The Soft-Start Analysis of a Full-Bridge LLC Converter with Hybrid Control Strategy

The current spike in full-bridge LLC resonant converter during start-up may destroy the circuit. Usually, the frequency modulation is used to solve this problem. However the waveform at the start-up is still not good enough. In this paper, a hybrid modulation method of phase-shift and frequency control is employed for the start-up process of LLC converter. By using the phase-shift modulation, the start-up process can be optimized and there is no need to reduce switching frequency. Furthermore, all the MOSFETs can achieve zero-voltage switching (ZVS) easily. Finally, the feasibility of the control method is verified by simulation.