Common-Mode EMI Cancellation in Full-Bridge Based Isolated DC-DC Converters

This paper introduces an approach to substantially reduce common-mode (CM) electromagnetic interference (EMI) generated by isolated DC-DC converters using full-bridge inverters and/or rectifiers. By minimizing CM noise, CM chokes can be miniaturized and converters can achieve higher power density. The proposed technique is applicable to all isolated converter topologies having at least one full-bridge that is operated with 180⁰ phase-shift between its two legs. In this paper, a capacitor-inductor-inductor-inductor-capacitor (CLLLC) converter is used to demonstrate the approach. First, the CM noise model of the CLLLC converter is developed. Then, capacitors with values derived from closed-form expressions are added at appropriate locations to make the CM noise current generated by one leg of the full bridge inverter and rectifier cancel with that generated by the other leg. Finally, the proposed cancellation scheme is validated on the Texas Instruments reference design TIDM-02002. Experimental results show that the proposed approach can achieve about 17dB noise reduction at fundamental switching frequency and as much as 30dB noise reduction at higher frequencies.