By Donna Schaefer, Engineer,
BI Technologies Magnetic
Components Division,
Fullerton, Calif.
The latest generation
of powdered-iron and ferrite materials
for inductor cores provide power designers with new options
for increased efficiency or power density in dc-dc converters.
P
ower architectures with nonisolated voltage
regulation continue to evolve, and power inductor designs are fundamental to the success of the
new product designs. The trend toward two-stage
conversion with nonisolated point-of-load (POL)
modules is fueling the demand for low-profile, high-power
surface-mount inductors with current ratings up to 25 A.
Power system design engineers have a wide variety of power
inductors at 8 A and higher to choose from when working
on a new design. This can be rather overwhelming for a
new designer because datasheets can be confusing, and
an experienced designer may not be aware of some of the
newer core materials available such as powdered iron and
ferrite materials. These materials can help reduce size, cost
and power losses.
As power densities and current levels continue to increase
and more competitors enter the market, product performance becomes critical for successful product sales. For
the magnetics design engineer seeking to minimize power
losses, the core material selection and physical package size
are critical. The job becomes even more difficult as the size
of the device decreases and the current increases.
These materials have been around for a long time, but manufacturers are continuously developing new ones. The basic
core material ingredient is iron, but it is the alloy blends or
oxide formulations and the process controls that give materials their unique performance characteristics. Even though
there are many categories, the majority of the materials are
considered powdered irons. Categories 1 through 5 in Table
1 outline powdered irons.
Each material category occupies a niche where it is the best
material for a given design application. If cost is the critical
factor in the design application, then powdered iron is the
traditional choice. Table 2 depicts a core material cost comparison using standard powdered iron as the benchmark.
The cost multiplier has been declining in recent years for
the ferrite, powdered alloy and high flux materials, because
more vendors are developing new materials and selling this
type of product. The molypermalloy (MPP) material is costrestrictive for most applications; however, it is often used for
low-volume, height-restrictive applications where a toroidal
shape is required. MPP manufacturers are starting to expand
their MPP offerings into EIR core shapes, but this product
will probably remain too expensive for most high-volume,
high-current inductor designs.
For power module designs, the inductor’s saturation
level is critical. Saturation level is typically defined as the
The Right Material for the Right Job
The most common core materials are listed in Table 1.
Power Electronics Technology October 2006
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Category number Material trade name
Composition
1
Powdered iron
Fe
2
High flux
Fe-Ni alloy
3
Powdered alloy
Fe-Si alloy
4
Sendust
Fe-Si-Al alloy
5
MPP
Ni-Fe-Mo alloy
6
Ferrite
Fe-Mn-Zn oxide
Table 1. Inductor core materials.
Material
Cost multiplier
Powdered iron
1.0
Powdered alloy
3.4 to 4.0
Ferrite (ungapped)
3.4 to 4.3
High flux
3.1
MPP
15 to 20
Table 2. Core material cost comparison.
Material
B at 25°C (G)
B at 100°C (G)
Powdered iron
11,000 to 14,000
11,000 to 14,000
High flux
15,000
15,000
Powdered alloy
9000 to 15,000
9000 to 15,000
Sendust
10,000 to 10,500
10,500 to 10,500
MPP
7000 to 7500
7000 to 7500
Ferrite
4300 to 5800
3700 to 4800
Table 3. Core material saturation levels.
overlooked in the customer’s product specification. If ambient temperatures are 70°C maximum and the inductor has a
maximum temperature rise of 40°C, then the inductor must
be able to operate above 100°C. Many inductor datasheets list
saturation current ratings at room temperature conditions
only; often the customer must specifically request current
saturation level rating information.
dc current level at which the device’s inductance declines to
75% to 80% of its nominal inductance. The dc current can
saturate the inductor quickly if an air gap is not introduced
into the core’s magnetic path. The powdered iron materials have an inherent air gap that is distributed throughout
the core, which gives them a soft saturation curve. Ferrite
material must have an air gap physically inserted or ground
between the mating surfaces of the core halves. The saturation curve is steeper and saturation is abrupt. Typical material saturation levels, or maximum flux density levels, are
shown in Table 3.
If the most critical design parameter is current saturation
level and the customer’s pc board size has either height- or
board-area constraints, a high-flux material or a powderedalloy is a good choice. Ferrite-core materials have typically
struggled in this area because their maximum flux density
levels are about one-half to one-third the level of powderediron materials. In addition, the ferrite material’s saturation
level will decrease at higher operating temperatures, while
the powdered-iron-alloy saturation level does not decrease
at higher operating temperatures. This consideration is often
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A New Ferrite Material
Within the last five years, manufacturers have introduced a new type of ferrite material that has increased the
maximum flux density level from a previous high of 5000 G to
5800 G at 25°C, and from 3800 G to 4800 G at 100°C. The
material is often referred to as a high-BSAT material. An
example of how this material can help is shown in Fig. 1.
Two devices were tested for inductance versus current
at 130°C. Both inductors were constructed with an EIR12
core and a four-turn coil with a nominal inductance of
1.2 µH. The only difference between the parts tested was the
core material. One used a high BSAT material and the other
used a standard ferrite-powder material. The difference in
saturation levels is approximately 4 A, which represents a
significant improvement in performance.
Comparing the high-BSAT ferrite material to the high-flux
material, the latter material’s saturation performance is still
15
Power Electronics Technology October 2006
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CORE MATERIALS
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Fig. 1. The effect of saturation in an inductor using a high-BSAT material
is significantly lower than that in an identical conductor using standard
ferrite-powder material.
Fig. 2. In two 1.2-µH inductors having identical structures, high-flux
core material, which saturates at 22 A, improves upon BSAT , which
saturates at 18 A.
much better. Given 1.2-µH inductor designs with an EIR9.5
core size, the high BSAT ferrite-core material will saturate at
18 A versus the high-flux material saturating at 22 A. Fig. 2
shows inductance versus current at 25°C.
The equivalent saturation performance in a ferrite part
would require the core package to be 50% larger than the
high-flux design. At 100°C, the difference in saturation levels
is even more
pronounced.
CKE-HVCDC
7-06
7/12/06 11:00 AM Page 1
High-performance voltage regulators require a fast tran-
sient response, which has increased switching frequencies
to 350 kHz and higher. Following this trend, the inductor’s
inductance value has decreased to 0.50 µH to 1.5 µH, with
peak currents pushing to 30 A and ripple currents rising
to 30% and higher. Under these conditions, core losses are
becoming a significant factor in the selection of the inductor. Most magnetics design guides recommend that the
17mm_ad 9/14/06 10:50 AM Page 1
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CORE MATERIALS
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Fig. 3. Several different core materials clearly indicate that the power
loss for each case is a function of frequency.
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Fig. 4. This graph compares 1-µH inductors using various materials,
showing the design based on ferrite materials produces lower losses
than any of those based on powdered iron.
the winding versus the core material, as copper has a higher
thermal conductivity than either ferrite or powdered iron.
Core losses are a function of frequency and flux swing, and
are defined in the following formula:
PV = k  (fx)  (By),
where PV is the core loss density (kW/m3), f is the frequency (Hz), B is the change in flux density in Gauss (G),
and k, x and y are constants derived from actual test data.
Ferrite materials have a higher resistivity than powdered-iron
materials, which permits them to operate more efficiently
at higher frequencies. Core loss will vary with the swing
in flux density, frequency and temperature. No two ferrite
materials or powdered iron materials are alike when it comes
to core-loss performance. In general, the less expensive the
material, the higher the core losses. Most empirical core-loss
data is derived using a sinusoidal waveform, which makes
the testing setup straightforward. Fig. 3 shows a comparison
of core losses per volume for various materials.
The tolerance of this data is typically 15%. This data is
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CORE MATERIALS
Molded
powdered alloy
inductor. The core in this
inductor is pressed around
the coil instead of assemDimensions:
910.55.6
910.55.6
10.511.54
12.513.56
LWH (mm)
bling two core halves. The
design parameters for the
Volume (mm3)
529
529
483
1013
inductor were as follows:
Typical DCR (m)
2.2
2.2
2.6
1.7
1 µH nominal at 0 Adc,
Table 4. Package size comparison for 1-µH inductors.
peak current equals 16 A,
based on the core manufacturer’s standard material characripple current equals 6 APK-PK. The various cores were asteristics and is taken at a flux density of 100 G at 25°C.
sembled with flat ribbon wire coils and the finished package
At switching frequencies of 300 kHz and higher, many
sizes are listed in Table 4.
power modules use ferrite-core inductor designs, requiring
Fig. 4 shows the power losses that were measured on a
significant pc board area to accommodate the inductor.
Clarke-Hesse V-A-W meter. The ferrite design has the lowest
However, there are new powdered-iron materials that perpower losses. However, the package size is much larger than
form well with relatively low core loss levels up to 500 kHz
the powdered-iron options.
and higher. The 60 perm-powdered alloy shown in Fig. 3
If board area is critical, a powdered-iron alloy may be
is close in performance to a MPP material. It is difficult to
a good choice. Conversely, if height is critical, a molded
compare core losses in various types of materials because
powdered alloy may be a good choice. However, either will
the flux-density level is inversely related to the turns and
not significantly increase the ac power losses at switching
core area. These parameters can vary because the material
frequencies up to 500 kHz. Finally, pricing is comparable
permeability levels are so different, especially between a
between all of these options.
ferrite design and a powdered-iron design. With that said, a
Given that most engineers designing voltage regulators
benchmark study was conducted to determine the difference
are looking for smaller size in both footprint and height, as
in ac power losses (coil plus core) for several powdered-iron
well as higher current-handling capability, the new ferrite
core inductor designs versus a high BSAT ferrite-core design.
and powdered-iron materials will provide new solutions to
Included in this comparison was a molded powdered-alloy
meet this challenge.
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High flux
High-BSAT ferrite
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