| 摘要 |
A series circuit of the primary winding (21) of a transformer (2), a switching element (3) and a current-detecting resistor (4) is connected between a pair of DC terminals (18, 19). A smoothing capacitor (7) is connected with the secondary winding (22) of the transformer (2) through a diode (6). A resonant capacitor (5) is connected in parallel with the series circuit of the switching element (3) and the current-detecting resistor (4). A control pulse for the switching element (3) is provided in accordance with the combination of the detected current signal, the detected output voltage signal and the detected switch voltage signal (11). A control circuit (13) includes a circuit for determining first and second minimum OFF periods with hysteresis. The OFF period of the switching element (3) is limited to a minimum OFF period for light load. The zero-volt switching when the switching element (3) turns on is carried out after a minimum OFF period has passed and the transformer (2) has completely discharged energy.
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