Switching power supply circuit

A switching power supply circuit, comprising:

• a smoothing rectifier for receiving an AC power supply, producing and outputting rectified smoothed DC input voltage having a level equal to that of the AC power supply;
• an insulating converter transformer having a gapless core for obtaining a predetermined coupling efficiency and a primary winding and a secondary winding wound on said core with polarities such that an additive operation mode is provided;
• a switching circuit for switching the DC input voltage on and off and outputting the switched voltage to said primary winding of said insulating converter transformer, thereby creating an alternating voltage at said secondary winding of said insulating converter transformer;
• a primary side parallel resonance circuit formed from a leakage inductance component of said primary winding of said insulating converter transformer and a capacitance of a parallel resonance capacitor for operating said switching circuit in a voltage resonance type operation;
• a DC output voltage circuit for receiving the alternating voltage at said secondary winding of said insulating converter transformer and performing a half-wave rectification on the alternating voltage to produce a secondary side DC output voltage having a level substantially equal to that of the rectified smoothed DC input voltage;
• and
• a constant voltage control circuit for varying a switching frequency of said switching circuit in response to a level of the secondary side DC output voltage to control a resonance impedance of said primary side parallel resonance circuit and vary a continuity angle of said switching circuit so as to perform constant voltage control of the secondary side DC output voltage.