Low Permeability Ferrite with Low Losses at High Frequency

Abstract

In Switching Mode Power Supplies, ferrites with low permeability and low losses are required to make inductors. (NixZnyCuz)1-εCoεFe2O4 ferrites are good candidates to meet this objective. Their permeability can be controlled by adjusting the Ni/Zn ratio and the Co content. Cobalt induced anisotropy lowers the core losses and leads to a more resonant behavior of the complex permeability spectra [1]. Cobalt substitution also modifies the variation of the magnetocrystalline anisotropy versus temperature, leading to a minimum loss at the compensation temperature, depending on the cobalt content and the Ni/Zn ratio [2]. By controlling those parameters, ferrites low core losses at high frequency and permeability lower than 100 have been obtained. Core losses have been measured between 1 and 2 MHz, for induction level up to 25 mT, and the contribution of the winding to those losses has been evaluated, from less than 10% for the highest permeability values to around 20%. Core losses lower than the state of the art were obtained, for materials with a permeability range from 40 to 135.