Abstract: A power supply unit includes an input terminal, an output terminal, a main circuit, an ORing field effect transistor, a driving circuit and a switch. The main circuit is connected between the input terminal and the output terminal. The ORing field effect transistor is connected between the output terminal and the main circuit. The driving circuit is connected with a control terminal of the ORing field effect transistor. The driving circuit generates a control signal to control the ORing field effect transistor. The switch is connected between the driving circuit and the control terminal of the ORing field effect transistor. Before the power supply unit is plugged into a system bus of a power supply system, the control signal from the driving circuit is bypassed by the switch. Consequently, the ORing field effect transistor is turned off.

Abstract: A synchronous rectification control method for a resonant power supply includes steps as follows. First, setting an initial conduction width of a synchronous rectification switch, which is less than a maximum conduction width. Afterward, detecting a voltage waveform across two ends of a body diode of the synchronous rectification switch. Afterward, calculating a resonant frequency according to the voltage waveform. Finally, determining the maximum conduction width of the synchronous rectification switch.