Analysis of Isolated Bi-Directional CLLC DC-DC Converters for Fast Electric-Vehicle (EV) Battery Charging

Bidirectional onboard chargers play a vital role in electric vehicles (EVs), contributing to smart grid applications by enabling functions such as peak load management and supporting grid stability through frequency and voltage regulation. However, traditional full-bridge dual-active bridge (FBDAB) resonant converters commonly used in these systems face considerable efficiency losses, particularly during reverse power flow and when operating away from their resonant frequency-a condition often caused by fluctuations in battery voltage. To overcome these challenges, this paper presents a bidirectional battery charger based on a CLLC resonant converter with efficient tracking mechanism. The converter's steady-state behavior is analyzed under both grid-to-vehicle (G2V) charging and vehicle-to-grid (V2G) discharging conditions. By maintaining the switching frequency close to the ideal resonant point, this design successfully lowers circulating currents in the resonant tank and transformer, decrease losses in MOSFETs, and limits the rate of current change in the diodes. Simulations comparing this approach to a FBDAB resonant converter with fixed DC demonstrate the enhanced efficiency and superior performance of the proposed system.