| 摘要 |
PROBLEM TO BE SOLVED: To provide a switching power unit which can avoid the breakdown of a three-terminal commutating element SW2 in a light load state. SOLUTION: A synchronous rectifying circuit 31 comprises a three-terminal rectifying element SW1 and a thre-terminal commutating element SW2. The three-terminal rectifying element SW1 and the three-terminal commutating terminal SW2 synchronously rectify the voltage induced in a second coil N2, by the synchronous control signal supplied from a transformer T to a control electrode. A decision means 33 monitors the voltage signal which has a time duration corresponding to the intermittence of switching output, on the side of an output circuit 3, and decides it as being a light load, when the time duration corresponding to the ON period of the three-terminal commutation element becomes a certain value or smaller, and switches off the three-terminal commutation element SW2. COPYRIGHT: (C)2005,JPO&NCIPI
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