Variation of Core Loss in High-Frequency Transformers Under Different Load Conditions

Typically, the core loss of transformers is considered constant, regardless of the load conditions on the secondary side. However, this assumption is no longer valid for high-frequency transformers (HFTs). This article reveals that different load conditions can lead to the core loss in the HFTs deviating from the no-load case due to the variation of voltage drop on the windings' impedance and, subsequently, the magnetic field in the core, which is not commonly assessed and considered in engineering practice. To accurately estimate the core loss, the main challenge in the case of HFTs, compared to the inductors, is that the unneglectable ac winding resistance deviates depending on the load conditions, which makes the voltage across the magnetizing branch (or flux density within the core) difficult to find. The core loss variation is quantitatively evaluated in this article against various load conditions through a case study of one representative HFT. Both experimental measurements and three-dimensional finite element analysis results confirmed this phenomenon, with the physical causes of this variation discussed and investigated.