Shielding-Cancelation Technique for Suppressing Common-Mode EMI in Isolated Power Converters

Abstract

In an isolated power converter, the interwinding capacitance of the transformer is the critical coupling path for the generating of common-mode (CM) noise. The shielding technique, for the purpose of blocking the electric coupling between the adjacent windings, has been widely used to deal with the CM noise. For the transformer with a single shielding layer, the displacement current that flows between the shielding layer and its adjacent windings still causes CM noise. In this paper, the shielding-cancelation (SC) technique is proposed by combining the shielding technique and the winding cancelation method together: The shielding layer is adjusted to have the same turns number and winding direction with its adjacent windings. In this way, the $dv/dt$ on the interwinding capacitance is always zero; thus, the CM noise can be effectively suppressed. A flyback converter prototype is built in the laboratory, and the performance of the SC technique on suppressing the CM noise is verified by experiments.