A Study on Stray Capacitance of Ferrite Common Mode Chokes for EMI Filters

The stray capacitance of CM chokes consists of the material capacitance and the winding parasitic capacitances. The material capacitance is determined by the intrinsic material properties of the ferrite core. The influence of ferrite core material on stray capacitance of CM chokes are researched. The complex magnetic permeability (CMP) of a ferrite core is measured and the CMP characteristic is analyzed. The behavior of the material capacitance is inversely proportional to the square of turn N. Winding parasitic capacitances include turn-to-turn capacitance and turn-to-core capacitance. An analytical method is adopted to calculate the winding parasitic capacitances taking into the core physical construction and material properties. Based on the capacitance network model, the stray capacitance can be derived. The impedance characteristic of a CM choke is tested using impedance analyzer to extract the stray capacitance, and the analytical result is consistent with the measured result.