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8/12/2019 Ferrites - The Most Important Properties
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Ferrites - "The Most Important Properties"
1994 Soft Ferrite Users Conference
By : George OrenchakFerrite International Company
http://www.docudesk.com/8/12/2019 Ferrites - The Most Important Properties
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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