JPH07212968A - Low-voltage sensitive circuit breaker and sequential circuit - Google Patents

Abstract

PURPOSE:To provide a circuit breaker operable on low input voltage, which cuts off its output transistor to prevent overheat if its output terminals are short-circuited. CONSTITUTION:A charging capacitor C3 is inserted between the base of a transistor for cut-off Q1 and an input terminal Vin and a Zener diode ZD1' is so inserted between the base of the transistor for cut-off Q1 and an output terminal Vout that the anode of the Zener diode ZD1' may be connected to the base of the transistor for cut-off. The capacitor C3 installed at the input side is charged only at the time of start-up to drive a transistor for driving Q2 and the transistor for cut-off Q1 and does not operate afterwards. After the start-up, the Zener diode ZD installed at the output side detects the voltage reduction of the input voltage and the short of the output terminal and cuts off the output transistor. When the voltage reduction of the input voltage and the short of the output terminal are detected, the transistor for cut-off is surely turned OFF and output is stopped and therefore the transistor for cut-off does not generate heat as in the conventional method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply used in a power supply unit of various electric devices, particularly devices equipped with a microcomputer.

[0002]

2. Description of the Related Art A conventional technique will be described with reference to FIG. FIG. 4 is a circuit diagram of a low voltage detection cutoff circuit used as a switching power supply.

As for the detection of the reduced voltage in the conventional power supply circuit, as shown in FIG. 4, the Zener diode ZD1 provided on the emitter side of the cutoff transistor Q1 detects the input voltage V _in . Zener diode ZD in this circuit
Since the voltage applied to 1 is usually higher than the Zener voltage,
The Zener diode ZD1 is turned on, the base current flows from the Zener diode ZD1 and the resistor R1 to the driving transistor Q2, and the driving transistor Q2 is turned on. As a result, the emitter-collector of the driving transistor Q1 is also turned on, and the output V _OUT is output.

[0004] When the input voltage V _in is reduced by dividing the voltage applied Zener voltage the Zener diode ZD1, the Zener diode ZD1 is OFF, the driving transistor Q2, the transistor Q1 is also turned OFF, the output V
_OUT is cut off.

In the figure, R2 to R4 are resistors.

FIG. 5 is a circuit diagram of an output fall sequence circuit to which the undervoltage detection cutoff circuit of FIG. 4 is applied, and FIG. 6 is an output waveform diagram of the output fall sequence circuit when AC input is cut off. The output falling sequence circuit is shown in FIG.
As shown in, when the AC input of the switching voltage unit is cut off, the power supply having two or more outputs has a holding time of ΔT1 or more, and the output A is ΔT2 or more after the output B becomes the voltage value B ′. It is a circuit having a specification that holds the output A. Here, the time of ΔT1 and ΔT2 is, for example, about 5 to 10 ms.

The operation will be described below with reference to the circuit diagram of FIG. As shown in FIG. 5, this circuit has a first output circuit section that outputs an output B and a second output circuit section that outputs an output A.

During normal operation, the Zener diode ZD2
Is turned on to turn on the diode of the photocoupler (photocoupler, hereinafter referred to as a photocoupler) PC1 to turn on the transistor of the photocoupler PC1. As a result, the base current of the driving transistor Q2 flows through the resistor R8 and the transistor of the photocoupler PC1, the driving transistor Q2 is turned on, and the cutoff transistor Q1 is also turned on.
However, the output B is output.

On the other hand, when the AC input is turned off,
The voltage applied to the capacitor C2 gradually decreases, and ΔT1
When the voltage applied to the Zener diode ZD2 divided by the resistors R5 and R6 has passed the Zener voltage,
The Zener diode ZD2 is turned off, and the diode of the photocoupler PC1 stops emitting light. Therefore, the transistor of the photocoupler PC1 is OFF, the driving transistor Q2 is OFF, and the cutoff transistor Q1 is also OFF.
The output B is cut off.

On the other hand, the output A continues to be output as long as the energy that can be output to C2 remains after the elapse of ΔT1. In this way, the above output falling sequence is realized.

In FIG. 5, C1 and C2 are capacitors and D1.
Is a diode, Q3 is a transistor, BD1 is a diode bridge, T1 is a transformer, and TH1 is a fuse.

[0012]

By the way, the above-mentioned FIG.
In the conventional example shown in (1), when the output is short-circuited, the voltage of Vin is applied between the emitter and collector of the cutoff transistor Q1, and the Zener diode ZD1, the resistor R1,
A collector current corresponding to the base current of the cutoff transistor Q1 defined by the driving transistor Q2 and the resistor R3 flows, and the loss of the cutoff transistor Q1 is significantly increased, which causes a great amount of heat generation and is dangerous. Therefore, there is a method of protecting the cut-off transistor Q1 by connecting a thermal fuse, but this is not preferable in terms of cost and maintenance of the fuse.

Further, in the conventional output falling sequence circuit shown in FIG. 5, a resistor R5 for dividing a high voltage is used.
R6 is required, and a photocoupler extending between the primary and secondary sides is required, resulting in poor packaging density. Further, a Zener diode ZD2 is arranged on the primary side, and a photocoupler PC1
In this configuration in which the transistors Q1 and Q2 on the secondary side are cut off via the transistor Q1 and Q2 by changing the voltage setting of the Zener diode ZD2 and the like.
There is a problem that it is very difficult to adjust the circuit because the timing of shutting off of No. 2 changes greatly. Further, as in the case of FIG. 4, when the output terminal B is short-circuited, the cutoff transistor Q
1 has a problem of heat generation, and the thermal fuse TH1 is necessary as a countermeasure against the problem.

Therefore, an object of the present invention is to provide a transistor Q for shutting off when the output end is short-circuited with a simple structure.
It is an object of the present invention to provide a reduced voltage detection / interruption circuit capable of suppressing heat generation of the transistor itself by turning off 1 and detecting and interrupting the input reduced voltage.

Further, by using the above-mentioned voltage reduction detection cutoff circuit, it is possible to provide a highly reliable output falling sequence circuit which can simplify the circuit configuration as compared with the prior art and can easily set the voltage cutoff timing. is there.

[0016]

In order to achieve the above-mentioned object, a reduction voltage detecting / interrupting circuit according to the present invention comprises a blocking transistor inserted between an input terminal and an output terminal and a base of the blocking transistor. A driving transistor that supplies current to the cutoff transistor, and a charging capacitor is inserted between the base of the driving transistor and the input terminal, and the base of the driving transistor and the A Zener diode is inserted between the output terminal and the output terminal so that the anode side is connected to the base of the driving transistor.

Further, with respect to the charging capacitor,
It is characterized in that the base and emitter of the reset transistor are connected in parallel.

Further, the present invention has a first output circuit section and a second output circuit section for obtaining two outputs from an AC input,
A sequence in which when the input is cut off, the first output circuit section cuts off the output after a time lag from the AC input cutoff, and the second output circuit section cuts off the output after a further time lag from the first output circuit section. In the circuit, the first output circuit section is provided with the above-described voltage reduction detection cutoff circuit.

[0019]

In the reduced voltage detection cutoff circuit of the present invention, the capacitor provided on the input side is charged only at the time of start-up to drive the drive transistor and the cutoff transistor, and thereafter it does not operate. After the start-up, the Zener diode provided on the output side detects the reduced voltage of the input voltage and the short circuit of the output terminal, and cuts off the output transistor.

In this structure, when the input voltage is reduced and the output terminal is short-circuited, the cutoff transistor is surely turned off and the output is stopped, so that the cutoff transistor does not generate heat unlike the conventional case. , High reliability can be guaranteed. Moreover, this circuit needs only the addition of a charging capacitor on the input side and a Zener diode on the output side, as compared with the conventional circuit, and the above effects can be achieved by an extremely simple circuit configuration.

Further, if a resetting transistor is added to the charging capacitor, it is possible to easily restart after the output is cut off.

In addition, in the output falling sequence circuit using the above-mentioned voltage reduction detection cutoff circuit, not only the output short circuit on the secondary side but also the voltage reduction on the primary side is detected. The photo coupler can be omitted and the mounting density can be improved. Furthermore, the timing of the fall of the two outputs will be controlled by the Zener diode on the secondary side. Compared to the conventional circuit that uses the divided voltage on the primary side via the photocoupler for secondary side control. And accurate timing control can be performed.

In particular, in the circuit in which the resetting transistor is added to the sequence circuit, the first output circuit is cut off before the output of the second output circuit is stopped. The output circuit of can be restarted. That is, it is possible to realize a useful sequence circuit when it is necessary to continuously output the output of the second output circuit while detecting the reduced voltage and the output short circuit.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. FIG. 1 is a circuit diagram of a reduced voltage detection cutoff circuit according to the present embodiment. The same functional parts as those of the conventional example shown in FIG. 4 are designated by the same symbols.

The difference between this embodiment and the conventional example shown in FIG.
A capacitor is inserted in place of the Zener diode ZD1 of FIG. 4, and a Zener diode ZD1 ′ is provided between the base of the driving transistor Q2 and the output terminal Vout.
Is inserted so that the anode side is connected to the base of the driving transistor Q2. With this circuit configuration,
When the output terminal Vout is short-circuited, the cut-off transistor Q1 can be turned off and heat generation can be prevented, so that high reliability can be obtained.

A detailed description will be given below with reference to the drawings.

In FIG. 1, when the input voltage Vin rises, a charging current flows to the base of the driving transistor Q2 through the charging capacitor C3 and the resistor R1 until the charging capacitor C3 is completely charged, and the driving transistor Q2 is turned on. . The base current of the cutoff transistor Q1 flows through the resistor R3 and the collector and emitter of the driving transistor Q2, and the cutoff transistor Q1 is turned on.
An output voltage appears at the output terminal Vout.

When the voltage applied to the Zener diode ZD1 'exceeds the Zener voltage, the Zener diode ZD1' is turned on, and the resistor R5 and the Zener diode ZD are turned on.
The current passing through 1 ′ flows to the base of the driving transistor Q2, the driving transistor Q2 maintains the ON state, and thus the cutoff transistor Q1 also maintains the ON state.

On the other hand, the current flowing through the capacitor C3 and the resistor R1 to the base of the driving transistor Q2 does not flow after the capacitor C3 has been charged. Therefore, the current supplied to the base of the driving transistor Q2 is , The resistor R5 and the Zener diode ZD1 'are the only currents.

When the input voltage Vin drops, the output voltage Vout also drops, and the Zener diode ZD1 '.
The Zener diode ZD1 'is turned off at the time when the voltage applied to the voltage drops below the Zener voltage. As a result, the current to the base of the drive transistor Q2 is cut off, the drive transistor Q2 is turned off, and the cutoff transistor Q1 is also turned off.

When the output terminal is short-circuited, the Zener diode ZD1 'is turned off and no current is supplied to the base of the driving transistor Q2. Therefore, the driving transistor Q2 is turned off, and thus the cutoff transistor Q1 is also turned off. It turns off, and as a result, the shut-off transistor Q
The heat generation of 1 can be prevented. To release this short-circuit protection,
Turn off in to remove the charge from the capacitor C3,
It can be achieved by adding n.

The specific constants of the capacitor C3 and the Zener diode ZD1 'are 18 when the output B is 9V and the output A is 5V, respectively.
0μ and about 6V.

As described above, in the present embodiment, the output is cut off when the input voltage reduction is detected as in the conventional case by the extremely simple structure in which the capacitor C3 which operates only at the time of starting and the Zener diode ZD1 'are added. In addition, it is possible to provide a highly reliable low voltage detection cutoff circuit capable of suppressing heat generation of the cutoff transistor Q1 when the output terminal is short-circuited.

If the reduced voltage detection and interruption circuit of the present embodiment is used, the output falling sequence circuit of FIG. 5 uses the reduced voltage detection and interruption circuit in the first output circuit section, so that the circuit of FIG. The circuit configuration is as follows. The characteristic of this circuit is that, as shown in FIG. 2, the parts on the primary side are unnecessary and the photocoupler can be eliminated.

The operation of the circuit of FIG. 2 will be described below with reference to FIG. 6 which is an output waveform diagram when AC input is cut off.

When the output B is higher than the output A, the output A is taken from the output B by a chopper circuit or the like, and the capacity of the capacitor C1 is set to a capacity sufficient to satisfy the time ΔT1 of FIG. When this happens, both outputs A and B are output until the output B becomes B ', that is, for the time ΔT1. Here, if the output B is lower than B'determined by the Zener diode ZD1 ', the Zener diode ZD1' is turned off.
The driving transistor Q2 is turned off, the cutoff transistor Q1 is also turned off, and the output B goes down.

Output A continues to be output even if output B becomes B'or less, if there is an input voltage that can be output, and capacitor C
By adjusting the capacitances of C1 and C4, it is possible to output at time ΔT2 or more.

In the above circuit, when the input voltage (here, the secondary voltage of the transformer T1) is reduced or the output B is short-circuited, the shut-off transistor Q1 operates in the same manner as the circuit of FIG. It is turned off and short circuit protection without heat generation can be realized.

Further, in the present embodiment, the timing of the fall of the two outputs is controlled by the Zener diode ZD1 'on the secondary side, and the divided voltage on the primary side as in the prior art is passed through the photocoupler. As compared with the circuit used for the secondary side control, accurate timing control can be performed. The inventor has also confirmed that a change in the setting of the voltage value of the Zener diode ZD1 ′ provided on the secondary side does not significantly change the cutoff timing of the transistors Q1 and Q2. As described above, there is an excellent advantage that the timing adjustment as the output falling sequence circuit can be performed more easily than the conventional example shown in FIG.

FIG. 3 is a circuit for detecting and reducing a reduced voltage according to another embodiment of the present invention. The circuit shown in FIG. 3 is obtained by adding a reset circuit 1 for overcurrent protection to the circuit shown in FIG. The purpose of the circuit of this embodiment is to turn off AC and then turn on AC again when releasing the overcurrent protection state in the circuit of FIG. 1. However, at this time, the output A is also cut off. To avoid.

That is, when the reset terminal RST1 is dropped to GND, the reset transistor Q4 is turned on and the transistors Q2 and Q1 are turned on to output. Then, if the reset terminal RST1 is opened again, the circuit becomes similar to that of FIG.

Therefore, by applying this circuit to the falling sequence circuit of FIG. 2, it is possible to realize a sequence circuit which continuously outputs the output of the second output circuit while detecting the reduced voltage and the output short circuit.

[0043]

As described above, the circuit for detecting and reducing voltage of the present invention has the conventional structure that the interruption transistor is surely turned off and the output is stopped when the input voltage is reduced and the output terminal is short-circuited. As described above, the cutoff transistor does not generate heat, and high reliability can be guaranteed.

In addition, this circuit needs only the addition of a charging capacitor on the input side and a Zener diode on the output side as compared with the conventional circuit, and the above-mentioned effect can be achieved by a very simple circuit configuration.

Further, in the output fall sequence circuit using the above-mentioned voltage reduction detection interruption circuit, the parts on the primary side and the photo coupler can be omitted, and the packaging density can be improved. Further, it is possible to accurately control the falling timing of the two outputs.

Further, by adding a resetting transistor to this sequence circuit, it is possible to realize a sequence circuit which continuously outputs the output of the second output circuit while detecting a reduced voltage and an output short circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a low voltage detection cutoff circuit according to an exemplary embodiment of the present invention.

FIG. 2 is a circuit diagram of an output falling sequence circuit to which the circuit of FIG. 1 is applied.

FIG. 3 is a circuit diagram of a low voltage detection cutoff circuit according to another embodiment of the present invention.

FIG. 4 is a circuit diagram of a conventional low voltage detection cutoff circuit.

5 is a circuit diagram of an output falling sequence circuit to which the circuit of FIG. 4 is applied.

6 is an output voltage waveform diagram when the AC input is cut off in the circuit of FIG.

[Explanation of symbols]

 1 1st output circuit part 2 2nd output circuit part 3 Reset circuit C3 Charging capacitor ZD1 'Zener diode Q1 Breaking transistor Q2 Driving transistor Q4 Reset transistor Vin Input terminal Vout Output terminal

Claims (3)

[Claims] 1. A drive circuit comprising: a cutoff transistor interposed between an input terminal and an output terminal; and a drive transistor connected to the base of the cutoff transistor and supplying a current to the cutoff transistor. A charging capacitor is inserted between the base of the transistor and the input terminal, a Zener diode is connected between the base of the driving transistor and the output terminal, and the anode side is connected to the base of the driving transistor. A low voltage detection cutoff circuit characterized in that it is inserted as follows. 2. A reduced voltage detection cutoff circuit comprising a base and an emitter of a reset transistor connected in parallel to the charging capacitor according to claim 1. 3. A first output circuit section and a second output circuit section for obtaining two outputs from an AC input, wherein the first output circuit section has a time difference from the AC input cutoff when the AC input is cut off. Output is cut off, and the second output circuit section is
In the sequence circuit that cuts off the output after a further time difference from the output circuit section, the sequence is characterized in that the first output circuit section is provided with the undervoltage detection cutoff circuit according to claim 1 or 2. circuit.