Seamless Control of Full-Bridge and Half-Bridge Topology Morphing LLC Converter Based on State Plane Analysis

The topology morphing (TPM) strategy, which involves switching between full-bridge and half-bridge modes, offers a promising solution to broaden the voltage gain range of
LLC
converters without adding an extra device. However, due to the distinct voltage gain curves of these two modes, the existence of a stable operating point after TPM is not promised, resulting in system instability. To address this issue, this article proposes a design procedure based on peak gain trajectory, incorporating geometrically simplified steady-state state plane analysis. In this way, the predicted result shows a great agreement with the exact peak gain trajectory. In addition, a state plane–based topology morphing control (SPTMC) is introduced to mitigate significant output voltage transients caused by the abrupt TPM. With such a strategy, the desired switching frequency after TPM is forecasted and feedforward to the conventional linear control to ensure tight regulation of the output voltage during TPM. It is noted that neither a lookup table nor control scheme switching is required for the proposed control strategy. A prototype has been constructed to validate the feasibility of the proposed design procedure and the proposed SPTMC.