Noniterative Design of Litz-Wire High-Frequency Gapped-Transformer (Lw-HFGT) for LLC Converters Based on Optimal Core-Geometry Factor Model (OKGM)

Litz-wire high-frequency gapped-transformer (Lw-HFGT) is a vital component that facilitates efficient operation of LLC converters. The converter designers go through cumbersome multiobjective optimization techniques, with many iterations, to obtain an optimal Lw-HFGT design. High reliance on such techniques is due to deficiencies in existing analytical core and winding selection (CWS) methodologies; most analytical CWS models do not focus on optimization. Therefore, this article proposes noniterative analytical CWS methodology for Lw-HFGT based on an innovative optimal core-geometry factor model (OKGM). The aim is to obtain Lw-HFGT design with minimized losses and size, integrated magnetizing inductance, and temperature rise within limits. The method incorporates application requirements (excitation-voltage waveform, LLC circuit-parameters, thermal limit), along with Lw-HFGT physical characteristics [core geometrical features, peak flux density (B-pk), current density, core-material parameters, air-gap, effective permeability, and Litz-wire-sizing (LwS)] in the CWS process. Analytical models with improved accuracy for core geometrical features extraction from core-geometry factor, optimal-B-pk, and LwS are also proposed. The complete methodology is improved based on proposed models, optimality criteria, application requirements, and energy storage inside gapped transformer. Optimal values of initial setup parameters, calculated using optimal-B-pk, enable OKGM to carryout optimal CWS in single iteration. The methodology is experimentally validated by designing Lw-HFGT for the 110-kHz, 200-W, 400-12 VDC LLC converter. The PC40-material-based Lw-HFGT design achieves up to 67% reduction in volume-loss product, in comparison to various existing methods with the same input.