Abstract: Disclosed is a synchronous rectifier type DC-to-DC converter capable of preventing an increase in losses occurring when a rectifier synchronous rectification switch and a flywheel synchronous rectification switch, which are responsible for rectification on the side of a secondary winding of a transformer, are turned on simultaneously, and the destruction of FETs or windings due to large currents. The DC-to-DC converter comprises a transformer, a primary switching device connected in series with a primary winding of the transformer, a control circuit for controlling turning on or off of the primary switching device, a rectifier synchronous rectification switching device, and a flywheel synchronous rectification switching device, and converts a voltage of a DC power source into another voltage.

Abstract: The present invention relates to a DC-DC converter, and aims at preventing a flywheel switch from being destroyed. A PWM control circuit outputs an oscillation signal in order to alternately switch between a main switch and a rectification switch, and a flywheel switch. A state detecting unit generates a control instruction signal for a controlling unit, if the DC-DC converter is powered down or if it is overloaded. The controlling unit turns off the flywheel switch by controlling a driving circuit which controls the on/off status of the flywheel switch. An electric current I.sub.Q2 flows via a parasitic diode D.sub.Q2, when the flywheel switch is turned off. With the above described configuration, the flywheel switch can be prevented from being destroyed, when the DC-DC converter is overloaded, or when an overcurrent occurs due to power-down.