Study of Frequency-dependent Parasitics' Effects on Resonance Frequencies of a Two-winding Medium/High-frequency Transformer

A medium/high-frequency (MF/HF) transformer has become an integral part of many high-power conversion systems. Therefore, it is important to study the frequency response analysis (FRA) of a MF/HF transformer to ensure a reliable and safe converter operation. In this paper, frequency-dependent behavior of the transformer parasitics and their effects on the natural resonance frequencies are analyzed. A transfer function based π-model is employed to compute the frequency-dependent FRA of a two-winding transformer and estimate its natural resonance frequencies. It concludes that at high frequency, increased winding ac resistance exhibits higher damping effect at the resonance frequencies; whereas reduced leakage inductance shifts the resonance frequencies towards the high-frequency region of frequency response. Analytical calculations are verified with the help of experimental results obtained from a 1.5 kW, 15 kHz ferrite core transformer. The predicted and measured values of the resonance frequencies agree well, indicating a good modeling accuracy and aptness of the proposed methodology.