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Ferrites
in HF applications (published in
Electron # 9, 2001) Introduction
In general ferrites are being applied because
of magnetic-field concentrating qualities. As a consequence, inductors of comparable
value will consist of much less turns when ferrite cores are applied,
therefore acquire much less parasitic capacitance and may be applied as an
inductor over a much higher frequency range than without this core material.
Application as in wide-band transformers, baluns and EMC chokes may be
familiar. Ferrite is a ceramic product, consisting of a
composite of iron-oxide with a different metal such as manganese (Mn), zinc
(Zn), nickel (Ni), cobalt (Co), copper (Cu), iron (Fe) or magnesium (Mn). Powdered
materials are mixed and molded in an initial form and thereafter heated up to
1300 ˚C (sintered). Specific (electro-magnetic) qualities are
being obtained from a specific mixture and the heating and cooling process.
At the end of the manufacturing process, a very hard, brittle and chemically
inert component has been obtained with a more or less uniform dark grey or
black color. After this manufacturing process, the type of processing or
materials composition may not be recognised any more from the looks of the
component. Global material qualities
In electro-magnetic applications usually
compositions MnZn and NiZn are being selected with high 'field concentrating'
properties (permeability, m > 1000) for a lower frequency
range (< 3 MHz; Ferroxcube code 3xx) and lower permeability (100 < m < 1000) for the higher frequency range
(> 1 MHz; Ferroxcube code 4xx).
Later we will discuss this in more detail. Ferroxcube being the name
of the company after this department became independent of the Philips group
of companies. At first sight one would prefer high
permeability materials together with a high frequency applicability but
unfortunately these two are to some extend mutually exclusive. At the
ferrimagnetic resonance frequency, where permeability and material loss are
equal, the product of this frequency and initial permeability appear to be
more or less constant for all ferrite materials; when the application call
for a maximum frequency, the materials permeability more or less follows. Depending on manufacturer, ferrite materials
may be color coded to distinguish various types. Unfortunately such color
coding schemes are not standardized and even within one manufacturing process
the same product may vary color from batch to batch. On top of this the same
manufacturer may decide to completely change (or do away) color coding so the
specific material has to be guaranteed by the reseller or will have to be
established locally. Core materials exhibit a range of specific
electrical resistances, changing from less than a few Ω.m (iron powder,
MnZn ferrite) to (much) more than 100
kW.m (NiZn ferrite). The color
coding layer (parylene-C nylon by
Ferroxcube) therefore is also to ensure good electrical isolation to prevent
the often sharp core edges of low-ohmic materials to shorten the windings. In
case of uncoated materials, the user will have to isolate low-ohmic cores
first before winding. An other effect of low-ohmic materials may be
found in increased parasitic capacitance, lowering the maximum usable
frequency of an inductor on such core. Core materials are selected because of
permeability. This property however is temperature dependent up to 10+ 'permeability units' per ˚C
for some ferrites. This effect may be beneficial in case of a choking application
but is less desirable when operating in a (resonant) inductor. Above a
certain maximum temperature, permeability will drop sharply (Curie
temperature) and this should be avoided, unless specifically requested as an
indicator (effect is reversible). Almost all ferrites exhibit a Curie
temperature above 100 ˚C, many even above 200 ˚C. In 'normal'
situations this will not be a problem, as other components usually give up
before. Barely distinguishable from ferrites are
powder-iron cores. Permeability is lower than ferrites (2 < m < 100) but these materials tend to be
more tolerant to induced flux. For this group, flakes or powdered iron is
mixed with a binder material and cured at comparatively low temperatures.
Therefore core temperatures may not exceed about 70 ˚C to prevent
permanent deformation of shape. In contrast to ferrites, powder-iron cores
exhibit a negative temperature coefficient, making these materials prone to
thermal run-away under high load conditions. Formerly, and older materials around still
do, powder-iron cores exhibit a low qualify factor (Q < 20); this type is
applied in LF chokes, transformers and power supplies especially because of
the high flux tolerance and not so good HF qualities. More specialized
powder-iron cores (Carbonyl type) also exhibit a low permeability (m < 15) but a much higher Q - factors, up
to high(er) frequencies than other powder-iron materials. This is making
carbonyl cores very suitable for higher HF to VHF applications. Color
coatings usually are of a darker hue, this time also applied for rust
prevention. More on powder-iron
cores in a different chapter. In table 0 we may find an overview of some
regularly applied materials and their general properties. The Manganese-Zinc (MnZn) ferrite group
typically exhibits (very) high permeability (mi) and low
ferrimagnetic resonance frequency (fr), and is regularly applied
in LF systems (formerly in telephony) and for wide band EMC purposes. The Nickel-Zinc (NiZn) ferrite group exhibits
a high mi and high(er) fr and is
applied in inductors and transformers in HF frequencies, where these
materials are performing best of class. Powder-Iron group exhibits a moderate mi and low maximum
application frequency. Relatively high saturation flux tolerance makes these
materials suitable for low frequency applications like (mains) transformers. Carbonyl powder-iron group exhibits lowest
temperature coefficient (Tco) for permeability and also lowest permeability
of all (mi < 15) but with highest frequency applications.
Applications will be found in (resonant) inductors in the HF range and
transformers up to and over 100 MHz.
In the picture below an impression may be found
of shapes and sizes of ferrite core materials. This is by no means an
extensive overview of all possibilities. Furthermore, dedicated shapes are
being manufactured to customer specification, e.g. deflection yokes,
accelerator tiles, cable sleeves etc.
Ferrite toroides and inductance factors In table 1 below one may find an impression
of some well known toroide coil forms and inductance factors. The table again
is by no means an extensive overview. Colors as mentions have been used for
some time by Ferroxcube, but this manufacturer is applying a uniform beige
color now more frequently. Right below the table an example may be found on
how to apply the numbers.
Table
1: Toroides and inductance factors Applying table 1 The inductance factor
_____ _________________
n = \/ L / With this comparably low number the wire does
not have to be too thin to fit, making this inductor capable of carrying a
practical amount of current. Not all manufacturers handle the same
definition for AL. Outside main stream we may also find The inductance factor
High(er) frequency application At frequencies above 1 / 10 ferrimagnetic resonance,
table 2 is insufficient for a reliable design. Not only is permeability
(μ') frequency dependent but also a ferrite 'loss factor (μ")
' has to be taken into account which is frequency dependent again, but in a
different way. Table 2 is giving an impression of these factors and frequency
dependencies. Background to these factors and how to apply these may be found
in "Ferrite
materials and qualities" |
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