Mixed Resonant Frequency Control Improving Light-Load Efficiency of Full-Bridge LLC Converter

This letter introduces a mixed resonant frequency control improving the light-load efficiency of a full-bridge LLC converter. The proposed control significantly decreases the switching frequency under light-load condition, by inserting the sub-resonant freewheeling period caused by the magnetizing inductor of the transformer. By doing so, the operation of LLC converter becomes like a continuously operating burst mode, increasing the effective loading current. The advantages of the proposed control distinguished from the previous approaches are simple implementation, less switching losses, and zero impact on the dynamics. The effectiveness of the proposed control is verified through a prototype converter with a 400–300 V input and a 50 V/10 A output.