A primary-side switching circuit generates switching signals for switching a transformer. A secondary-side switching circuit is coupled to an output of the power converter to generate pulse signals in response to the switching signals and an output voltage of the power converter. Pulse signals are generated to rectify and regulate the power converter. A synchronous switch includes a power-switch set and a control circuit. The control circuit receives the pulse signals via capacitors for turning on/off the power-switch set. The power-switch set is connected in between the transformer and the output of the power converter. Furthermore, a flyback switch is operated to freewheel the inductor current of the power converter. The flyback switch is turned on in response to the off state of the power-switch set. The on time of the flyback switch is programmable and correlated to the on time of the power-switch set.