Start up Process for LLC Resonant Converter

There is a current spike in LLC resonant converter during start-up and frequency modulation method is adopted to limit the resonant current usually, which controls switching frequency decreasing from specific high frequency to steady state frequency in exponential pattern with a time constant tau. However, it’s hard to determine the crucial parameter of time constant of the exponential switching frequency sweep curve, due to the complexity in analyzing dynamic process of the converter.

In this paper, the operation modes and characteristics under continued conductive mode and discontinued conductive mode are analyzed based on state-plane analysis and the boundary of two conductive modes is figured out. Then the iteration calculation model of LLC converter is proposed to predict the resonant current during start-up progress, and time constant of the existing soft start method is optimized using iteration algorithm. Using the proposed iterative design method, the current stress of LLC resonant converter is limitted, as well as guaranteeing the response speed of output voltage.