Radio frequency characteristics of high power common-mode chokes

Toroid EMC-ferrites are used to design chokes for low-pass filters. They provide high permeability up to a corner frequency in the range of kHz to MHz. At and above this corner frequency the damping effect is strongly supported by magnetic loss. Modelling this behaviour using complexe µ is a problem, because µ is often known only for one exemplar of the material and only up to the corner frequency. The filter designer also has to deal with huge tolerances of the materials of up to 25%. In this paper measurements of µ up to 100 MHz of MnZn-cores are presented and a modelling approach is shown to be accurate in the frequency domain. Besides parasitic capacitance depending mainly on wire distance to core, with EMC-ferrites magnetic loss is a predominant predicate at higher frequencies.