Is intentional overlap in the secondary theoretically possible?

[MinuteMylar]

Hi there!
I believe it's considered best practise when winding a secondary to never allow any overlap, due to high voltage differential between turns making arcing likely.

But assuming we lived in a world where perfect insulation could be achieved, would there be other issues with overlapping the secondary intentionally (allowing it to be smaller)? I guess there would be increased self-capacitance between windings, for one, but I don't know if that would be significant.

[davekni]

I guess there would be increased self-capacitance between windings, for one, but I don't know if that would be significant.

Of course, depends on amount of overlap.  Presuming you're thinking of every other turn being on top of the previous two turns, added capacitance should be insignificant.

Other possible issue that comes to mind is less uniform winding surface topology.  Could lead to more racing sparks from local electric field concentration on high spots.  Varnish coating several times thicker than wire diameter should minimize that potential issue.

May be more issues that haven't come to mind yet.

Have fun finding perfect insulation

[Mads Barnkob]

What is the end goal of making overlapping? I can only think of minimizing physical layout, because often we do not see increased performance from higher ratios secondary/primary wise, better performance comes from close impedance matching and driving envelope.

[TiagoBS]

I imagine the main reason is precisely to reduce physical dimensions, especially for coils that need to operate at lower frequencies (100-200 kHz).

It might be possible to use tubes of different diameters, with the first layer wound on the inner tube like a traditional coil, then covered by a second, larger-diameter layer. Epoxy resin could easily be used for insulation between these two layers, while the outer layer could still use varnish or any other existing insulation method.



I'm not sure if this layout would negatively impact the self-capacitance.

[MinuteMylar]

Thanks for the input everyone!
Yes, Tiago hit the nail on the head – I'm curious about minimising the physical size of a lower frequency coil.

I guess I should just go ahead and start experimenting then!

[Mads Barnkob]

I do not believe that a two layer coil will work. It has to be a segmented winding like a DIY HV flyback transformer.

Two single layers would have a HUGE voltage potential between the windings in the ends, not even taking leakage capacitance into account.

[MinuteMylar]

Hi Mads, love your website btw
Yes that was definitely the plan: very small segments on a well insulated bobbin.

(Unrelated note, your gate drive transformer spreadsheet is very helpful, but is it possible there's a mistake calculating Ipeak? It looks like the spreadsheet uses V*f/L instead of V*t/L)