JPH08228477A - Protective circuit for multi-output switching power source - Google Patents

Abstract

PURPOSE: To prevent the burning of the transformer coil of a short circuit by short-circuiting the output circuit of low impedance when the AC output circuit of high impedance short-circuits, and putting the power source in overload drooping condition. CONSTITUTION: If high-impedance output (AC3W) short-circuits, a transistor Q30 is turned on. As a result, it turns on a thyristor THY 30, so the line between -5V and -27V being low-impedance output is short-circuited, and the power source E of a switching main circuit MA gets in overload condition, and a drooping circuit works, and throttles the full output voltage to minimum. Hereby, the burning of the transformer T1 coil of a short circuit can be prevented.

Description

Detailed Description of the Invention

[0001]

[Field of the Invention]

(2) The present invention relates to an output short circuit protection circuit for a multi-output switching power supply.

[0002]

2. Description of the Related Art A conventional circuit is obtained by removing the protection circuit A1, A2, A3 or A4 surrounded by a dotted line from the circuit of the present invention shown in FIGS.

That is, the multi-output circuit has a low impedance such as a -27V display tube bias load as shown in FIGS.
In addition to the dance output, a ± 5V memory load and a high impedance load such as an AC3V display tube heater are connected.

In this case, even if an output circuit with a high output impedance such as ± 5V or AC3V is short-circuited, the switching power supply does not cause an overload condition, and therefore the short-circuited circuit is burned out. There was a problem such as being lost.

As a countermeasure against these problems, a temperature fuse is put in a transformer coil and wound into the transformer, and the circuit is cut off by the temperature fuse before burning, or a coil with a thick wire is used for a high impedance circuit of output. It was done to prevent burning damage.

However, these conventional techniques have drawbacks such as a large transformer and high cost.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above drawbacks, and is a circuit having a small output capacitance in a multi-output circuit, that is, a circuit having a high impedance (especially, an output voltage of 5 V).
The output voltage of the following circuit) is detected, and the switching power supply is brought into a drooping state at the time of abnormality, and the output voltage is narrowed down to the minimum width in order to prevent burnout of the transformer coil of the short circuit. Therefore, it is possible to provide an inexpensive and highly reliable multi-output switching (3) power supply that does not cause a safety standard such as UL.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 to 4 are embodiments of a protection circuit for a multi-output switching power supply according to the present invention. FIG. 1 is a protection circuit that detects a high impedance AC output (AC3V) and short-circuits a low impedance output. FIG. 2 shows a protection circuit that detects a high-impedance DC output (DC5V) and short-circuits the low-impedance output.

FIG. 3 is a protection circuit for detecting the high impedance AC output (AC3V) to minimize the ON width of the control circuit. FIG. 4 is a high impedance DC output (DC).
5V) is a protection circuit that minimizes the ON width of the control circuit.

In each drawing, E is a DC power source, R1 ...
R4 is a resistor, MA is a switching main circuit including a control circuit, C1 to C2 are capacitors, T1 is a transformer, D1 is a diode, and PC1 is a photocoupler, which constitutes the primary side of the switching power supply and the detection feedback circuit. .

D20 to D22 are rectifying diodes, R21 to R
25 is the resistance of the detection circuit, C20 is a capacitor, C21 to C23 are smoothing capacitors, D23 is a Zener diode and IC21.
Is a voltage detection IC, which constitutes a multi-output secondary side and a detection circuit.

R30 to R34 are resistors, C30 is a capacitor, and D
30 is Zener diode, D31 is diode, Q30 to Q
Reference numeral 31 is a transistor and THY 30 is a thyristor. With the above configuration, a multi-output protection circuit which is a part of the present invention is configured.

The protection circuit will be described. (4) The portion A1 surrounded by the dotted line in FIG. 1 is the first embodiment of the protection circuit. Explaining the A1 portion in FIG. 1, when the high impedance output (AC3V) is short-circuited,
The transistor Q30 turns on. As a result Thyristor TH
Low impedance output because Y30 is conducted-
Short-circuiting between 5V and -27V causes the switching power supply to become overloaded and the drooping circuit works, limiting the total output voltage to the minimum width and protecting the short-circuit circuit from burning.

A portion A2 in FIG. 2 is a second embodiment of the protection circuit. In the figure, DC5V which is a high impedance output is detected, and the low impedance output is -27V as described above.
Is short-circuited to perform drooping operation to protect the circuit.

Similar to FIG. 1, the A3 portion of FIG. 3 detects a high impedance AC3V output, but when the transistor Q30 is turned on due to the short circuit of this output, the primary current of the photocoupler PC1 increases. By this signal, the control circuit in the MA is narrowed down to the minimum width and the output voltage is reduced to the minimum, thereby protecting the short circuit from burning.

The A4 portion of FIG. 4 detects a high impedance DC5V output as in the case of FIG. 2, but the ON width of the control circuit is narrowed down to the minimum by the same operation as in FIG. It is carried out.

Although four embodiments have been described above, a high impedance
Turns on the switching element by detecting a short circuit in the dance output
It goes without saying that the present invention falls within the scope of the present invention irrespective of the above embodiment as long as it has the function of performing the operation of narrowing the width to the minimum.

[0018]

According to the present invention, the protection circuit portion is made into a hybrid IC or is assembled with insert parts to meet the UL standard or the like with only a slight cost increase.
We can provide highly reliable multi-output switching power supplies, etc.
The effect is extremely large. (5)

[Brief description of drawings]

FIG. 1 is a multi-output switching power supply having a protection circuit according to a first embodiment.

FIG. 2 is a multi-output switching power supply having a protection circuit according to a second embodiment.

FIG. 3 is a multi-output switching power supply having a protection circuit according to a third embodiment.

FIG. 4 is a multi-output switching power supply having a protection circuit according to a fourth embodiment.

[Explanation of symbols]

 E DC power supply R1 to R4 resistance MA Switching circuit including control circuit C1 to C2 capacitor T1 transformer D1 diode PC1 photocoupler D20 to D22 rectification diode R21 to R25 resistance of detection circuit C20 capacitor C21 to C23 smoothing capacitor D23 Zener diode IC21 Voltage detecting IC R30 to R34 Resistor C30 Capacitor D30 Zener diode D31 Diode Q30 to Q31 Transistor (6) THY30 Thyristor A1 First embodiment Protection circuit section A2 Second embodiment Protection circuit unit A3 Third embodiment Protection circuit unit A4 Fourth embodiment Protection circuit unit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H02M 7/48 9181-5H H02M 7/48 T

Claims (4)

[Claims] 1. A multi-output circuit for supplying an output side of a switching power supply comprising a DC power supply, a primary winding of a transformer, and a series circuit of main switching elements to a load through a secondary to n-th winding of the transformer. An output switching power supply, which has a high impedance of the multi-output circuit
The voltage of the output circuit is detected, and when the AC output circuit is short-circuited, the low-impedance output circuit of the multi-output circuit is short-circuited to bring the switching power supply into the overload drooping state. A protection circuit for a multi-output switching power supply, which is characterized by 2. A low-impedance output circuit of the multi-output circuit, which detects a voltage of a DC output circuit having a high impedance of the multi-output circuit, and when the DC output circuit is short-circuited. The protection circuit for a multi-output switching power supply according to claim 1, characterized in that the switching power supply is brought into an overload drooping state by short-circuiting. 3. A multi-output circuit for supplying an output side of a switching power supply comprising a DC power supply, a primary winding of a transformer and a series circuit of main switching elements to a load through a secondary to n-th winding of the transformer. An output switching power supply, which has a high impedance of the multi-output circuit
A protection circuit for a multi-output switching power supply, which is configured to detect the voltage of the output circuit and control the ON width of the switching power supply control circuit to a minimum when the AC output circuit is short-circuited. 4. A voltage of a DC output circuit having a high impedance among the multi-output circuits is detected, and when the DC output circuit is short-circuited, an ON width of a switching power supply control circuit is controlled to a minimum. A protection circuit for a multi-output switching power supply according to claim 3, characterized in that it is configured.