| 摘要 |
PROBLEM TO BE SOLVED: To prevent an abnormal oscillation from being caused, by superimposing sawtooth voltage on detection voltage caused by detecting current flowing through a switching circuit for obtaining summed voltage, and involving overcurrent protection circuit which determines a timing for turning off switching circuit so that the summed peak voltage may be maintained constant. SOLUTION: This switching power supply is provided with a compensation circuit constituted of a sawtooth wave generating circuit 20 and an adder 21. The current Ip of the primary winding 10a of a transformer 10 is detected by a current transformer 5 to convert it into detection voltage Vp and a sawtooth voltage Vn which is synchronous with a PWM control circuit 24 and has a fixed tilt is superimposed on the detection voltage Vp by means of the adder 21 to obtain summed voltage Vw. At the timing when the summed voltage Vw becomes equal to reference voltage Vr, a signal of turning off switching elements 6, 7, 8, 9 is outputted to an AND circuit 25. It is thus possible to reduce an open loop gain involved in abnormal oscillation, thereby preventing generation of abnormal oscillation.
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