A power factor correction circuit includes a power transistor, an inductor, and detection circuitry. The inductor is coupled to a drain terminal of the power transistor. The detection circuitry is coupled to the drain terminal of the power transistor. The detection circuitry is configured to determine an input voltage applied to the inductor based on resonant ringing of voltage at the drain terminal, and to detect a valley in the voltage at the drain terminal based on the input voltage applied to the inductor.