Calculation of effective impedance of common-mode choke made of Mn-Zn ferrite

Abstract

Mn-Zn ferrite is used widely in the design of common-mode chokes. Their effective impedance is the main design concern. The effective impedance is determined by the material intrinsic properties, including complex permeabilities, complex permittivity and conductivity. The complex permeabilities found in manufacturers' data sheet are measured values. These values may deviate severely from their intrinsic values, due to the fact that Mn-Zn ferrites have high permeabilities as well as high permittivities within kHz-to-MHz frequency range. High permittivities result in the deviations of measured permeabilities from their intrinsic values. Ferrite cores with different dimensions have the same intrinsic permeability, but the measured permeabilities vary when cores with different dimensions are used. When the intrinsic complex permeability of Mn-Zn ferrites is known, the effective impedance of common-mode chokes can be calculated, irrespective of the dimensions of the ferrite cores used to construct the common-mode chokes. This paper presents a general method to calculate the effective impedance of the common-mode chokes.