Study on Improving Line Frequency Ripple of Bidirectional DC-DC CLLC Resonant Converter

The output of the traditional battery charging system contains high double line frequency ripple content, which causes thermal cycling and short life cycle of the battery. In this paper, a digital controlled isolated bidirectional resonant converter for battery charging system with low double line frequency ripple in charging stage is implemented. The full-bridge CLLC converter with silicon carbide power devices can achieve soft switching characteristic and higher efficiency. In addition, an adaptive frequency modulation method with the input voltage ripple feed-forward (RFF) control is proposed to reduce the double line frequency ripple. The operating principles of bidirectional resonant converter are analyzed, the steady-state equivalent models and the voltage gain curves are derived, then appropriate frequency variation for output voltage regulation is designed. Also, the resonant tank of CLLC is carefully designed for achieving symmetrical operations as CLLLC in charging and discharging stages. A digital signal processor (DSP) is used to implement laboratory prototype with rated power 1.5 kW, DC bus voltage 390 V-DC, and battery voltage 130-170 V-DC to verify the feasibility of the proposed method. The maximum conversion efficiency of CLLC converter in the charging stage and discharging stage are 96.7% and 95.6%, respectively. Moreover, the double line frequency ripple voltage can be reduced by 70% with the proposed algorithm.