A New Transformer Model with Separate Common-Mode and Differential-Mode Capacitance

A new transformer equivalent circuit model is presented containing separate common-mode and differential-mode capacitors, allowing for direct and simple analysis of the transformers influence. The representation of common-mode and differential-mode signals in two-winding transformers have been described and presented. Problems related to two commonly used transformer equivalent circuit models are shown. Operation scenarios of an isolated full bridge phase-shift DC/DC converter is used to compare the new transformer model with a commonly used model. Three transformers with different turns-ratio are constructed, with the purpose of evaluating the transformer models. Both models provided accurate impedance prediction for transformers with turns-ratio equal to one. However, for turns-ratios different from one, the commonly used model has an error of 40%-132% in the predicted equivalent capacitance of the measurements, whereas the proposed model only has an error of 0.6%-7.3%. The proposed model is therefore the only model that predicts accurate transformer behavior for any turns-ratio.