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DIY Extreme performance Output Transformers - AMSOL
DIY Extreme performance Output Transformers - AMSOL
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Old 25th April 2005, 10:46 PM   #11
smoking-amp is offline smoking-amp  United States
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Default Light at the end of the tunnel is not an oncoming train!

Hi Peter,
The permalloy wire might work fine for a SE xfmr where one needs a little air gap. The problem for a normal xfmr is that the magnetic flux needs to make a loop around the wire so has to jump the gap from turn to turn along the length of wire. So would lower effective permeability of permalloy covering.


AV8R, "spent 40 years on the holy grail of transformer designing"
"And still have not succeded in DC to light!"

The solution was published in Wireless World a while back, and I've seen it publihed elsewhere too. Well, near DC to microwave and no isolation. Here it is:

For transformation from Zi to Zo=n*n*Zi one uses n equal length transmission lines of n*Zi characteristic impedance. The Zi end of the lines all get connected in parallel and the Zo ends of the lines all get connected in series.
Common mode inductors are required on the outside of the lines to handle the signal swing at the high impedance ends. This can be done using a single large E core with the trans. lines all wound on it with the outer lines having more turns with linearly decreasing turns as one approaches the center lines. (ie. the common mode voltages must all show up as same volts per turn on the core) Ferrite beads get put on the lines also as HF common mode inductors too. Equal length lines are required so all signals traverse in the same time. No round the loop transmission is required as in some simpler broadband xfmrs.



Hooray!!!!!!!!!!!!! Party Time!!!!!!!!!

I think I have solved the original Litz problem! But don't quote me on this till I have a chance to sober up and re-check my calculations again. The problem was due to not being able to get the Litz wire diameter down enough (for shorter magnetic path length) to get the required self inductance of the primary. But I believe there IS a way. We start with a Litz wire containing only two wires. For the 10 to 1 turns ratio example, we cut one of the wires into 10 equal length sections and bring the ends out for access. Now coil the whole thing up as 10 turns so that the access wires all come out conveniently at the same position around the loop. The 10 sections get wired in parallel. (Of course, practically speaking, we would use 10 sections of Lits to do this)

The reduced Litz wire diameter now makes it possible to get the same self inductance on the primary winding, as in the E core model, using the same amount of magnetic material volume. Only problem is that the Litz is so small in diameter now that any wrapping tape will have to be ultra thin and flexible. This leaves out amorphous. But permalloy is still possible I think. Should be possible to reduce distributed capacitance a little better this way two. Another approach might be to use ferrite beads strung on the Litz instead of permalloy tape, but it will take an awful lot of them.

Don

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Old 26th April 2005, 12:19 AM   #12
Peter M. is offline Peter M.  Norway
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Hi smoking-amp,

You got me thinking now Obviously i dont know as much as you do about magnetics...

I`m just throwing out some ideas ... Maybe i learn something


Just a thought...

A sandwich of a very thin mylar insulated copperfoiltape and permalloy tape... the permalloy tape slightly wider than the copperfoiltape.

A sandwich of - permalloy-primary-secondary-permalloy- ten of this sandwich stacked on top of eachother(10:1) primarys in series ,secondarys in parrallell. Coil this stack to a flat round cake, and bridge the gap with a sheet of permalloy on the flat sides.

Will this work at all?
The permalloy will be verrrry close to the copper, and it will have the same distance to the windings over the whole length and width.
The capasitance might be a problem?

Regards,
Peter
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Old 26th April 2005, 12:59 AM   #13
smoking-amp is offline smoking-amp  United States
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Hi Peter,

Likely would need to do a little better completion of the magnetic path than just close spacing of the permalloy sheets. Might be able to continuos spot weld the edges of the permalloy.

The distributed capacitance is definately a major concern in the Litz or analogous designs due to the continuous proximity of primary/secondary and permalloy. It affects bandwidth and would sum up to a capacitance across the primary for the tubes to drive. For this reason I suspect a teflon or foam insulator between the wires may be required with some thickness. I'm thinking maybe two twisted wires of typical teflon insulated wire. But maybe just something like twisted kynar wire wrap wire would work. Have to calculate capacitance per foot or measure a sample. I'm also wondering if there might already be a product out there that fits this design, might be called shielded twisted pair, but has to use permalloy shielding.

The smaller the overall Litz wire diameter (due to thinner dielectric), the shorter the magnetic path length, so the shorter the length of Litz assembly required to reach a given primary inductance. Looks like the resultant capacitance of the primary may then be a constant no matter what insulation thickness is used, although thicker permalloy coating allows shorter length independently.

Another technique to consider is the possiblility to plate permalloy onto copper. This was done in the past to make plated wire memory for computers. In this case one would probably use a coax configuration for the primary/secondary with plating on the outer surface. But coax has a much higher capacitance per foot than twisted pair, so is at a disadvantage here.

Don
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Old 26th April 2005, 02:17 AM   #14
Peter M. is offline Peter M.  Norway
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Hi Don,

Continous spotwelding, i can`t see how that could be done with the insulated copperfoil inside, the insulation would melt. And how would that affect the permeability of the permalloy. i guess annealing would fix that, and melt the insulation completely.(maybe kapton insulation could withstand the high temp?)
What about making the coil perfectly flat to accept the sheet with no gap, and use a thicker permalloy tape?

Yes, that would be like continous interleaving, no (low) leakage inductance, but high capacitance.

Electrostatic shielding between primary and secondary? Hmm... Increases the leakage inductance

What about foam just between primary an secondary(Cu foil coil)? Shouldn`t affect the primary inductance too much?

Pimary inductance, parasitic capacitance, leakage inductance, a lot of tradeoffs here.

Regards,
Peter
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Old 26th April 2005, 02:50 AM   #15
jlsem is offline jlsem  United States
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I am seeing only zero turns with this technique, even it it were remotely physically possible. Without the wires wrapping around the core, no magnetic flux is induced. No flux induced = no magnet coupling. However, if you were able to anneal the permalloy after wrapping, the wire would be well shielded from external magnetic fields.

John
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Old 26th April 2005, 03:43 AM   #16
smoking-amp is offline smoking-amp  United States
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Hi Jlsem,

I don't follow your reasoning. The topology is really no different than an ordinary xfmr. A turn in this case is the length of wire thru the outer covering. The magnetic core is the cylindrical permalloy covering. It just looks strange since the core is stretched out dramatically in length. But can visualize this as a geometrically distorted toroid with one turn of wire thru it.

Or maybe the problem you are seeing is the lack of multiple turns to perform the N to 1 turns ratio? A simple way of seeing the impedance transform is to consider each of the 10 sections as a 1 to 1 xfmr between the two wires passing thru that core section. The impedance transform is done by connecting the secondaries in parallel and the primaries in series. This is a pretty well known technique.

Did I miss something? Edit: OH!, maybe you are talking about one of Peter's configurations.

Don
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