Relationship between Resistivity, Core Size and Core Loss of Fe-Si Compressed Cores

Abstract

SynopsisCompressed cores made with Fe-Si-Al or Fe-Si alloys are widely used for smoothing and boost choke coils of switching regulators and DC-DC converters. In this study, the relationship between resistivity and eddy current loss was investigated for Fe-3 mass% Si compressed cores by changing heat treatment temperature and height. Furthermore, eddy current losses were calculated electromagnetically by finite element method, and they were compared to the experimental results. Following conclusions were obtained.(1) Eddy current loss rapidly increases with increase in heat treatment temperature above 1173 K.(2) Resistivity decreases monotonically with increase in heat treatment temperature.(3) Eddy current loss is almost constant and independent of core height in higher resistivity than about 0.1 m. When the resistivity is lower than 0.1 m, eddy current loss increases with decrease in resistivity and increase in core height. (4) The above behavior of eddy current loss was explained as follows. Eddy current generates only inside of each particle and eddy current loss does not depend on core size in higher resistivity than 0.1 m. On the other hand, eddy current flows particle to particle and causes large eddy current loss dependent on resistivity and core size in lower resistivity than 0.1 m. (5) Calculated inter-particle eddy current losses based on homogeneous bulk model show good agreements with experimental data. The fact suggests that the behavior of inter-particle eddy current in the compressed core is same as that of homogeneous bulk material.