Switching power supply circuit

A switching power supply circuit comprising:

• a switching unit formed with switching devices for performing switching so as to interrupt a direct-current input voltage input to the switching unit;
• a driving unit for switching-driving said switching devices;
• an isolated converter transformer for transmitting a switching output of said switching unit from a primary side to a secondary side thereof, said isolated converter transformer having wound thereon at least a primary winding and a secondary winding;
• a primary side resonant capacitor connected to a predetermined part on the primary side so as to form a primary side resonant circuit for converting an operation of said switching unit into a resonance type operation with at least a leakage inductance component of the primary winding of said isolated converter transformer and a capacitance of the primary side resonant capacitor;
• a primary side partial voltage resonant circuit for performing a partial voltage resonant operation during a period that the switching devices forming said switching unit are turned off, said primary side partial voltage resonant circuit being formed by a capacitance of a partial resonant capacitor connected in parallel with at least one of the switching devices forming said switching unit and the leakage inductance component of the primary winding of said isolated converter transformer;
• and
• a synchronous rectifier circuit for providing a secondary side direct-current output voltage as a voltage across a secondary side smoothing capacitor by performing a rectifying operation on an alternating voltage induced in the secondary winding of said isolated converter transformer and charging said secondary side smoothing capacitor with a rectified current;
• wherein said synchronous rectifier circuit includes:
• a first field-effect transistor inserted between one end of said secondary winding and a negative electrode terminal of said secondary side smoothing capacitor;
• a second field-effect transistor inserted between another end of said secondary winding and the negative electrode terminal of said secondary side smoothing capacitor;
• a first driving circuit for detecting, by a resistive element, a secondary winding voltage corresponding to a half-wave period in which said first field-effect transistor makes a rectified current flow, and outputting a gate voltage for turning on said first field-effect transistor;
• and
• a second driving circuit for detecting, by a resistive element, a secondary winding voltage corresponding to a half-wave period in which said second field-effect transistor makes a rectified current flow, and outputting a gate voltage for turning on said second field-effect transistor, wherein
• a magnetic flux density of said isolated converter transformer is set to a predetermined value so that the secondary side rectified current flowing through the synchronous rectifier circuit as a result of said rectifying operation is in a continuous mode irrespective of a variation in a condition of a load connected to said secondary side direct-current voltage.