Extraction of Magnetic Parameters for Elements of a Planar EMI Filter

The planar electromagnetic interference (EMI) filter has good application in aviation for its easy magnetic integration, small parasitic parameter, and strong anti-EMI ability. Based on the annular structure, the common-mode (CM) and difference-mode (DM) modules are proposed. The former is formed by “capacitance and inductance” (LC unit), while the latter is formed by an independent DM capacitor and the leakage inductance of the LC units. The extraction of the unit-length electromagnetic parameters is the foundation of analyzing the characteristics of the modules and further the whole EMI filter. The axisymmetric electrostatic field model of LC unit is established to extract the turn-to-turn partial capacitances. Considering the skin effect, proximity effect, and core loss, the model of harmonic magnetic field of LC unit is also established to extract the inductances and resistances. To obtain the leakage inductance that serves as the DM inductance, the model of harmonic magnetic field with a leakage layer between LC units is studied. The DM capacitance can be calculated directly using the approximate expression of a parallel plate. Finally, the impedances of the LC units can be drawn out with the obtained distributed parameters under the appreciate boundary conditions, and they are compared with the measured results. The results verify the accuracy of the extracted parameters and the validity of the proposed extraction method.