Current ripple reduction in 4kW LLC resonant converter based battery charger for electric vehicles

Electric vehicles rely on efficient charging techniques to maximize battery performance, efficiency and lifetime. Most EV on-board chargers are supplied from single phase AC mains, and contain 2^nd order mains frequency harmonic in the battery current. This harmonic current incurs extra loss in the battery, increases battery temperature, and hence reduces charging efficiency and battery lifetime. Conventional battery charger controllers are unable to reject this harmonic current completely. This paper describes a resonant controller employed to suppress the low-frequency current ripple in an LLC resonant converter for EV battery chargers. A small-signal model of the LLC resonant converter based battery charging system has been established from time-domain simulations by injecting a small perturbation signal into the converter switching frequency. A resonant controller is subsequently designed and the closed-loop charger system performance is analyzed. The effectiveness of the proposed resonant controller on suppressing the harmonic current is assessed by extensive simulations and experimental tests.