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From: John Larkin <jjSNIPlarkin@highNONOlandtechnology.com>
Newsgroups: sci.electronics.design
Subject: Re: CCFL transformer
Date: Fri, 26 Apr 2024 19:34:19 -0700
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On Sat, 27 Apr 2024 02:17:23 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

>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

Leakage inductance?