An LTI Transformer Model via Admittance Formulation and Hybrid Parameter Extraction for Parasitic-Aware Analysis of High-Frequency Isolated Power Converters

This paper proposes a physics-based lumped-element circuit model for transformers using a linear time-invariant (LTI) admittance formulation with hybrid parameter extraction, aimed at predicting parasitic effects in isolated high-frequency power converters. Requiring only standard open and short-circuit impedance measurements performed as a function of frequency, the model employs a compound admittance matrix that integrates magnetic and electrostatic coupling effects, enabling the decoupling of low-frequency (inductive–resistive) and high-frequency (capacitive) behaviors. The parameter extraction is hybrid, as analytical equations derived from the parallel association of magnetic and capacitance matrices are combined with a numerical optimization procedure. In particular, the interwinding capacitance is estimated by maximizing the coefficient of determination ( $R^{2}$ ) of regressions between measured and modeled responses, considering both logarithmic magnitudes ( $\log _{10}|Z|$ ) and linear phase angles. Validation was carried out on three distinct transformer designs: a high-coupling flyback transformer on a planar core, a high-leakage integrated magnetic for a DAB on an EE core, and a split-bobbin integrated magnetic for an LLC on an ETD core. Across all DUTs, the model achieved $R^{2} \geq 0.960$ for magnitude responses and an average $R^{2}$ of 0.874 for phase, demonstrating strong agreement with experimental frequency response data. The extracted model was also benchmarked against PLECS frequency-domain simulations, further confirming its accuracy. Overall, the method enhances capacitance estimation and provides a comprehensive mathematical framework and circuit model to account for parasitic effects in transformer-isolated power converters, covering steady-state performance (voltage transfer and soft-switching boundaries), dynamic behavior, and high-frequency phenomena (ringing, EMI, and common-mode coupling paths).