An Optimized Dual Duty-Cycle Modulation for Three-Phase LLC Converter With Improved Light-Load Performance

The three-phase LLC (3ph-LLC) converter is a promising choice for battery chargers facing the growing power demand. However, under light-load conditions, traditional pulsefrequency modulation (PFM) fails to regulate the output voltage properly, leading to high switching frequency and low efficiency. Though several new modulation strategies have been proposed to improve light-load efficiency, challenges such as narrow voltage regulation range and increased hardware complexity still exist. To overcome these issues, this article proposes a novel optimized dual duty-cycle modulation (DDCM) for the 3ph-LLC. By separately controlling the duty cycle of primary and secondary switches, the converter has two degrees of freedom. Time-domain analysis of DDCM proves that the 3ph-LLC can achieve a wide voltage gain range under light-load conditions due to the loadindependent characteristics. Furthermore, based on theoretical analysis, a closed-form optimization strategy is introduced to enhance light-load efficiency. The strategy maintains zero-voltage switching and minimizes the circulating current. Finally, a 4.5- kW experimental setup was built to verify the effectiveness of DDCM. The results show that DDCM improves efficiency over the entire load range compared with PFM. At 10% load, DDCM maintains high efficiency over a wide voltage range, reaching up to 93%. In addition, detailed loss analysis and performance comparison are conducted to verify the superiority of DDCM.