JPH01198214A - Detector for low voltage - Google Patents

Abstract

PURPOSE:To prevent a malfunction before the rise of a power supply when the power supply is closed by applying voltage between an emitter and a collector of a transistor after a fixed delay time when the power supply is closed. CONSTITUTION:When a power supply is closed, an electrolytic capacitor 15 is charged by currents from constant voltage Vc. When the charging voltage of the electrolytic capacitor 15 reaches the Zener voltage or more of a Zener diode 16, transistors 17, 20 are turned ON, and a transistor 13 is charged instantaneously and voltage is applied between an emitter and a collector of the transistor 13. When voltage from an input terminal 11 is a specified level or more, the transistor 13 is turned ON, an SCR 24 is not conducted, and fixed voltage is output from an output terminal 25. When voltage from the input terminal 11 lowers, the transistor 13 is turned OFF, the SCR 24 is conducted, and an output from the output terminal 25 is interrupted.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention detects when the output voltage from a switching regulator, etc. drops or becomes zero, and cuts off the output of the switching regulator, etc. The present invention relates to a low voltage detection circuit.

[Conventional technology and problems]

Conventionally, for example, in switching regulators, etc.
The desired output voltage is obtained by pulse-controlling the input DC voltage, but if this output voltage drops or becomes zero due to a short circuit, damage, etc., this is detected by the low voltage detection circuit. The switching regulator, etc. is protected by detecting the pulse control, cutting off the output to the control circuit that performs the pulse control, and stopping the oscillation of the control circuit, thereby cutting off the output of the switching regulator, etc. ing.

As such a low voltage detection circuit, for example, one having the configuration shown in FIG. 3 is known. That is, in FIG.
The low voltage detection circuit 1 includes a transistor 3 whose base receives an output voltage from a switching regulator or the like (not shown) via an input terminal 2, a positive side connected to the emitter of the transistor 3, and a negative side connected to the collector of the transistor 3. An electrolytic capacitor 5 is connected to the + side of the electrolytic capacitor 5 and a constant voltage Vc is applied to the + side of the electrolytic capacitor 5 via the resistor 4.
The side is connected to the gate via a Zener diode 6 and a resistor with a predetermined Zener voltage, and the constant voltage Vc
It consists of a 5CR7 whose anode is input via a resistor, and an output terminal 8 connected to the anode side of the 5CR7.

According to the low voltage detection circuit 1 configured in this way, when the output voltage of a switching regulator or the like inputted to the input terminal 2 is higher than a predetermined voltage, an H level signal is sent to the base of the transistor 3. Therefore, since the emitter and collector of the transistor 3 are turned on, the constant voltage Vc input to the + side of the electrolytic capacitor 5 flows to the ground via the transistor 3, so 5CR7 No signal is input to the gate of
Therefore, since this 5CRT is in a non-conducting state, the 5CRT
The constant voltage V, which is input to the anode side of 7, is input from the output terminal 8 to a control circuit that performs pulse control of a switching regulator, etc. (not shown), and this input causes the control circuit to continue operating, so that the switching regulator, etc. outputs continuously.

Here, when the output voltage of the switching regulator, etc. decreases due to a short circuit or break in the load, or becomes zero, the output voltage applied to the input terminal 2 decreases, so that the output voltage remains at a predetermined level. When the voltage drops below the voltage, an L-level signal is input to the base of the transistor 3, and the emitter and collector of the transistor 3 are turned off, thereby charging the electrolytic capacitor 5 with the current from the constant voltage. Therefore, when the charging voltage of the electrolytic capacitor 5 becomes equal to or higher than the predetermined Zener voltage of the Zener diode 6, 5CR is transferred from the electrolytic capacitor 5 through the Zener diode 6 and the resistor.
A current flows through the gate of T, which brings conduction between the anode and cathode of SCR7.
The constant voltage Vc input to the anode side of the S CR7
, and thus no signal is output to the control circuit (not shown).
The operation of the control circuit is stopped, and the output of the switching regulator and the like is cut off.

However, the low voltage detection circuit 1 configured in this way
, the time constant is determined by the resistor 4 and the electrolytic capacitor 5 so that the output of the switching regulator, etc. is not cut off due to the activation of the low voltage detection circuit l before the output voltage of the switching regulator, etc. rises when the power is turned on. It has a delay time.

Therefore, even if the above output voltage drops or becomes zero due to an abnormality, the 5CRT becomes conductive after the above delay time has elapsed, stopping the signal input to the control circuit, stopping the control circuit and switching. Since the output of the regulator etc. is cut off, even if a sudden accident such as a load short circuit occurs, the output of the switching regulator etc. will only be cut off after a predetermined delay time has elapsed.

Furthermore, in the case of a short-circuit accident, it is normal for the short-circuit state to occur after repeating several short-circuits and open-circuits, rather than for the short-circuit state to occur immediately. On the other hand, in the low voltage detection circuit 1 configured as described above, devices such as a switching regulator may be destroyed due to the presence of the delay time.

(Purpose of the Invention) In view of the above points, the present invention does not operate before the output voltage of the output whose voltage drop should be detected rises when the power is turned on, and moreover, when a drop in the output voltage is detected, the present invention immediately It is an object of the present invention to provide a low voltage detection circuit that can cut off the above output.

[Means and actions for solving problems] The above purpose is to
According to the present invention, a signal for detecting a voltage drop is input to the base and the emitter.

The transistor includes a transistor to which a predetermined voltage is applied between the collectors, and a switching element connected between an output terminal to which a constant voltage is supplied and ground, and the collector of the transistor is connected to the gate via a Zener diode. , the predetermined voltage applied to the transistor is achieved by a low voltage detection circuit that is supplied from a constant voltage power supply via a delay circuit.

According to this invention, first, when the power is turned on, a predetermined voltage is applied between the emitter and collector of the transistor after a predetermined delay time has elapsed from the constant voltage power supply via the delay circuit, so that the signal to be detected reaches the predetermined voltage. Before the low voltage detection circuit starts up, a predetermined voltage is not applied between the emitter and collector of the transistor, so even if the transistor is turned off, no signal is input to the gate of the switching element, so the switching element is in a non-conducting state, and the constant voltage output from the output terminal is not cut off. However, when a predetermined voltage is applied to the transistor after that, if the signal to be detected is higher than the reference voltage, the transistor is turned on. Therefore, the predetermined voltage passes through the transistor and falls to the ground, so no signal is input to the gate of the switching element, so the switching element is in a non-conducting state, and a constant voltage remains output from the output terminal. In addition, if the signal to be detected in this state drops in voltage due to a short circuit or breakdown of the load and becomes below the reference voltage, the base of the transistor becomes L level, which turns off the transistor, so that the predetermined voltage is applied to the switching element. Therefore, the switching element becomes conductive, and the constant voltage applied to the output terminal passes through the switching element and falls to the ground, so that the output from the output terminal is output without any delay time. It will be immediately blocked.

In this way, the low voltage detection circuit according to the present invention does not operate before the output voltage of the output whose voltage drop should be detected rises when the power is turned on, and moreover, when a drop in the output voltage is detected, it immediately cuts off the output. It is possible.

〔Example〕

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

1st. , the figure shows an embodiment of a low voltage detection circuit according to the present invention, and this low voltage detection circuit 1O is configured such that an output voltage from a switching regulator (not shown) or the like is input to an input terminal 1.
A transistor 13 whose base is connected to the resistor 1 through a resistor 1 and whose emitter is connected to ground through a Zener diode 12;
an electrolytic capacitor 15 to which voltage is applied to the + side through the electrolytic capacitor 15; and a second transistor 17 whose + side is connected to its base via a Zener diode 16 with a predetermined Zener voltage and whose emitter is connected to ground.
The constant voltage VC is input to the base via the resistor 18, and the collector of the second transistor 17 is connected to the resistor 1.
9 and the above constant voltage ■ to the emitter,
and a third transistor 20 to which is inputted,
The collector of this third transistor 20 is the capacitor 2
1 to ground, and is connected to the collector of the first transistor 13 via a resistor 22, and the collector of the transistor 13 is further input to the anode via a Zener diode 23 and the resistor. 5CR24, and an output terminal 25 connected to the anode side of the 5CR24 and to the collector of the 30th transistor 20 via a resistor.

The low voltage detection circuit 10 according to the present invention is configured as described above, and when the power is first turned on, the constant voltage VC is
As shown in Figure (A), since the voltage rises instantaneously, the electrolytic capacitor 15 is charged by the current from the constant voltage VC,
When the charging voltage of the electrolytic capacitor 15 exceeds the Zener voltage of the Zener diode 16, the electrolytic capacitor 1
A current flows from 5 to the base of transistor 17 via the Zener diode 16, and as a result, the base of transistor 17 becomes H level, and 8 becomes transistor 1.
7 is turned on, and at the same time, transistor 20 is also turned on. Further, since the Zener diode 12 is present, no current flows to the ground through the transistor 13 even if the transistor 13 is turned on, so that the capacitor 21 is instantly charged.

Thus, after a predetermined delay time tIl, a predetermined voltage is applied to the collector of the transistor 13 via the resistor 22.

Note that the above-mentioned predetermined delay time t is determined by taking into account the rise time of the output voltage of a signal such as a switching regulator that generates a signal to detect a voltage drop (see FIG. 2 (A)). After rising, a predetermined voltage is applied to the collector of the transistor 13. Therefore, before the signal to be detected rises to a predetermined voltage, the low voltage detection circuit 10 of the present invention detects that the base of the transistor 13 is at the L level because the predetermined voltage is not applied between the emitter and the collector of the transistor 13. As a result, even if the transistor 13 is turned off, no signal is input to the gate of 5CR24, so 2
1S CR24 is in a non-conductive state, and the output of the predetermined voltage from the output terminal 25 is not cut off.

When a predetermined voltage is applied to the collector of the transistor 13, if the signal to be detected is higher than the reference voltage, the signal input from the input terminal 11 to the base of the transistor 13 via the resistor is at H level. Therefore, the transistor 13 is on, and the predetermined voltage passes through the transistor 13 and falls to the ground. Since the voltage at the collector of the transistor 13 is low, a signal is input to the gate of the 5CR 24 due to the action of the Zener diode 23. Therefore, 5CR24 is in a non-conductive state, and the predetermined voltage continues to be output from output terminal 25.

Here, when the signal to be detected becomes lower than the reference voltage due to a voltage drop due to a short circuit or breakdown of the load, the signal inputted from the input terminal 11 to the transistor 13 through the resistor goes to L level, which causes the transistor 13 is turned off, the predetermined voltage is applied to the Zener diode 2.
3 to the gate of 5CR24, therefore, 5CR24 becomes conductive, and the output terminal 25
Since the constant voltage applied to the output terminal 25 falls to the ground through the 5CR 24, the output from the output terminal 25 is immediately cut off without any delay time (see FIG. 2(C)).

Therefore, when the output terminal 25 of the present low voltage detection circuit 10 is input to the control circuit as a signal for operating the control circuit for performing pulse control of the input voltage of a switching regulator, for example, this When the output voltage of the switching regulator drops due to some reason such as a short circuit or breakdown of the load, the low voltage detection circuit 10 detects this and immediately detects the voltage drop at its output terminal 25.
This stops the oscillation of the control circuit (see Fig. 2 (D)) and interrupts its pulse control, so that the switching regulator immediately stops when the output voltage drops. The output is cut off, and damage to the switching regulator and the load circuit can be prevented.

〔Effect of the invention〕

As described above, according to the present invention, there is provided a transistor to which a signal for detecting a voltage drop is input to the base and a predetermined voltage is applied between the emitter and the collector, an output terminal to which a constant voltage is supplied, and a ground terminal. and a switching element whose collector is connected to its gate via a Zener diode, and a predetermined voltage applied to the transistor is supplied from a constant voltage power supply via a delay circuit. Since the low voltage detection circuit is configured as shown in FIG.
Before the signal to be detected rises to a predetermined voltage, the predetermined voltage is not applied between the emitter and collector of the transistor in this low voltage detection circuit, so even if the transistor is turned off, the signal is not applied to the gate of the switching element. is not input, so the switching element is in a non-conducting state,
Constant voltage output is not cut off from the output terminal.

However, when a predetermined voltage is subsequently applied to the transistor, if the signal to be detected is equal to or higher than the reference voltage, the transistor is on, and the predetermined voltage passes through the transistor and falls to ground, causing switching. No signal is applied to the gate of the element, so the switching element is in a non-conducting state, and a constant voltage is still being output from the output terminal. When the voltage drops below the reference voltage, the base of the transistor becomes L level,
This turns off the transistor, so the predetermined voltage is input to the gate of the switching element, so the switching element becomes conductive, and the constant voltage applied to the output terminal passes through the switching element to ground. As a result, the output from the output terminal is immediately cut off without any delay time.

Thus, according to the present invention, when the power is turned on, the device does not operate before the output voltage of the output whose voltage drop is to be detected rises, and when a drop in the output voltage is detected, the output can be immediately shut off. As a result, it is possible to provide an extremely excellent low voltage detection circuit that can prevent damage to the switching regulator or the like that outputs the signal to be detected.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of a low voltage detection circuit according to the present invention, and FIGS. 2(A) to (D) are time charts showing operation timings of the low voltage detection circuit of FIG. 1. FIG. 3 is a circuit diagram showing an example of a conventional low voltage detection circuit. 10...Low voltage detection circuit: 11...Input terminal; 1
2.16.23... Zener diode; 13゜1
7.20... Transistor; 14, 1B, 19°22... Resistor; 15... Electrolytic capacitor; 21
...Capacitor; 24...SCR; 25...Output terminal. Patent applicant: Stanley Electric Co., Ltd. Representative: Patent attorney Hirayama - Office

Claims (1)

[Claims] A transistor is connected between the base of which a signal to detect a voltage drop is input and a predetermined voltage is applied between the emitter and collector, and the output terminal to which a constant voltage is supplied and ground, and whose collector is a Zener. a switching element connected to the gate via a diode, and the predetermined voltage applied to the transistor is supplied from a constant voltage power supply via a delay circuit. detection circuit.