I'm a bit late to the table on this issue, because I've been on the road almost
continuously for three weeks. But I have some comments that might shed some
light.
First, I agree with John's comments re: misuse of the word "saturation."
Second, I
strongly suggest that anyone interested in the use of toroids for any purpose
go to the
source, www.fair-rite.com and download their pdf catalog. These are the people
who MAKE
the ferrite materials and form them into the parts that people like Amidon and
Palomar
re-sell. The curves in that catalog are very useful. They show the series
equivalent
values for complex permeability for all of the materials they make.
Third, I strongly suggest that anyone interested in understanding ferrites
download the
tutorial on my website, which includes a lot of measured data and a new
equivalent
circuit for a ferrite coil. This equivalent circuit should go a long way
towards helping
us understand how ferrites work, which parts should be used, how to use them in
circuits, and how to analyze their performance.
http://audiosystemsgroup.com/SAC0305Ferrites.pdf
In short, the loss component seen by any coil wound around a ferrite part can
be quite
significant, and becomes dominant with increasing frequency. This is true
whether the
coil is part of a transformer, a balun, or a "choke" used for suppression or
decoupling.
If the core is handling significant power, this loss component must be taken
into
account, and can often be the most important design parameter. For this reason,
the
Fair-Rite #61 material is the most practical one for high power applications at
HF and
above. And even this material begins to show significant loss above about 20
MHz.
What defines high power? Any application that causes significant heating of
the core,
which in turn reduces permeability, burns transmitter power, and can cause
overheating
of the wire (or coax)!
One place where I disagree with John's post is his advice with respect to use
of the
core in a transformer. The loss component IS quite important IF the
transformer, and
thus the core, is carrying significant power. The transfer of energy from one
winding of
a transformer to another is through the core, so any power being transferred
WILL heat
that core and power will be dissipated. One watt in the component as large as
the one
that Amidon and Palomar call an FT240-61 or FT240-43 is no big deal, but 100
watts can
be, depending on the frequency. If you doubt this, wind 6-8 turns around both
components
and transmit through them at HF. The FT240-43 will get HOT. The FT240-61 won't.
Jim Brown K9YC
, On Thu, 02 Feb 2006 11:54:21 -0500, John Popelish wrote:
>First of all, there is some confusion using the word "saturates".
>Magnetic saturation involves a sharp drop in permeability as the
>magnetic flux passes through some large positive or negative value.
>It has almost nothing to do with frequency.
>The two important aspects of any ferromagnetic material for high
>frequency use are permeability versus frequency and loss versus
>frequency and flux swing.
>If a core is use for energy storage, for instance as the inductor in a
>filter or tank, it stores that energy by accumulating flux. So
>significant flux swing is implied, and all the loss mechanisms that
>take place during a flux swing cycle. But if a core is used as a
>transformer core that only couples energy from one turn to another,
>then there may be very little flux involved (which goes up by the
>volts*seconds of each half cycle, so higher frequencies imply lower
>flux swing) permeability and conductivity (that provides a one turn
>sneak load across the transformer), are about all that matter.
>That is why they say that type 61 is usable for wide band transformers
>(turns coupling applications) up to 200 MHz, where its permeability
>goes to crap, but is usable for high Q (energy storage, since Q is the
>energy stored divided by the energy lost, each cycle) only up to 15
>MHz, because its loss per cycle for large flux swings goes to crap there.
>So the first stem in defining an appropriate material is to put the
>application in the turns coupling or energy storage category.
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