# Help with choosing transformer wire type and sizes for high frequency transformer

I have been using the following reference to design a HF transformer https://www.ti.com/lit/ml/slup126/slup126.pdf, which I have found very helpful and useful for the core loss section. However when it gets to the winding losses, page 4-10 onwards, the author assumes that the reader has knowledge of their winding structure, winding methods and the like. Unfortunately I do not. I know that Litz wire is used for high frequency, and that one can choose solid wire and foils etc, but I have no understanding how to continue from this point onwards.

He mentions winding breadth of 1.3cm, allowing maximum insulated wire diameter of 0.87mm, and that AWG21 - 0.72mm copper will be used. I am lost at this point. Where do I even begin to find out what thickness of copper and what type of wires I will need to use? How much insulation will my wires need? My application is high voltage also, with max output voltage being 6kV. So I know I at least will need to know how to calculate the insulation thickness that I need - but I cannot find any useful documents that walk me through the process of choosing the wiring structure for the transformer.

Can anyone help with this, or alternatively supply me with a thorough but uncomplicated reference which can do so? I am at a loss as there seems to be so many different options and I do not have the slightest clue where to start.

• What frequency? That affects whether Litz is useful or not. Generally fill the winding window as much copper as you can. Enamel is usually good for adjacent turn insulation. You need thin tape for inter-layer insulation, and a plastic former or lots of tape for winding to core insulation. Mar 19, 2020 at 15:20
• 250kHz transformer frequency, 500kHz switching frequency, push-pull circuit. Possibly higher. I appreciate your answer, but how do I choose what gauge AWG wire? What about diameter? Can I calculate the insulation thickness? Usually the transformers are wound on a custom bobbin to encapsulate the entire thing in silicon - is there a need to take this into account? Mar 19, 2020 at 18:31
• Don't calacuate wire insulation thickness, use wire that's rated for your voltage. Even the thinnest enamel is rated for 50V (you won't be running 50V per turn), and you can get double or triple insulated if you need. Don't use AWG (strange US units), use mm diameter. You know the number of turns. Aim to fill 50% of the winding space with copper, that gives you copper area and hence diameter. The rest will be air and insulation. Those frequencies will benefit from Litz wire, but it doesn't make or break a design. Current flows on the surface of the wire, so we maximise diameter. Mar 19, 2020 at 19:17

As far as wire specs go, you can use any "magnet wire" catalog as a reference. I like https://prod.essexwire.com/sites/essexwire.com/files/2017-08/Essex-Wire-Engineering-Data-Handbook-EN.pdf for general wire physical properties, but you will need to check each of the many insulation materials to determine the breakdown voltage for a specific insulation build. Wire is listed in both AWG and metric diameter formats. It's good to have a familiarity with both because you can usually find stock in the AWG sizes.

Sorry to say that after 40+ years I still don't always get my high voltage designs right on the first try. So expect to do some iterating.

First, assuming your core will support the required power, your push-pull primary must be wound carefully to prevent the "flux walking" described in your paper. Assuming this is a step-up transformer, your primary should probably be wound bifilar so that the turns are as symmetrical as they can be, and your drive circuit must provide an exactly 50% duty cycle.

The wire size for the primary should be governed by the required current. You are switching at 250 kHz, but the frequency content of the switched current is higher, so you are limited to pretty fine wire. For round wire, the skin depth at 250 kHz is 0.13 mm, and there is a "rule of thumb" saying to stay below two skin depths, so this is probably a good maximum starting wire diameter. If the resistance is too high and you need more current on your primary, you can consider Litz wire or foil (or just using a heavier gauge of round wire and realizing that you will not get the full effect of the copper in the center of the conductor). Using foil may make it harder to avoid the "flux walking".

Place a layer of Kapton between primary and secondary to keep the high voltage out of your low voltage. Also the winding technique is important. @Neil_UK is correct that a properly wound high voltage transformer will not have high voltage between adjacent windings. However, consider the picture below:

Both are 77-turn windings. However, in example "A" ("layered" winding) turn 41 is adjacent to turn 1. Turn 41 will have more than half (41/77) of the output voltage between the two turns, or more than 3000 volts in your case. So if you use the layered approach, put a layer of Kapton between each layer to provide extra insulation and space. The extra insulation will also reduce interwinding capacitance.

Example "B" is called "bank winding" and is hard to accomplish without a machine. But there are never more than 5 turns between adjacent windings, resulting in reduced inter-turn voltage.

An even worse mistake would be to wind on a toroid and have the high voltage end overlap the low voltage end. When winding a toroid, leave a gap.

You will also need to put extra insulation on the outside of the coil and on the wires leaving the coil.

As a general rule, try to keep the number of turns in both primary and secondary as low as you can to reduce winding capacitance. If you have the space, spreading out the windings is the best approach to prevent interwinding capacitance. Too much capacitance will cause your transformer to ring at its resonant frequency, and your components will have to deal with a spiky, decaying oscillation each time you switch.

Other than these issues, it's pretty much the same as any low voltage switching transformer. Good luck!

• Forgot to thank you for this response John. I have put it on hold temporarily but your answer is very helpful and I'm sure will help me when I get back around to looking at my transformer design! Thank you very much. Mar 27, 2020 at 19:06