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 pulse-frequency 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 load-independent 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.