Iron Losses in a Medium-Frequency Transformer Operated in a High-Power DC-DC Converter

The three-phase dual-active bridge is a dc-dc converter, which is highly suitable for high-power applications. Among others, this is due to the medium-frequency transformer in the ac link, which provides galvanic isolation. The transformer is operated with a square-shaped voltage waveform. The flux density in the transformer core is piecewise linear. However, for the sake of simplicity, the magnetic flux is often assumed sinusoidal. Thereby, the actual iron losses generated in the core material are misinterpreted. This paper discusses the difference between the exact piecewise linear and the sinusoidal course in terms of iron losses. Silicon steel with a thickness of 0.18 mm is measured at a frequency of 1000 Hz, comparing the sinusoidal excitation with the actual one. The measurements are validated using the improved generalized Steinmetz equation. Finally, the transformer core losses are evaluated when the dc-dc converter is operated under load. The results are confirmed through measurement.