A DC converter has a transformer with loosely coupled primary and secondary windings, a main switch connected in series with the primary winding of the transformer, and a series circuit connected to ends of one of the primary winding and main switch. The series circuit includes a clamp capacitor and an auxiliary switch. The main and auxiliary switches are alternately turned on/off so that a voltage of the secondary winding of the transformer is synchronously rectified with synchronous rectifiers and is smoothed with smoothing elements, to provide a DC output. The DC converter also includes a tertiary winding tightly coupled with the primary winding of the transformer, a voltage source to supply a voltage lower than a voltage generated by the tertiary winding of the transformer, and clamp diodes to clamp the voltage generated by the tertiary winding with the use of the voltage source. The clamp diodes provide voltage-clamped signals to drive the synchronous rectifiers.