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-, 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. 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