Common-Mode Noise Reduction in Full-Bridge LLC Resonant Converter with Split Primary Winding Transformer

This paper addresses the issue of common-mode (CM) conducted electromagnetic interference (EMI) noise in full-bridge (FB) LLC resonant converters. In conventional FB LLC resonant converters, the position of the resonant tank can affect the symmetry of the entire circuit, leading to different dv/dt values at the two winding terminals of the transformer. This results in non-cancelable displacement currents generated in the parasitic interwinding capacitance of the transformer, leading to severe CM noise issues. In this paper, a low CM noise FB LLC resonant converter with the Split Primary Winding Transformer (SPWT) configuration is proposed. The transformer primary winding is split into two windings and the resonant tank is connected between these two windings. With a symmetrical winding structure, the CM noise current generated in the transformer can be canceled completely. The concept of complementary couple-turns is proposed to ensure a symmetrical winding arrangement for the planar transformer in the printed circuit board (PCB) layout stage before it is fabricated physically. A 360 W FB LLC prototype with a planar transformer is built to verify the feasibility and validity of the proposed methods. The CM noise is reduced by around 11 dBμV (a reduction of around 4 times) below 5 MHz.