Chopper-type direct-current converter comprising a magnetic core (M), which comprises a first and a second side leg (MS1, MS2), the ends of which are connected to each other with end pieces, and a center leg provided with an air gap and connected to the end pieces between the first and the second side legs. According to the invention, the secondary side filter coil is wound around the center leg. The primary and secondary windings are wound around the side legs so that the magnetic flux produced by them flows in the same direction as the magnetic flux of the filter coil.