Patent ID: 7755915
Filing Date: 2010-07-13
Classification: H02M

Abstract:
1. A power supply circuit, comprising: a first commutating and filter circuit; a transformer comprising a primary winding, a secondary winding, and at least one auxiliary winding, the at least one auxiliary winding comprising a first auxiliary winding comprising two taps; a second commutating and filter circuit; a transistor, a drain electrode of the transistor being connected to ground via a current limiting resistor; a pulse width modulation circuit comprising a first feedback port and a control port, the control port configured for outputting one of a high level signal and a low level signal to effect a selective one of turning on or turning off the transistor; and at least one feedback circuit comprising a first feedback circuit and a second feedback circuit; the first feedback circuit comprising a first resistor, a second resistor, and a diode, a cathode of the diode being connected to the first feedback port via the first resistor, an anode of the diode being connected to one of the two taps of the first auxiliary winding, the other tap of the first auxiliary winding being connected to ground, and the first feedback port being connected to ground via the second resistor; the second feedback circuit comprising a third resistor and a first capacitor, the third resistor being connected between the first feedback port and the drain electrode of the transistor, and the first capacitor being connected between the first feedback port and the drain electrode of the transistor; wherein an external alternating current voltage is converted into a first direct current voltage via the first commutating and filter circuit; when the transistor is turned on, electrical energy stored in the primary winding linearly increases until the electrical energy reaches a maximum value; when the transistor is turned off, the electrical energy stored in the primary winding is transmitted to the secondary winding, and is then converted into a second direct current voltage via the second commutating and filter circuit; electrical energy stored in the first auxiliary winding is fed back into the first feedback port of the pulse width modulation circuit; and the pulse width modulation circuit compares a value of a feedback signal of the first feedback port and a reference value stored in the pulse width modulation circuit, and when the value of the feedback signal is larger than the reference value, the control port of the pulse width modulation circuit outputs said one of the high level signal and the low level signal to effect turning off the transistor.