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We investigated the electrical and magnetic properties of cobalt-substituted manganese-zinc soft ferrite by using the equivalent lumped elements acquired from the appropriate equivalent electrical circuit of polycrystalline ferrite. We applied the equivalent lumped circuit, combined with equivalent lumped resistances and capacitance, to determine the effect of microstructure on electrical and magnetic properties of cobalt-substituted manganese-zinc ferrites. Both the hysteresis loss and the eddy-current loss of soft ferrites account for a major proportion of iron loss in high-frequency switching mode power supplies. Replacing a small portion of Fe2+ with Co2+ remarkably increases the bulk resistivity of the MnZn ferrite and decreases the core loss by lowering eddy-current loss. A longer isothermal duration causes grain growth and forms a larger equivalent capacitance, which leads to a reduction in hysteresis loss. However, excess substitution of Co2+ for Fe2+ is disadvantageous to magnetic permeability and raises core loss. We measured the dc resistivity by the four-probe method on sintered disks with both sides polished and coated with a thin layer of silver paste as a good contact material. We measured the magnetic permeability by an impedance analyzer at room temperature. The total loss of Mn0.58Zn0.37 Co0.01Fe2.04O4 ferrite core does not exceed 420 mW/cm3 at 300 kHz/70 mT.
Date of Publication: Oct. 2007