Optimum EMC design of flyback and load resonant converters using toroids with air gaps

In this paper the external magnetic stray field is investigated in transformers realized on gapped toroids with a rectangular cross section. An analytical method is used in which the field distribution is calculated by means of orthogonal expansion. As a conclusion it is shown that optimum EMC design for the case of a flyback converter is given by the results for a simple inductor. In case of a load resonant converter minimum parasitic induction is obtained, if the winding conducting maximum current is located exactly above the air gap. The second winding has to be located as close as possible to the first one with respect to the desired value of the total leakage inductance.