A switch-mode power supply includes a power transistor, a transformer, and detection circuitry. The transformer includes a primary winding that is coupled to a drain terminal of the power transistor. The detection circuitry is coupled to a source terminal of the power transistor. The detection circuitry is operable to monitor signal present on the drain terminal via parasitic drain-source capacitance of the power transistor while the power transistor is switched off, and to detect demagnetization of a secondary winding of the transformer via the monitored signal.