Winding for high voltage

[Nixie]

In very high voltage transformers (such as flybacks), I've noticed often the windings are not neat layer over layer, but seems to have some strange pattern, probably to minimize voltage between adjacent turns. Does anyone have any information on the actual patterns they use to wind these?

[poobah]

Flybacks rely on mutual inductance to achieve good cross regulation with multiple secondaries. You will often see the primaries interleaved in flybacks for this reason.

[AndrewT]

Hi Nixie,
yes you need to think about turn to turn voltage difference.

Side by side windings will only have a volt (or much less) between them. But winding one layer to the end and then winding a second layer returning back to the start could have many tens of volts across the enamel insulation.

For this reason two or three layers are wound together gradually moving across the layer and then applying a secondary insulation sheet before returning to the start side. This keeps the turn to turn voltage down to the voltage across four or five turns and again does not overstress the enamel.
This all relies on two flaws in the enamel insulation never coinciding with each other.

You are right to be cautious when voltages are high.