JPH09215331A - Switching power source apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely provide an over-current protection without relation to a coupling degree of coils when an over-load condition is generated due to a floating output voltage. SOLUTION: Under the steady condition, a capacitor C4 is charged with an output voltage V2 from a secondary coil 522 via capacitors C3, C5. Thereby, the capacitor C4 is set to a voltage higher than that of a zener diode ZD. Therefore, the diode D5 does not become conductive. If an output of the secondary coil 522 becomes over-loaded, potential of a capacitor C4 is reduced and becomes lower than the voltage of the zener diode ZD, a base current of the transistor Q increases causing a photocoupler Pct to emit light and the photocoupler PCr operates. Thereafter, with the operation of the photocoupler PCr, a control IC3 reduces a coil voltage of the transformer 5 up to the voltage in which a tertiary coil 53 cannot drive the control IC3 to stop the operation of the power source for the operation of the over-current protection circuit.

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 device, and more particularly to a switching power supply device provided with an overcurrent protection circuit for a floating output voltage whose ground side is insulated from other loads.

[0002]

2. Description of the Related Art In a conventional switching power supply device, as shown in FIG. 3, a current value flowing through a primary side detection resistor Rs for detecting total output power exceeds an upper limit of a current value set by a control IC 3. Then, the ON duty of the switching element 4 is suppressed, the secondary winding voltages V1, V2, and the tertiary winding voltage are reduced, and the control IC 3 that uses the tertiary winding voltage as a driving means is turned off to stop the power supply. I am letting you.

[0003]

However, the conventional switching power supply device is provided with the output of the first secondary winding 521 and the output of the second secondary winding 522 which requires DC isolation. In this case, when the output voltage V2 is overloaded when the coupling with the first secondary winding 521 is low, the voltage of the first secondary winding 521 and the tertiary winding 53 does not decrease and the control IC 3 stops. However, an overcurrent continues to flow to the side of the second secondary winding 522 to cause a problem such as abnormal heat generation, and the output voltage V2 cannot share the ground G2 with the ground G1 of the output voltage V1. It is impossible to form an overcurrent protection circuit using the feedback circuit 207 provided on the side of the secondary winding 521 of No. 1 of FIG.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching power supply device that reliably operates overcurrent protection regardless of the degree of coupling of windings when the floating output voltage is overloaded.

[0005]

In order to solve the above-mentioned problems, a switching power supply device of the present invention is a rectifying / smoothing circuit for rectifying and smoothing an input AC power supply to output a DC voltage. A switching element that switches a DC voltage that has been rectified and smoothed by a rectifying and smoothing circuit, and a DC voltage that has been switched by this switching element are input to a primary winding, and a first secondary winding and a second secondary winding are input. Line and a transformer for generating an output voltage to the tertiary winding, a control IC driven by the output voltage from the tertiary winding of the transformer and controlling the switching element, and the second secondary winding A capacitor and a diode are connected to the output terminal of and the voltage generated in proportion to the output of the second secondary winding is compared with the output voltage of the first secondary winding to prevent overcurrent. Comparison circuit to detect Controlling the output voltage of the first secondary winding to be fed back to the control IC and feeding back to the control IC based on the comparison result of the comparison circuit to stop the output voltage. And a feedback circuit that operates.

[0006]

Next, a switching power supply device according to a first embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a switching power supply device according to a first embodiment of the present invention.

The switching power supply device according to the first embodiment of the present invention, as shown in FIG.
i and smoothing Ci, rectifying and smoothing circuit 2 for outputting a DC voltage
And a switching element 4 for switching DC voltage
Then, the rectangular wave voltage switched by the switching element 4 is input to the primary winding 51 to input the first secondary winding 521.
A transformer 5 for generating an output voltage in the second secondary winding 522 and a voltage in the tertiary winding 53; and a control IC 3 for controlling the switching element 4 according to the current value flowing in the detection resistor Rs. A diode Ds for rectifying the voltage generated by the tertiary winding 53 of the transformer 5, a capacitor Cs for smoothing the voltage rectified by the diode Ds, and an output voltage V1 of the first secondary winding 521 are rectified. A diode D1, a capacitor C1 that smoothes the output voltage V1, and an output voltage V2 of the second secondary winding 522.
A diode D2 for rectifying the output voltage, a capacitor C2 for smoothing the output voltage V2, output voltages V1 and V2 of the first secondary winding 521 and the second secondary winding 522, and feedback to the control IC3. A feedback circuit 7 to perform and a comparison circuit for detecting an overcurrent by comparing the voltage generated in proportion to the output of the second secondary winding 522 with the output voltage V1 of the first secondary winding 521. 6 and 6.

Further, the feedback circuit 7 includes a resistor R
3, a transistor Q biased by R4, and a zener diode ZD, a photocoupler PCt, and a resistor R2 connected between the output voltage V1 via the transistor Q are provided, and the comparison circuit 6 has a second secondary circuit. Capacitors C3, C5 and diode D connected to both ends of the winding 522
3, D4, and the output of the transistor Q, the resistor R1, the capacitor C4, and the diode D5 connected to one end of the diode D4.

Next, the operation of the switching power supply device according to the first embodiment of the present invention will be described with reference to the drawings.

The operation of the switching power supply unit according to the first embodiment of the present invention is, as shown in FIG. 1, an output voltage V2 output from the second secondary winding 522 via the capacitors C3 and C5 in a steady state. As a result, when the capacitor C4 is charged and set to a voltage higher than the Zener diode ZD during steady operation, the diode D5 does not conduct, and the output of the second secondary winding 522 is overloaded, the capacitor C4 When the potential decreases and becomes equal to or lower than the voltage of the Zener diode ZD, the base current of the transistor Q increases, the photocoupler PCt emits light, and the photocoupler PCr operates.

After that, the control IC 3 operates to lower the winding voltage of the transformer 5 by the operation of the photocoupler PCr, lowers the voltage until the tertiary winding 53 cannot drive the control IC 3, and stops the power supply. Then, the overcurrent protection circuit operates.

Next, a switching power supply device according to a second embodiment of the present invention will be described with reference to the drawings.

FIG. 2 is a circuit diagram of a switching power supply device according to a second embodiment of the present invention.

As shown in FIG. 2, the switching power supply device according to the second embodiment of the present invention is different from the first embodiment of the present invention in that a shunt regulator TD is used instead of the Zener diode ZD of the feedback circuit 7. A resistor R5 and a transistor Q1 are used instead of the diode D5 of the circuit 6, and the other configuration is the same as that of the first embodiment of the present invention.

Next, the operation of the switching power supply device according to the second embodiment of the present invention will be described with reference to the drawings.

The operation of the switching power supply device according to the second embodiment of the present invention is, as shown in FIG. 2, output from the second secondary winding 522 through the capacitors C3 and C5 and the diodes D3 and D4 in a steady state. The output voltage V2 charges the capacitor C4 and is set to a higher voltage value than the resistors R2 and R5 that divide the output voltage V1 during steady operation, and the potentials of the resistors R2 and R5 that divide the output voltage V1 are shunted. If it is set higher than the reference of the regulator TD, the transistor Q1 does not conduct.

When the output of the second secondary winding 522 is overloaded, the voltage of the capacitor C4 drops and the base voltage of the transistor Q1 becomes lower than the potentials of the resistors R2 and R5 which divide the output voltage V1. When the voltage becomes low, the transistor Q1 becomes conductive, the photocoupler PCt emits light, the photocoupler PCr operates, the control IC3 lowers the winding voltage of the transformer 5, and the tertiary winding 53 cannot drive the control IC3. When the voltage drops to a certain point, the overcurrent protection circuit that stops the power supply operates.

[0019]

As described above, according to the switching power supply device of the present invention, the overcurrent protection can be reliably operated even when the floating output voltage is overloaded regardless of the degree of coupling of the windings. There is.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching power supply device according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a switching power supply device according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a conventional switching power supply device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 AC power supply 2 Rectifying / smoothing circuit 3 Control IC 4 Switching element 5 Transformer 6,106 Comparison circuit 7,107 Feedback circuit 51 Primary winding 53 Third winding 521 First secondary winding 522 Second secondary winding Line C1 First secondary winding (capacitor) C2 Second secondary winding (capacitor) C3, C4, C5 Comparison circuit (capacitor) Ci Rectification smoothing circuit (input capacitor) D1 First secondary winding ( Diode) D2 Second secondary winding (diode) D3, D4, D5 Comparison circuit (diode) Di Rectification smoothing circuit (bridge diode) PCr Control IC (photocoupler) PCt Feedback circuit (photocoupler) Q Feedback circuit (transistor) ) Q1 comparator circuit (transistor) R1, R5 comparator circuit (resistor) R2, R3, R4 feed bar Circuit (resistor) TD feedback circuit (shunt regulator) ZD feedback circuit (zener diode)

Claims (1)

[Claims] 1. A rectifying / smoothing circuit for rectifying and smoothing an input AC power source to output a DC voltage, a switching element for switching a DC voltage rectified and smoothed by this rectifying / smoothing circuit, and this switching element. A transformer for inputting the switched DC voltage to the primary winding to generate an output voltage to the first secondary winding, the second secondary winding and the third winding, and the tertiary winding of this transformer. A control IC driven by an output voltage from the line and controlling the switching element, and a capacitor and a diode are connected to the output terminal of the second secondary winding, and the output of the second secondary winding is connected. And a comparison circuit for detecting an overcurrent by comparing the voltage generated in proportion to the output voltage of the first secondary winding and the output voltage of the first secondary winding for detecting Feedback to the control IC, A switching power supply device comprising: a feedback circuit that feeds back to the control IC to stop the output voltage based on a comparison result of the comparison circuit.