Comprehensive Conducted Emission Analysis of the Three-Phase and Single-Phase LLC Resonant Converters for EV Application

Resonant
LLC
converter is a preferred candidate for emerging electric vehicle (EV) applications, thanks to its high efficiency and high power density characteristics, which are achieved due to high switching frequency and soft-switching operation. On the other hand, planar transformers (PTs) for EVs offer power density in a small package and withstand vibrations. However, large parasitic capacitance is a characteristic of PT, which along with the large
dv/dt
of the converter leads to severe conducted emission problems. Therefore, multifold complicated and bulky electromagnetic interference (EMI) filters are required to suppress conducted noise and meet the requirements of the relevant standards. In this article, a comprehensive analysis has been conducted to model and study both single-phase and three-phase
LLC
resonant converters from a conducted noise perspective. With this study, a better understanding of the EMI behavior of these converters can be obtained. Owing to the symmetrical design of the three-phase converter, several harmonics cancel out each other, resulting in lower common-mode (CM) emissions. It is also shown that differential-mode (DM) current generated from the three-phase converter contains lower peak amplitude at high frequency. For comparison purposes, two prototype converters based on the conventional single-phase and the three-phase configuration are fabricated and their conducted emissions are measured and compared experimentally. Simulation and experimental results verify that the three-phase converter effectively suppresses CM and DM noise by a considerable amount.