Characterization of three winding transformers for the reduction of switching mode power supply conducted emission

The paper focuses the attention on the modelling of transformers for the prediction of conducted emissions from SMPS. The transformer equivalent circuit component values are extracted by impedance measurements with proper load conditions. Beside a traditional two-winding transformer, also a three winding version is analysed: the third winding is used to realize a shield, in order to reduce the conducted emission. This is a quick and cheap way to realise the shield, without appreciably increase the production time. Finally, the simulation of a flyback SMPS, allows to evaluate the benefits obtained in using the shielded transformer. All simulations are experimentally validated showing a satisfactory agreement.