An ISOP LLC Resonant DC–DC Converter with Wide Voltage Range and High Step-Down Ratio for Electric Vehicle Auxiliary Power Systems

Electric vehicles (EVs) employ high-voltage battery systems to improve drivetrain efficiency, while numerous auxiliary loads still require low-voltage power supplies, typically at 12 V. This creates a demand for isolated DC–DC auxiliary power modules (APMs) with high step-down ratios, wide operating ranges, and high energy conversion efficiency. In this paper, a high-efficiency DC–DC converter based on an input-series output-parallel (ISOP) LLC resonant architecture is proposed for EV auxiliary power applications. The proposed converter adopts dual LLC modules connected in an ISOP configuration to distribute stress, reduce the transformer turns ratio, and inherently achieve output current sharing. Full-bridge and half-bridge LLC operating modes are combined with hybrid pulse-frequency modulation (PFM) and phase-shift modulation (PSM) control strategies to enable wide voltage operation while maintaining soft-switching characteristics. A two-phase interleaved scheme further suppresses output current ripple. A 1000 W prototype demonstrates stable operation over 200–400 V input and 10–16 V output ranges with a peak efficiency of 97.87%. In this paper, PSM denotes phase-shift modulation, defined as the intentional delay between primary-side switching legs for power regulation.