An Optimal Frequency-Modulated Control for Bidirectional CLLC Resonant Converters

This paper proposes a bidirectional CLLC resonant converter for battery charging applications. In order to enhance the charging performance as well as to extend the battery life, five charging methods are adopted, which are (1) the constant-current (CC) charging, (2) the pulse-ripple-current (PRC) charging, (3) the sinusoidal-ripple-current (SRC) charging, (4) the bidirectional pulse-ripple-current (BPRC) charging and (5) the bidirectional sinusoidal-ripple-current (BSRC) charging. In addition, an optimal frequency-modulated control (OFMC) strategy will be developed. With the OFMC, the switching frequency will dynamically be regulated to achieve fast system response. Under the BPRC and BSRC charging scenarios, the charging and discharging current magnitude might be different. Therefore, the resonant tank gain of the dc-link side and the battery side will be designed independently. As a result, the charging/discharging performance could be increased. In this paper, detailed operational principles and mathematical derivations will also be revealed. The proposed multi charging methods and the OFMC could be realized by the digital signal processor (DSP) without adding extra circuit and components. Finally, simulation results obtained from a 720W CLLC prototype circuit verify the performance and feasibility of the proposed circuit and control strategies.