A power supply circuit applied to a central processing unit (CPU) of a computer includes a pulse width modulation (PWM) controller, a control circuit, a first switch circuit, a second switch circuit, a first compensation circuit, a second compensation circuit, a first power circuit, and a second power circuit. When the computer is turned on, the PWM controller regulates duty cycles of pulse signals and outputs the regulated pulse signals to the first and second power circuits according to a first compensation signal, to provide a stable voltage to the CPU. When the computer is in a standby mode, the PWM controller regulates duty cycles of pulse signals and outputs the regulated pulse signals to the first power circuit according to a second compensation signal, to provide a stable voltage to the CPU.