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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: CCFL transformer
Date: Sat, 27 Apr 2024 02:17:23 +0200
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On 25-04-2024 09:02, Bill Sloman wrote:
> 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.
> 

Measurement:

https://www.electronicsdesign.dk/tmp/FL2015-4D_primaryL.png

FL2015-4D, primary inductance is 43uH. Resonance is 332kHz, reflected 
capacitance to primary is 5nF. Reflected to secondary 100mH is 2.3pF