Study Of MnZn Power Ferrites For MHz Application

The development of power electronics industry requires the miniaturization,integration and lightweight of power transmission devices?such as inductions,transformers and converters?.As the main components of various power transmission devices,ferrite cores,especially MnZn ferrite cores,are subsequently required to exhibit low losses and high permeability in MHz high frequencies according to energy transmission theory.However,the low-resistivity MnZn ferrites are hardly used in high-frequency applications of more than 4 MHz to avoid excessive eddy current losses and residual losses,which result in the rapid temperature rise and damage the devices.This study uses solid-state reaction method to prepare low-power manganese-zinc power ferrite materials for 1-10 MHz application.Based on the optimized process parameters,the respective effects of Ca-Si-La doping,Co-Ti doping and Nb-Ti doping were studied.As a result,several kinds of MnZn ferrite materials capable of using at 1-10 MHz were successfully prepared.Meanwhile,we modified the classic power loss separation model for high frequency application,which was charactered by its new effective empirical formula for the calculation of residual loss.In summary,the specific conclusions are as follows:1)The co-doping of La₂O₃-CaO-SiO₂ can remarkably changes the grain size,grain boundary structure and the subsequent resistivity of MnZn ferrites.The grain growth and resistivity can be enhanced by low La₂O₃ concentration,while unexpectedly suppressed by doping excessive La₂O₃.It is speculated that the addition of a small amount of La₂O₃ reduces the melting point of the Ca-Si-based grain boundary glass phase,which improves the wettability of the grain boundary glass phase and improves the resistivity.However,the introduction of excessive La³⁺will induce the formation of LaFeO₃ at the peripheries of the grains,this compound reduces the resistivity because of its semiconductor property but suppresses the diffusion of Fe element during sintering,making it hard for grain boundary migration and grain growth.2)On the basis of the Ca-Si-La doping,the Ti-Co doping is further optimized to improve the comprehensive magnetic properties of MnZn ferrites.As a result,The material doped with 500 ppm Co₂O₃ and 2000 ppm TiO₂ exhibits a initial permeability value of 710 at 60°C,a cut-off frequency value of 9.2 MHz,and a core loss value 122 KW/m³ at 5 MHz,10 mT,60°C.The material doped with 1000 ppm Co₂O₃ and 1500 ppm TiO₂ exhibits an initial permeability value of 534,a cut-off frequency value of 12.7 MHz,and a magnetic loss value of about 520 KW/m³ at 10MHz,10 mT,25°C.3)Common loss separation models for high frequency are summarized,then a new model is proposed.In this new model,the residual loss is separated according to the traditional Wijn model,and its relationship with frequency,magnetic induction intensity,and magnetic spectrum is experimentally explored.4)The effects of TiO₂-Nb₂O₅ co-doping on the structure and properties of Mn-Zn power ferrites used for 1 MHz were studied.A proper amount of Nb₂O₅ can help increase the density,promote the uniform growth of grains,and increase the resistivity,which ultimately improves the electromagnetic performance of the material.The solution of TiO₂ in spinel phase can introduce extra cation vacancies,which provide extra diffusion channel for the migration of Nb⁵⁺to the triple joint points,thereby accelerates the effect of Nb₂O₅ doping.The final optimized material has an initial permeability value of more than 1500 at 80°C,a saturation magnetic induction value of more than 490 mT at room temperature,a density value of 4.82 g/cm³,and a power loss value of 394 KW/m³ at 1 MHz,50 mT,80°C.