Study On The Key Manufacturing Technology And Loss Mechanism Of High-Frequency MnZn Power Ferrite

High-efficiency power conversion devices,such as AC-DC,DC-DC converters,switching power supplies,etc.,are inseparable from high-performance core materials.MnZn power ferrite has high initial permeability,high saturation magnetic induction,high resistivity and low core loss,so it is very suitable as a core material for power conversion devices.According to the transformer principle,increasing the operating frequency of the switching power supply can reduce the volume of the magnetic core,which is conducive to the miniaturization and integration of the switching power supply.At present,the frequency of switching power supply has broken through MHz,and with the rapid development of electronic information technology,in the near future,the operating frequency of the power supply will inevitably reach a higher 2?4MHz or even higher.For MnZn power ferrite cores,the increase in operating frequency will cause the core loss to increase sharply,reduce the power supply efficiency,and even burn the device.Therefore,MnZn power ferrite material with high frequency and low loss is a key factor restricting the development of miniaturization and integration of switching power supplies.At present,the research and development of high-frequency MnZn power ferrite materials is relatively backward in China,and there is no high-frequency product that can match the international top products.In addition,in recent years,foreign developed countries have imposed scientific and technological sanctions on Chinese enterprises,and the development of independent high-frequency MnZn power ferrite materials is very urgent for China.Based on the above content,this thesis carried out the research on the key preparation technology and loss mechanism of high frequency MnZn power ferrite.In terms of material preparation,with the goal of reducing the high frequency loss of MnZn power ferrite,the effects of the main formula,sintering temperature,additives,and ion substitution on the performance of MnZn power ferrite were studied.In terms of loss mechanism,the relationship between magnetic domain morphology and high-frequency loss,as well as DC superposition characteristics were studied.First,the effect of Fe₂O₃ and ZnO content in the main formula on the performance of MnZn power ferrite was studied.The research results show that by optimizing the Fe₂O₃ content,the magnetic properties of MnZn power ferrites can be improved and the high frequency loss can be reduced.The appropriate Fe₂O₃ content is 54.5mol%.After determining the Fe₂O₃ content,continue to optimize the ZnO content.When the ZnO content is 11mol%,the high-frequency core losses of MnZn power ferrite are further reduced.So far,the main formula of MnZn power ferrite has been determined,namely:54.5mol%Fe₂O₃,34.5mol%Mn₃O₄,and 11mol%ZnO.In addition,the effect of sintering temperature on the performance of high frequency MnZn power ferrites was studied.When the sintering temperature is 1150^oC,MnZn ferrite has the best microstructure,the most uniform crystal grains,the least pores,and the lowest high-frequency loss.Secondly,using the above optimized formula and process,the influence of additives and ion substitution on the performance of MnZn ferrite was studied.The research results are as follows:?1?A proper amount of Co₂O₃ doping can increase the saturation magnetic induction and initial permeability,and reduce high-frequency core losses.When the Co₂O₃ content is 0.30wt%,the core losses of MnZn ferrite are the lowest.At room temperature,the core losses of 3MHz 10m T and 30m T are 61k W/m³and 887k W/m³,respectively.?2?On the basis of Co₂O₃ doping,the combined doping of Ca CO₃/V₂O₅ can further reduce the high-frequency core losses of MnZn ferrite.Because the melting points of Ca CO₃ and V₂O₅ are different,they form a competition mechanism for the growth of grains during the sintering process,which improves the microstructure and resistivity.When the doping amounts of Ca CO₃ and V₂O₅ are 0.1wt%and 0.001wt%,respectively,at room temperature,the core losses at 3MHz 10m T and30m T are 46k W/m³ and 664k W/m³,respectively.?3?Based on the joint doping of Ca CO₃ and V₂O₅,by doping an appropriate amount of dielectric material Ba Ti O₃?BTO?,the temperature characteristics of resistivity of MnZn power ferrite are further improved,thereby reducing the loss of MnZn ferrite at high temperature and high frequency.When the BTO content is 0.04wt%,from room temperature to 120^oC,MnZn ferrite has the lowest high frequency loss.At room temperature 3MHz 30m T,the loss of this sample is302k W/m³.The reduction in room temperature loss is mainly due to the refinement effect of BTO on the grains,which reduces the residual loss;at 100^oC,3MHz 30m T,the loss of this sample is only 890k W/m³.The reduction of high-temperature core losses is mainly due to the improvement of the temperature characteristics of the grain boundary resistance by BTO,which reduces the eddy current loss at high temperature.?4?In addition to Ba Ti O₃,another dielectric material,Ca Cu₃Ti₄O₁₂?CCTO?,can also reduce the high frequency loss of MnZn power ferrite.Due to its high resistivity,CCTO can increase the resistivity of MnZn ferrite and reduce high-frequency core losses.When the CCTO doping amount is 0.2wt%,the core losses of MnZn ferrite are 31k W/m³ and495k W/m³ at room temperature 3MHz,10m T and 30m T,respectively.Unlike BTO,CCTO can promote grain growth and increase the saturation induction and initial permeability.?5?Ti ion substitution will reduce the resistivity of MnZn ferrite and deteriorate the magnetic properties.A proper amount of Sn ion substitution can increase the resistivity of MnZn ferrite,increase the resonance frequency,and reduce high-frequency loss.However,when the Ti/Sn substitution is excessive,impurity phases will be introduced into the MnZn ferrite and the magnetic properties will drop sharply.When the substitution amount of Sn is 0.003,the core losses of MnZn ferrite is457k W/m³ at room temperature,3MHz 30m T.Through additive and ion substitution studies,the MnZn power ferrite with low loss at high frequency and high temperature was finally prepared,that is,the sample when BTO doped.Finally,for the loss mechanism,the high-frequency core losses are analyzed from the magnetic domain morphology,and the DC superposition characteristics of MnZn power ferrite are studied.The research results are as follows:?1?The Lorentz transmission electron microscope was used to characterize the magnetic domain morphology,and the relationship between the crystal grain size and the magnetic domain morphology was studied.When the grain size increases,the grains will change from a single domain state to a multi-domain state.Through the fitting and separation of the magnetic spectrum,the influence of the magnetic domain morphology on the dynamic magnetization process is analyzed,and then the magnetic domain morphology is related to the high-frequency loss.When the grain size increases,the contribution of the domain wall displacement to the permeability increases,and the domain wall resonance frequency gradually decreases.When the test frequency is close to the domain wall resonance frequency,the residual loss will increase rapidly due to the resonance phenomenon.?2?Under the DC bias magnetic field,the performance of MnZn ferrite will change greatly.When the DC bias magnetic field increases,the incremental permeability first increases and then decreases.As the DC bias magnetic field increases,the low-frequency core losses first decreases and then increases,while the high-frequency core loss increases monotonically.The variation of loss at different frequencies is related to the composition and ratio of loss.For the direct current superposition characteristics of magnetic permeability,a high anisotropy field can be introduced to improve the ability of magnetic permeability to withstand a direct-current bias magnetic field.Regarding the DC superimposition characteristics of losses,the ratio of each loss can be adjusted to keep the loss low under a certain DC bias magnetic field.