EMI reduction with near field coupling suppression techniques for planar transformers and CM chokes in switching-mode power converters

In switching-mode power converters, planar transformers are one of the most important near magnetic field sources and CM chokes is one of the most important victims. This paper first analyzes the near magnetic field distribution of planar transformers and the effects of near magnetic field couplings on CM chokes. Based on these analyses, three techniques to suppress near magnetic field coupling´s effects on EMI noise are proposed. They include the optimization of the position and orientation of planar transformers, the reduction of near magnetic field of planar transformers with magnetic shielding and a stacked CM choke structure with external EMI magnetic field immunity. Experimental results show these techniques can successfully suppress the EMI resulted from near magnetic field couplings.