Study and Implementation on Start-Up Control of Full-Bridge LLC Resonant Converter

This paper aims to study and develop a start-up surge-current-restraining strategy for a full-bridge LLC resonant converter. The idea of this thesis is to first operate the full-bridge LLC converter at half-bridge mode, therefore halve the resonant tank voltage into half of input voltage. Afterwards, frequency modulation control and phase-shift modulation control are also applied to suppress resonant tank energy. In addition, the relationship between resonant current, output voltage, and switching frequency based on state plane analysis is investigate to obtain the frequency adjusting curve under the consideration of resonant current limitation. The operating frequency consequently is modulated according to the curve to limit resonant current. At the end of this thesis, a full bridge series resonant converter with 480W, input voltage 400 V, the output voltage 48 V, and the resonant frequency 100 kHz is used as the test platform to compare the method with general method. The experimental results show that this method is effective to suppress the surge current during start-up moment, and lower initial frequency can be used.