An Optimal VF-TPS Modulation Scheme for CLLC Converter With Minimized RMS Current

The CLLC converters are gradually becoming the most popular topology of isolated bidirectional dc-dc converters due to their high efficiency, high power density, and low electromagnetic interference. However, the commonly utilized pulse frequency modulation (PFM) faces efficiency degradation and unregulated output voltage at light loads. Phase-shift modulations such as extended phase shift and triple phase shift are often adopted to improve light-load efficiency, but their performance is not satisfactory when the voltage gain deviates from 1. To address this issue, a time-domain analysis model-based optimal variable frequency-TPS (VF-TPS) modulation scheme for the CLLC converter is proposed in this article. An offline particle swarm optimization algorithm is introduced to obtain the optimized combination of all four modulation variables for different operating conditions to ensure soft-switching and minimum RMS current. Finally, a 1 kW experimental platform was built, and comparative experiments were conducted. The results show that the proposed VF-TPS modulation scheme can improve efficiency across the entire power range, compared with PFM, the half-load and full-load efficiencies have been improved by 0.78% and 0.63% to 97.85% and 97.27%, respectively, at the minimum voltage gain.