Light-Load Efficiency Improvement of MHz RDCX Based on Output Voltage Ripple and Resonant Pulse Number Control

Regulated DC transformer (RDCX) achieves high-efficiency, high-power-density power conversion due to fully soft switching, transformer optimization, and partial-power regulation. However, the load-independent losses, especially the core loss at high switching frequency, make it difficult for RDCX to meet the increasing light-load efficiency requirements. Burst mode operation is an effective way to reduce the load-independent losses of the resonant DC transformer (DCX) in RDCX. However, when applying the conventional burst mode strategies in DCX, soft switching will be lost during the burst-on time, or the implementation have high requirements on the hardware or software. In this paper, a simple burst-mode strategy is proposed. Pulse sequence with fixed pulse widths in DCX maintains soft switching and simplifies the DSP calculation. To improve the efficiency in burst-mode operation, average output current during the burst-on time is controlled, and extra losses in the burst mode operation is minimized. Cooperating with the auxiliary DC-DC, automatic adjustment of average output current during the burst-on time is realized. Finally, a 1-MHz 66V/48V 500W prototype based on silicon devices is established for verification. The efficiency at 10% load is improved from 92.6% to 95.3%, which means a 37% loss reduction.