Comparative analysis of high step-down ratio isolated DC/DC topologies in PEV applications

In plug-in electric vehicles (PEVs), an isolated dc/dc converter with high step-down ratio is required to supply power to the onboard appliances. The wide battery pack voltage range on the input side brings significant challenges to the design and optimization of this dc/dc converter. This study comprehensively compares LLC resonant topology, series-resonant topology with delay-time control (DTC-SRC), and LLC cascaded with buck topology (LLC-Buck) in terms of the power density, system efficiency and control complexity. It is found that the LLC-Buck topology maintains the best overall conversion efficiency performance. A 400 W LLC-Buck converter prototype is designed and tested to verify the proof of concept.