Technical Bulletin BULLETIN NO. FC-S1

Ferrite CoreMaterial Selection Guide

IntroductionIn the design of ferrite core

t r a n s f o r m e r s f o r u s e inswitchmode power supplies, onemust take into considerationswitching frequency, operatingflux density, the resulting coreloss, and temperature of opera-tion. Temperature rise aboveambient is a direct result of corelosses in the ferrite material andof copper losses in the windings.To limit the temperature rise due

to core losses, the designer mustlimit the operating flux density forthe specified switching frequency.

The type of ferrite materialchosen will influence the corelosses at the given operating con-ditions. The different ferrite ma-terials are designed to have mini-mum core losses within specifictemperature ranges (see Figures1 and 2). From these curves it

can be generalized that F mate-rial has its lowest losses at roomtemperature to 40°C, P materialhas lowest losses at 70°C to80°C, R material has lowestlosses from 100°C to 110°C, andK material has lowest losses at40°C to 60°C at elevated fre-quencies. Please reference Mag-netics Ferrite Catalog FC-601 foradditional characteristics of thepower materials.

Figure 1Core Loss vs. Temperature

@ 100 kHz, 1K Gauss

Figure 2Core Loss vs. Temperature

@ 500 kHz, 500 Gauss

Performance FactorFor material selection based on

frequency, one recent trend has beento plot curves of “performance factor”(B x f) versus frequency at some de-fined core loss density. The perfor-mance factor is a measure of materialutility derived by multiplying the oper-ating frequency by the correspondingflux density level that would yield thepredefined core loss value, where:

B = Flux Densityf = Operating Frequency

MAGNETICS power materials areplotted in this manner in Figures 3through 6. By observation of thesecurves, it appears that an optimummaterial can be selected for a particu-lar operating frequency. However, thiscomparison method only yields a rela-

tive figure of merit for the chosen ma-terial, and the design engineer mustperform further analysis to determinea usable value of flux density for agiven frequency and core loss densitylevel that will limit the temperature risevalue to acceptable levels.

FERRITE MATERIAL UTILITY(B x f) vs. FREQUENCY

@ constant core loss = 300mW/cm³

Fig. 3 Fig. 4

Fig. 5 Fig. 6

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Temperature Rise ConsiderationIt should be noted at this point that

the designer is not always free tochoose the switching frequency of thepower supply so as to optimize theusage of the chosen core material.The upper limit of the frequency maybe dictated by the individual charac-teristics and economics of other com-ponents in the power supply.

As operating frequency increases,it is necessary to adjust the flux den-sity in order to limit core temperaturerise. MAGNETICS has developed aset of curves that best illustrates this.Figures 7 through 10 are based on lim-iting the core loss density to 100 mW/cm³, a figure that would keep the tem-perature rise at approximately 40°C for

USABLE FLUX DENSITY vs. FREQUENCY@ constant core loss = 100 mW/cm³

Fig. 7 Fig. 8

Fig. 9

medium sized cores. Using these

choose the flux density for his deviceat any frequency while maintaining thecore loss at 100 mW/cm³. Multipliersare provided with the curves to accountfor a higher flux density value whendesigning for a higher core loss density.

Fig. 10

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curves, the designer can quickly

@150 mW/cm³, multiply B by 1.15@200 mW/cm³, multiply B by 1.30@300 mW/cm³, multiply B by 1.45

ExamplesFor example, see Figure 8. If K

material is used at about 150 kHz at60°C, an operating flux density of 1000gauss is recommended to keep a limiton the temperature rise above anambient of 20°C to 25°C. At 500 kHzand 60°C, the design operating fluxdensity should be about 550 gauss.

As another example, if an engi-neer has a requirement for a mediumsized core used at 200 kHz, and thedesign can be operated at 100°C(75°C temperature rise above ambi-ent), what material and operating fluxdensity should be chosen? The firststep is to locate 200 kHz on the 100°Ccurves (Figure10). Keep in mind thatthe design is for a temperature rise of

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75°C. From the curves one can seethat R material will yield the highestusable flux density for 100 mW/cm³(i.e., 800 gauss). But this only gives atemperature rise of approximately40°C; a loss density of 200 mW/cm³will produce about 70°C ∆ T. There-fore, from the multipliers, the R mate-rial core can be operated at 1.30 x 800,or 1040 gauss. The design efforts cannow be centered on using R materialat the flux density in this example.

SummaryFigures 7 through 10, and the two

examples above, provide a method forchoosing among the various MAG-NETICS power ferrite materials. The

actual temperature rise for a given coreloss density will vary with core size,core geometry, winding losses and themethod of heat removal. Small coreswill develop a much lower temperaturerise than large cores at the same coreloss density, and the open shapes ofE cores will dissipate heat more readilythan enclosed shapes of pot cores.

Magnetics provides a wide rangeof power ferrites that have been opti-mized for various frequency and typi-cal temperature ranges. A completedescription of each of these materialsand the available geometries is pro-vided in Magnetics Ferrite CatalogFC-601.

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