Patent ID: 7697307

Claim:
A power supply circuit, comprising: a transformer having a primary coil and a secondary coil; a rectification circuit coupled to one terminal of the primary coil; a transistor coupled to the other terminal of the primary coil; a communicating and filter circuit coupled to the secondary coil; a sampling circuit having a first resistor and a first capacitor connected in series; a pulse width modulation circuit coupled between the transistor and the sampling circuit; and a diode and a second capacitor, wherein an anode of the diode is coupled to the first capacitor, and a cathode of the diode is grounded via the second capacitor; wherein an alternating current (AC) voltage is applied to the rectification circuit and is converted into a direct current (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, and the pulse width modulation circuit adjusts a gating time of the transistor so as to adjust the DC voltage output by the power supply circuit.