A CLLC resonant converter based bidirectional EV charger with maximum efficiency tracking

On-board bi-directional chargers for electric vehicles (EV) are important elements of smart grid infrastructure enabling peak load shaving and grid frequency/voltage support. Conventional LLC resonant converter based on-board EV charger incurs large losses in reverse mode of operation and when operating frequency deviates from the resonant frequency which is often the case as battery voltage changes. To over this limitation, this paper presents a bi-directional CLLC resonant converter based battery charger with maximum efficiency tracking. Steady state performance of the converter is first evaluated using fundamental harmonic approximation (FHA) method, in both forward and reverse power flow modes. In order to maintain the operating frequency close to optimum series resonant frequency corresponding to the maximum efficiency point as the battery voltage varies, the DC link voltage is adjusted in the same ratio. Thus the resonant converter is always operating with less circulating current in the resonant tank and transformer, less turn off current in the primary side MOSFETs and less di/dt in the secondary side diodes. Simulation and comparison with the conventional LLC resonant converter with fixed DC link voltage are presented to verify the effectiveness of the proposed converter.