| 摘要 |
PROBLEM TO BE SOLVED: To provide a switching power supply that limits frequency by setting a pseudo voltage resonance time longer. SOLUTION: The switching power supply circuit comprises a transformer 1 consisting of a primary coil and a secondary coil, a semiconductor switch 2 connected in series to the primary coil, and a control circuit 4 for periodically turning on/off the semiconductor switch 2. A valley point detecting circuit 10 detects the magnetic energy stored in the transformer 1 while the semiconductor switch 2 is on, and the voltage valley point of the voltage resonance that occurs after total outputting through the secondary coil during turning-off. The control circuit 4 comprises on-timing determining circuit 40 that turns on at the valley of preset fixed frequency and detected by the valley point detecting circuit 10 for each switching circuit. The on-timing determining circuit 40 comprises a valley voltage count circuit 41 for controlling turning-on at the valley of the fixed frequency by counting frequency of the signal from the valley detecting circuit 10. COPYRIGHT: (C)2006,JPO&NCIPI
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