Quasi-resonant switching power supply

A quasi-resonance switching power supply that utilizes resonance phenomenon in a circuit including an inductance and a resonance capacitor connected in series to an ON-OFF driven switching element, and that turns ON the switching element at a timing of a bottom of a resonant voltage, the quasi-resonance switching power supply comprising:

• a bottom detecting circuit that detects a bottom of the resonant oscillation voltage developed upon turning OFF of the switching element;
• a bottom skipping number determining circuit that determines a number of bottom skips corresponding to a load of the switching element;
• and
• a turn ON control circuit that turns ON the switching element when a number of bottoms detected by the bottom detecting circuit reaches a number of bottom skips determined by the bottom skipping number determining circuit;
• wherein
• the bottom skipping number determining circuit comprises a capacitor that holds a voltage corresponding to the load of the switching element over one switching period of the switching element, and sequentially compares the voltage held on the capacitor with a plurality of reference voltages for determining the number of bottom skips in one switching period of the switching element to determine the number of bottom skips corresponding to the voltage held on the capacitor.