A first switching control circuit (CNT1), triggered by a timing when a rectifier diode (Ds1) enters a nonconducting state and a transformer voltage Vt is inverted, turns on a first switching element (Q1) after specified delay time. A second switching control circuit (CNT2), triggered by a timing when the first switching element (Q1) is turned off and the transformer voltage Vt is inverted, turns on a second switching element (Q2). A third switching control circuit (CNT3), triggered by a timing when the second switching element (Q2) is turned off, turns on a third switching element (Q3). The (CNT1) determines the interval ton1 of the first switching element (Q1) such that a first output voltage Vo1 has a specified value, the (CNT2) determines the on-interval ton2 of the second switching element (Q2) such that a second output voltage Vo2 has a specified value, and the (CNT3) determines the on-interval ton3 of the third switching element (Q3) such that a third output voltage Vo3 has a specified value.
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