Quasi-resonant switching power supply

A quasi-resonance switching power supply quickly determines the number of bottom skips corresponding to the load condition even in abrupt load change. The quasi-resonance switching power supply is provided with a bottom skipping control function and a capacitor to hold a voltage corresponding to the load condition of the switching element over one switching period of the switching element. The quasi-resonance switching power supply comprises a bottom skipping number determining circuit that compares the voltage held on the capacitor with comparison reference voltages selected from a plurality of reference voltages for determining the number of bottom skips, and revises the comparison reference voltage according to the comparison result. The processing of comparison and revision is executed multiple times in one switching period of the switching element. Thus, the bottom skipping number determining circuit determines the number of bottom skips corresponding to the voltage held on the capacitor.