Design_World_-_Internet_of_Things_Handbook_April_2020

12 DESIGN WORLD — EE NETWORK 4 • 2020 eeworldonline.com | designworldonline.com

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content typically makes them 5-25% less costly
than MPP. High fl ux cores have a higher core
loss than MPP and Kool Mμ. But they have a
higher Bsat which leads to a low inductance shift
under high dc bias or high ac peak current. Like
MPP cores, high-fl ux cores are generally toroid-
shaped only.
Core manufacturers may mix proprietary
combinations of materials to produce cores
with special qualities. Examples include Kool
Mμ (or, sendust) cores. These are distributed
air gap cores employing iron, aluminum, and
silicon alloy powder. Kool Mμ material has dc
bias performance resembling that of MPP. But
the absence of nickel in the formulation helps
keep the cost down. The main trade-off is that
Kool Mμ has ac losses exceeding those of
MPP. It is designed for use when iron powder
is too lossy, typically because the frequency is
moderate or high.
Another proprietary formulation is the Xfl ux
distributed gap cores made from a silicon-iron
alloy powder. The XFlux material exhibits slightly
better dc bias performance than High Flux
cores and much better than than that of MPP
or Kool Mμ. Again, the absence of nickel in the
formulation helps keep down costs. But XFlux
has higher ac losses than High Flux. It targets
applications where iron powder is too lossy or
lacking dc bias or where nickel alloys are too
expensive or lack dc bias.
Iron-powder cores have higher core
losses than MPP or Kool Mμ but generally

INDUCT

ANCE

CURRENT FERRITE

CURRENT

POWDER CORE

CURRENT

FERRITE

The gapped ferrite must be kept a safe

distance away from the sudden rolloff.

Small shifts in the rolloff curve, or in the

operating point, could have a distastrous

effect. This curve shifts to the left with

increasing temperature.

The powder core is safely

designed to operate part way

down the curve. The curve

does not shift appreciably

with increasing temperature.

POWDER CORE

So saturation e ects

A graph from Magnetics Inc. showing

how powder materials saturate gradually

and still maintain a useful, predictable

inductance even at high current loads. A

gapped ferrite will maintain an inductance

closer to the unbiased value until saturation,

at which point inductance suddenly drops.

References

Magnetics Inc., http://www.mag-inc.com/

Ferroxcube Inc. (Div. of Yaego), http://www.ferroxcube.com/

Micrometals Inc., http://www.micrometals.com/

Amidon Inc., http://www.amidoncorp.com/

Fair-Rite Products Inc., http://www.fair-rite.com/

TDK U.S.A., http://www.tdk.com/ferrites.php/

cost less. Iron powder tends to fi nd use when

the frequency is quite low or when the ac

ripple current is minimal (resulting in fairly low

ac losses). Most iron-powder cores contain

an organic binder that can eventually break

down in high temperatures, so thermal aging

qualities (available from published curves) are

a consideration. Iron-powder cores come in a

variety of shapes including toroids, E-cores, pot

cores, U-cores, and rods.

Gapped ferrite cores are marketed as an

alternative to powder cores. Powder materials

saturate gradually even when the current

load rises signifi cantly. A gapped ferrite will

maintain an inductance closer to the unbiased

value until saturation, where inductance

suddenly drops. Another point to note is

that the fl ux capacity of any power ferrite

drops signifi cantly as temperatures rises while

the fl ux capacity of powder cores remains

essentially constant over temperature.

The operating point of powder cores

doesn’t shift much with temperature or material

tolerances. And these cores have a

natural swinging inductance – high L

at low load, controlled L at high load.

Finally, powder cores not susceptible

to fringing losses and gap EMI effects,

and that they have higher inherent Bsat

levels than ferrites.

Finally, there is a small change

in dimension (generally on the

order of a few parts per million)

when a magnetic material is magnetized. The

effect is called magnetostriction. The resulting

mechanical motion can produce an audible hum

if it takes place in the audio range. Magnetic

materials that include Permalloy 80, KoolMμ and

MPP powder cores have low magnetostrictive

properties and frequently get specifi ed when

audible noise is a possibility.

CORE SIZE

There are two dimensions that primarily impact

the size of a magnetic core: the core window

(winding) area and the core cross¬ sectional

area. The product of these two is generally

called the area product, or WaAc and relates

to how much power the core can handle. The

larger the WaAc, the higher the power capacity.

The area product can drop as operating

frequencies rise, thus reducing the necessary

core size. Core suppliers often publish fi gures

for the area products of their products.

Curie temperature is the temperature

at which a material loses all of its magnetic

properties and thus become electrically useless.

Many cores incorporate an insulated coating

which melt well below the Curie temperature.

Similarly, exposure to the Curie temperature

permanently alters the qualities of tape-wound

cores. Tape-wound cores and powder cores

generally have Curie temperatures exceeding

450°C, but their materials can oxidize well

below this temperature. Ferrites, however, have

low Curie temperatures (120 to 300°C) and

temperatures somewhat above these levels

won’t alter the structure of the ceramic material.

In general, the core magnetic properties return

when the temperature drops below the Curie

temperature as long as the material hasn’t

oxidized or been held at high temperature for

extended periods.