| 摘要 |
PROBLEM TO BE SOLVED: To provide a switching power supply unit wherein even when its DC input voltage is increased to a higher voltage than a predetermined voltage, its power switching element is not destroyed by the saturation of its transformer in the case it is loaded heavily. SOLUTION: In a switching power supply, sensing by a voltage sensing circuit 6 the voltage of its first DC output obtained through a transformer T and a rectifier-filter circuit, a control circuit 10 generates from the sensed voltage and its second DC output a control signal VC1 proportional to the first DC output. Also, the control circuit 10 generates a second control signal VC2 proportional to a voltage VDC of its DC input power supply to control the ON-width of a power switching element Q1 by the comparison results of these first and second control signals VC1, VC2 with a triangular-wave signal VR.
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