| 摘要 |
PURPOSE: To provide a switching power supply unit capable of attaining size, weight, and cost reductions as well as high efficiency of the switching power supply thereof, by realizing a two-crystal flyback switching power supply unit which operates in a current continuous mode in a self-excited oscillating manner. CONSTITUTION: In this switching power supply unit which alternately turns on/off a first switching device Q1 and a second switching device Q2 for self-excited oscillation, there is provided an on-time control circuit 4 which is preset at a time constant for turning off the second switching device Q2 so that voltage may be developed at an inductor L to reverse transformer voltage, by resonating the inductor L and a capacitor C during an off-period of the first switching device Q1, and cutting off resonance current flowing through a series circuit consisting of the second switching device Q2 and the inductor L before energy release from a secondary winding T2 after the turning-off of the second switching device Q2.
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