JPH1155946A - Protection circuit against overcurrent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protection circuit against overcurrent, which varies the output power value where protection against overcurrent works, according to the load current of the output of a secondary transformer. SOLUTION: A protection circuit against overcurrent, which uses a conventional control IC having a protection function against overcurrent, is provided with a circuit composed of a Zener diode ZD1, resistors R6, R7, and R8, and transistors Q2, Q3, and a photocoupler PC2, and the voltage of the emitter terminal of the switching transistor Q1 is raised to approximately the specified voltage value where the protection against overcurrent works, by the transistor Q2 only in the case of working the protection against overvoltage in a low voltage system V1 when the high-voltage system V2 of the secondary transformer is in light load. Hereby, the load current which should have been let flow excessively to the low-voltage system V1 can be reduced by lowering the output power value where the protection against overcurrent works, and the bad influence which were exerted on the element power line of the low voltage system V1 can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an overcurrent protection circuit, and more particularly, to an overcurrent protection circuit used for a switching regulator.

[0002]

2. Description of the Related Art A conventional overcurrent protection circuit comprises a detection resistor R1 connected to an emitter of a switching transistor Q1.
, And when the terminal voltage of the resistor R1, that is, the emitter terminal voltage of the transistor Q1 exceeds a predetermined voltage value, the control is performed by a control IC which is set so that the overcurrent protection works. The side transformer output is composed of a high-voltage system V2 for driving a motor and the like and a low-voltage system V1 for driving a CPU and the like. The feature is that as the load becomes lighter, the voltage of the high voltage system V2 also rises.

[0003]

In the conventional overcurrent protection circuit, the output of the secondary transformer is composed of a high voltage system V2 for driving a motor and the like and a low voltage system V1 for driving a CPU and the like. In order to activate the overcurrent protection in the low-voltage system V1 when the high-voltage system V2 is in a light load state, a constant voltage value set inside the control IC having the overcurrent protection function is used. Since it is necessary to keep the load current of the low-voltage system V1 flowing until the terminal voltage of the detection resistor R1 connected to the transistor Q1, ie, the emitter terminal voltage of the transistor Q1, exceeds the load current of the low-voltage system V1. In other words, there is a problem that the elements in the power supply line of the low-voltage system V1 are adversely affected.

Accordingly, it is an object of the present invention to provide an overcurrent protection circuit that varies an output power value at which overcurrent protection operates in accordance with a load current of a secondary transformer output.

[0005]

In order to solve the above-mentioned problems, an overcurrent protection circuit according to the present invention comprises: a rectifier and a smoother for rectifying and smoothing an input AC power; And a switching transistor for switching the smoothed AC power supply, and the AC power supply switched by the switching transistor is input to a primary winding to generate an output voltage on a secondary winding and a flyback voltage on a tertiary winding. A transformer, control means for controlling the switching transistor by a voltage obtained by rectifying and smoothing a flyback voltage generated in a tertiary winding of the transformer, and a control means for increasing the voltage with respect to the secondary control winding voltage. DC generation means for rectifying and smoothing the next winding voltage, and detecting the DC voltage generated by the DC generation means based on a predetermined voltage value Output means, a transmission means for feeding back a signal detected on the secondary side of the transformer, a control IC for activating overcurrent protection when the emitter voltage of the switching transistor exceeds a predetermined voltage value, and Variable means for varying the current flowing to the emitter of the switching transistor based on the magnitude of the transmitted signal, and varying the resistance value connected to the emitter to vary the emitter voltage. I do.

Further, the overcurrent protection circuit of the present invention comprises a Zener diode for controlling the cathode to be higher than the secondary control winding voltage, a photocoupler in which the light emitting side of the anode of the Zener diode is connected in series, A detecting means having a resistor connected in series to a cathode of the photocoupler, a transmitting means for feeding back a signal detected by the detecting means by a photocoupler on a light receiving side, and a resistor connected to a power supply voltage of the control means; And a variable means comprising a transistor connected to a collector of a biased current amplifying transistor and an emitter of a switching transistor to vary an emitter voltage by a photocoupler on a light receiving side serially connected to the light receiving side. .

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An overcurrent protection circuit according to one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of an overcurrent protection circuit according to one embodiment of the present invention.

As shown in FIG. 1, an overcurrent protection circuit according to an embodiment of the present invention comprises a rectifier 2 for rectifying and smoothing an input AC power supply 1 with an input diode D1 and an input capacitor C1, and this rectifier. A switching transistor Q1 for switching the AC power supply 1 rectified and smoothed by the means 2 and the AC power supply 1 switched by the switching transistor Q1 are input to a primary winding 51, a secondary control winding 52, a secondary winding Output voltage and tertiary winding 5
4, a transformer 5 for generating a flyback voltage, control means 3 for controlling the switching transistor Q1 with a voltage obtained by rectifying and smoothing the flyback voltage generated by the tertiary winding 54 of the transformer 5, and a secondary control winding. Line 52,
The output voltage sent to the secondary winding 53 is converted to an output diode D
3, DC generating means 6 and 7 for rectifying and smoothing by D4 and output capacitors C3 and C4, and the voltage of the DC generating means 7 configured to be higher than the voltage of the DC generating means 6 to a predetermined voltage value. Detecting means 8 for detecting based on the input signal, and transmitting means 9 for feeding back the detected signal to the input side.
And the emitter voltage of the switching transistor Q1 is
The current flowing through the transistor Q2 connected to the emitter of the switching transistor is variable based on the control IC 31 set so that the overcurrent protection is activated when the voltage exceeds a predetermined value, and the magnitude of the signal transmitted by the transmission means 9. And a variable means 4 for varying the resistance value connected to the emitter of the switching transistor Q1 to vary the emitter voltage.

The control means 3 includes a control IC 31 controlled by the switching transistor Q1, a starting capacitor C2 for charging the AC power supply 1 smoothed by the input capacitor C1 through the input resistor R2 and smoothing a flyback voltage, When the power supply 1 is input, the control IC 31 supplies the ground signal to the photocoupler PC1 and the output voltage of the DC generation means 6 to the reference voltage I
Photocoupler PC that detects and emits light via C1 and resistor R3
1 and a start-up diode D2 for rectifying the flyback voltage generated in the tertiary winding 54 of the transformer 5.

Further, based on the Zener voltage of the Zener diode ZD1 connected to the DC generation means 7, the detection means 8 includes a photocoupler PC2 for detecting the state of the voltage of the DC generation means 7 to the Zener diode ZD1, and a Zener diode. ZD1 and a resistor 8 for limiting the current flowing through the photocoupler PC2.
Is constituted by a photocoupler PC2 that transmits a signal detected on the secondary side to the input side, and the variable means 4 is connected to the emitter of the switching transistor Q1.
A current varying transistor Q2, a transistor Q3 for amplifying a signal transmitted by PC2, and a resistor R
6, R7.

The operation of the overcurrent protection circuit according to one embodiment of the present invention will be described with reference to the drawings.

In the overcurrent protection circuit according to one embodiment of the present invention, as shown in FIG. 1, the output of the secondary side transformer is for driving a high voltage system V2 for driving a motor and the like and a CPU and the like. When the voltage of the low-voltage system V1 is controlled, the voltage value of the high-voltage system V2 increases as the high-voltage system V2 becomes lighter in load, and the high-voltage system V1
2 and the Zener diode ZD
The current flowing through 1 to the diode of the secondary-side photocoupler PC2 increases.

When the current flowing through the diode of the secondary photocoupler PC2 increases, the current flowing through the transistor of the primary photocoupler PC2 increases.
When the current flowing through the transistor of the primary side photocoupler PC2 increases, the base current of the biased transistor Q3 increases due to the resistor R7 connected to the power supply voltage VCC of the control IC, and the collector current of the transistor Q3 increases accordingly. I do.

Further, when the collector current of the transistor Q3 increases, the base current of the transistor Q2 biased by the resistor R6 decreases, the collector current of the transistor Q2 also decreases, and when the collector current of the transistor Q2 decreases, the transistor The resistance value between the collector and the emitter of Q2 increases, and the emitter terminal voltage of the switching transistor Q1 rises, thereby approaching a predetermined voltage value at which the overcurrent protection of the control IC works and the output at which the overcurrent protection works. When the power value decreases and the high voltage system V2 is in a heavy load state, the Zener diode ZD
1 does not exceed the zener voltage of the photocoupler PC1.
, No current flows, and the transistor Q3 does not operate. Therefore, an operation of reducing the base and collector currents of the transistor Q2 cannot be performed, and there is no change in the output power value at which the overcurrent protection works.

[0016]

As described above, the overcurrent protection circuit according to the present invention includes a Zener diode and a resistor in a control IC in which overcurrent overprotection is performed when the emitter voltage of a conventional switching transistor exceeds a predetermined voltage value. And a circuit formed from a transistor and a photocoupler, which is connected to the emitter of the switching transistor only when the high voltage system of the secondary transformer output is in a light load state and the overcurrent protection is activated in the low voltage system. By making the transistor variable, increasing the detection resistance value connected to the emitter of the switching transistor, and increasing the emitter voltage of the switching transistor, the output power value at which the overcurrent protection works is lowered, and the low voltage system becomes excessive. Devices that can reduce the load current that had to flow and are in the low-voltage power supply line There is an effect that can be reduced have had adverse effects exerted.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an overcurrent protection circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional overcurrent protection circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 AC power supply 2 Rectifying means 3 Control means 4 Variable means 5 Transformer 6 DC generation means 7 DC generation means 8 Detecting means 9 Transmission means 31 Control IC 51 Primary winding 52 Secondary control winding 53 Secondary winding 54 Tertiary Winding C1 Input capacitor C2 Starting capacitor C3, C4 Output capacitor D1 Input diode D2 Starting diode D3, D4 Output diode IC1 Reference voltage IC Q1 Switching transistor Q2, Q3 Transistor R1 Detection resistor R2 Input resistance R3 to R8 Resistance ZD1 Zener diode PC1, PC2 photo coupler

Claims (2)

[Claims] 1. A rectifying and smoothing means for rectifying and smoothing an input AC power supply, a switching transistor for switching the AC power supply rectified and smoothed by the rectifying and smoothing means, and an AC power supply switched by the switching transistor And a transformer for generating an output voltage in the secondary winding and a flyback voltage in the tertiary winding, and rectifying and smoothing the flyback voltage generated in the tertiary winding of the transformer. Control means for controlling the switching transistor with a voltage; DC generation means for rectifying and smoothing the secondary winding voltage so that the voltage is higher than the secondary control winding voltage; Detecting means for detecting the detected DC voltage based on a predetermined voltage value, and a transmission means for feeding back a signal detected on the secondary side of the transformer. Control means for activating overcurrent protection when the emitter voltage of the switching transistor exceeds a predetermined voltage value, and a current flowing to the emitter of the switching transistor based on the magnitude of a signal transmitted from the transmission means. And a variable means for varying an emitter voltage by varying a resistance value connected to the emitter. 2. A Zener diode for controlling a cathode to be higher than a secondary control winding voltage, a photocoupler having a light emitting side of an anode of the Zener diode connected in series, and a photocoupler connected in series to a cathode of the photocoupler. The detecting means having a resistance, the transmitting means for feeding back a signal detected by the detecting means by a photocoupler on a light receiving side, and a photo on a light receiving side connected in series to a resistor connected to a power supply voltage of the control means. 2. The overcurrent protection circuit according to claim 1, wherein said overcurrent protection circuit comprises a transistor connected to a collector of a biased current amplification transistor and an emitter of a switching transistor to vary an emitter voltage by a coupler. .