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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: CCFL transformer
Date: Sat, 27 Apr 2024 18:49:06 +0200
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On 27-04-2024 08:18, Bill Sloman wrote:
> On 27/04/2024 10:17 am, Klaus Vestergaard Kragelund 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
> 
> There's no "reflection" involved. The resonance reflects the oscillating 
> flux in the core, and the parallel capacitances of the primary and 
> secondary windings both get charged up and discharged during the cycle.
> 
> The parallel capacitance of the secondary will be higher, and the 
> voltages across it much higher, so it is dominant.
> 
> The resonant current is flowing through the capacitances so may not heat 
> the insides of the winding wires.
> 
> Measuring the self-heating of a transformer being resonated might be an 
> interesting exercise.
> 

I wrote "reflected", since the inductance on the primary was the 
measurement. The resonance of the transformer is the same on all 
windings, if the coupling is reasonable good.

So like you wrote, the secondary is dominant, which is why the primary 
resonance is due to reflection from the secondary.