application guide presents some general guidelines for the optimum
choice of powder core materials ( MPP, Sendust, Kool Mu®, High
Flux or Iron Powder ) for different inductor, choke and filter
design requirements. The choice of one type of material over another
often depends on the following:
DC Bias Current through the inductor
2) Ambient Operating Temperature and acceptable temperature rise.
Ambient temperature of over 100 deg C is now quite common.
3) Size constraint and mounting methods ( through hole or surface
4) Costs : iron Powder being the cheapest and MPP, the most expansive.
5) Electrical stability of the core with temperature changes
6) Availability of the core material. For example, Micrometals
#26 and #52 are mainly available from stock. Most commonly available
MPP cores is the 125 permeability materials, etc.
As a result of recent advances in ferromagnetic technology, a
greater choice of core materials for design optimization is now
available. For switch mode power supplies (SMPS), inductors, chokes
and filters, typical materials are MPP (molypermalloy powder),
High Flux , Sendust, and Iron Powder cores. Each of the above
power core materials has individual characteristics suitable for
common manufacturers of the above powder cores are:
Micrometals for iron powder cores. Only Micrometals cores are
tested for thermal stability and CWS only uses Micrometals cores
in all its designs.
2) Magnetics Inc, Arnold Engineering, CSC, and T/T Electronics
for MPP, Sendust ( Kool Mu®), and High Flux cores
3) TDK, Tokin, Toho for Sendust Cores
With powder cores, high permeability material is ground or atomized
into powder. The permeability of the cores will depend on the
particle size and density of the high permeability materials.
Adjustment of the particle size and density of this material leads
to different permeability of the cores. The smaller the particle
size, the lower the permeability and better DC bias characteristics,
but at a higher cost. The individual powder particles are insulated
from one another, allowing the cores to have inherently distributed
air gaps for energy storage in an inductor.
This distributed air gap property ensures that the energy are
stored evenly through the core. This makes the core have a better
temperature stability. Gapped or slitted ferrites stores the energy
in the localized air gap but with much more flux leakage causing
localized gap loss and interference. In some cases, this loss
due to localized gap can exceed the core loss itself. Due to the
localized nature of the air gap in a gapped ferrite core, it does
not exhibit good temperature stability.
Optimum core selection is to choose the best material with a minimal
compromise while meeting all design objectives. If cost is the
primary factor, iron powder is the choice. If temperature stability
is the primary concern, MPP will be the first option. The attributes
of each type of material are discussed briefly.
the 3 types of powder cores can be purchased on-line in small
volume from stock (immediate delivery) at the following website:
More technical data of these materials can be found in www.bytemark.com
MPP (Molypermalloy Powder Cores)
MPP cores has the lowest overall core loss and best temperature
stability. Typically, inductance variance is under 1% up to 140
deg C. MPP cores are available in initial permeabilities (µi)
of 26, 60, 125, 160, 173, 200, and 550. MPP offers high resistivity,
low hysteresis and eddy current losses, and very good inductance
stability under DC bias and AC conditions. Under AC excitation,
inductance change is under 2% (very stable) for µi=125 cores
at AC flux density of over 2000 gauss. It does not saturate easily
at high DC magnetization or DC bias condition.The saturation flux
density of MPP core is approximately 8000 gauss ( 800 mT)
Compared to other materials, MPP cores are the costliest, but
highest quality in terms of core loss and stability. For application
involving DC bias condition, use the following guidelines. To
get less than 20% decrease in initial permeability under DC bias
condition:- For µi= 60 cores, max. DC bias < 50 oersted;
µi=125, max. DC bias < 30 oersted; µi=160, max. DC bias
core loss among all the powder materials. Low hysteristics loss
resulting in low signal distortion and low residual loss.
temperature stability. Under 1%.
maximum saturation flux density is 8000 gauss (0.8 tesla)
tolerance: + - 8%. (3% from 500 Hz to 200 Khz)
commonly used in aerospace, military, medical and high temperature
readily available as comapred to high flux and sendust.
High Q filters, loading coils, resonant circuits, RFI filters
for frequencies below 300 kHz, transformers, chokes, differential
mode filters, and DC biased output filters.
High Flux Cores
High Flux cores is composed of compacted 50% nickel and 50% iron
alloy powder. The base material is similar to the regular nickel
iron lamination in tape wound cores. High Flux cores have higher
energy storage capabilities, and higher saturation flux density.
Their saturation flux density is around 15,000 gauss ( 1500 mT),
about the same as iron powder cores. High Flux cores offers slightly
lower core loss than Sendust. However, High Flux's core loss is
quite a bit higher than MPP cores. High Flux cores are most commonly
used in application where the DC bias current is high. However,
it is not as readily available as MPP or Sendust, and are limited
in its permeability choices or size selections.
1) In Line Noise filters where the inductor must support large
AC voltages without saturation.
2) Switching Regulators Inductors to handle large amount of DC
3) Pulse Transformers and Flyback Transformers as its residual
flux density is near to zero gauss. With the saturation flux density
of 15K gauss, the usable flux density ( from zero to 15K gauss)
is ideally suited for unipolar drive applications such as pulse
transformer and flyback transformers.
Kool Mu® / SENDUST
Sendust cores are also known as Kool Mu® from Magnetics Inc., Sendust
material was first used in Japan in an area called Sendai, and
it was called the 'dust' core, and thus the name Sendust. In general,
sendust cores have significantly lower losses than iron powder
cores, but have higher core losses than MPP cores. Compared to
iron powder, sendust core loss could be as low as 40% to 50% of
Iron powder core loss. Sendust cores also exhibits very low magnetostriction
coefficient, and it is therefore suitable for applications requiring
low audible noise. Sendust cores has a saturation flux density
of 10,000 gauss which is lower than Iron powder. However, sendust
offers higher energy storage than MPP or gapped ferrites.
Sendust cores are available in initial permeabilities (Ui) of
60 and 125. Sendust core offer minimal change in permeability
or inductance (under 3% for ui=125) under AC excitation. Temperature
stability is very good at the high end. Inductance change is less
than 3% from ambient to 125 deg C. However, as the temperature
decreases to 65 deg C, its inductance decreases by about 15% for
µi=125. Also note that as temperature increases, sendust
exhibits a decrease in inductance versus an increase in inductance
for all the other powder materials. This could be a good choice
for temperature compensation, when used with other materials in
a composite core structure.
Sendust cores cost less than MPPs or high fluxes, but slightly
more expensive than iron powder cores. For application involving
DC bias conditions, use the following guidelines. To get under
20% decrease in initial permeability under DC bias condition:
For µi= 60 cores, max. DC bias < 40 oersted; µi=125,
max. DC bias < 15 oersted.
core loss than Iron Powder.
magnetostriction coefficient, low audible noise.
temperature stability. Under 4% from -15 'C to 125 'C
flux density: 10,000 gauss (1.0 tesla)
regulators or Power Inductors in SMPS
and Pulse transformers (inductors)
control circuits (low audible noise) light dimmers, motor speed
Iron powder is the most cost effective of all the powder cores.
It offers a cost effective design alternative to MPP, High Flux
or Sendust cores. Its higher core loss among all the powder materials
can be compensated by using bigger size cores. In many applications,
where space and higher temperature rise in the iron powder cores
are insignificant compared to savings in costs, iron powder cores
offers the best solution. Iron Powder cores are available in 2 classes
: carbonyl iron and hydrogen reduced iron. Carbonyl iron has lower
core losses and exhibits high Q for RF applications.
Iron Powder cores are available in permeabilities from 1 to 100.
The popular materials for SMPS applications are #26 (µi=75),
#8/90 (µi=35), #52 (µi= 75) and #18 (µi=55). Iron
powder cores have saturation flux density of 10,000 to 15,000 gauss.
Iron powder cores are quite stable with temperature. The #26 material
has temperature stability of 825 ppm/C (inductance change of approximately
9% with temperature change of up to l25 deg C).The#52 material is
650 PPM/C (7%). The #18 material is 385 PPM/ C (4%), and the #8/90
material is 255 PPM/C (3%).
Iron powder cores are ideal in lower frequency applications. Since
their hysteresis and eddy current core loss are higher, the operating
temperature should be limited to below 125 deg C.
For application involving DC bias conditions, the following guidelines
are recommended. To get under 20% decrease in initial permeability
under DC bias condition:
For Material #26, max DC bias < 20 oersteds;
For Material #52, max DC bias < 25 oersteds;
For Material #18, max DC bias < 40 oersteds;
For Material #8/90, max DC bias < 80 oersteds.
for low frequency application (<10OKhz).
maximum flux density: 15,000 gauss
tolerance ± 10%
Biased Inductor Operation.
frequency DC output chokes
Hz differential mode EMI Line Chokes
Factor correction Chokes.
noise filters. Able to withstand large AC line current without
20% Permeability Limits
DC Bias (Oersteds)
Under DC magnetizing conditions, all powder materials exhibit
a reduction in permeability as shown in the charts. The data above
assumes an AC flux density of 20 gauss. For application such as
output chokes, where the inductors are DC biased, the magnetization
force (H=0.4*PHI*N*l/l) needs to be calculated, and the
number of turns increased to account for the reduction in permeability.
If the magnetization force (H) calculated is within the above
maximum DC biased limits, the designer only needs to increase
the turns by a maximum of 20%.
Relative Cost Comparison Table
The relative costs of each material is based on prevailing products
pricing and raw material costs. These numbers should be used as
a guide only. In general Micrometal's Iron Powder #26 is most
cost effective, and MPPs are the costliest materials.
are many manufacturers and importers of iron powders cores, and
most of them do not exhibit the quality level as those offered