Modeling Transformer Winding Behavior of Multi Output Power Supplies using Mutual Impedance Effects

Abstract:
Transformer equivalent circuits based on modeling magnetic coupling or mutual inductance describe important aspects of transformer behavior such as leakage inductance and cross regulation. Transformer equivalent circuits based on adding L-R networks typically focus on winding losses predicted by Dowell's method, but they often don't model coupling effects, and they typically don't work well at predicting winding losses for transformers that supply independent loads or for transformers that have windings connected in parallel unless very complicated models are used. This presentation describes an equivalent circuit for transformers that models both magnetic coupling effects and winding losses while using a surprisingly simple structure. The key to this approach is that it is based on mutual impedances that have both inductive and resistive components. This equivalent circuit models the well-known magnetic coupling coefficients, but also models the lesser-known resistance coupling coefficients. This allows the model to accurately model frequency-dependent losses as well as frequency-dependent leakage inductances. The performance of a simulated phase-shifted bridge converter with two independent loads is compared to hardware results. Various effects of mutual impedance are illustrated including cross regulation, how an external ZVS inductor affects the winding losses, and how the frequency dependence of leakage inductances affects diode reverse recovery and the energy that needs to be snubbed.

About the Speaker: