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new idea for ferrite output xfmr
I have a whole box here of salvaged ferrite cores, and I think I've
found a use for them.
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I have come to believe that the major disadvantage of using an output
transformer of the type that I outline in my solid state driver is
that the large number of secondary turns makes for a less than perfect
transformer. I don't think its the leakage inductance (which becomes
part of the coil series inductance at resonance), as much as the self
capacitance of the transformer windings.
I say that since I notice that the driver is under much more stress
without any load applied, than with the coil applied (cap is absorbed
at resonance).
[This leads to a driver secret that I forgot to include in my circuit
description (stolen from the ham radio old-timer): take a 250 watt
light bulb and put in series with the output transformer. If anything
goes wrong, it provides a nice destruction dissipating load, and also
a nice indicator of coil tuning.]
Thus much higher performance, especially with higher frequency coils
can be expected if the number of turns in the transformer is kept to a
minimum.
Since Bmax is proportional to 1/Cross Sectional Area of Core; instead
of having 10 primary turns, one could get away with 1 primary turn
with 10 cores in use. Then you could have only 20 secondary turns,
but would have to figure out some different way of insulating turns.
(I refer people to the Chryssis book or others for discussion of
saturation of magnetic cores)
This type of transformer would look very similar to the ones in the
Motorola RF Databooks (by Granburg, I believe) where there is one turn
of silver plated copper pipe thru the cores and the secondary is
looped thru the inside of the pipe. (in the RF application, this
makes a fractional ohm to 50 ohm output transformer).
Whem I get my mosfets, I will try this out with the lightbulb ballast.
Gee, maybe I'd better call Digikey and order more fets. Rob.