Reducing common mode EMI noise in two-switch forward converter

This paper addresses the common mode (CM) electromagnetic interference (EMI) noise issues in the two-switch forward converter. Two-switch forward converter has low CM noise with its symmetric primary side structure. However, the asymmetric parasitic capacitances and secondary side noise source have adverse effects on the symmetry and degrade the CM noise reduction. In order to solve the issues, CM noise model of the converter and transformer is first derived. Symmetry conditions are then summarized as the guideline for achieving CM noise reduction. Methods are proposed to minimize the CM noise by controlling the transformer termination, parasitic capacitances and structure. Experimental results validated the proposed techniques.