| 摘要 |
A control circuit 8 of a switching power source comprises a voltage detector 31 for detecting a voltage V<SUB>DC </SUB>of a DC power supply 1 to produce a detection voltage V<SUB>DT</SUB>; a comparator 41 for producing an output signal V<SUB>CP </SUB>when detection voltage V<SUB>DT </SUB>from voltage detector 31 exceeds a reference voltage V<SUB>R2</SUB>; a bottom voltage detector 51 for detecting a bottom point of voltage V<SUB>DS </SUB>across a MOS-FET 3 after energy has been discharged from transformer 2 ; and a switching controller 61 for selectively turning MOS-FET 3 on depending on existence or absence of the output signal V<SUB>CP </SUB>from comparator 41 . When the input voltage V<SUB>DC </SUB>from DC power supply 1 so rises that detection voltage V<SUB>DT </SUB>of voltage detector 31 is above the reference voltage V<SUB>R2</SUB>, switching controller 61 serves to late turn MOS-FET 3 on at the time bottom voltage detector 51 detects the second or later bottom point of the voltage V<SUB>DS </SUB>across MOS-FET 3 , extending the off period of MOS-FET 3 to reduce switching frequency of MOS-FET 3.
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