via the transistor 3, Since no signal is input to the gate of SCR7 and therefore this SCR7 is in a non-conducting state, the constant voltage V _C input to the anode side of the SCR7 performs pulse control of a switching regulator or the like (not shown) from the output terminal 8. It is input to the control circuit, and the control circuit continues to operate by this input, so that the switching regulator or the like continuously outputs.

Here, when the output voltage of the switching regulator or the like decreases or becomes zero due to a load short circuit, damage, etc., the output voltage applied to the input terminal 2 decreases, so that the output voltage is equal to or lower than a predetermined voltage. Then, an L-level signal is input to the base of the transistor 3, and therefore the emitter-collector of the transistor 3 is turned off, whereby the electrolytic capacitor 5 is charged by the current from the constant voltage V _C. When the charging voltage of the electrolytic capacitor 5 becomes equal to or higher than the predetermined Zener voltage of the Zener diode 6, the electrolytic capacitor 5 becomes
A current flows from the SCR7 to the gate of the SCR7 through the Zener diode 6 and the resistor, and the anode and the cathode of the SCR7 become conductive, so that the constant voltage V _C input to the anode side of the SCR7 is the SCR7. Since the signal is not output to the control circuit (not shown), the operation of this control circuit is stopped and the output of the switching regulator or the like is cut off.

However, the low voltage detection circuit 1 configured in this way
When the power is turned on, the time constant of the resistor 4 and the electrolytic capacitor 5 determines that the output of the switching regulator or the like is not interrupted by the operation of the low voltage detection circuit 1 before the output voltage of the switching regulator or the like rises. It has a delay time.

Therefore, even if the output voltage drops or becomes zero at abnormal times, the signal input to the control circuit is stopped due to SCR7 becoming conductive after the delay time described above, and this control circuit is stopped and switched. Since the output of the regulator or the like is cut off, the output of the switching regulator or the like is cut off only after the elapse of a predetermined delay time even when a sudden accident such as a load short circuit occurs.

In addition, in the case of a short-circuit accident, it is generally the case that the short-circuit condition does not occur at one time, but rather the short-circuit condition is settled after repeating short-circuiting and opening several times. On the other hand, in the low voltage detection circuit 1 configured as described above, a device such as a switching regulator may be destroyed due to the presence of the delay time.

Further, when the output voltage of a device such as a switching regulator is on / off controlled by an on / off signal of a control circuit that performs pulse control, that is, a signal to be detected which is the output voltage of the switching regulator or the like is on / off controlled by an on / off control signal. In this case, even when the signal to be detected is turned off by the on / off control, the low voltage detection circuit detects this,
There is also a problem in that the output is cut off, and the signal does not rise to ON even after the ON control.

[Object of the Invention]

In view of the above points, the present invention does not operate before the output voltage of the signal for which the voltage drop should be detected at power-on, and immediately shuts off the output when the output voltage drop is detected. It is an object of the present invention to provide a low voltage detection circuit which does not block the output in the off state when the signal to be detected is on / off controlled.

[Means for solving problems]

According to the present invention, there is provided a low voltage detection circuit for detecting a voltage drop at the time of abnormality of a device such as a switching regulator which outputs a voltage in an on state or an on / off state according to an on / off signal of a control circuit for performing pulse control. , A first transistor whose output voltage of a device such as a switching regulator is input from an input terminal to a base through a resistor, and an emitter of which is connected to the ground through a zener diode, and a constant voltage of a constant voltage power supply. Is applied via a switching circuit that turns on and off based on the on / off signal of the control circuit that performs pulse control, the output of this delay circuit is connected to the base via a Zener diode, and the emitter is grounded. The constant voltage is input to the connected second transistor and the base through a resistor. And a third transistor the constant voltage is input to the emitter with the collector of the second transistor is connected via a resistor, the third
The collector of the transistor is connected to the ground through the capacitor and the collector of the first transistor through the resistor, and the collector of the first transistor is connected to the gate through the Zener diode and the resistor. A control circuit including a switching element to be connected and an output terminal connected to the anode side of the switching element and having the collector of the third transistor connected via a resistor, for performing pulse control The second transistor is turned on by delaying the on signal of the on / off signal through the switching circuit and the delay circuit that are turned on / off based on the on / off signal of
Transistor turns on and the predetermined voltage of the constant voltage power supply becomes the first
Are sequentially applied to the collectors of the transistors, and the output voltage of the device such as the switching regulator in the ON state or the ON / OFF state is a voltage drop when there is an abnormality, and when the first transistor is OFF based on this voltage drop, the switching element is Is turned on and the output of the output terminal is cut off by a low voltage detection circuit.

Hereinafter, an output voltage in an on / off state of a device such as a switching regulator that generates a desired voltage by an on / off signal of a control circuit that performs pulse control is referred to as a signal to be detected.

[Work]

According to the present invention, first, when the power is turned on, the predetermined voltage is applied between the emitter and the collector of the transistor from the constant voltage power supply through the delay circuit after the predetermined delay time, so that the signal to be detected has the predetermined voltage. Since a predetermined voltage is not applied between the emitter and collector of the transistor in the low-voltage detection circuit before the rising, a signal is not input to the gate of the switching element even when the transistor is turned off. Is a non-conducting state, the output of the constant voltage from the output terminal is not cut off,
After that, when a predetermined voltage is applied to the transistor, if the signal to be detected is equal to or higher than the reference voltage, the transistor is on, and thus the predetermined voltage drops through the transistor to the ground. No signal is input to the gate, so the switching element is in a non-conducting state, and a constant voltage is still output from the output terminal.

Also, when the signal to be detected from this state drops below the reference voltage due to a load short circuit, breakdown, etc., the base of the transistor becomes L level, which turns off the transistor. Therefore, the switching element becomes conductive and the constant voltage applied to the output terminal falls to the ground through the switching element, so the output from this output terminal immediately without delay time. Since the switching circuit is turned off by the on / off control signal when the signal to be detected is in the off state by the on / off control signal while being cut off,
Since the predetermined voltage is not applied to the transistor, the output from the output terminal is not cut off as described above.

Thus, the low-voltage detection circuit according to the present invention does not operate before the output voltage of the output for which the voltage drop should be detected at power-on, and immediately shuts off the output when the output voltage drop is detected. In addition, when the signal to be detected is in the off state, the low voltage detection circuit does not operate and the output from the output terminal is not cut off.

〔Example〕

The present invention will be described below based on the embodiments shown in the drawings.

FIG. 1 shows an embodiment of a low voltage detection circuit according to the present invention. In this low voltage detection circuit 10, an output voltage from a switching regulator or the like (not shown) is input to the base from an input terminal 11 via a resistor. A first transistor 13 whose emitter is connected to the ground via a Zener diode 12, and an electrolytic capacitor 15 to which a constant voltage V _C is applied to the + side via a switching circuit A and a resistor 14 described later. , The second side of the electrolytic capacitor 15 is connected to the base via the Zener diode 16 of a predetermined Zener voltage and the emitter is connected to the ground, and the constant voltage V _C is connected to the resistor 18 at the base. Input via the resistor 19, the collector of the second transistor 17 is connected via a resistor 19, and the constant voltage V _{C is} applied to the emitter.
Is input to the collector of the third transistor 20 and the collector of the third transistor 20 is connected to the ground via the capacitor 21 and to the collector of the first transistor 13 via the resistor 22. The SCR24 connected to the collector of the first transistor 13 is input to the anode via the Zener diode 23 and the resistor.
And an output terminal 25 connected to the anode side of the SCR 24 and the collector of the third transistor 20 is connected via a resistor.

Further, in the switching circuit A, a constant voltage V _C is input to the drain through a resistor 26 and a protection diode 27, the source is connected to the ground, and the gate is on / off controlled to control the signal to be detected on / off. A FET 29 to which a signal is input from the input terminal 28 and this FET via a resistor 30
The base of the drain of 29 is connected to the constant voltage V _C via the resistor 31, the emitter is connected to the constant voltage V _C , and the collector of the transistor 32 is connected to the resistor 14. .

The low voltage detection circuit 10 according to the present invention is configured as described above, and when the power is turned on and the on / off control signal input to the input terminal 28 is turned on (see FIG. 2 (A)). , FET 29 is turned on, and the constant voltage V _C rises instantaneously, so that the signal input to the base of the transistor 32 becomes L level, so that the transistor 32 is turned on, and therefore the current from the constant voltage V _C causes electrolysis. When the capacitor 15 is charged and the charging voltage of the electrolytic capacitor 15 becomes equal to or higher than the Zener voltage of the Zener diode 16, a current flows from the electrolytic capacitor 15 to the base of the transistor 17 through the Zener diode 16, thereby causing the transistor 17 to pass through. The base becomes H level, the transistor 17 is turned on, the transistor 20 is also turned on, and the zener diode 12 is turned on. For because transistor 13 is also turned on no current flows to ground through the transistor 13, so that the capacitor 21 is charged instantly.

In this way, after a predetermined delay time t _D, a predetermined voltage is applied to the collector of the transistor 13 via the resistor 22, and thus the low voltage detection circuit 10 is in the operating state.

The predetermined delay time t _D is set in consideration of the rise time (see FIG. 2B) of the output voltage of a signal such as a switching regulator that generates a signal whose voltage drop should be detected. After the voltage rises, the transistor 13
A predetermined voltage is applied to the collector of the.
Therefore, before the signal to be detected rises to a predetermined voltage, the low voltage detection circuit 10 detects that the emitter of the transistor 13
Since a predetermined voltage is not applied between the collectors, it is in a non-operation state (see FIG. 2 (C)). Even if the base of the transistor 13 becomes L level and the transistor 13 is turned off, the gate of the SCR 24 is turned off. Since no signal is input to the SCR 24, the SCR 24 is in a non-conductive state, and the output of the predetermined voltage from the output terminal 25 is not cut off (see FIG. 2 (D)).

In the state where a predetermined voltage is applied to the collector of the transistor 13, if the signal to be detected is equal to or higher than the reference voltage, the transistor 13 is connected from the input terminal 11 via the resistor.
Since the signal input to the base of the transistor is at the H level, the transistor 13 is on and the predetermined voltage is
Since the voltage at the collector of this transistor 13 is low as it will pass through 13 to ground, the Zener diode
By the action of 23, no signal is input to the gate of SCR24,
Therefore, the SCR 24 is in a non-conducting state, and the output terminal 25 keeps outputting the predetermined voltage (see FIG. 2 (D)).

When the on / off control signal input to the input terminal 28 is turned off, the FET 29 is turned off, and therefore the transistor 32 is also turned off. Therefore, a predetermined voltage is not applied to the collector of the transistor 13, and this low voltage detection circuit 10 is in a non-operating state (see FIG. 2 (C)). In this case, SCR2
Since no signal is input to the gate of 4, the SCR 24 is in a non-conducting state. However, since a predetermined voltage is not applied to the output terminal 25, there is no output from the output terminal 25. When the on / off control signal input to the input terminal 28 is turned on again, the above operation is repeated, and the low voltage detection circuit 10 is put into the operating state again (see FIG. 2 (C)).

Here, when the signal to be detected drops below the reference voltage due to a voltage drop due to a load short circuit, breakdown, etc., the signal input to the base of the transistor 13 from the input terminal 11 via the resistor becomes the L level, which causes Since the transistor 13 is turned off, the predetermined voltage is input to the gate of the SCR 24 through the Zener diode 23, and therefore the SCR 24 is
24 becomes conductive and the low voltage applied to the output terminal 25 passes through the SCR 24 to the ground.
The output from is immediately cut off without delay time (see FIG. 2 (D)).

Therefore, when the output terminal 25 of the low voltage detection circuit 10 is input to this control circuit as a signal for operating the control circuit for performing pulse control of the input voltage of the switching regulator, for example, When the output voltage of the switching regulator causes a voltage drop due to some reason such as load short-circuit or breakdown, the low voltage detection circuit 10 detects this and immediately shuts off the output from its output terminal 25. As a result, the oscillation of the control circuit is stopped (see FIG. 2 (D)) and the pulse control is interrupted, so that the output of the switching regulator is cut off immediately when the output voltage drops. Therefore, the breakdown of the switching regulator and the load circuit can be prevented.

〔The invention's effect〕

As described above, according to the present invention, first, when the power is turned on, the predetermined voltage is applied between the emitter and the collector of the transistor from the constant voltage power supply through the delay circuit after the predetermined delay time, so that the detection is performed. Since the low voltage detection circuit does not apply the predetermined voltage between the emitter and collector of the transistor before the power signal rises to the predetermined voltage,
No signal is input to the gate of the switching element even when this transistor is turned off, so that the switching element is in a non-conducting state and the output of the constant voltage from the output terminal is not cut off.

However, when a predetermined voltage is applied to the transistor after that, when the signal to be detected is equal to or higher than the reference voltage, the transistor is on, and thus the predetermined voltage drops through the transistor to the ground, and thus switching occurs. No signal is input to the gate of the element, therefore the switching element is in a non-conducting state, and a constant voltage is still output from the output terminal. Also, when the signal to be detected from this state drops below the reference voltage due to a load short circuit, breakdown, etc., the base of the transistor becomes L level, which turns off the transistor. Therefore, the switching element becomes conductive,
Since the constant voltage applied to the output terminal passes through the switching element to the ground, the output from this output terminal is immediately cut off without delay time, and the signal to be detected is turned off by the on / off control signal. At a certain time, the switching circuit is turned off by the on / off control signal, so that a predetermined voltage is not applied to the transistor, so that the output from the output terminal is not cut off as described above.

Thus, according to the present invention, when the power is turned on, the output does not operate before the output voltage of the output for which the voltage drop should be detected rises, and the output can be shut off immediately when the output voltage drop is detected. This prevents damage to the switching regulator that outputs the signal to be detected, etc., and when the signal to be detected is in the off state, this low voltage detection circuit does not operate and the output from its output terminal is output. Therefore, it is possible to provide an extremely excellent low voltage detection circuit in which the low voltage detection circuit does not malfunction and the output thereof is cut off by turning on / off the signal to be detected.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a low voltage detection circuit according to the present invention, and FIG. 2 is a time chart showing the operation timing of the low voltage detection circuit of FIG. FIG. 3 is a circuit diagram showing an example of a conventional low voltage detection circuit. 10 …… Low voltage detection circuit; 11,28 …… Input terminal; 12,16,23…
… Zener diode; 13,17,20,32 …… Transistor; 1
4,18,19,22,26,30,31 …… Resistance; 15 …… Electrolytic capacitor; 2
1 …… Capacitor; 24 …… SCR; 25 …… Output terminal; 27 …… Protection diode; 29 …… FET.

Claims (1)

[Claims] 1. A low voltage detection circuit for detecting a voltage drop at the time of abnormality of a device such as a switching regulator which outputs a voltage in an on state or an on / off state by an on / off signal of a control circuit for performing pulse control, said switching regulator. The output voltage of the device such as the above is input to the base from the input terminal via the resistor, and the constant voltage of the constant voltage power supply is connected to the first transistor whose emitter is connected to the ground via the Zener diode. A delay circuit that is applied via a switching circuit that turns on and off based on the on / off signal of the control circuit that performs control, the output of this delay circuit is connected to the base through a Zener diode, and the emitter is connected to ground. And the constant voltage is input to the base via a resistor, And,
The collector of the second transistor is connected through a resistor, and the constant voltage is input to the emitter of the third transistor.
And the collector of the third transistor are connected to the ground via a capacitor and the collector of the first transistor via a resistor, and the collector of the first transistor is connected to the zener. A switching element connected to the gate through a diode and a resistor, and an anode side of the switching element, and
An output terminal to which the collector of the third transistor is connected via a resistor is provided, and via a delay circuit and a switching circuit that is turned on and off based on an on / off signal of a control circuit that performs the pulse control. The second transistor is turned on with a delay from the on signal of the on / off signal, whereby the third transistor is turned on and a predetermined voltage of the constant voltage power source is sequentially applied to the collector of the first transistor, and the switching is performed. When the output voltage of a device such as a regulator in the on state or the on / off state is a voltage drop at the time of an abnormality and the first transistor is off based on this voltage drop, the switching element is in a conductive state and the output A low voltage detection circuit that shuts off the output of the terminal.