SWITCHING POWER SUPPLY DEVICE

摘要

PROBLEM TO BE SOLVED: To provide a high efficiency switching power supply device which can reduce its switching frequency when a load is light and obtain the switching frequency equivalent to a conventional frequency. SOLUTION: In a switching power supply device 1, a delay circuit 6 including a transistor Q3 and a time constant circuit comprising a capacitor C5 and a resistor R8 is connected to the gate of an FET Q1. A voltage generated in the feedback winding Nb of a transformer T and the charging voltage Vc1 of a capacitor C6 of which a rectifying/smoothing circuit 7 is composed are applied to the capacitor C5 for charging. The turning-on of the FET Q1 is delayed until the charging voltage Vc2 of the capacitor C5 reaches a voltage Von by which the transistor Q3 is turned on.