Analysis and Optimized Design of Conducted Common-Mode EMI Characteristics of CLLC Circuits

The CLLC converter offers advantages such as high efficiency, high power density, bidirectional Buck-Boost capability, and soft-switching operation, and has been widely applied in automotive onboard chargers and renewable energy systems.Electromagnetic compatibility (EMC) is one of the critical performance indicators for CLLC circuits.To investigate the conducted common-mode (CM) EMI characteristics of the CLLC converter, a CM EMI equivalent circuit model was established based on the noise transmission mechanism, and an equivalent model of the CLLC converter was derived, clarifying the noise source characteristics and the key CM noise transmission path impedances.The noise source characteristics under different operating conditions were analyzed to reveal the CM noise behavior in each condition.Based on this analysis, a symmetry-based design method for optimizing the CM EMI performance of the CLLC converter was proposed, and the impacts of resonant inductor and capacitor symmetry were examined.The correctness of the CM characteristic analysis for the CLLC converter is verified through simulations and experimental tests.By constructing a symmetric resonant cavity structure, the CM noise is attenuated by 20-30 dB, which further validates the feasibility of the symmetric structure-based optimization design method.