Smooth Structure Transfiguration Methodology for Half- and Full-Bridge LLC Resonant Converter via State Plane Approach

The structure transfiguration (STF) methodology of LLC resonant converter can switch the LLC converter’s structure between full bridge configuration and half bridge configuration, enabling a wider voltage gain range without additional circuits. However, significant transient processes occur during structure transfiguration due to voltage gain mismatch before and after the transfiguration, potentially causing system instability. To solve this issue, a smooth structure transfiguration feedforward control based on steady-state state plane analysis is proposed. In the proposed method, the ideal switching frequency of LLC resonant converter after structure transfiguration can be obtained via state plane analysis is then added to the linear controller. This ensures precise control to the output voltage during the STF. Importantly, the proposed strategy does not require a lookup table or control scheme switching. To verify the performance of the proposed STF control, a prototype is built, which demonstrates the practicality and effectiveness of the STF control.