Patent ID: 7656686
Filing Date: 2010-02-02
Classification: H02M,Y02B,Y02P

Abstract:
1. A switching power supply circuit comprising: a rectifying and smoothing section for converting an alternating input voltage from an alternating-current power supply into a primary side direct-current voltage; a converter section for converting the primary side direct-current voltage from said rectifying and smoothing section into an alternating voltage and further converting the alternating voltage into a secondary side direct-current voltage; and a power factor improving section for improving a power factor; wherein said rectifying and smoothing section includes a primary side rectifying element for rectifying said alternating input voltage and a smoothing capacitor for smoothing rectified output from said primary side rectifying element, and supplies said primary side direct-current voltage, said converter section includes: a choke coil having one terminal connected to said smoothing capacitor, a converter transformer having a primary winding on a primary side, the primary winding being connected to another terminal of said choke coil, and a secondary winding on a secondary side, the secondary winding being magnetically loosely coupled with said primary winding, a switching element for switching said primary side direct-current voltage supplied via the primary winding of said converter transformer, an oscillating and driving circuit for on-off driving of said switching element, a control circuit for supplying said oscillating and driving circuit with a control signal to make a value of said secondary side direct-current voltage output by a secondary side rectifier circuit connected to the secondary winding of said converter transformer a predetermined value, a primary side series resonant capacitor having a predetermined capacitance, the primary side series resonant capacitor having one terminal connected to a point of connection between said choke coil and said primary winding, whereby the primary side series resonant capacitor is connected in series with one of said choke coil and said primary winding, said predetermined capacitance being set such that a primary side first series resonance frequency of a primary side first series resonant circuit, the primary side first series resonance frequency being governed by an inductance of said choke coil and said predetermined capacitance, is substantially twice a primary side second series resonance frequency of a primary side second series resonant circuit, the primary side second series resonance frequency being governed by a leakage inductance occurring at said primary winding and said predetermined capacitance, and a primary side parallel resonant capacitor having a predetermined capacitance, the primary side parallel resonant capacitor being connected in parallel with said switching element, said predetermined capacitance being set such that a primary side parallel resonance frequency governed by the inductance of said choke coil, the leakage inductance occurring at said primary winding, and said predetermined capacitance is substantially twice the primary side first series resonance frequency, and said power factor improving section adds and passes a current corresponding to a voltage generated in said primary side series resonant capacitor to said smoothing capacitor via said primary side rectifying element.