Dual-Variable Maximum-Efficiency Tracking Control for a High-Step-Up Boost-Integrated Dual-LLC Resonant Converter

This paper presents the analysis and maximum-efficiency tracking control of a high-step-up boost-integrated dual-LLC resonant converter. The proposed converter combines one boost stage and two LLC resonant stages using a shared half-bridge, while the series-connected outputs provide a high voltage-conversion ratio. Since the converter gain is affected by both the duty ratio and switching frequency, their coupled effects on voltage gain and loss characteristics are analyzed. Conduction, switching, and magnetic-core losses are considered to determine the maximum-efficiency operating point under different load conditions. The analysis shows that the optimal duty ratio and switching frequency vary with output power, making fixed-duty or fixed-frequency operation insufficient for maintaining maximum efficiency over a wide load range. A two-loop control strategy is therefore employed, in which the duty ratio regulates the output voltage and the switching frequency tracks the maximum-efficiency point along the constant-output-voltage trajectory. The method is evaluated by PSIM simulation, including closed-loop tracking under load steps, and by frequency sweeps on a 500 W, 60–380 V prototype. The measured efficiency at the sweep-identified optimum reaches 97.44% at 300 W and 96.60% at 500 W, and the observed but not statistically established differences relative to single-variable control are up to 0.27 percentage points.