Yiren Wang, The University of Manchester
High-frequency operation is an obvious way to reduce the size of the magnetic components, which typically account for around 50% of the weight of DC-DC converters. However, the losses in the magnetic components tend to increase at high frequencies, in particular some AC losses associated with the fringe field around air gaps. The core loss due to the air gaps, called gap loss, typically occurs in laminated cores. The fringing flux has a component that is normal to the lamination planes, and therefore will create eddy current and losses within the laminations.
Nanocrystalline cores are attractive for the size reduction of magnetic components. They offer a high saturation flux density and low core loss, but their performance may be limited by the gap loss at high frequency. The analytical prediction of the gap loss is very complicated and difficult. This is due to the high non-linearity of the fringe field and the 3D nature of the eddy currents. Finite element analysis is often used for eddy current problems, but it is still very challenging to model the gap loss especially at high frequencies. The finely laminated structure of the core, 18 µm lamination thickness for nanocrystalline cores, will result in an enormous computational effort across the core dimension. In addition, the skin depth of nanocrystalline materials is very small at high frequency, around 35 µm at 100 kHz, which will require a very fine mesh in order to fully capture the eddy current effect.
In this work, the gap loss effect has been studied in Opera 3D simulation package using the ELEKTRA/SS solver. The homogenisation technique is used to model the finely laminated core as a solid bulk with anisotropic core properties, and an equivalent skin depth is calculated from the homogenised core properties to determine the mesh size. Mosaic meshing that combines different mesh shapes and sizes is used in the inductor model for a faster and more accurate solution. The results show that the gap loss is significant at high frequency and it is highly concentrated around the edge of the gaps. The excess loss may cause local overheating if untreated, which may break down the insulation material between the lamination layers. A sensitivity study has also been carried out in Opera 3D to explore the dependency of the gap loss on the key inductor design parameters and operating conditions. An empirical equation is derived to provide a design-oriented prediction of high frequency gap loss. The results provide a sound basis for the designs of high frequency gapped inductors and their thermal management techniques.
The FE modelling of gap loss is experimentally validated by temperature measurements on a 300 A, 60 kHz foil-wound nanocrystalline inductor on a 25 kW DC-DC converter, showing good agreement between predictions and measurements.