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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: CCFL transformer
Date: Fri, 26 Apr 2024 01:36:06 +1000
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On 26/04/2024 12:52 am, legg wrote:
> On Wed, 24 Apr 2024 01:57:36 +0200, Klaus Vestergaard Kragelund
> <klauskvik@hotmail.com> wrote:
> 
>> Hi
>>
>> I need a low distributed capacitance winding transformer, for a HV
>> stepup function (3.5kV)
>>
>> I am zeroing in on similar concept as CCFL transformers with
>> sectionalized bobbin.
>>
>> For example:
>>
>> https://www.coilcraft.com/en-us/products/transformers/power-transformers/ccfl-transformers/fl/
>>
>> Possibly using Triple Insulated Wire to create some distance between the
>> individual turns.
>>
>> Not many sells CCFLs these days.
>>
>> Guess I will keep it alive....
>>
>> Regards
>>
>> Klaus
> 
> Stress between turns is limited by v/n limit of core. It's layer
> stress and section stress that you have to deal with.
> That's what the multisection bobbin and pancake windings do.

They also reduce the parallel capacitance of the windings, and give you 
are higher resonant frequency for the transformer as a whole.

"Layer stress" and "section stress" aren't specific electronic 
engineering terms, and the "v/n" limit of the core is pretty vague.

There is a volt per turn limit imposed by the magnetic field that 
saturates the core - but at higher frequencies you can tolerate more 
volts per turn before the core saturates - it's a linear function of 
switching frequency, up to the point where resistance around the current 
loops inside the core lets enough current circulate to heat the core 
above its Curie temperature.

> If the CCFL transformer will allow only 1600V, imagine the
> precautions required for 3x that stress. I'm not sure you
> can avoid vacuum impregnation / potting in anything 'small'.

Imagination does seem to be what's being applied here.

There's a least one truly horrible 1969 text book on transformer design

https://www.amazon.com.au/Soft-Ferrites-Applications-C-Snelling/dp/0408027606

and it took me years to realise quite how confusing it was.

-- 
Bill Sloman, Sydney