Analytical Method for Extraction of Stray Capacitance in Single-Layer CM Chokes

Common mode (CM) chokes are widely used to reduce EMI noise of Switched-mode power supplies (SMPS) and meet with electromagnetic compatibility standards. However, the stray capacitance of CM chokes deteriorates its high frequency filtering performance. It can provide a guideline for EMI filters design to study the stray capacitance of CM chokes. In this paper, an analytical method suitable for accurate calculation of the stray capacitance for a single-layer CM choke is presented. The distributed stray capacitance network model is established and the model consists of turn-to-turn and turnto-core capacitances. The analytical equations of parasitic capacitance are derived taking into the CM choke physical construction and material properties. Based on two connection modes of CM choke's coils, CM and DM stray capacitances are calculated. What is more, parasitic capacitances are extracted using the 3D finite element method (FEM) to validate the analytical method. Finally, the presented analytical method is verified by the impedance measurement for a CM choke with nanocrystalline core. The experimental results show the calculated and measured values have a good agreement.