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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: CCFL transformer
Date: Sat, 27 Apr 2024 16:04:55 +1000
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On 27/04/2024 12:34 pm, John Larkin wrote:
> 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?]

Wrong question. What you care about is the coupling between the primary 
inductance (which you drive) and the secondary inductances (from which 
you take your output). This can vary from 0.98 to about 0.999 in 
practical transformers.

The usual way of measuring it is to short one winding and measure the 
apparent inductance of the other, but this doesn't allow for resistive 
loses in either winding.

If you know the winding resistances you can correct for it. 
Sophisticated users presumably do this. I wasn't all that sophisticated 
when I did it, and didn't bother.

You can document the measured inductance with the other winding shorted 
as the "leakage inductance" but it creates a misleading impression about 
what it going on.

And you will write down a much higher leakage inductance if you short a 
primary with a only a few turns and measure a secondary with a lot of 
turns than when you go the other way, when both measurements reflect the 
same mutual coupling.

-- 
Bill Sloman, Sydney