Ferrites - "The Most Important Properties"

1994 Soft Ferrite Users Conference

By : George Orenchak Ferrite International Company

Ferrites - "The Most Important Properties" Slide 1

Inductance - Electrical property that opposes any change in current because of a magnetic field. L = ((.004) (π) (N2) (Ae) (106)) / Le (in Henries)

• Material Permeability, core dimensions & number of turns all affect inductance and 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 unit turn in nH/N2

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 saturation

Br = Residual InductionThe magnetic Induction remaining in a magnetized

material after the magnetizing force (H) has been removed-B

ua

ui

Flux per unit area induced by a field strength (H)

The externally applied force that induces magnetic

Initial Permeability at low amplitudes (ui)

Amplitude Permeability at high amplitudes (ua)

Ferrites - "The Most Important Properties" Slide 3

Initial Permeability vs. Temperature

Temperature in Centigrade

Per

mea

bili

ty

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

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

TSF-5000TSF-7070TSF-8040

• Permeability varies with temperature and drops to unity above the curie temperature.

Ferrites - "The Most Important Properties" Slide 4

Permeability vs. Flux DensityTSF-7070 material

Flux Density in Gauss

Per

mea

bili

ty

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

5 1000 2000 3000 4000

25 centigrade100 centigrade

• Permeability varies with Flux Density and drops to unity when saturated. Soft Ferrite materials saturate sooner at elevated temperatures.

Ferrites - "The Most Important Properties" Slide 5

Saturation Flux Density - The value of magnetic flux density at saturation. A materials maximum magnetic induction. B = (Erms)(108) / (4.44)(f)(N)(Ae)

Saturation Flux Density vs. TemperatureTSF-7070 material

15 oersteds

Temperature in Centigrade

Flu

x D

ensi

ty (

gau

ss)

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 decreases with increasing temperature. Often a material's Saturation Flux Density is a constraint on the minimum core size. f = frequency N= turns Ae = effective core area

Ferrites - "The Most Important Properties" Slide 6

B

H-H

-B

ungapped

ungapped

Gapped

Gapped

• The hysteresis loop shears over with increasing gap depths. The gapped structure results in lower effective permeabilities but requires more magnetizing force to saturate the core.

Ferrites - "The Most Important Properties" Slide 7

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

Gap Depth

AL

val

ue

0

1000

2000

3000

4000

5000

6000

0.0000 0.0010 0.0020 0.0030 0.0040 0.0050

•Inductance decays exponentially as the air gap increases. The slope is steep for small gaps which have large AL values. For this reason larger tolerances are needed compared to the AL tolerances for deep gaps that have shallow slopes and small AL values. AL=(4πµAeAg)/(µAeLe)+Ag(Le-LG) times (1+Lg/Square root Ae ln(2G/Lg)) to account for fringing flux

Ferrites - "The Most Important Properties" Slide 8

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

Ampere Turns

AL

val

ue

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 material saturates. Small gaps (Large AL values) saturate sooner than large gaps (small AL values)

Ferrites - "The Most Important Properties" Slide 9

Losses Loss Factor - Figure of merit of a material at low levels of magnetizing force ( Tan δ / µQ = 1 / µQ )

• Often a materials core loss characteristics is a constraint on the minimum core size.

Core Loss - A measure of the efficiency of a material at high levels of magnetizing force. Dissipated energy in the form of heat.

Ferrites - "The Most Important Properties" Slide 10

Core loss vs. Temperature

25kH, 2000 gauss

Temperature in Centigrade

mW/cc

0102030405060708090

100110120130140

20 30 40 50 60 70 80 90 100 110 120

TSF-5080 TSF-7070 TSF-8040

• A number of material grades have been designed so that their minimum core loss occurs at specific temperatures.

Ferrites - "The Most Important Properties" Slide 11

Core loss vs. Flux Density

TSF-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 with increasing Flux Density.

Ferrites - "The Most Important Properties" Slide 12

Core loss vs. Frequency

TSF-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 with increasing frequency.

Ferrites - "The Most Important Properties" Slide 13

DisaccommodationInductance vs. TimeTSF-7070 material

Time in minutes

% c

han

ge

in p

erm

eab

ility

-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 of permeability with time. Mechanical, magnetic or thermal disturbances cause the initial permeability to be raised to an unstable value from which it returns as a function of time. This process is indefinitely repeatable.

Ferrites - "The Most Important Properties" Slide 14

Ferrite Material Constants

Specific Heat 0.25 cal / g / °c Thermal Conductivity 10 x 10-3 cal / sec / cm / °c Coefficient of Linear Expansion 8 to 10 x 10-6 / °c Compressive Strength 60 x 103 lbs / in2 Young's Modulus 18 x 103 lbs / in2 Hardness (Knoop) 650 Density 4.6 to 4.8 gm / cm3