| 摘要 |
PROBLEM TO BE SOLVED: To prevent malfunction of a power-supply protective circuit by a method wherein, when the rectified output of the secondary winding of a transformer is short-circuited, a secondary-side detected voltage is transmitted, via a photocoupler, to a pulse-width control circuit for driving switching pulses of the primary winding of the transformer, a constant voltage is controlled and a noise component is removed. SOLUTION: When a DC voltage +B1 is short-circuited, the emitter current of a switching transistor Q5 is increased. A voltage across a resistor R5 due to an increase in the current is detected. A noise component which is superposed on the voltage across the resistor R5 is removed by a filter circuit 8. The output of the filter circuit 8 is rectified by a diode D10, and a voltage is divided by a resistor R6 and a resistor R7 so as to be applied to the base of a thyristor- connection transistor Q2. The thyristor-connection transistor Q2 and a thyristor- connection transistor Q3 are turned on, and they absorb the output of an operational amplifier 2 via a diode 9. Thereby, the input of a pulse modulator 5 is cut off, the operation of the switching transistor Q5 is stopped, and the supply of electric power to the secondary side of a switching transformer T is stopped.
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