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Re: Solid-state TC - transformer design
On Sat, 19 Oct 1996, Tesla List wrote:
> Problem if you interweave primary and secondary then you increase
> the capacitance problem. The arrangement that I use: primary on
That can happen. You got that right. One sure needs to look when it
can be done and when not. It is always a compromise. :(
> the bottom then secondary (ground end), 3 layers, at least minimises
That is a very good point. That will mean primary/fist layer capasitance
is at minimum. (It all gets back to energy in capasitor finelly).
> the voltage difference between the layers and hopefully the lowest layer
That is a very good point too. A good example: if you wind transformer
in a "traditional" way ie. layer 1 from left to right, leyer two then
than back from right to left and so on you will end up with some
interlayer voltage difference and capasitance. At the other end voltage
difference between layers is two times voltage in one layer! That means
you need two times as much insulator too!
On the other hand, if you wind 1st layer from left to right, then bring
the wire back to the left side somehow, then wind the second layer again
from left to right there will be voltage difference equal to the
voltage induced in one layer. A great advantage! If you calculate
the interlayer capasitance (through energy balance) you will find
you the interlayer capasitance is 1/4 of the capasitance you did get
with the fist method!
Keeping interlayer voltage difference can also be affected by some
very complicated techniques such as winding the transformer in
"two thinner slices" side by side. Therefore all layers have lot
less voltage difference and capasitance goes down. That one is surely
a painfull thing to wind and even more painfull to insulate!
Then you have to always think of leakage inductance too. If you
wind primary on the bottom and many layers on top of it you will get
very large leakage inductance! Sometimes you better do some kind
of compromise.
One good compromise with many layers is indeed to put the primary
between the secundary halves. That will halve the leakage inductance.
As Alan pointed out capasitance is increased. However, it is increased
only as much as the capasitance of primary to the two closest layers.
That is not often the same as doubling the interlayer capasitance.
If you have many layers you may well approximate that the primary
secundary interleaving did double the frequency that self resonance
occurs in the transformer.
Then there is always the insulation. Low dielectretic constant will
keep capasitance lower. More insulator between windings will decrease
capasitance but it will also make coil bigger and increase leakage
inductance. Make a good compromise on that one! I think best easy
compromise might be to interleave prim/sec and put plenty of insulator
at that point to keep capasitance down. Usually you have a very good
insulation at that point anyway.
> of the secondary will screen the effects of the upper layers. Could a layer
> of aluminium foil be wrapped around the primary - insulating the overlaping
> ends to avoid a shorted turn. This would halve the capacitance, but add an
> extra layer.
Copper foil is quite often used between windings as a grounded protective
shield. There is no reason why aluminum should not work as well.
> The only way I can see out of this is to use a lower ratio output transformer
> and then connect via a LC resonator as someone suggested earlier. Probably
> yeilds a different set of problems! I remember that someone on the list was
> discussing doing this - anyone tried it - does it work? How do you tune the
> LC resonator?
I remember someone talking about it as well. I might even myself have
said something about the idea. At least I have been thinking about
it quite a lot.
The idea itself is great. You get away with low output voltage step-up
transformer (whatever is practical, perhaps 1:2 or 1:3). However,
the tuned LC-circuit itself is a real pain. It can be done in theory
with a single L and single C. However, C will need to be rated for
high current and high voltage. L as well but it is lot easier to make.
Remember that when you have Q of say 10, you will have at resonance
output voltage multiplied by 10. You will also be circulating 10 times
as much current in the LC tank you will put out! Therefore that really
needs to be a low loss one.
Then there is the tuning problem: LC-tank will have to be tuned at
the same frequency the tesla coil is. As the coil frequency can
even vary during operation an optinum would be some way to alter
the LC-tank freq. on a fly.
Altering LC-tank freq. is not easy. The best way I can think of is
to take advantage of the nonlinearity of ferrites. That is, feed
some syncronous current to the coil, therefore altering its effective
permeability, therefore altering the frequency. That, however, is not
an easy task to do. Using adjustable cores with high power devices
is not a good choise either.
If anyone figures out a good solution for that one, I'd certainly
like to hear about it! Anyway, I will try the idea out in some form
some day. My prototype is almost ready for testing and has even
the spare plases for installing LC-tanks in various points. :)
--
"99% of the pain in the world is due to misunderstanding" -M. Jarvelainen
Harri.Suomalainen-at-hut.fi - PGP key available by fingering haba-at-alpha.hut.fi