Patent ID: 8213189
Filing Date: 2012-07-03
Classification: H02M,Y02B

Abstract:
1. A switching power supply comprising: a DC power supply comprising a positive terminal and a negative terminal; a first series circuit comprising a first switching device and a second switching device connected in series to each other, the first series circuit being connected between the positive terminal and the negative terminal of the DC power supply; a second series circuit comprising a primary winding of an insulation transformer, an inductor, and a capacitor, the second series circuit being connected in parallel to the first switching device or the second switching device, wherein the switching power supply turns on and off the first switching device and the second switching device alternately for generating a voltage across a secondary winding of the insulation transformer, the switching power supply rectifying and smoothing the voltage generated across the secondary winding of the insulation transformer for obtaining a DC output; and a control circuit controlling ON and OFF of the first switching device and the second switching device, the control circuit comprising an error amplifying circuit, an oscillator circuit, a frequency fixing circuit, and a pulse width control circuit, wherein the error amplifying circuit controls a DC output voltage of the DC output at a constant preset value; the oscillator circuit controls a switching frequency corresponding to an output signal level of the error amplifying circuit; the frequency fixing circuit fixes the switching frequency to a predetermined frequency; and the pulse width control circuit conducts pulse width control for adjusting pulse widths of an ON-period of the first switching device and an ON-period of the second switching device; wherein the control circuit controls the ON and OFF of the first switching device and the second switching device based on an output from the oscillator circuit when the output signal level of the error amplifying circuit is higher than a threshold level; and the control circuit makes the frequency fixing circuit fix the switching frequency when the output signal level of the error amplifying circuit is lower than the threshold level, the control circuit controlling the ON and OFF of the first switching device and the second switching device based on an output from the pulse width control circuit, and wherein a turn ratio of the primary winding and the secondary winding in the insulation transformer is set at a value that makes the ON-period of the first switching device and the second switching device shorter than a half wave period of a series resonance current that flows through a series resonance circuit comprising the inductor, leakage inductance of the insulation transformer, and the capacitor.