A power supply circuit includes a pulse width modulation (PWM) signal generator, a current adjustment unit, a drive unit, and a feedback unit. The PWM signal generator outputs a PWM voltage signal to the drive unit. The drive unit outputs a first control signal to control the current adjustment unit to work, and outputs a first direct current signal and a direct voltage signal. The current adjustment unit outputs a second direct current signal. A working current of the electrical device comes from the first and second direct current signals. The feedback unit generates a feedback voltage signal according to the direct voltage signal, and transmits the feedback voltage signal to the PWM signal generator to adjust the PWM voltage signal.