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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Date: Tue, 21 Oct 2008 21:40:50 +0100
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Subject: LF: Ferrites - why low frequency limit?
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Dear John, LF Group,

There is no strict limit on the frequency range of a particular type of
ferrite, rather there is an optimum frequency range depending on the
application.

All ferrites have losses that increase with frequency. At low frequency this
is mainly due to magnetic hysteresis in the core, which results in a loss
that rises with operating frequency at a given level of magnetic flux, and
at high frequencies things like eddy currents and dielectric losses increase
in significance too. The lower permeability ferrite materials tend to have
lower overall loss in the magnetic core material in the LF/MF/HF range, but
require more turns of wire to achieve a given inductance, or a particular
maximum level of flux in the core, resulting in higher losses in the
windings. So there is a trade-off, favouring high permeability materials at
low frequency where the loss due to hysteresis is relatively low and the
smaller number of turns needed is a benefit, and low permeability materials
at higher frequency where fewer turns are required.

Whether a material is suitable at a particular frequency depends a lot on
what it is being used for. If one looks at the impedance of a particular
winding, the core losses result in a resistive component that increases with
frequency, and an inductance that is constant at low frequencies, but
decreases rapidly at high frequencies. At very high frequencies, the
resistive component may also reduce. So there comes a crossover point when
the coil impedance becomes mostly resistive, and at higer frequencies still
the overall impedance of the coil actually reduces (this is ignoring the
effect of stray capacitance, which will also cause the impedance to reduce
at frequencies above resonance). The crossover tends to occur at higher
frequencies for lower permeability materials. For a signal transformer, one
does not usually care too much about the resistive component, provided the
overall winding impedance is high, which favours high permeability cores.
For a high Q coil in a tuned circuit, one wants to minimise the resistive
component as much as possible, which tends to favour low permeability cores.
For noise supression, one wants to maintain a large impedance over a wide
frequency range, and a resistive impedance is actually quite useful in
damping out resonances. Here the upper limit is where the overall impedance
starts to decrease. For power applications such as SMPSUs and transmitters,
the trade-off becomes more complicated, because one also must consider flux
density, temperature rise, size and cost of the core, the effect of a DC
bias current, etc.

So the reccomended frequency range of a ferrite material is really rather a
vague notion. Nothing terrible happens at low frequencies, but the windings
tend to get unmanageably large. At high frequencies, there comes a point
where the losses are too high for the circuit requirements. But in both
cases, the frequency limits will depend a lot on what the core is being used
for.

Cheers, Jim Moritz
73 de M0BMU