An exemplary power supply circuit includes a transformer (21) having a primary coil (211) and a secondary coil (212); a rectification circuit and a transistor (26) respectively coupled to two terminals of the primary coil; a communicating and filter circuit (22) coupled to the secondary coil; a sampling circuit (13) having a first resistor (231) and a first capacitor (232) connected in series; and a pulse width modulation circuit (25) coupled between the transistor and the sampling circuit. Direct current (AC) voltage is applied to the rectification circuit and is converted into DC voltage via the transformer and the communicating and filter circuit. The DC voltage is fed back to the pulse width modulation circuit via a voltage applied to the first capacitor. The pulse width modulation circuit adjusts a gating time of the transistor so as to adjust the output DC voltage output by the power supply circuit.