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Ferrites - "The Most Important Properties"

1994 Soft Ferrite Users Conference

By : George OrenchakFerrite International Company

http://www.docudesk.com/

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Ferrites - "The Most Important Properties" Slide 1

Inductance - Electrical property that opposes anychange in current because of a magnetic field.

L = ((.004) ( ) (N 2) (Ae) (10 6)) / Le (inHenries) Material Permeability, core dimensions

& number of turns all affect inductanceand therefor affect component size

Ae = effective cross sectional area

Le = effective magnetic path length

N = turns on coil.

Inductance Index Al is the Inductance per unitturn in nH/N 2

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Ferrites - "The Most Important Properties" Slide 2

B

Br

HHc

Bs

- H

- B

- Hc

- Br

Hysteresis CurveB = Magnetic Flux Density

H = Magnetizing Force

flux in a magnetic material

u = B / H = Permeability

Bsat = Saturation Flux DensityThe value of magnetic flux density at

Br = Residual InductionThe magnetic Induction remaining inmaterial after the magnetizing force (removed-B

ua

ui

Flux per unit area induced by a field

The externally applied force that indu

Initial Permeability at low amplitudesAmplitude Permeability at high amplit

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Ferrites - "The Most Important Properties" Slide 3

Initial Permeability vs. Temperature

Temperature in Centigrade

P e r m e a b

i l i t y

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

-30 -10 10 30 50 70 90 110 130 150 170

TSF-5000

TSF-7070TSF-8040

Permeability varies withtemperature and drops to unityabove the curie temperature.

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Ferrites - "The Most Important Properties" Slide 4

Permeability vs. Flux DensityTSF-7070 material

Flux Density in Gauss

P e r m e a b

i l i t y

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

5 1000 2000 3000 4000

25 centigrade100 centigrade

Permeability varies with FluxDensity and drops to unity whensaturated. Soft Ferrite materialssaturate sooner at elevatedtemperatures.

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Ferrites - "The Most Important Properties" Slide 5

Saturation Flux Density - The value of magnetic flux density at saturation. Amaterials maximum magnetic induction.

B = (Erms)(10 8) / (4.44)(f)(N)(Ae)Saturation Flux Density vs. Temperature

TSF-7070 material15 oersteds

Temperature in Centigrade

F l u x

D e n s

i t y

( g a u s s

)

0

1000

2000

3000

4000

5000

6000

-30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250

Saturation Flux Density decreaseswith increasing temperature.Often a material's Saturation FluxDensity is a constraint on theminimum core size.

f = frequencyN= turnsAe = effective core area

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Ferrites - "The Most Important Properties" Slide 6

B

H-H

-B

ungapped

ungapped

Gap

Gapped

The hysteresis loop shears over withincreasing gap depths. The gappedstructure results in lower effectivepermeabilities but requires moremagnetizing force to saturate thecore.

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Ferrites - "The Most Important Properties" Slide 7

Inductance Index AL vs. Gap DepthTSF-7070-41-16-13 E core

Gap Depth

A L v a

l u e

0

1000

2000

3000

4000

5000

6000

0.0000 0.0010 0.0020 0.0030 0.0040 0.0050

Inductance decays exponentially asthe air gap increases.The slope is steep for small gaps whichhave large AL values. For this reasonlarger tolerances are needed comparedto the AL tolerances for deep gaps thathave shallow slopes and small ALvalues.

AL=(4 AeAg)/( AeLe)+Ag(Le-LG)times (1+Lg/Square root Ae ln(2G/Lg))to account for fringing flux

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Ferrites - "The Most Important Properties" Slide 8

Inuctance Index (AL value) vs. Ampere TurnsTSF-7070 material

Ampere Turns

A L v a

l u e

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 60 80 100

.005" gap

.010" gap.015" gap

Inductance rolls-off as the materialsaturates. Small gaps (Large ALvalues) saturate sooner than largegaps (small AL values)

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Ferrites - "The Most Important Properties" Slide 9

Losses

Loss Factor - Figure of merit of a

material at low levels of magnetizingforce ( Tan / Q = 1 / Q ) Often a materials core loss

characteristics is a constraint onthe minimum core size.

Core Loss - A measure of theefficiency of a material at high levelsof magnetizing force. Dissipatedenergy in the form of heat.

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Ferrites - "The Most Important Properties" Slide 10

Core loss vs. Temperature25kH, 2000 gauss

Temperature in Centigrade

mW/cc

0102030405060708090

100110120130

140

20 30 40 50 60 70 80 90 100 110 120

TSF-5080 TSF-7070 TSF-8040

A number of material grades havebeen designed so that their minimumcore loss occurs at specifictemperatures.

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Ferrites - "The Most Important Properties" Slide 11

Core loss vs. Flux DensityTSF-7070 material

Flux Density in gauss

mW/cc

0

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500 2000 2500 3000 3500

25 kH 100 kH

Core Loss increases exponentially withincreasing Flux Density.

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Ferrites - "The Most Important Properties" Slide 12

Core loss vs. FrequencyTSF-7070 material

Frequency in kHz

mW/cc

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70 80 90 100

1000 gauss 2000 gauss

Core Loss increases exponetially withincreasing frequency.

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Ferrites - "The Most Important Properties" Slide 13

DisaccommodationInductance vs. TimeTSF-7070 material

Time in minutes

% c

h a n g e

i n p e r m e a

b i l i t y

-3

-2.5

-2

-1.5

-1

-0.5

0

0 10 20 30 40 50 60 70 80 90 100

Disaccomodation is the variation ofpermeability with time. Mechanical, magneticor thermal disturbances cause the initialpermeability to be raised to an unstable valuefrom which it returns as a function of time.This process is indefinitely repeatable.

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Ferrites - "The Most Important Properties" Slide 14

Ferrite Material ConstantsSpecific Heat 0.25 cal / g / c

Thermal Conductivity 10 x 10 -3 cal / sec / cm / cCoefficient of Linear Expansion 8 to 10 x 10 -6 / cCompressive Strength 60 x 10 3 lbs / in 2 Young's Modulus 18 x 10 3 lbs / in 2 Hardness (Knoop) 650Density 4.6 to 4.8 gm / cm 3