Ripple Behavior of LLC EV Charger in Boost Mode With Two-Capacitor Series Filter

This article aims to explore the ripple behavior of the recommended connection method between the converter output and the filter, especially the connection to a two-capacitor series filter (TCSF) used to boost LLC output voltage as an electric vehicle (EV) charger. The study confirmed that the form of ripple current in the capacitor significantly determines the integral ripple voltage and the equivalent series resistance (ESR) ripple voltage. The most effective way to reduce ripple current is to phase shift the converter output prior to connecting to the filter. This will effectively achieve a smaller ripple current to improve and maintain the reliability and quality of the capacitor. In this study, the ripple behavior of the three connection modes between two interleaved LLC converters (ILLC) and TCSF is studied, analyzed, and compared, including general connection, in-phase parallel (IPP) connection, and out-of-phase cross (OPC) connection. To facilitate the analysis, the first-harmonic approximation (FHA) method and graphical description are used to model the LLC output as a quasi-sinusoidal current source and analyze the ripple behavior. In addition, we attempt to connect the third capacitor in parallel between TCSF and the load. Experiments proved that the load energy is adjustable, the ripple current in TCSF is halved, the reliability and quality of the capacitor are further augmented and maintained, and electromagnetic interference (EMI) immunity is improved. Finally, a 4-kW/400-V EV charger composed of two ILLCs connected to TCSF was implemented to verify the estimated ripple performance in various modes. Compared with the experimental results, the recommended OPC-connection mode 3 has the most significant ability to achieve the lowest ripple current and ripple voltage.