Analysis of Grounding Impedance on CM EMI Filter Performance in Onboard Chargers and Optimized Grounding Design

The high-frequency attenuation performance of Electromagnetic Interference (EMI) filters is significantly affected by their parasitic parameters. However, current filter designs fail to adequately account for the parasitic parameters in the grounding path, resulting in substantial discrepancies between actual and expected high-frequency performance. This paper first proposes a novel method for extracting grounding impedance and analyzes the magnitude of parasitic impedance under various grounding configurations. Next, the study simplifies the model and evaluates the critical impact of different grounding point configurations on the common-mode (CM) performance of filters. An optimized grounding scheme is then proposed, in which, for onboard chargers (OBCs), the common-mode capacitors located on the LISN side are directly connected to the chassis surface. Finally, the CM noise current and CM insertion loss of several commercial OBCs are measured. The results indicate that the proposed optimized grounding method is easy to implement and, in the tested commercial OBCs, can effectively reduce the CM noise spectrum in the megahertz range compared with non-optimized grounding layouts, without adding any additional cost or volume.