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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: CCFL transformer
Date: Sun, 28 Apr 2024 16:51:09 +1000
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On 28/04/2024 2:49 am, Klaus Vestergaard Kragelund wrote:
> 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.

There's no "reflection" involved. Both the primary and the secondary 
winding take part in the same process, and make their own - more or less 
independent - contributions.

It's a pedantic point, but getting a proper grip on what going on in 
transformer does seem to be difficult, and it does take a while. Getting 
close to precisely the right point of view probably helps.

-- 
Bill Sloman, Sydney