Re: CCFL transformer

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.
--
Bill Sloman, Sydney