Abstract: A switching mode power transformer for operation as feedback converter has a core which is made in only one piece and which does not require any particular finishing operations. The core has a center core part with windings thereon and outer core parts which are chamfered on their mutually remote sides to form outer surfaces which converge towards each other.

Abstract: A coil arrangement comprising two coils on a single coil former, termed a twin coil, is characterized in that the two coils (W1, W2 and, respectively, W3A), with any respective individual magnetic core (10 or, respectively, 9) required, are mounted with a substantial distance from each other on the coil former (1) and in that the respective residual coupling of the coils (W1, W2 and W3A) is eliminated by an oppositely wound or connected compensation winding (W3B). The compensating winding (8) is located in the one winding compartment (3) and has current from the coil in the other winding chamber (4) flowing through it. As a result the twin coil in accordance with the invention leads to some advantages over known twin coils in which the coupling between the two coils is nearly 100%.