Patent ID: 7054167

Claim:
A switching power supply circuit, comprising: a rectification smoothing section for receiving an ac voltage as an input to produce a rectified smoothed voltage; a switching section including a switching element for receiving the rectified smoothed voltage as a dc input voltage to perform a switching operation thereon; a switching drive section for driving said switching element at a predetermined switching frequency to perform the switching operation; an insulating converter transformer including a primary winding wound thereon for receiving a switching output obtained by the switching operation of said switching section and a secondary winding wound thereon for exciting an alternating voltage using a switching output obtained from said primary winding; a primary side series resonance circuit formed from a leakage inductance component of said primary winding of said insulating converter transformer and a capacitance of a primary side series resonance capacitor connected in series to said primary winding for causing said switching section to perform an operation of the current resonance type; a secondary side smoothing capacitor; a dc output voltage generation section for rectifying the alternating voltage obtained by said secondary winding of said insulating converter transformer and charging said secondary side smoothing capacitor with resulting rectification current to obtain a secondary side dc output voltage as a voltage across said secondary side smoothing capacitor; a constant voltage control section for variably controlling said switching frequency of said switching drive section in response to the level of the secondary side dc output voltage to perform constant voltage control of the secondary side dc output voltage; and a power factor improving circuit for feeding back the alternating voltage based on the switching output of said switching section to a rectification current path formed in said rectification smoothing section and utilizing the alternating voltage based on the switching output to selectively interrupt the rectified current component by a rectifying element provided in said rectification smoothing section thereby improving a power factor; said insulating converter transformer being formed so as to have a magnetic flux density set at a predetermined level with which secondary side rectification current flowing in said dc output voltage generation section has a continuous mode irrespective of a load condition connected to the secondary side dc output voltage and a variation of said ac voltage.