Optimal Trajectory Control of LLC Resonant Converters for Soft Start-Up

In this paper, the soft start-up process for the LLC resonant converter is investigated and optimized based on a graphical state-trajectory analysis. By setting a current limitation band, several optimal switching patterns are proposed to settle the initial condition. After that, by sensing the output voltage, the optimal switching frequency is determined within the current limiting band. This virtually guarantees that there will be no current and voltage stress in the resonant tank during the soft start-up process. Meanwhile, the output voltage is built up quickly and smoothly.