PURPOSE: To provide a DC-DC converter which has high efficiency, a small size, and a simple circuit configuration and can supply a stabilized DC voltage to industrial equipment for information, communication, OA, etc., and electronic equipment of consumer appliances, such as VTR, TV, etc., by using a single-winding transformer which is well coupled as compared with insulated transformers and requires a less amount of winding.

CONSTITUTION: This DC-DC converter is provided with a switching means 3, secondary winding 4b of a transformer 4 which is connected to the primary winding 4a of a transformer 4 and added to the winding 4a, and a rectifying and smoothing circuit which is composed of diodes 6 and 7, a choke transformer 8, and a capacitor 9 and transmits the output of the secondary winding 4b to a load and the switching means 1, winding 4b, and rectifying an smoothing circuit are connected in series with a DC power source 1. The converter is also provided with a reset circuit which resets the transformer 4 when a switching means 3 composed of resetting winding 4c and diode 5 is turned off and a control means 11 which controls the on/off ratio of the means 3 so that the voltage across both ends of the capacitor 9 can become constant. In addition, the converter is constituted so that an arbitrary output voltage can be outputted from the capacitor 9 by generating a high voltage by superimposing a voltage induced across the secondary winding 4b upon an input voltage applied across the primary winding 4a.