A switch mode power supply (200, FIG. 2; 600, FIG. 6) includes a transformer (208), a transistor (212) that drives the primary winding of the transformer, and a controller (210, 610). The controller senses whether or not the transformer is reset and applies a switching control signal to a control terminal (220) of the transistor, accordingly. The signal is produced at a switching frequency, and the signal has a duty cycle that is limited based on whether or not the transformer is reset.