Modulation Transition Methods Based on Trajectory Control for $LLC$ Resonant Converters Operating in Wide Input- and/or Output-Voltage Range

The serial-half-bridge (SHB) LLC resonant converter can block high input voltage, realize soft switching, and achieve a wide gain range. This article presents a control method for fast modulation transition of the SHB LLC resonant converter operating with a wide input-voltage and/or output-voltage range based on the trajectory analysis on the state plane. The proposed transition methods enable on-the-fly modulation transition at the optimal conditions so that the transitions are made in only one switching cycle without noticeable transients on the resonant components as well as output voltage. The performance is analyzed in detail and the generalization of the proposed methods for other resonant converters is also discussed. The performance of the proposed modulation transition method is verified experimentally on a 15-kW LLC dc/dc converter prototype designed for a 200–1000 V output-voltage range.