JPH0564351A - Overcurrent protection circuit for switching power supply equipment - Google Patents

Abstract

PURPOSE:To make it possible to easily fix the operating point for protection during overcurrent by stopping the operation of a switching transistor at the primary side by detecting a load current at the secondary side not being affected by the input voltage fluctuation at the primary side. CONSTITUTION:When a supply current/(io) to a load Z at the secondary side of a transformer exceeds a predetermined level, the resistance value of a current detecting resistor RT comprising a PTC thermister increases suddenly, and the voltage at both the ends of this is also increased suddenly. As a result, a photodiode PD of a photocoupler PC irradiates light and a phototransistor PT is turned on, switching stop transistor Q2 is turned on, the base and emitter of a switching transistor Q1 are short-circuited, and the transistor Q1 is turned off.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RCC (Ringing Choke Converter) used as a power supply device for various electronic devices.
, Also known as Flyback Converter) switching regulator and other switching power supply device, relates to an overcurrent protection circuit at the time of output short-circuit or the like. In the following figures, the same reference numerals indicate the same or corresponding parts.

[0002]

2. Description of the Related Art FIG. 2 shows a circuit configuration example of a conventional RCC type switching regulator. In the figure, Ei is an input voltage, T is a transformer, and W1, W2 and W3 are the primary winding, secondary winding and base winding of this transformer T, respectively. Q1 is a switching transistor that opens and closes the input voltage Ei via the primary winding W1, and R1 is an emitter resistor inserted in series with the emitter of this transistor Q1. The base winding W3 is connected to the transistor Q via the resistor R3.
A base-emitter circuit including one emitter resistor R1 is connected so as to supply a base current. D is a diode that rectifies the voltage of the secondary winding W2, C is a capacitor that smoothes this rectified output, and Z is a load. Also Q2
Is a switching stop transistor, and R2 is a starting resistor.

Next, the operation of FIG. 2 will be described. Input voltage Ei
Is applied, the starting current is flows to the base of the switching transistor Q1 through the starting resistor R2, and the transistor Q1 is turned on. Transistor Q1 is once O
When entering the N state, the input voltage E is applied to the primary winding W1 of the transformer.
i is applied, and as a result, a voltage according to the turns ratio of the transformer is generated in the base winding W3 on the primary side. Since this voltage is connected to the polarity in which the transistor Q1 is further turned on, the transistor Q1 continues to be kept on, and the base current iB of the transistor Q1 becomes a constant current and continues to flow. Therefore, the current of the primary winding W1 (that is, the transistor Q
1 collector current) ic gradually increases, and at the same time, the magnetic flux of the transformer T also increases. At this time, a voltage is also generated in the secondary winding W2, but this voltage is in the blocking direction of the diode D, and no current flows in the secondary winding W2.

When the magnetic flux of the transformer T eventually becomes saturated, the voltage of the base winding W3 disappears, the switching transistor Q is turned off, the magnetic flux is switched to the downward direction, and the transformer T is turned off.
Each winding voltage is inverted. The voltage of the secondary winding W2 at this time is rectified by the diode D1, smoothed by the capacitor C, and supplied to the load Z. During this time,
The energy stored in the primary winding W1 is discharged to the secondary winding W2. When the discharge of this energy ends and the voltage of the secondary winding W2 disappears and the secondary current i2 becomes 0, a slight reverse recovery current flows through the diode D due to the charging voltage of the capacitor C, and then the diode D is blocked. Transition to the state. At this time, a voltage for turning ON the switching transistor Q1 is generated again in the base winding W3, and the switching operation of the transistor Q1 is repeated.

The following method has been taken as a circuit protection method for an overcurrent such as an output short circuit in the circuit of FIG. That is, when the load current io on the secondary side increases, the switching frequency (frequency) of the transistor Q1 increases, the collector current ic on the primary side also increases, and the voltage drop of the emitter resistor R1 connected to the emitter side of the transistor Q1 increases. When this value rises and exceeds the threshold value of the base-emitter voltage of the switching stopping transistor Q2, this transistor Q2 turns on. As a result, the base current iB of the transistor Q1 is bypassed, and the transistor Q1 stops switching. In this way the transistor Q
1 was protected from damage due to overcurrent.

[0006]

However, the overcurrent protection method described in FIG. 2 uses the voltage drop of the emitter resistor R1 due to the primary side current ic, so that the primary side input voltage Ei may fluctuate. When an overcurrent is detected in a rising state, the input power on the primary side is large, so the corresponding thermal power on the secondary side is also large. At this time, the magnitude of the secondary overcurrent is the input on the primary side. It has a larger value than that when the voltage Ei is normal. In other words, since the secondary overcurrent value at the overcurrent protection operating point varies depending on the primary input voltage, it is difficult to determine the protection operating point. Therefore, an object of the present invention is to provide an overcurrent protection circuit for a switching power supply device that can solve this problem.

[0007]

In order to solve the above-mentioned problems, an overcurrent protection circuit according to claim 1 has an input DC voltage (E).
primary winding (W1 etc.) of transformer (T etc.)
A first transistor (Q1 or the like) which is intermittently connected via the
In a switching power supply device that rectifies and smoothes a generated voltage of a next winding (W2, etc.) (via a diode D, a smoothing capacitor C, etc.) and supplies it to a load (Z, etc.), the drive voltage to the first transistor is short-circuited. And a means for turning on the second transistor when the supply current (io, etc.) to the load exceeds a predetermined level. And again

According to another aspect of the present invention, there is provided an overcurrent protection circuit.
In the overcurrent protection circuit described in (1), the means for turning on the second transistor is provided in series with the load, and the resistance value rapidly increases when the supply current to the load exceeds the predetermined level. It comprises a body (such as a current detection resistor RT), a photodiode (such as PD) and a phototransistor (such as PT), the photodiode being energized by a voltage including at least the voltage drop of the resistor, and the phototransistor is connected to the second transistor. And a photocoupler (such as PC) connected to transmit the drive voltage (such as the generated voltage of the base winding W3) to the transistor.

[0009]

The overcurrent of the secondary side load current of the RCC type switching regulator is directly detected to stop the switching operation of the primary side switching transistor.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit diagram showing a configuration as an embodiment of the present invention and corresponds to FIG. 1, a current detection resistor RT that detects the load current io is provided in series with the secondary load Z in FIG. 2, and a resistor R4 is connected in parallel to the series circuit of the load Z and the current detection resistor RT. The photodiode PD of the photocoupler PC is inserted. On the primary side, the emitter resistor R1 is omitted, and the base of the switching stop transistor Q2 that bypasses between the base and emitter of the switching transistor Q1 is driven by the voltage of the base winding W3 via the phototransistor PT of the photocoupler PC. It is configured to be driven. The current detection resistor RT is PT in this example.
It consists of a C thermistor, a non-linear resistance with a positive temperature coefficient of resistance.

Next, the operation of FIG. 1 will be described. Output voltage Eo
When the load current io on the side becomes an overcurrent state due to a short circuit of the load Z or the like and becomes equal to or more than the set load current value, the resistance value of the current detection resistor RT rapidly increases and becomes a non-conduction state, and both ends of the smoothing capacitor C are connected. The voltage between a and b rises. Due to this voltage increase, the current of the photodiode PD of the photocoupler PC increases and turns on the phototransistor PT. This turns on the switching operation stop transistor Q2.
Then, the switching operation of the switching transistor Q1 is stopped. As a result, the switching transistor Q
1 and the rectifier D are prevented from being destroyed and the circuit is protected.

[0012]

According to the present invention, in the RCC type switching power supply device, the load current is detected on the secondary side which is not affected by the fluctuation of the primary side input voltage, and the operation of the switching transistor on the primary side is detected. Since it has been stopped, it is possible to easily determine and fix the protection operating point at the time of overcurrent such as load short circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration as one embodiment of the present invention.

FIG. 2 is a conventional circuit diagram corresponding to FIG.

[Explanation of symbols]

 Ei Input voltage Eo Output voltage T Transformer W1 Primary winding W2 Secondary winding W3 Base winding Q1 Switching transistor Q2 Switching stop transistor D Diode C Smoothing capacitor R2 Starting resistance Z Load RT Current detection resistance PC Photocoupler PD Photodiode PT Photo Transistor

Claims (2)

[Claims] 1. A first transistor for connecting and disconnecting an input DC voltage through a primary winding of a transformer, wherein a voltage generated in a secondary winding of the transformer based on the operation of the transistor is rectified and smoothed to a load. In the switching power supply device for supplying, a second transistor to which a main circuit is connected so as to short-circuit the drive voltage to the first transistor, and the second transistor when the supply current to the load exceeds a predetermined level. And a means for turning on the transistor, the overcurrent protection circuit for the switching power supply device. 2. The overcurrent protection circuit according to claim 1, wherein the means for turning on the second transistor is provided in series with the load, and the supply current to the load exceeds the predetermined level. And a photodiode and a phototransistor, which are energized by a voltage including at least the voltage drop of the resistor, the phototransistor supplying its driving voltage to the second transistor. An overcurrent protection circuit for a switching power supply device, comprising: a photocoupler connected so as to transmit it.