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Iron-powder materials in HF applications (published in Electron #1, 2006) Introduction This chapter on iron-powder materials is an extension to the
article-series on Ferrites
in HF applications. Next to ferrites iron-powder cores are regularly
applied in comparable conditions and often regarded as just an other HF core
material. Qualities of iron powder-cores however are different enough to
dedicate an additional chapter, especially since these materials are
sometimes applied when ferrite-cores where intended, which may lead to
undesired and often dangerous situations, either to the equipment or to the
operator. Two main groups of iron-powder materials may be
distinguished for application 'around' HF circuits, identified by the
manufacturing process: 1. Electrolytic iron powder Flake-like iron particles are formed by an electrolytic
process and cut to microscopic size. This 'flake powder' is mixed with an
isolating binder material and pressed / cured to a high-density material of
the desired shape. Permeability up to 100 may be obtained by this type of
processing. Electrical losses usually will be high since the iron flakes are
sharing mutual contact points that allow relatively large Eddy-currents.
Magnetic domains are somewhat larger than in ferrite materials allowing for
higher flux density before saturation will cut in. Application of
electrolytic iron-powder materials therefore may be found at high(er) DC
/ low(er) AC current applications,
e.g. chokes in switched-mode power
supplies (SMPS). Ferroxcube is manufacturing electrolytic iron-powder
toroides, intended for application below one megahertz. Material 2P40 for
instance is characterized by permeability mi = 40 and Q = 16.7, always measured and specified at 10
kHz. and intended for low-frequency applications. These materials may also be
found in ignition coils for
fluorescent lamps. MicoMetals and Amidon also are well known sources for
this material e.g. 'mix-26' type, color-coded yellow-white, and again
intended for SMPS applications. 2. Carbonyl-iron powder Iron particles are formed in a chemical vapor deposition
process from iron-tetra-carbonyl and are much smaller that in the above
process. Again iron particles are mixed and processed with an isolating
binder material but to a lower density. This process yields a lower
permeability material but with very low loss (high Q) since iron particles
are not in mutual contact. Saturation qualities are lower than in
electrolytic iron powder and are comparable or lower than those for ferrites.
Ferroxcube used to apply these materials in tuning rods for filters in
telephony applications, especially because of the low temperature
coefficients as compared to ferrite materials. Other Ferroxcube applications
may be found in HF inductors, usually in rod-type shapes. For this type of
application permeability does not need to be too high and saturation may not
quickly be a problem in these rod-shapes. Materials Q-factor usually will be
high to over 10 MHz., allowing for inductor applications for a large portion
of the HF range. Low permeability values in general are related to the small grain size in the carbonyl process. At higher field strength the magnetic domain borders (Bloch walls) will be displaced. When these displacements are crossing the magnetic domain borders, additional energy will be lost. Therefore low permeability iron powder will saturate earlier than higher permeability materials and certainly earlier than ferrites. General carbonyl
properties At the MicroMetals
and Amidon
website most carbonyl materials may be found. Higher permeability material
(hydrogen reduced iron powder says Amidon) in the range of 35 - 100 is
recommended for low-frequency applications which is typical for electrolytic
iron powders. Carbonyl material permeability is ranging from 3 - 35, with
highest application frequencies for lowest permeability types. In general carbonyl type materials will be priced in the
ferrite range or somewhat lower since the first do not require the expensive
sintering process. Iron powder materials in general are mold-pressed after
which a mild-temperature oven process removes / hardens the binder material.
This process may differ somewhat for each manufacturer as Ferroxcube
guarantees its materials up to MicroMetals is currently one of the important iron-powder
manufacturers with a wide range of materials including a 'high performance'
range, which is their indication of carbonyl types. This is not unrealistic
since e.g. 'mix-2' type of material is specified at an initial permeability tolerance
of +/- 5 % and temperature stability of -95 ppm/C, which translates to high
precision and high stability when compared to equivalent ferrite parameters. It should be noted though Eddy currents to increase at
raised temperatures which means higher materials loss at high(er) power
applications. This may start an avalanche of increasing loss that eventually
may destroy the component; 'thermal runaway' according to MicroMetals. When comparing above 'high performance' materials to
widely applied ferrite types like 4C65 (61), permeability tolerance at the
latter is +/- 20% with a temperature coefficient between 0 and As a first conclusion it may be decided carbonyl iron powder
is the better material for HF resonant applications with the lower permeability ferrites (4C65, 61) to excel in
wide-band / high power circuits. This is in line with MicroMetals,
suggesting: " Broadband transformers with iron powder cores will
not have the wide bandwidth attainable with high permeability ferrite
cores". Also in the professional world, iron-powder materials
will not be regarded as competitors to (NiZn) ferrites as permeability of the
first is rather low and materials loss at higher flux densities is higher
than with ferrites. It should be noted that all permeability measurements are
performed at a very low measuring flux of 0,1 mT, to avoid hysteresis
effects. This is a generally accepted technique which also applies to ferrite
materials. Q-factor will be defined as the 'bare-materials' properties and
defined as μ' / μ", disregarding other loss mechanisms e.g.
copper loss. A practical inductor therefore will always exhibit a somewhat
lower quality factor. When discussing these Q-factors it should be noted that
some manufacturers may be using different definitions. Especially when using
the inverse Q as the materials 'Loss-factor', these manufacturers are
defining this to the initial permeability in stead of the permeability at
frequency! Most
manufacturers are presenting materials overviews as a selection mechanism.
MicroMetals is presenting a materials overview that gives a fair impression
of a large portion of the carbonyl iron powder field. Unfortunately US manufacturers
do not often specify permeability (μ' ) and
loss (μ") curves over frequency as
in In impression of the materials application area may be obtained from the resonant range figures. This range represents the higher Q-values, usually over 75. MicroMetals Carbonyl iron powder materials
Table 1: Carbonyl iron-powders by MicroMetals
This winding effect is not present
at the (usually much) higher permeability ferrites. Since magnetic
'resistance' is much lower in the latter, no flux will leak outside the core. Manufacturers
usually present a winding factor for a particular core shape and material
type, usually expressed in nano-Henry per turn squared (nH/n2). Some popular -
T200-2 (2", mix-2 toroide) is
specified at -
TN36/23/15-4C65 ( For
a 10 turn inductor, the first will show an inductance of around 1,2 μH,
depending on winding style (figure 1), while the second will exhibit an
inductance of 17 μH, independent of the winding style. With the To
arrive at the more technical definition of nH/n2 , the μH/100
figure should be divided by a factor of In general a number of specific
measures should be taken care of when constructing a high-Q inductor on
low-permeability materials: - Make turns stay close and next to each
other (avoid leakage inductance) - Use the core efficiently (use full winding
space). An inductor that is fully filling the winding space will have a higher Q at equal inductance as an
inductor this will not fully occupied winding space (on a differently shaped coil former) -
Mind the For all inductor types one
should note - Apply - It is not useful (and
should be avoided) to apply litze (multiple isolated strands) type of wire on
HF frequencies above 2 MHz. The gain
of the higher surface area is more than lost by the increasing parasitic
parallel capacitance of this type of wire,
which also will have a diminishing effect on system Q. - Do not apply more than one layer of wire.
Parasitic parallel capacitance will increase excessively with each new layer. Core
resistivity Resistivity for iron powder
materials is in the order of 0,5 Ohm.m., as compared to at least 50 kOhm.m. at
NiZn ferrite materials, all measured at 1 MHz. This basic resistivity
difference is showing in more than one way at practical inductors. When making a coil on an iron
powder coil-former, one should always start to apply an isolating layer
before putting on turns. Depending on the coil former finishing, edges could
be sharp and may cut through the wire coating, shortening the inductor. This
additional isolating layer may will add to the leakage flux of these low
permeability materials. The highly conductive,
iron-powder coil former will have an increasing effect on the parasitic
capacitance of an inductor on this material. Increased capacitance will lower
maximum operational frequency. Some
practical measurements Various inductors have been constructed and measured on
popular iron-powder coil formers in the carbonyl range, especially on T200-2,
T68-2 and T50-2 toroides. In table 2 some of these measurements may be found
as made on a 6 turns inductor on a T200-2 toroide.
Table 2: Measurements to mix –2 carbonyl toroide inductor Surprisingly the materials permeability (μ') is
higher than specified by the manufacturer. This permeability is nicely
constant over a wide frequency range which also shows at the inductance
column. The second observation is the low value of the equivalent series
resistor (second column), that is related to the materials loss factor of
μ". Above 20 MHz. loss is increasing and the effects of loss and
series resistance are showing in the last column for the Q-values. For this
inductor, highest quality factors will be obtained around 10 MHz. to drop off
rather sharply at higher frequencies. These measurements show this material
may be applied in resonating circuits up to 20 MHz. A series of comparable measurements have been made at a
Table 3: Measurements to a 4C65 (61) ferrite toroide Again permeability is nicely constant across a wide range
of frequencies. Up to 10 MHz. this material also is very suitable for high-Q
(low loss) applications. Above this frequency the quality factor is
dropping-off sharply and this effect is starting at a lower frequency when
compared to mix-2 material. Because of the much higher permeability, total impedance
is seven times higher at all HF frequencies and will stay that way for a long
time thereafter in spite of the lowering Q-values. This effect allows a choke
or a transformer at 4C65 material to consist of less turns for a required
impedance and therefore to also show a lower parasitic capacitance. Ferrite
materials therefore in general offer wider band-width for this type of
application. In the article on Ferrites for HF
applications we derived a formula for the maximum voltage across an
inductor on ferrite core materials, based on core loss mechanisms. With
ferrite materials the maximum induction is in the order of 300 mT, where
carbonyl materials may only be saturated at 1 T. The formula for maximum
voltage in the latter material may therefore be adapted tot this new situation,
by changing the loss-correction factor.
______________________ Umax = Ö(Pmax.core . (Q/2 + 1/Q) .
XL)
(1) with: Umax =
maximum voltage across the inductor-on-core Pmax =
maximum loss power in the core
Q = quality
factor (XL / r , also m’ / m”) XL =
reactance of the inductor After measuring a For the following calculation it has been assumed these
thermal considerations also to apply to iron-powder materials. It should be
noted though, these materials to have been constructed differently
(press-molded and dried at low(er) temperatures) and so the thermal
resistance may be different (lower). In the chapters on ferrite materials we found that
roughly above 1 MHz. maximum allowed voltage across an inductor is determined
by the material loss; at lower frequencies the maximum allowed voltage for
linear application will determine the limits. While taking thermal conditions for mix 2 and 4C65
materials to be equal we have calculated maximum allowed system power in a 50
Ohm system to be applied to a
Graph 1 is
showing 4C65 ferrite material to be applied at (much) higher system power between
1 and 30 MHz. compared to the iron-powder core. This is mainly due to the
(much) higher impedance of the inductor with the same number of turns. Only
at around 30 MHz. the two cores may handle system power equally, mainly
because of the lowering Q of 4C65 ferrite, meaning loss factors are becoming
more important. This also applies to the mix-2 material. The graph is also showing that at a maximum system power of 100 Watt, the 4C65 component to be fit for a frequency range below 1 MHz. to 30 MHz., with mix 2 to handle this amount of power only at around 30 MHz. The calculation does not take into account the higher parasitic capacitance at mix-2 material because of low core material resistivity. This will influence the high-frequency cut-off.
What if we
were to apply the bigger T200 toroide ( To start off, the impedance would go down by around 7 % because of the less favorable relation between core-area versus magnetic path length. The T220 core however has a bigger volume and since thermal resistance is scaling with the root of the volume ratio, core dissipation will go up by 29 %. Taking both effects into account, maximum allowable system power would go up by 20 %, which is low pay-off for this 40 % size increase. Increasing the
number of turns would be much more effective. The All in all graph 1 illustrates the effectiveness of high permeability materials (ferrites) in power applications. Even at low quality factors, impedances are easily much higher than the same number of turns on a iron powder core and will allow higher power / bandwidth ratio's. In general one should be very much aware when designing baluns for non-resonating antenna systems and / or in high impedance environments like symmetrical (300 - 600 Ohm) feed-lines. To still be effective, equivalent parallel inductances of the particular balun should be very high indeed. To still be efficient over a wide band width, the number of turns of this balun should be low enough to not have parasitic effects spoil the upper frequency limit, while still guarantee a high enough impedance at the lowest operating frequency. Low permeability materials therefore are not very much suited for this kind of applications with damaged components and / or transceiver as a result should this advice be neglected. At the end of our discussion on iron power materials it
may be useful to generate a global overview on the various core materials,
the specific application aria and the frequency range of choice. This may be
found in table 4.
Figure 4: Application area's for inductor core materials The table in figure 4 presents a global overview of
application area's that should be regarded in an un-dogmatical way. The
columns for m, Q, Tco and Bsat are indicating which parameter is the
more important in that particular application area, with '+' for 'important'
and '~' for 'no dominant factor'. The composition column is showing MnZn for
manganese / zinc ferrite materials, the high permeability ferrites (μ'
> 1000), NiZn for nickel / zinc ferrite materials, the lower permeability
ferrites (100 < m' < 1000), electrolytic iron for high
permeability iron-powder materials (35 <
m' < 100) and Carbonyl for low permeability iron-powder
materials (2 < m' < 25). It should be noted that each materials group represents a
large 'community' with indicated frequency limits for the best materials in
that community. In this overview the general tendency is clear for
electrolytic iron-powder materials to be found in LF applications like
switch-mode power supplies, MnZn to be applied up to the lower HF area (and
higher for choking purposes), NiZn for the greater part of the HF area (and
up to 200 MHz in choking applications) and carbonyl for the higher HF area
and above. Although most important parameter have been showed in the
table, for each application all parameters should be regarded to prevent
unpleasant surprises in the final system. Bob J. van Donselaar, |
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