PURPOSE:To perform a rush current preventing operation which is not affected by influence of variation in a secondary side output circuit by phase-controlling an input AC voltage based on lapse of time after a power source is energized and charge amount stored in a capacitor of a smoothing circuit. CONSTITUTION:A full-wave rectifier 76, a phase detector 77 for detecting a zerocross point from the waveform S3 of a fully-rectified voltage, a time forming circuit 78 for forming a time gradually extending from a lapse time from energization of a power source, and a controller 79 for controlling to close a switching element of a rush current preventing circuit 71 by a phase detection signal S1 and to continuously close it during a period of a time signal S2, are provided. Accordingly, since the main switching operation and a rush current preventing operation are separated, the rush current preventing operation is not affected by the configuration of a transformer and the voltage to be applied to the transformer. Thus, an inconvenience for reducing the stable range of the rush current preventing operation due to variation in an output side load capacity is prevented.
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