JPH05252735A - Power circuit - Google Patents

Abstract

PURPOSE:To prevent a malfunction in a short-circuit protective circuit at the power-on operation with respect to a power supply circuit having the diode- detection-type short-circuit protective circuit. CONSTITUTION:A microcomputer-reset signal highly responsive to power on-off operation is provided with a base of an inverse transistor 22, and is transmitted through a resistor 21 to an OR diode when a switching power supply is started up. Then, the OR diode causes a photo-coupler 19 to stop its function. In this way, a malfunction is prevented even when power on-off operation is repeated. In case of current shortage for load, a short-circuit protective circuit functions promptly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention drops the base voltage of a switching transistor on the primary side to the ground when the load of a switching control constant voltage power supply is short-circuited or is in a state close to that, and oscillates the switching transistor. The present invention relates to a power supply circuit accompanied by a diode detection type short protection circuit for stopping the power supply and protecting the power supply.

[0002]

2. Description of the Related Art Conventionally, when a short protection circuit is attached to a switching control constant voltage power source as shown in FIG. 3, there is a delay in outputting a voltage to the rectifying / smoothing sections 8 and 9 when the power source is started. Since this is the same state as when the load is short-circuited, the short protection circuit operates. In order to prevent this, the electrolytic capacitor 25 is provided and the voltage applied to the light emitting diode of the photo coupler 19 is delayed until the voltage is output to both the rectifying and smoothing units 8 and 9.

[0003]

However, when the short protection circuit as shown in FIG. 3 is attached, when the power is turned on and off repeatedly, electric charges are accumulated in the electrolytic capacitor 25 as shown in FIG. A voltage is applied to the light emitting diode of 19 and the short protection circuit malfunctions (see FIG. 3). Therefore, the capacity of the electrolytic capacitor 25 must be sufficiently large. However, when the load is short-circuited, detection is delayed due to the large capacity of the electrolytic capacitor 25, which destroys the elements forming other circuits. There is a risk that

[0004]

[Means for Solving the Problems] When power is turned on, a reset signal from a microcomputer is used to construct a circuit that prevents the operation of a photo coupler that feeds back a load short-circuited state on the secondary side to the primary side. .

[0005]

Function: The power ON / OFF can be known by detecting the reset signal of the microcomputer. Therefore, when the power supply is turned on, if the short-circuit protection circuit does not operate only during the period when all the secondary power supply voltage of the switching power supply rises, the power supply can be accurately started.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A short protection circuit according to an embodiment of the present invention will be described below with reference to FIGS.

In the following description, when a voltage is output to each part, it is represented by H, and when it is not output, it is represented by an L symbol.

A circuit 1 is a rectifying / smoothing circuit for a commercial power supply, and a transistor 2 is a switching constant voltage power supply for switching.
Transistor, winding 3 is primary winding, winding 4 is control winding,
The protector circuit 5 has a thyristor structure, and when the photo coupler 19 operates, the base voltage of the transistor 2 is dropped to the ground. The windings 6 and 7 are secondary windings, the circuits 8 and 9 are secondary winding rectifying / smoothing circuits, and the reset signal generating circuit 10 is a circuit for generating a signal for initializing the microcomputer. The output signal is output from the rectifying / smoothing unit 9 as shown in FIG.
Is L when the output voltage is L, and the output voltage of the rectifying / smoothing unit 9 is H after a certain period of time. Diode 1
Reference numerals 1 and 12 are for supplying power to the short-circuit protection circuit on the secondary side, diodes 13 and 14 are for detecting a load short circuit, and diode 15 surely biases the base of the transistor 18 when the power supply is activated. The diode 20 reliably biases the base of the transistor 18 in a steady state. The resistor 16 controls the base resistance of the transistor 19, the resistor 17 controls the collector resistance of the transistor 18, and the transistor 18 controls the photo coupler 19. The photo coupler 19 feeds back the load short circuit on the secondary side to the primary side. Resistance 2
1 is the collector resistance of the transistor 22 in the steady state, and is the base resistance of the transistor 18 when the power is started. The transistor 22 is included in the malfunction prevention circuit of the short protection circuit when the power is turned on.

The operation of the short protect circuit configured as described above will be described. (1) Voltage of α in a steady state: H voltage of cathodes of the diodes 13 and 14: H voltage of anode of the diode 15: H reset signal: H collector voltage of the transistor 22: L voltage of anode of the diode 20: L voltage of the transistor 18 Base voltage: H The logic of the anode voltage of the light emitting diode of the photo coupler 19 is: L, and the base of the transistor 18 is biased by the alternate long and two short dashes line in FIG. Therefore, since the photo coupler 19 does not operate, the primary side short protection circuit body also does not operate. (2) When the load of the secondary winding 6 is short-circuited: voltage of α: voltage of cathode of H diode 13: voltage of anode of L diodes 13 and 14: voltage of anode of L diode 15: L reset signal: H transistor 22 collector voltage: L voltage of anode of diode 20: L base voltage of transistor 18: L anode voltage of light emitting diode of photo coupler 19: logic H such that the transistor 18 is OFF. The photocoupler 20 is biased through the path shown by the one-dot chain line in FIG. 1, and the main body of the short-circuit protection circuit operates, lowering the base voltage of the switching transistor 2 and stopping the oscillation of 2. (3) When the power supply is started: Anode voltage of diode 11: H voltage of anode of diode 12: voltage of L α: voltage of cathode of H diode 13: voltage of anode of L diodes 13 and 14: voltage of L anode of anode 15 Voltage: L Reset signal: L Collector voltage of the transistor 22: H Voltage of the anode of the diode 20: H Base voltage of the transistor 18: H Anode voltage of the light emitting diode of the photo coupler 19: Logic like L: Power supply start At this time, the base of the transistor 18 is biased through the path indicated by the broken line in FIG. 1 and the transistor 18 is turned on. Therefore, photo coupler 19
Does not work, and the main body of the short-circuit protection circuit does not work either.

As described above, according to the present embodiment, only when the power is turned on, a reset signal for a microcomputer, which has good followability for turning the power on / off, is added to the base of the inverting transistor 22 and the OR diode is turned on via the resistor 21. After that, by making the operation of the photo coupler 19 stop, even if the power is turned on / off repeatedly, a short circuit is generated.
We were able to achieve that the short-circuit protection circuit operates quickly when the load is short-circuited, without the protection circuit malfunctioning.

[0011]

As described above, when the power is turned on, the reset signal for the microcomputer controls the main body of the short-circuit protection circuit, and the short-circuit protection circuit is temporarily stopped only when the power is turned on. There is no malfunction with repeated power ON / OFF, and the short-circuit protection circuit can be activated quickly when the load is short-circuited.

[Brief description of drawings]

FIG. 1 is a power supply circuit diagram in an embodiment of the present invention.

FIG. 2 is a timing chart of a reset signal for a microcomputer in the embodiment of FIG.

FIG. 3 Conventional power supply circuit diagram

4 is a repetitive power supply O of the electrolytic capacitor 25 of FIG.
Charge waveform diagram when N / OFF

[Explanation of symbols]

 10 Reset signal generation circuit for microcomputer 20 Diode 21 Resistor 22 Transistor

Claims (1)

[Claims] 1. A protection circuit for detecting an output voltage on a secondary side of a switching constant voltage power supply to stop oscillation of a switching transistor on a primary side, and a microprocessor driven by the power supply circuit (hereinafter referred to as a microcomputer). ), And the operation of the protection circuit is stopped during the reset period by a reset signal for initializing the microcomputer.