A power supply circuit for providing power and detecting a plurality of loads' input voltages on a motherboard includes a pulse width modulation (PWM) controller, a voltage output circuit and a voltage feedback circuit electrically connected to the PWM controller and the plurality of loads. The PWM controller outputs PWM control signals. The voltage output circuit receives the PWM control signals and outputs working voltage to the plurality of loads according to the received PWM control signals. The voltage feedback circuit detects the plurality of loads' input voltages and outputs feedback signals to the PWM controller according to the detected input voltages. The PWM controller adjusts its PWM control signal outputs, according to the received feedback signals, and adjusting working voltages to the plurality of loads.