Active-Clamp ZVZCS Resonant Forward DC Transformer (DCX) With Load-Adaptive ON-Time Control

Targeting at low-power high-density bus converter applications, a megahertz active-clamp resonant forward converter with whole-load-range zero-voltage switching and zero-current switching (ZVZCS) characteristics is proposed in this paper, which improves system efficiency performance. Compared with previous resonant forward converters, the proposed converter can effectively reduce the root-mean-square current and the conduction loss. To realize zero-voltage-switching turn on of switch over the entire input voltage range, a dead-time control is adopted to account for the nonlinear capacitors of switches. Besides, given fixed resonant parameters, the ZVZCS feature can be maintained at various combinations of operating frequency and duty cycle, which is a unique advantage over conventional resonant converters. Based on this concept, a load-adaptive on-time control that can significantly reduce core loss is proposed to improve the light-load efficiency. A 66-W 24-V-to-12-V prototype is built up to verify the effectiveness of the proposed converter, which achieves 96% full-load efficiency and 93.5% 10%-load efficiency.