Study On Magnetic Core And Winding Characteristics In High Frequency Transformers With Amorphous And Nanocrystalline Alloys

With the demand of high power electrical power conversion,the high frequency transformers coupled with switching power converters have attracted significant attentions in the recent years.Thanks to the excellent electric,magnetic and electromagnetic characteristics,amorphous and nanocrystalline alloys have been widely used for the core of high frequency transformers.In this dissertation,high frequency transformers with amorphous and nanocrystalline cores were analyzed.The major obstacles of design and optimization of high frequency transformers were discussed,the magnetic properties of amorphous and nanocrystalline alloys,calculation of winding parameters,calculation of losses,and optimization of high frequency transformers were analyzed.Firstly,Two amorphous and nanocrystalline cores were wound,high frequency magnetic properties of amorphous and nanocrystalline alloys are measured by using the annular ring samples.The magnetic hysteresis and specific core loss curves under excitations of various frequencies of amorphous and nanocrystalline cores were drawn with the experimental data.The core loss coefficients of amorphous and nanocrystalline alloys were calculated by the Steinmetz and loss separation method.It was shown that nanocrystalline alloy had lower specific core losses than amorphous alloy under the same situation.The additional core loss by the effect of end stripe was calculated and analyzed by finite element method(FEM).The influence of core loss by the effect of end stripe was studied by considering the factors which affect the effect of end stripe.It could be found that the core loss by considering the effect of end stripe was more than that without considering the effect of end stripe.The influence of high frequency transformer by the effect of end stripe would be analyzed by considering the thickness of magnetic core and the exciting frequency.Secondly,the eddy current effect of copper foil and solid wire were analyzed by the analytical models.The analytical model of Litz-wire was set up,and modified by considering the coefficient of proximity effect.The current density distribution of foil winding,solid wire and Litz-wire were calculated by FEM models,the AC resistances of foil winding,solid wire and Litz-wire were calculated by two foil windings analytical model,two solid wires analytical model and Litz-wire analytical model and compared with them by FEM models.It could be found that the result of analytical method for Litz-wire was just so so,while the result of modified analytical method matched well with the FEM.The relative error was no more than 1%.Thirdly,the capacitances of foil winding,solid wire and Litz-wire in high frequency transformer were calculated by using the static capacitance models.A new static capacitance model is proposed considering that the actual electric field lines changed gradually in the windings.By comparing with the finite element model,the proposed model is more accurate than the traditional models.A dynamic capacitance analytical model is present by considering the charge distribution uneven.It is found that the dynamic capacitance is lower than the static capacitance according to the static capacitance and dynamic capacitance analytical model.Finally,the parameters of Litz-wire in high frequency transformer were measured by setup measurement system according to the equivalent circuit.A method is proposed for design optimization of high frequency transformers,and four high frequency transformers were optimally designed with amorphous and nanocrystalline alloy cores.The performances were evaluated by the normalized index value.It could be found that the performance of high frequency transformers with nanocrystalline cores were better than those with amorphous cores.The performance of high frequency transformer with nanocrystalline alloy FT-3M was best.