Research On Core Loss Model With Dead Zone Voltage Excitation Waveform

Power ferrite core has the advantages of high permeability,high resistivity and low cost.It is a common magnetic core material for power magnetic components in switching converters.The magnetic element of switching converter usually works under non-sinusoidal excitation,and the sinusoidal excitation loss data given by manufacturer cannot be directly applied.With the demand of miniaturization,the modeling and prediction of core loss are becoming more and more important due to the increasing switching frequency.The overheating of magnetic components is one of the main reasons for switching converter failure.In this paper,the loss components of power ferrite core are analyzed firstly:hysteresis loss,eddy current loss and residual loss,and their physical mechanism is introduced respectively.At present,the classical core loss model cannot explain the additional core loss phenomenon under the zero-voltage excitation waveform.Aiming at this theoretical and engineering problem,the physical mechanism of excess loss and its corresponding loss model under zero-voltage excitation waveform are studied in this paper from the microstructure of power ferrite core.In view of the microscopic distribution characteristics of grain and insulating layer in power ferrite cores,a method of constructing dielectric loss component based on the average size and length of grain and insulating layer is proposed in this paper,and the calculation process of relaxation constants in dielectric loss model is systematically described.The dielectric loss discharge model can effectively reflect the effect of working conditions such as flux density change rate and duty cycle on the additional core loss.Secondly,according to the nonlinear distribution characteristics of flux density in the magnetic core,the core size equivalent transformation method from nonlinear distribution to linear distribution is proposed,and the equivalent lumped parameter circuit model including eddy current loss effect and dielectric loss effect is derived.The calculation method of related circuit parameters is analyzed in detail,which is helpful to construct magnetic loss circuit simulation model based on SPICE core.At the same time,it is proved that the superposition of the parasitic inductance current and the excitation inductance current of the core material itself is the root cause of the current spike at the initial stage of the zero-voltage waveform.Although energy in the parasitic inductor is also released during the zero voltage period,it contains energy is so small and the effect on the extra magnetic loss is negligible.Finally,it is proved in detail that the extra loss caused by the zero-voltage excitation waveform mainly comes from two aspects:the main one is the excessive dielectric loss caused by the interface polarization effect between the core grains and grain layers;The secondary cause is heat loss due to the release of energy stored in the equivalent parasitic inductance of the core material itself.Taking full-bridge switching converter as an example,two typical ferrite working magnetic rings made by Ferroxcube company 3C90(TX25/15/10)and EPCOS company N87(TR36/23/15)are selected to construct a core loss testing platform based on average input power.The core losses of N87 cores at f of 100k Hz and 200k Hz and B_p of 50m T and 100m T were tested respectively.The core loss of 3C90 is calculated under the combined condition that f is 50k Hz,100k Hz,200k Hz and B_p is 153m T.In this paper,dielectric loss fitting function is constructed on the basis of experimental data,and dielectric loss component is included on the basis of traditional core loss separation model,which can effectively reflect the core loss characteristics under the zero-voltage excitation waveform.The experimental results are in good agreement with the circuit simulation results,which verifies the correctness of the proposed method.