对新结构的高频间隙电感的设计和绕组损耗机理的分析

4036IEEE TRANSACTIONS ON MAGNETICS,VOL.41,NO.10,OCTOBER2005 Winding Loss Mechanism Analysis and Design for New Structure High-Frequency Gapped InductorXingkui Mao1,Wei Chen1;2,and Yunxiu Li2College of Electrical Engineering and Automation,Fuzhou University,Fujian Province,350002,China Research and Development Center,Delta Electronics(Shanghai)Company Ltd.,Pudong,Shanghai,201209,China Winding eddy current loss mechanism of high-frequency gapped inductor is studied deeply by two-dimensionalfinite element analysis. It reveals that concentrated distribution of the magneticfield in the winding window due to the air gap obviously leads to higherflux strength and then higher winding eddy current loss.Based on the analysis,a new simple inductor structure is proposed to make thefield in the winding window more uniform by introducing a low permeability magnetic shunt between the gapped core leg and the winding, resulting in a significant winding loss reduction.Moreover,field analysis and design for the new structure are also performed.Index Terms—Eddy current loss,inductors,magnetic devices,power conversion,winding.I.I NTRODUCTIONI NDUCTORS have enormous applications in high-frequencyswitch-mode power supply for energy storage andfiltering. For inductors,all magneto-motive force(MMF)produced by exciting current must be supported in magnetic core or air gaps. Low permeability(permeability)cores have benefits of lower winding loss because no air gap is needed and the magnetic field in the winding window is more uniform.Unfortunately, low permeability cores,whether they are iron powder,high-flux,Kool-mu,or molypermalloy powder(MPP),exhibit much higher core loss than that of the ferrite cores in large alternating current(ac)flux density and high frequency applications,such as resonant inductors and power-factor correction(PFC)induc-tors working in discontinuous construction mode(DCM).So high permeability ferrite cores gapped with air gap are usually adopted in such applications for reducing core loss.But an in-ductor constructed with high permeability ferrite gapped core used in high-frequency switch-mode power supply will lead to high extra winding eddy current loss due to the introduction of air gap,resulting in low efficiency and high temperature rise in the inductor.In order to reduce this extra winding loss,many techniques were proposed,including winding shape design[1], core dimension consideration[2],segmenting single air gap[3], staggering air gap arrangement[4],and distributing air gap by combination of high permeability and low permeability mag-netic core[5].But all these techniques make the winding and core structures much more complex.In this work,by two-dimensional(2-D)eddy currentfinite element analysis(FEA),winding eddy current loss mechanism in the inductor constructed with ferrite core and air gap is deeply investigated and then a new very simple inductor structure is proposed to reduce the winding loss greatly.Moreover,analysis and design for the new structure are also performed.Digital Object Identifier10.1109/TMAG.2005.854991Fig.1.Magneticfield of gapped inductor by2-D FEA(halfcore).Fig.2.Simplified magneticfield of gapped inductor(half core).II.W INDING L OSS M ECHANISM A NALYSIS FORH IGH-F REQUENCY G APPED I NDUCTORFlux distribution of an inductor constructed with high per-meability ferrite core and air gap in central core leg workingat frequency300kHz is shown in Fig.1byfield simulation.It can be seen that thefluxes can be classified into three por-tions:mainfluxes which pass through high permeability mag-netic core and air gap,fringingfluxes which diffuse into thewinding window near the air gap,and bypassfluxes which passacross the winding window between the two adjacent legs,as inFig.2.Among thesefluxes,mainfluxes corresponding to mostof the stored magnetic energy of the inductor do not penetrateinto winding conductors and induce no extra winding loss.But 0018-9464/$20.00©2005IEEEMAO et al.:WINDING LOSS MECHANISM ANALYSIS AND DESIGN4037Fig.3.New structure inductor proposed (half core).the other two-portion fluxes will induce high eddy current loss at high frequency because they cut into winding window.In mech-anism investigation on the winding eddy current loss,the effects by these two-portion fluxes are focused for study.It is well known that magnetic fluxes are caused by MMF applying across magnetic path.In an inductor gapped with air gap,almost all MMF or ampere-turn product (NI)should drop across the air gap.So,the fringing fluxes near air gap are caused by MMF produced by all inductor current.But bypass fluxes are caused by MMF applying across between the two adjacent core legs.This MMF producing bypass fluxes increases with in-crement of winding ampere-turnproductand drop across air gap along winding window height,so staggering air gap ar-rangement is helpful in reducing the winding loss [3].It is just the concentrations of the two MMF due to the air gap result in the concentrations of the magnetic fluxes,including fringing and bypass fluxes,in the winding window and then much higher winding eddy current loss at high frequency.So finding some new and simple structure to distribute the two MMF and make magnetic field more uniform in winding window will reduce the winding loss.III.N EW S TRUCTURE TO D ISTRIBUTE M MF FORH IGH -F REQUENCY G APPED I NDUCTORThe new structure proposed is shown in Fig.3.A thin thick-ness and low permeability magneticmaterial is introduced into between the gapped core leg and the winding to form two parallel magneticpathsand for all fluxes to pass through.With this structure,MMF producing by all current is dis-tributed along magneticpathconsisting of air gap Acf1,Acf2,and low permeability magnetic material ,while the MMF is concentrated along magneticpath consisting of high permeability magnetic core and airgap .So,magnetic filed in winding window will become more uniform by the introduction ofpath ,resulting in great winding loss reduction.Moreover,path supports the main fluxes or most of the magnetic energy,whilepathjust acts as a magnetic shunt to support a very small portion of the fluxes,including fringing fluxes and bypass fluxes.So,a very small extra core loss is introduced in magnetic path .Low permeability magneticmaterial can be available by flexible ferrite polymer composites (FPC)provided by EPCOS or other vendors and then the new structure can be very easilyconstructed.Fig.4.Example inductor of the new structure under studying (halfcore).Fig.5.Normalized winding loss of new structure versus of the material F and length d of air gap Acf1,Acf2at frequency 300kHz.Fig.6.Magnetic field strength along the surface of the most left winding layer (line A-A)at =20for the material,length d =5mm for air gap Acf1,Acf2,and frequency 300kHz.IV .A NALYSIS AND D ESIGN FOR THE N EW S TRUCTURE In order to achieve the more uniform magnetic field in the winding window and minimize the winding loss,factors af-fecting the MMF along magneticpath or the design parame-ters,such as permeability value and size of the magnetic mate-rial need to be analyzed and properly designed.An example inductor,as in Fig.4,is used to study the new structure by Ansoft 2-D FEA.The simulation results in Fig.5show the normalized winding loss versusrelativeand the size of the magneticmaterial at frequency 300kHz.Thelength of air gap Acf1and Acf2indicate the size here.In Fig.5,the normalized winding loss is with respect to thesewithout the magnetic material .Withof the material ,the minimum normalized winding loss are only 0.41atlengthmm,and the magnetic field strength in winding window is shown in Fig.6,showing more uniform magnetic field of the new structure compared with that of the structure without ma-terial .Although winding loss reaches its minimumat mmand from Fig.5,here is set to be 5mm because4038IEEE TRANSACTIONS ON MAGNETICS,VOL.41,NO.10,OCTOBER2005Fig.7.Winding loss versus distance a at =20for the material F ,length d =5mm for air gap Acf1,Acf2and frequency 300kHz.Fig.8.Winding loss versus relative permeability of the material F at length d =5mm for air gap Acf1,Acf2,and frequency 300kHz.value of flexible FPC now available is equal or less than 20andwith,the loss is minimumwith mm.So,mm is de fined in the following analysis.Inaddition,can not be zero,because some space is also needed for winding bobbin.Moreover,the MMF distribution along magneticpathis affected greatly bypath because thatdistance between the two magnetic paths is fairly small.Keeping distance between the gapped core leg and the winding constant,we get Fig.7byvarying .Fig.7shows thatsmaller will lead to less lossreduction.Especiallyduringmm,loss is even higher than that of single air gap.It is because the magneticmaterial will tend to bring more air-gap fringing fluxes to the winding conductorswhen is too small.It must be noted that the low permeability magneticmaterial can not be regarded as a magnetic shielding to prevent the air gap fringing fluxes from going into winding window [6].If function of thematerial is wrongly regarded as shielding,the higher permeability of thematerial should lead to better shielding effect and then lower winding loss.But from Fig.8,ifis too high,thematerial will lose its function to reduce the winding loss.It is because althoughhighercan reduce the field strength near airgap ,it will bring much higher field strength near the air gap Acf1and Acf2.Fig.9of magnetic fieldwithof thematerialand mm clearly shows the phenomena.So,in the new structure design,we can first setdistance equal to thickness of winding bobbin,then select fairlyhigher for thematerial to get minimum loss byvarying .How-ever,even though the winding loss reach their minimum valueat,as in Fig.8,the tradeoff between the winding loss reduction and the saturation of the magneticmaterial should also be considered in the design,because higherpermeabilityFig.9.Magnetic field strength along line A-A at =1500for the material F ,length d =5mm for air gap Acf1,Acf2,and frequency 300kHz.material is likely to attract more fluxes and become saturated.The design for avoiding saturation of the low permeability ma-terial can be achieved by considerationsof value and size of the material ordistance between gapped core leg and the material by 2-D eddy current filed simulation.For example,if flux density of thematerial is beyond the saturation value,one should modify the design with lower permeability and more thickness of the material or larger length d.V .C ONCLUSIONInvestigations on the winding eddy current loss mechanism reveal that the fringing fluxes and bypass fluxes are caused by the concentrated MMF distribution due to the air gap in gapped inductor.In the proposed new structure,low permeability mag-netic material introduced into between the gapped core leg and the winding builds an auxiliary magnetic path to make the MMF distribution more uniform in the winding window,resulting in great winding eddy current loss reduction of the high-frequency gapped inductor.With the new structure inductor,the parameter of the auxiliary magnetic path should be carefully designed to achieve the minimum winding loss.The effect of the new struc-ture inductor has been veri fied by 2-D FEA simulation.A CKNOWLEDGMENTThis work was supported in part by the Natural Science Foun-dation of Fujian Province,China under Grant A0210011and in part by the Science Research Foundation of Fuzhou University under Grant 2004-XY-02.R EFERENCES[1]J.Hu and C.R.Sulliivan,“Optimization of shapes for round-wire high-frequency gapped-inductor windings,”IEEE IAS ,pp.907–912,1998.[2]R.A.Jensen and C.R.Sullivan,“Optimal core dimensional ratios forminimizing winding loss in high-frequency gapped-Inductor windings,”IEEE APEC ,pp.1164–1169,2003.[3]W.Chen,J.He,and L.Henglian,“Winding loss analysis and new air-gaparrangement for high frequency inductors,”IEEE PESC ,pp.2084–2089,2001.[4]J.Hu and C.R.Sullivan,“AC resistance of planar power inductorsand the quasidistributed gap technique,”IEEE Trans.Power Electr.,pp.558–567,2001.[5]N.H.Kutkut and M.D.Divan,“Optimal air-gap design in high-fre-quency foil windings,”IEEE Trans.Power Electr.,pp.942–949,1998.[6]P.Wallmeier and H.Grotstollen,“Magnetic shielding applied to high-frequency inductors,”IEEE IAS ,pp.1131–1138,1997.Manuscript 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