IEEE TRANSACTIONS ON MAGNEIICS, VOL. 29, NO. 6, NOVEMBER 1993
3532
Low Loss MnZn-Ferrites : Frequency Dependence of M i n i " Power Loss Temperature
0.Incue. N. Matsutani and K. Kugimiya
Central Res. Lab.,
Matsushita electric Industrial Co., Ltd.
Moriguchi, Osaka 570, Japan
Abstract - A simple method was applied to process, to obtain fine and homogeneous powder
separate magnetic loss into hysteresis loss, eddy without imprities [41,[81. Chemical composition of
CaO and SiOr were
current loss and residual loss. Residual loss be- the sample was Mno.742no.2Fez.,2a.
comes dominant at high frequency, and is assumed to used as basic additives. The sample was sintered at
be a main cause of the frequency dependence of the 1250°C for 3 h in N2-02 atmosphere. Electrical
minimum loss temperature. The origin of the resistivity was 5.4 R e m (dc). Magnetic loss and
residual loss is thought to be domain wall permeability ,u were measured by a B-H loop
resonance. Experiments along this hypothesis have analyzer at 50mT (AB=lOOmT).
led to new low loss ferrites with magnetic loss of
111. RESULTS and DISCUSSION
less than 100 klA/m3 at I&, 50mT and 80°C.
I. IKTRdDUcTIoN
Recently, low magnetic loss MnZn-ferrites have
been intensively studied because their high
electrical resistivity and low magnetic loss, compared to metallic magnetic materials [ll-[4], allow
for even smaller switching power supplies. In the
MHz range, however, the magnetic loss increases
even for high resistivity MnZn-ferrites, and also
other problems OCCUT.
The magnetic loss of MnZn-ferrite shows a well
known temperature dependence C31,[SI. The temperature of the minimum lass (Tmim) is preferably about
80"c, because the transformer core is usually
operated at these temperatures. However, it has
been observed that T m i m decreases far lower than
the desired temperature when the frequency is increased, especially in the MHz range.
Theoretically, magnetic loss PL is divided into
three parts, hysteresis loss Ph, eddy current loss
Pe and residual loss Pr [SI. Ph and Pe were reduced
by controlling the nano-structure of grain boundaries for polycrystalline MnZn-ferrites as already
reported [41. Pr was assumed to be ignored at high
induction levels (BdlOmT) for power applications
[31,El,[71. In the course of this study, however,
we have found that Pr is an influential factor for
the further reduction of PL and for the frequency
dependence of T m i n in the hiHz range.
11. EXPERIMENTS
Polycrystalline MnZn-ferrites were prepared by a
newly developed ceramic technique, the nano-powder
Manuscript recieved February 15, 1993.
Fig. 1 shows the temperature dependence of PL at
several frequencies. PL radically increases and
T. gradually decreases with increasing frequency.
It is generally assumed that T.i. corresponds to
the temperature of the maximum permeability Tp...,
because T P . ~ " is realized when the crystalline
magnetic anisotropy d e s zero [31,[51,[91. Fig.2
h
1500 - B m = 5 O m T
I. 5uiz
n
>
E
Y
1000'
v
,
CL
500 -
O'2o
40
'
60
'
'
80
I
100
'
120'
(C)
Temp.
dependence of magnetic loss (PL),
Fig. 1 T-rature
I . 5 Ullz.
3
j-=:-I-_I
LOO kllz
1000'
'
.
.
'
.
.
'
.
'
'
3533
shows the temperature dependence of p at 100 kHz
and 1.5 MHz. p shows maximum value at about 100°C.
The observed value of 1OO'c corresponds with T.im
at the low frequency of 100 kHz, Colt not with T m i n
at the high frequency of 1.5 MHz. Thus the discrepancy is not dissolved with the above assumption
of p , and some other effects have to be taken into
amt.
PL per cycle (PL/f) at several temperatures is
plotted against frequency and shown in Fig.3.
Linear relationships were obtained at lower
frequencies, but deviations from the linear behavior were observed at higher frequencies. The
deviation begins at lower frequencies as the temperature increases (marked by 4 in Fig.31, in good
agreement with the frequency dependence of Tmin.
Theoretically, PWf is expressed by the following
equation [41,[61.
PWf = Ph/f t Pe/f t Pr/f
=
Kh*B3-t o*n2*S*B2*f/C*pmi
t Pr/f
Frequency resonance of ferrites are composed by
domain wall, dimensional or natural resonances. We
assume that the observed resonance was caused by
domain wall movement because the resonance
frequency is considerably lower than the calculated
natural resonance frequency and because Pr was independent of the sample dimensions. The resonance
frequency f, for domain wall motion is expressed by
1.0
-
0.5
0
1.0
f
(1)
1.5
(MHz)
Fig3 Relationship between magnetic lass per cycle
(PWf) and frequency (f).
where Kh is a constant, B is the magnetic flux density, 7 is an anomaly factor, S is the cross section area, p.i is the micro electric resistivity
and C is the sample shape constant.
B m = 5 OmT:80C
PL/f at 25°C is directly proportional to f between 0.l MHz to l. 0 MHz (see Fig.3). This implies
that Pr is negligible and p.i is independent of f
0.5 according to (1). We have already reported that Pe
of the sample is independent of the macro electrical resistivity ( p 1 which shows a frequency dispersion, and that Pe is dependent on micro eddy
currents [l I, [41. Therefore, we conclude that the
0
0.5
1.0
1.5
deviations from the linear behavior correspond to
f
(MHz)
Pr as shown in Fig.4.
By the simple method described above, we have
Fig.4 Separation methcd of magnetic loss PL into
hysterisis loss Ph, eddy current loss Pe
separated PL at 80C as shown in Fig.3, into the
and residual loss Pr.
three separate loss categories as follows,
at 1.0 MHz, Ph=62klR/m3,Pe=167kW/m3, Pr= 95kW/m3
at 1.5 MHz, Ph=92kW/m3, Pe=443kW/m3, Pr=706klR/m3
1600
It is thus assumed that the large PL and the strong
frequency dependence of T m i n in the MHz range is
caused by Pr.
The frequency dispersion of p at several temperatures is shown in Fig.5. p shows a resonance
'\'* 40C
type frequency dispersion, and the resonance
25c
frequency f, decreases with increasing temperature.
The frequency at which p first increases (marked
by 4 in Fig.5) corresponds well with the deviation
*O0 0. I 0.2
0.5
1.0
2.0
frequency of the PWf vs. f plots for each temperaf (MHz)
ture (marked by 1 in Fig.3). This may be a good
indication that Pr is associated with some type of Fig.5 Frequency dependence of relative permeability p
resonance.
I
'\
3534
where Is is the saturation magnetic flux density
and C is a constant [lo]. Since Is decreases and p
increases with temperature, f, should decrease with
temperature as observed in Fig. 5.
According to (21, f, decreases as p increase,
and thus Pr is expected to increase with p . Fig.6
shows the relationship between Pr at 1.5 MHz and p
at 100 kHz and 25°C for samples sintered at various
sintering temperatures. Pr increases with p , as
expected from (2). All of these observations indicate that Pr is due to domain wall resonance.
Since Pr becornes dominant at high frequency, it
is important to reduce this resonance for realizing
lower loss materials in the MHz range. Based on
this concept, we have synthesized new low loss ferrites with relatively low ,u even at high temperature (p-1000 at 120°C). T.I. was atmt 80°C for
all the observed frequencies as shown in Fig.7. Observed losses at 80 "C were,
at 1.0 MHz, Ph=43kW/mS, Pe=19kW/m3, Pr=13kW/m3
at 1.5 MHz, Ph=68kW/m3, Pe=44kW/m3, Pr=87kW/m3
The result shows that residual loss plays an important role in reducing power loss in the MHz range.
Ph and Pe of this sample were also improved by total optimization of preparation conditions.
IV. CONCLUSIONS
The frequency dependence of the magnetic loss for
low loss MnZn-ferrites was analysed and the magnetic loss was separated into hysteresis loss, eddy
current loss and residual loss by a simple method.
Residual loss becomes dominant at high frequency
and high temperature, and creates a frequency dependence for the minimum loss temperature. The
origin of the residual loss was assumed to be the
domain wall resonance. An extremely low loss of
less than 100 kW/m3 was realized by a newly
developed ferrite based upon this concept.
ACKNOWLEDGMENT
The authors wish to thank Dr.T.Nitta and
Dr.K. Kanai for their encouragement throughout this
study, Mr. T.Maeoka, MI-.Y.Ikeda and Mr. H. Handa far
their cooperation in measuring are loss, and Mr.
0.Ishii and Mr. Y.Aono for sample preparation.
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>
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Fig.6 Relationship between residual loss Pr at 1.5 MIlz
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Y
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