Medium-Frequency Transformers for Fast Rise Time PWM Voltages: Modelling and Design Considerations

The PWM generated by wide bandgap devices can feature a shorter rise time than in the case of standard silicon switches. The resulting harmonic content can excite the resonance frequencies of Medium-Frequency Transformers (MFTs), causing internal overvoltages during continuous operation. If ignored, this phenomenon can cause unexpectedly high electric fields in the insulation, possible partial discharges and failure of the MFT. The occurrence of these overvoltages is determined by the high-frequency response of the MFT and it is significantly influenced by the winding topology. In this paper, the choice of conductor type and possible resulting topologies are analyzed. A multi-layer litz winding and a concentric disk foil winding are compared. The trade-off among the conductors used in the two topologies is followed by the analysis of the voltage distribution in the winding by the multi-conductor fully-coupled transmission line method. The continuous disk foil winding MFT does not present internal overvoltages and is therefore a preferable option in this respect. However, in light of the trade-off between litz and foil wire, the global optimal solution will be case-dependent relying on the design requirements, which concur in determining the best design choice.