Design And Implementation Of LLC Resonant Converter Efficiency Optimization Control Strategy

LLC resonant converter has been widly concerned because the advantages of simple structure、the ability to achieve zero voltage switching of the primary side switches and zero current turn-off of the secondary side rectifiers that makes excellent performance in high efficiency and high power density.However,under low load output power and near full load output,the efficiency of LLC resonant converter still has a certain improvement interval.Improving the efficiency of LLC in the full load range by optimized control strategy has great significance for reducing the size of the converter and reducing the heat dissipation cost of the converter.The efficiency improvement of the LLC resonant converter in the full load range is mainly focused.Firstly,it briefly analyzes the main reasons for the converter to limit its efficiency under light load and heavy load conditions,and the foundation of closed-loop voltage stabilization control is constructed for efficiency optimization control under corresponding load.Secondly under light load conditions,circuit modeling analysis of the underdamped oscillation of the resonant tank in the T_off stage of burst mode control is performed.Adding pre-adjustment at the T_on stage allows the converter to work quickly and stably on the state track of the highest efficiency point.Finally,the selection of the burst mode control period is limited.Thirdly under heavy load conditions,the synchronous rectification control of the converter is mainly optimized.In the rectifier device,the gallium nitride device is used to replace the traditional silicon device,and the synchronous rectification judgment mechanism of the gallium nitride device is proposed.At the same time,a time-division multiplexing adaptive synchronous rectification control strategy is proposed to save hardware resources.Finally,a multi-cycle interrupt synchronization algorithm is introduced to solve the digital implementation of high-frequency synchronous rectification.By combining the optimization of the control under different loads,the high efficiency of the LLC resonant converter in the full load range is finally achieved.The design content of the thesis is verified on a prototype of an LLC resonant converter with a switching frequency of 300 k Hz and a full-load output of 40A.The test results show that the efficiency of 3%-10%light load range is improved to above 91.85%by optimized burst mode control,compared with the efficiency of traditional frenquency control has increased by 27%.The converter reaches a peak efficiency of 97.13%at output power of 216W.Finally,the average efficiency in the 3%-50%load range is increased to 94.87%,which is 30%higher than the efficiency that is not optimized.