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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: CCFL transformer
Date: Thu, 25 Apr 2024 17:02:37 +1000
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On 24/04/2024 3:10 pm, Bill Sloman wrote:
> On 24/04/2024 12:25 pm, John Larkin 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
>>> step-up 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....
>>
>> Can you use a C-W multiplier?
>>
>> For low current, you can do resonant tricks too.
> 
> It's easier to use a voltage doubler or tripler that it is to find a 
> multi-section former off-the shelf. The occasional high voltage power 
> supply that I've dismantled clearly used proprietary formers, as do the 
> Coilcraft parts
> 
> I suppose one could use self-bonding wire to make a series of 
> self-supporting pancake windings, but I've never heard of anybody doing it.
> 
> The Baxandall configuration is definitely a resonant trick, and copes 
> with the interwinding capacitance by resonating it with the winding 
> inductance.
> 
> There's nothing "low current" about it, but if you are working at higher 
> currents and powers you can justify even more elaborate switching 
> arrangements.
> 
> http://sophia-elektronica.com/Baxandall1959JM.pdf
> 
> Jim Williams talked about it a lot - application notes AN45, AN49, AN51, 
> AN55, AN61, AN65 - but described it as a "a current driven Royer 
> inverter" which is simply wrong.
> 
> MOSFETs work better as switches than bipolar transistors, and don't seem 
> to "squeg".

The Coilcraft data sheets don't say anything much about the resonant 
frequencies of their transformers - except "The FL Series of 
transformers is designed for use in cold cathode fluorescent lamp (CCFL) 
power supplies at operating frequencies up to 100 kHz" where the "up to 
100kHz" gives them a lot of wriggle room.

A primary inductance of around 50uH with a 100:1 step-up implies a 0.5H 
secondary inductance. 10pF parallel capacitance would give a 71kHz 
resonant frequency, which is less than 100kHz.

Of course once you have one of the Coilcraft parts you can measure the 
resonant frequency.

-- 
Bill Sloman, Sydney