Flyback converter with a stepped air-gap transformer

In low power applications with isolation requirements (e.g. notebook adapters), the flyback converter is a commonly used converter topology. A key element of the flyback converter is its transformer. This paper introduces a flyback converter with a stepped air-gap transformer designed for a typical 65 W notebook adapter. Conventional flyback converters employ a transformer with a current independent (linear) magnetizing inductance. A stepped air-gap transformer, however, possesses a current dependent (nonlinear) magnetizing inductance. This current dependency can be influenced by the air-gap geometry. Thus, a stepped air-gap transformer offers the advantage of a greater design freedom. The use of a suitable designed stepped air-gap transformer enables an increase of the flyback converter efficiency. Measurements confirm a total loss reduction of up to 6.4%.