A power supply includes a transistor that is connected to a primary winding of a transformer. A controller controls a switching operation of the transistor by quasi-resonant switching. The controller receives a feedback voltage and adjusts the feedback voltage to adjust a blanking frequency, which is an inverse of a blanking time during which the transistor is prevented from being turned on. The controller turns on the transistor after expiration of the blanking time based on a level of a resonant ring.