development of a new soft ferrite core for power applications

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jmmm

Journal of Magnetism and Magnetic Materials 300 (2006) 500–505

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evelopment of a new soft ferrite core for power applications

Anjali Vermaa, M.I. Alama, Ratnamala Chatterjeea,�,T.C. Goelb, R.G. Mendirattac

aDepartment of Physics, Indian Institute of Technology, New Delhi 110016, IndiabBirla Institute of Technology & Science, Goa Campus, Zuari Nagar 403726, Goa, India

cNetaji Subhas Institute of Technology, Dwarka, New Delhi 110045, India

Received 15 March 2005; received in revised form 20 May 2005

Available online 15 August 2005

tract

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s with

. The

tivity,

ed at

Manganese-substituted nickel–zinc ferrites have been investigated as power core materials for applicatio

switched-mode-power supplies. High frequency operation of these power supplies requires high performance core

low power losses. The main contributors to the power loss are eddy current loss, hysteresis loss and residual loss

ferrites have been synthesized by the citrate precursor technique and their electromagnetic properties such as resis

permeability, saturation magnetization and Curie temperature studied. A power loss of 500mW/cc could be obtain

a frequency of 500 kHz, flux density of 50mT and temperature 100 1C.

r 2005 Elsevier B.V. All rights reserved.

PACS: 75.50.Gg

Keywords: Ferrites; Citrate precursor technique; Electromagnetic properties; Power loss; Temperature

Introduction

sizent osup1–3]ronicssor

to TV and video tape-recorders, the demand fornt ofly onies actionn ofo uselown of

In response to the current demand forreduction of electronic devices, the developmecompact and efficient switched-mode-powerplies has received considerable attention [Power supplies being an integral part of electequipments from computers and microproce

�Corresponding author. Tel: +9111 26591354;
rritesand
4-8853/$ - see front matter r 2005 Elsevier B.V. All rights

10.1016/j.jmmm.2005.05.040

+9111 6862037/26581114.

E-mail address: [email protected] (R. Chatterjee)

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ferrite cores, which are an essential componethe switching power supplies, is continuousthe rise. As the ferrite transformer core occupsubstantial volume of the power supply, reduin the core size is essential for miniaturizatiothe power supply. One way to achieve this is thigh switching frequencies, which requirespower loss cores to afford the transformatiohigh power.The main types of losses encountered in fe

are the eddy current loss, hysteresis loss

reserved.

.

www.elsevier.com/locate/jmmm

residual loss. Consequently the requirements of aeddyducecy toly oconse inouldransmay

thehighn alowd toe usehichthemresisbilityoachnickrmees o–ZnBothdivits onixedfindoweausethe

al tostatees iare

penicaarastoiwersok fog ofina

step in the processing of ferrites, one advantageeringse ofworkssingtionsixed-tes is

2. Experimental

withtrate[13].s ofzincerck,DH,wasrritevinylapese ofnessuterhick-for

per-ukerents,xideelec-partsjm00i ),encynaly-mea-mpleuriethodmea-lyzerto alifier

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A. Verma et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 500–505 501

power ferrite are high resistivity to keep thecurrent losses low, high permeability to rehysteresis losses and a high resonance frequenreduce the residual losses, which consist mainresonance-relaxation losses. The other majorsideration is decrease in power loss with increatemperature. A minimum in power loss shoccur at the operating temperature of the tformer to prevent thermal runaway, whichoccur due to heating of the core.

Manganese–zinc ferrites have hitherto beenmain power materials on account of theirpermeability [4–6]. However, their applicatiohigh frequencies is limited due to theirresistivity. The main method usually adopteimprove the resistivity of Mn–Zn ferrites is thof certain additives such as CaO and SiO2 wsegregate at the grain boundaries renderinginsulating. Although this method improvestivity, it may result in reduction of permeaand increase of hysteresis loss. Another appris to use a high-resistivity ferrite such asel–zinc. These ferrites however, have lower peability. In order to exploit the best propertiboth the ferrites, mixed combinations of Mnand Ni–Zn ferrites need to be investigated.these ferrites have been extensively studied indually [7–10], but there are hardly any reporthe power loss measurements of the mcombinations [11]. The aim of this work is tothe optimal chemical composition with low ploss at high frequencies. High power loss can cthe core temperature to rise and approachCurie temperature, which can prove detrimentthe ferrite performance. Also as the steady-operating temperature of most power devicaround 100 1C, higher Curie temperaturesrequired.

Because ferrite properties are strongly dedent on the preparation process, wet chemmethods are gaining prominence for the preption of homogeneous and compositionallychiometric ferrites [12]. In view of this, achemical method known as the citrate precumethod [13] has been used in the present worthe preparation of ferrites. Because sinterinpowdered compacts is the most important

f-

-

with use of Ni–Zn ferrites is that the sintatmosphere need not be nitrogen as in the cathe Mn–Zn ferrites [14]. The ferrites in thishave been sintered in air. The higher procetemperatures and prolonged sintering duranormally involved in the commonly used moxide route [15] for the preparation of ferriavoided by the use of the citrate method.

t

-

--f

-

r

s

-l--trrfl

MnxNi0.5�xZn0.5Fe2O4 compositionsx ¼ 0:1, 0.2 and 0.3 were prepared by the ciprecursor technique as described elsewhereThe raw materials used were the nitratemanganese (98.5%), nickel (99.0%) and(98.5%), citric acid (99.5%) (all from MGermany) and iron (III) citrate (99%) (BUK). The dried citrate precursor mixturecalcined at 500 1C in air to obtain the fepowder. This powder was granulated in polyacetate binder and compacted into different shsuch as disc and torroidal. The discs werdimensions 13mm diameter and 1.5mm thickwhile the torroids had dimensions of 16mm odiameter, 8mm inner diameter and 2.5mm tness. These samples were sintered at 1400 1C1h in air.The X-ray diffraction measurements were

formed in a powder X-ray diffractometer (BrAXS, Germany). For the electrical measuremthe discs were polished to remove the upper olayer and then coated with silver paste fortrical contacts. The real (m0i) and imaginary(m00i ) of the complex permeability, (mi ¼ m0i �were measured on torroidal cores in the frequrange 10 kHz to 13MHz in an Impedance Azer (HP 4192 A). Saturation magnetizationsurements were performed on a Vibrating SaMagnetometer (EG & G Princeton) and the Ctemperature was measured by a simple medescribed by Soohoo [16]. The power losssurements were carried out with a Power Ana(Norma model D 5235) operating upfrequency of 500 kHz and a Power Amp(ENI model 1040 L).

3. Results and discussion

thecubicinedd ataluesrkedcatesTheple, areof ‘anderionsighetticeon i

foseentes ie o

resistivity normally reported for the Mn–Zn2 that

7,18].r thewithainlyFe+2

stal-ns inn thethe

s thetatesctronalents due

ARTICLE IN PRESS

Table 2

Properties of MnxNi0.5�xZn0.5Fe2O4 ferrites

Ferrite composition x r (O cm) 4pMs Gauss Tc (1C) mi

0.1 6.4� 104 3000 322 400

0.2 1.2� 104 3800 323 885

0.3 8.9� 103 2700 312 180

A. Verma et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 500–505502

X-ray diffraction analysis revealed thatsamples crystallized in the single-phasespinel structure. The X-ray diffractogram obtafor Mn0.2Ni0.3Zn0.5Fe2O4 powder calcine500 1C is shown in Fig. 1. The (h k l) vcorresponding to the diffraction peaks are main the figure. The broadness of the peaks indithe fine crystalline nature of the powder.lattice parameters ‘a’ for the sintered samcalculated from the (3 1 1) diffraction peakshown in Table 1. The increase in the valuewith increase in Mn substitution can be ustood in terms of the ionic radii of Mn and NiBecause the ionic radius of Mn, 0.91 A, is hthan that of Ni, 0.78 A, the increase in the laparameter with increase in Mn concentratiexpected.

The electromagnetic properties obtainedMn–Ni–Zn ferrites are shown in Table 2. Asin the Table, the DC-resistivity, r, of the ferriof the order of about 104O-cm. The valu

hichs due%) ainel.rites.ionse the

ob-x ¼

ationionsl (B)ribu-f thenetn+2

he As arealsouallyively.pinel

Fig. 1. X-ray diffractogram of Mn0.2Ni0.3Zn0.5Fe2O4 powde

calcined at 500 1C.

Table 1

Lattice parameter ‘a’ for MnxNi0.5�xZn0.5Fe2O4 ferrites

Ferrite composition x Lattice parameter, ‘a’(A

0.1 8.365

0.2 8.384

0.3 8.398

s

’-.r

s

r

sf

ferrites is of the order of 10 O-cm, whilereported for Ni–Zn ferrites is 106O-cm [1Hence the value of 104O-cm obtained foMn–Ni–Zn ferrite compositions, is consistentthese results. Conductivity in ferrites is mattributed to electron hopping between theand Fe+3 states of iron on equivalent crylographic sites in the spinel lattice [19]. Iovalence states different from those required iideal spinel structure are formed duringsintering process. In case of Mn–Zn ferriteprobable presence of the Mn+2 and Mn+3 salso adds to the conductivity due to elehopping between the divalent and the trivmanganese ions. The presence of Mn+3 ions ito electron transfer from Mn+2 to Fe+3 wresults in the formation of Fe+2 ions. This ito the fact that manganese ferrite is partly (80normal spinel and partly (20%) inverse spThis results in the lower resistivity of these ferOn the other hand, in Ni–Zn ferrites, Ni+2

are not so easily oxidized to Ni+3 and hencresistivity in these ferrites is high [20].The saturation magnetization, 4pMs, is

served to be highest for the composition with0:2 as seen in Table 2. Saturation magnetizdepends on the type and the number oflocated at the tetrahedral (A) and octahedrasites in the spinel structure because this disttion affects the magnetizations, MA and MB oA and B sub-lattices, respectively. Themagnetization is given as MB–MA. The Zand Ni+2 ions have strong preferences for tand B sites, respectively, while the Fe+3 iondistributed over both the sites [21]. Mn+2 ionstend to occupy both the sites, the ratio usbeing 80:20 for the A and B sites, respectWhen the Mn ions are introduced in the s

r

)

lattice at the expense of the Ni ions, the Mn whichs theThint oe+

ThureaseitionalueMn

ationmbeB–BThiffectionthe

s thes itbovese T

–Zn, othanoundthe

d a

, mi

posie oty oZn0.ixedhose

obtained for the pure Mn–Zn and Ni–Zn ferrites,r theitionbilityy the

(1)

fieldion

(2)

ubichighlowwithalue.the

y theicalrritethe

] andwereores.posi-thesemea-uredtionsand

, thewestthat

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has a strong preference for the A sites, forcemigration of Fe+3 ions from the A to B sites.results in an increase in the magnetic momethe B sub-lattice, as the magnetic moment of Fions is higher than that of the Ni+2 ions [21].the B sub-lattice magnetization seems to incwith increasing Mn content up to the composwith x ¼ 0:2 as indicated by the higher 4pMs vfor this composition. Further addition of(x ¼ 0:3) results in increase in the Fe+3 migrfrom A to the B sites. The increase in the nuof Fe+3 ions at the B sites gives rise tocoupling, resulting in spin canting [21].reduces the B sub-lattice moments with the eof an overall decrease in the net magnetizaThis explains the lower 4pMs obtained forcomposition Mn0.3Ni0.2Zn0.5Fe2O4.

The Curie temperature, Tc, which representtemperature above which the ferrite loseferrimagnetic property, is measured to be a300 1C for all the compositions, Table 2. Thevalues are higher than those reported for Mnferrites, 150 1C, and Ni–Zn ferrites, 275 1Csimilar compositions [22] and are also higherthe temperature rises usually encountered (ar100 1C) in power transformers. This allowsmixed Mn–Zn and Ni–Zn ferrites to be usepower cores.

A look at the initial magnetic permeabilityof the ferrites in Table 2 shows that the comtion with x ¼ 0:2 exhibits the highest valupermeability. The reported initial permeabiliNi0.5Zn0.5 ferrites is 250, while that for Mn0.5ferrites is 1700 [23]. The values of mi for the mcompositions are expected to be in between t

Table 3

Power loss of MnxNi0.5�xZn0.5Fe2O4 ferrites under different c

Ferrite composition x Total power loss mW/cc

25 kHz, 200mT 100 kH

25 1C 100 1C 25 1C

0.1 492 492 1915

0.2 289 289 1445

sf3

s

r

st.

s

c

f

s

,-ff

5

which indeed is observed to be the case focompositions x ¼ 0:1 and 0.2. The composwith x ¼ 0:3, however shows a lower permeavalue. The permeability of ferrites is given brelation [24]:

mi �2Ms

3Ha,

where Ha is the induced crystal anisotropywhich for a cubic system is given by the relat

Ha ¼2K1

Ms,

where K1 is the anisotropy constant for the csystem. Hence, for a low Ms value, Ha iswhich means that mi would be low. Thus, thepermeability obtained for the compositionx ¼ 0:3 is in accordance with its lower 4pMs vThe permeability is greatly dependent on

chemical composition of the ferrite, especiallzinc content. Hence the choice of the chemcomposition of the ferrite is crucial. The fewith 50mol% of zinc is found to exhibithighest permeability at room temperature [23hence compositions with zinc 50mol%chosen for the preparation of power ferrite cAs the permeabilities exhibited by the com

tions with x ¼ 0:1 and 0.2 were higher,compositions were subjected to power losssurements. Table 3 gives the power loss meason these cores under different operating condiof the driving flux density, temperatureswitching frequency. As seen in the tablecomposition with x ¼ 0:2 exhibits the lopower loss. This is a consequence of the fact

onditions of frequency, magnetic field and temperature

z, 200mT 300 kHz, 100mT 500kHz, 50mT

100 1C 25 1C 100 1C 25 1C 100 1C

1915 2353 2408 1039 930

1387 1502 928 635 500

this composition is found to exhibit the bestationbilitythe

(3

mplis thethe

miÞ ithe

andencykHzf thees oeresilos

e thecomeThef thet andf thetan dmpletha

there is little variation in m0i at lower frequenciesigherf thes duey thepliedtan dpeak,eaterf theed torritesanceneti-ptionbeenancethe

ikely.z canss atneticloss

com-with

lossre ofhichthe

m innt onropy

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A. Verma et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 500–505504

properties of resistivity, saturation magnetizand permeability (Table 2). High permeaimplies low hysteresis losses according torelation [25]

oh ¼4nB3

ampl

3m3�f ðmiÞJm�3,

where Bampl is the material magnetization atude, n is a hysteresis loop constant, mo imagnetic permeability of vacuum, mi ismagnetic permeability of the material and f ð

a function proportionally dependent onmagnetic permeability.

According to Table 3, for all frequenciestemperatures power loss increases with frequup to 300 kHz and then decreases at 500Although the total power loss is a result osimultaneous contribution of all the three typlosses namely, the eddy current loss, the hystloss and the residual loss, the dominantmechanism at lower frequencies seems to bhystetresis loss as the eddy current losses bemore significant at higher frequencies [26].residual losses arise due to the superposition otwo components, the domain wall movemendomain rotation [8]. A plot of the real part ocomplex permeability m0i, and loss factor,(m0i=m

00i ) as a function of frequency for the sa

with x ¼ 0:2, is shown in Fig. 2. It can be seen

pera-ture.the

ccurshichum.hibit80 1Clexitylumen anatured bythe

ls atFig. 2. Frequency variation of initial perrmeability (m0i) and los

factor (tan d) of Mn0.2Ni0.3Zn0.5Fe2O4 .

)

-

s

.

fss

t

before it starts decreasing rapidly at hfrequencies. The loss factor tan d is low, oorder of 10�2 at lower frequencies and ariseto the domain walls, which under excitation bdriving AC field are not able to follow the apfield instantaneously. The rapid rise inbeyond about 1MHz is indicative of a losswhich probably would occur at frequencies grthan 13MHz (the maximum frequency oimpedance analyzer used) and can be attributthe magnetic resonance phenomenon in fe[27]. When the driving frequency is in resonwith the natural frequency at which the magzation rotates, a large peak in power absoroccurs. As the power loss measurements havemade at frequencies lower than the resonfrequency, significant contributions due toresidual loss to the total power loss are unlThe low power loss values obtained at 500 kHbe attributed to a decrease in the hysteresis lothese frequencies due to the low applied magfield. It seems that the decrease in hysteresiswith decrease in magnetic field more thanpensates the increase in the eddy current lossfrequency.It is observed in Table 3 that the power

obtained is lower at a measurement temperatu100 than at 25 1C. The temperature at wminimum power loss occurs depends ontemperature at which the secondary maximupermeability occurs which in turn is dependethe ferrite composition [23]. The anisotconstant k1 of the ferrite decreases with temture until it becomes zero at a certain temperaThis is the same temperature at whichsecondary maximum in permeability o[4,28]. Hence, it is also the temperature at wthe power loss exhibited by the ferrite is minimMn–Zn ferrites are normally designed to exthe minimum power losses at around[3,4,29–31]. However, with the design compof modern electronic devices, the high voloading of electronic components results iincrease in the steady-state operating temperof these devices. The low power loss exhibitethe Mn–Ni–Zn ferrites at 100 1C showspotentiality of these ferrites as power materia

s

higher temperatures and at a reasonably highss ofno

owe

[6] G.-M. Jeong, J. Choi, S.-S. Kim, IEEE Trans. Magn. 36

(5) (2000) 3405.

tions,

ys. 87

gn. 37

ARTICLE IN PRESS


frequency of 500 kHz. The probable usefulnethese ferrites at still higher frequencies couldbe ascertained due to the limitation of the panalyzer used.

4. Conclusions

of angahede oy oT inowere ostateoweus, aandctive

lkarni,

2211.

.

Eds.),

ce on

New

upta,

81 (7)

erials,

rrites,

3868.

tions,

ys-Bas

s, Ox-

The present work reports the developmentnew power ferrite material based on the manese–nickel–zinc ferrite composition for switcmode-power supplies. A power loss valu500mW/cc could be realized at a frequenc500 kHz and a driving flux density of 50mcores of small dimensions. This low value of ploss is obtained at a measurement temperatu100 1C, which is higher than the steady-temperature of 80 1C at which minimum ploss is usually reported for Mn–Zn ferrites. Thsuitable combination of ferrite compositionthe processing technique results in a prospematerial for the desired application.

Acknowledgements

ce &.

brary,

brary,

brary,

brary,

Financial help from Department of ScienTechnology, Govt. of India, is acknowledged

References

Eds.)

ce on

IBH

(Ed.)

ce on

Amer

bata

Sixth

yoto

1173

36 (5

ranc

tions,

1 (10)

brary,

Eds.),

ce on

owder

stava,

tional

xford,

IEEE

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development of a new soft ferrite core for power applications

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