Shielding Technique of Planar Transformers to Suppress Common-Mode EMI Noise for LLC Converter with Full Bridge Rectifier

The planar transformer shows excellent benefits when applied in a high efficiency and high power density LLC converter with increased common-mode (CM) noise caused by the large interwinding capacitances. For the shielding design of a half-bridge (HB) LLC converter with a full-bridge rectifier(FBR), it is essential to find the static-electric-potential (SEP) point in the physical primary or secondary windings. With the proper design of the ground connection of shielding winding, the voltage potential difference between shielding winding and primary/secondary windings is minimized, and the net displacement CM current diminishes. In this paper, the analysis of the SEP point of different transformers for the HBLLC converter with a FBR was discussed, and the net CM current under different ground connection strategies of shielding winding was compared. The CM noises of different strategies are measured based on a 1.5kW HBLLC converter with FBR hardware platform. The EMI measurement results show that it can attenuate the CM noise by 20 dB or more with proper shielding design.