Research On MnZn Power Ferrite And It’s Application Of MHz Switching Power Supply

In this dissertation,in order to meet the requirement of high frequency miniaturization of switching power supply,low loss MnZn power ferrites used for high frequency（MHz）switching power supply were prepared by conventional oxide ceramic process.The influences of main compositions,second milling time,sintering temperature and sintering oxygen partial pressure on the microstructure and magnetic properties of Mn Zn power ferrites were investigated.The Mn Zn power ferrites with high permeability,high saturation magnetic induction and low loss were prepared and the switch power transformers which are used at MHz frequency were completed.Firstly,effects of the formula and process conditions on MnZn power ferrites properties were analyzed.The results show that:when Fe₂O₃ content is in the range of53.5⁵5.6 mol%,the initial permeability first increases and then decreases,total loss first decreases and then increases,the temperature of secondary maximum peak ofμ_i-T curve and the minimum hysteresis losses temperature moved to lower temperature,eddy current losses and residual losses increase monotonously.When ZnO content is in the range of 9¹3 mol%,the initial permeability increases monotonously,total loss first decreases and then increases,the minimum hysteresis losses temperature moved to lower temperature,eddy current losses and residual losses first decreases and then increases,and the optimal molecular formula is Fe_2.13Mn_0.65Zn_0.22O₄.The second milling time influences the magnetic properties of MnZn ferrites by affecting the powder activity and the content of ZrO₂ in powders.Too short the second milling time will result in a decreased powder activity and the grain growth is incomplete,while too long will result in an excessive grain growth and an increased of content of ZrO₂.The suitable milling time is 4 h.Too low the sintering temperature will cause the grain growth to be incomplete,resulting a low permeability and high power loss of MnZn ferrites,while too high sintering temperature will cause the grain to grow abnormal and reduce the magnetic properties of MnZn ferrites.The proper sintering temperature is1150℃.The oxygen partial pressure will not only affect grain growth,but also affect the ion valence state of Fe elements directly.With the increase of oxygen partial pressure,density d and initial permeabilityμ_i of MnZn power ferrites monotonously decrease at room temperature,while the hysteresis loss P_h and eddy current loss P_e increase.Meanwhile,the temperature of secondary maximum peak ofμ_i-T curve and the minimum hysteresis losses temperature moved to higher temperature.With the increase of measurement temperature,P_e-T and P_r-T curve keep rising trend,and the optimum oxygen partial pressure is 2%.Secondly,we designed the winding structure of transformer,and the transformer for MHz switching power supply based on self-developed MnZn power ferrite was prepared by analyzing the loss characteristics of magnetic core and winding core.When frequency is in the range of 1³ MHz,the primary inductance and primary leakage of the transformer change insignificantly,and the transformer impedance increase monotonously.The transformer impedance increases significantly when the frequency exceeds 3 MHz.The transformer is suitable for operating at MHz high frequencies.