I take it, this is at 50/60Hz?
You don't want to put down an entire winding all by itself, because the magnetic field can't escape from the pile of turns. Within a section, the field from turns underneath are shielded by the overlaying turns, while adding on top of the field from those turns, vastly increasing eddy current losses (or skin effect losses, or whatever; this is broadly termed "proximity effect"). With copper of this size, I think even at 50Hz, you'll have issues with this, and I don't recommend it.
Wire-wound windings are normally done this way, i.e. in single sections, because the wire is smaller (up to, oh say, 10 AWG?). This is around where copper wire starts getting unfavorable due to proximity effect.
Another mitigation for proximity effect is to simply not have windings so tall -- reduce the number of layers that are stacked upon each other. If you're limited to "wasteless" style stampings, there's probably not much room to do this; but if you have the option to use a wider (and less tall) winding area, that would help.
A toroidal transformer may be attractive for this reason. The winding area is quite wide (the circumference of the core!). Rather difficult to use foil windings with! Multi-filar windings would be more attractive (i.e., using a ribbon of finer wires in parallel for the same total cross section).
Multi-filar also helps with proximity effect (you're making flat Litz, in a sense), which is another option. You may get a better fill factor with smaller enameled wire, than whole round wire.
Back to foil windings. What you can do, is wind both primary and secondary at the same time -- much as you would wind a capacitor with two plates and two dielectrics, this keeps the current balanced within the winding, so that currents aren't piling up. There's always one forward current (primary) beside an opposing current (secondary), alternating.
In this case, you might wind the primary and two secondaries, getting 6 and 6+6 turns respectively, then add another 4 turns (2 each at the top and bottom) to bring the total up to the desired 6:16. The additional turns aren't paired, so do exhibit proximity effect, but are a small part of the total so it's an acceptable compromise. Other divisions are possible but this is a simple one, and wouldn't be too bad.
In summary, that would be:
Pri, sec: full width foil; ampacity attained by selecting foil thickness appropriately.
Windup:
A: 2 turns sec
B: 6 turns of triple layer: sec-pri-sec (call these B1, B2, B3)
C: 2 turns sec
Wiring:
Secondary start = A start
Connect A end to B1 start
Connect B1 end to B3 start
Connect B3 end to C start
Secondary end = C end
Primary start = B2 start
Primary end = B2 end
BTW, if you have to neck down the foil, to get it out of the bobbin, that's fine. At the start/end of a winding, you'll do a 45° fold, to make a 90° bend, to escape vertically from the bobbin (as you already noted
). If this is too wide (the bobbin winding area width, and therefore foil width, may be wider than the slot in the bobbin cheek), the vertical lead can be cut narrower (increasing current density, but just for a short distance -- no big deal). Or actually, it could be folded in half, lengthwise, or thirds or whatever for that matter. Crush it down with pliers or hammer, it's going to get pretty thick pretty quickly...
You'll end up with a bunch of foil stubs sticking out of the bobbin, which need to be wired together. Just resolve these however you can. Expect to take up some space in the process. You'll probably fold over a few against the bobbin and solder them (if they're folded or rolled, make sure solder gets into the layers), or for that matter, drill holes and make bolted connections.
Or you can do some simplification right away, and, I think, assemble one single strip, with the other layers positioned on it exactly as needed. Start with a 10-turn length of secondary, and assemble the 6-turn lengths of primary and additional secondary, onto it, at the 2-turn mark. Now you only have six terminals to resolve. You need to get the lengths and positions just right, however!
By the way, regarding insulation: do use heavy plastic, or vulcanized paper ("fish paper"). Don't want that foil edge chafing through a weak layer of film tape or something! I would guess 10 mil PET should be effective here, or maybe two layers of 3-5 mil. Or anything equivalent in strength but higher rated in temperature, as needed.
There's actually kind of a downside to this construction, in that it may be
too good -- the characteristic impedance will be much lower than most mains-frequency transformers. If you were expecting some leakage inductance in the process (as might be beneficial in the below-mentioned example, converting MIG to stick -- the inductance improves arc stability), you'll get very little with such a tight windup, and may need to add an extra reactor (AC inductor) in series. In other words, the available short-circuit current will be very high, or the regulation will be very good. (These are also attributes that toroidal transformers share, and these are precisely the reasons why.
)
Somewhat more out-there ideas: is a transformer really the best solution? This sounds like a pretty unusual application, and maybe there is a better overall approach. The numbers roughly sound like, changing a 350A MIG welder to a 125A TIG/stick welder? Or, stepping up a UPS/inverter's primary (~24VAC) to some oddball voltage (UPS'd 1kW audio amp??).
In short, just to say: there may be a better (easier, cheaper), less clever alternative, e.g. selling the 350A MIG welder and buying a 125A welder.
Or you could design a switching supply which performs the transformation, preferably DC to DC, but AC to AC is also possible; but a switcher is considerably harder to pull off well, so I'm not going to recommend it without more background details.
No idea, these numbers are really oddball -- hence the custom transformer -- but when this happens, it's often a case of a missed assumption, and the reason no one uses this low-level solution, is because there is a better high-level alternative.
Tim
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