A power supply circuit for a south bridge chip includes a voltage sampling circuit, a control circuit, and an I/O controller. The voltage sampling circuit comprises an input terminal capable of receiving a first voltage, and an output terminal capable of outputting a control signal. The control circuit is capable of receiving the control signal from the voltage sampling circuit and outputting a power good signal when a high voltage level control signal is received. The I/O controller is capable of receiving the power good signal from the control circuit, adjusting time sequence for the power good signal to synchronize with the first voltage, and outputting the adjusted power good signal to provide power for the south bridge chip.