One embodiment includes a power supply system including a transformer comprising a primary, secondary, and auxiliary winding that are magnetically coupled. The system also includes a switch stage that generates a current through the primary winding in response to activation of a switch based on a control signal that is generated based on a feedback voltage associated with the auxiliary winding. The current can be induced in the secondary winding. The system also includes an output stage coupled to the secondary winding and that generates an output voltage based on the current induced in the secondary winding. The system further includes a feedback stage coupled to the auxiliary winding and comprising a discriminator configured to determine a zero-current condition associated with the current induced in the auxiliary winding based on monitoring a change in slope of the feedback voltage and to measure the feedback voltage during the zero-current condition.