A synchronous regulation circuit is provided to improve the efficiency of the power converter. A primary-side switching circuit generates a synchronous signal and a switching signal. The switching signal is used for soft switching a transformer. A secondary-side switching circuit is coupled to the output of the power converter to generate a pulse signal in response to the synchronous signal and the output voltage of the power converter. The pulse signal is a differential signal generated for the rectifying and the regulation of the power converter. A synchronous switch includes a power switch and a control circuit. The control circuit receives the pulse signal for turning on/off the power switch. The power switch is connected in between the transformer and the output of the power converter. Furthermore, a flyback switch is operated as a synchronous rectifier to freewheel the inductor current of the power converter. The flyback switch is turned on in response to the off of the power switch. The on time of flyback switch is programmable and correlated to the on time of the power switch.