Control Strategies for Wide Output Voltage Range LLC Resonant DC-DC Converters in Battery Chargers

In this paper, a control strategy is presented for a high-performance capacitively loaded loop (LLC) multiresonant dc-dc converter in a two-stage smart charger for neighborhood electric vehicle (NEV) 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 minimizes both low-and high-frequency current ripples 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 3-72 V dc at 650 W. The prototype achieves a peak efficiency value of 96%.