Reducing Common-Mode Noise in Two-Switch Forward Converter

This paper addresses the common-mode (CM) electromagnetic interference noise issues in the two-switch forward converter. The two-switch forward converter has low CM noise compared to other topologies because its symmetric primary-side circuit has two out-of-phase dv/dts that cancel each other. However, parasitic capacitances of the circuit significantly affect the symmetry and degrade the noise reduction. Moreover, the secondary-side circuit of the converter is not symmetric and still contributes to the CM noise. In this paper, the parasitic capacitances of the converter are first modeled. Different transformer structures and their parasitic capacitances are characterized. A general balance technique is introduced to further reduce the CM noise of the converter. For each transformer structure, balance can be achieved with proper connection of the windings terminals and control of the parasitic capacitances to minimize the CM noise of the converter. Experimental results validated the proposed techniques.