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 the graphical state-plane analysis. By setting a current limiting band, several optimal switch patterns are proposed to settle the initial condition. Then, by sensing the output voltage, the optimal frequency is determined within the current limiting band. Therefore, no current and voltage stress in the resonant tank is guaranteed during the soft start-up process. Meanwhile, the output voltage is built up quickly and smoothly.