A power supply circuit includes a basic input output system (BIOS), a super input output (SIO), a bivibrator, a logical selector, and a voltage converter. The basic input output system (BIOS) is configured for storing different operation modes of a computer. The super input output (SIO) is configured for generating standby mode signals according to the different operation modes. The bivibrator is configured for generating a reference signal when upon receiving a clock signal from the computer when the computer is turned on. The logical selector is configured for generating a standby control signal in response to the reference signal and one of the standby mode signals. The voltage converter is configured for transforming the first standby voltage into a second standby voltage to drive the SIO. The SIO receives the second standby voltage before the clock signal is delayed and provided to the SIO to start the computer.