Control strategies for a LLC multi-resonant DC-DC converter in battery charging applications

In this paper, a control strategy is presented for a high performance LLC multi-resonant dc-dc converter in a two stage smart charger for neighborhood electric vehicle applications. It addresses several aspects and limitations of LLC resonant dc-dc converters in battery charging applications, such as very wide output voltage range while keeping the efficiency maximized, implementation of the current mode control at the secondary side and optimization of burst mode operation for current regulation at very low output voltage. The proposed control scheme minimize both low and high frequency current ripple on the battery while maintaining stability of the dc-dc converter, thus maximizing battery life without penalizing the volume of the charger. Experimental results are presented for a prototype unit converting 390 V from the input dc link to an output voltage range of 48 V to 72 V dc at 650 W. The prototype achieves a peak efficiency of 96 %.