A circuit is provided. The circuit includes a DC/DC converter 100 and a power factor correction apparatus. The DC/DC converter 100 is for converting a rectified line power into an output power. The DC/DC converter 100 includes a first inductor and a converting switch. The power factor correction apparatus includes a mode switching circuit and a switch controller. The mode switching circuit is for comparing a detected signal of the output power with a threshold signal and outputting a ZCD signal. The switch controller coupled with the mode switching circuit for driving the converting switch based on the ZCD signal. When the detected signal is beyond the threshold signal, first switch signal is generated. When the detected signal is not beyond the threshold signal, a second switch signal is generated.
