Re: Solid State

To: Tesla List <tesla-at-poodle.pupman-dot-com>
Subject: Re: Solid State
From: Alan Sharp <100624.504-at-CompuServe.COM>
Date: 15 May 96 18:54:02 EDT
Malcolm wrote:

>Have you thought about using copper strap for the primary? It is a
>preferred conductor choice for high current windings in SMPS 
>transformers.
>   Secondly, you said the core was O.K. I believe you're using 3C85
>material. An even better one is the Philips 3F3 material (good for 
>the MHz range and quasi-resonant supplies.

Yes but Farnell (the only suppliers I've found for these) only stock
little dinky ones in 3F3. 

I will have to experiment with the windings, I hadn't considered copper
strap, push pull, with the need for two identical windings makes it a
little harder. I am going to reduce the transformer ratio - I'm not getting
the benifit even of 7:1 because the current limiting kicks in at about
50% pwm ratio (and it needs to). (12:1 gave a slightly longer spark
but only by drawing lots of current)
I took up Harri's suggestion and 
tried making up some Litz wire - it wasn't very regular but I could
make it work. 
I got a slight shock of the core when I checked it for heat so 
something's leaking corona. 

>to the extent of re-reading the Corum's notes again (80 pages 
or so for about the 6th time).

Well done I got it, looked at it, - decided that I probably could plough
through - but it was far too much like work, and my maths circuits are
rusty.

By the way Malcolm that was an excellent article in Wireless World - 
you really crammed a lot in. 

I asked about potential hazards - Jim suggested pacemakers.
There could easily be one in our 200+ congregation, but they will
be some distance 15' min from the 500W solid state coil - is this
a serious risk?

Phil asked for details or a schematic. I'll post as soon as I overcome
the transformer heating problems. A few details:

Get hold of the data sheets for the
UC3825 Unitrode, PWM chip hi speed
(Farnell Components data free)
The Motorola equivalent chip is MC34025, their data sheet is
worth having for the application circuits. (contact them on
the net)
Because I'm driving 3 irf740's on each side I buffer the
outputs with a couple of TC4422cpa  (Farnell or RS)
Also on the net is Harris Semiconductors - download
AN7244.2 Understanding Power Mosfets
The transformer is the EFD49 (Farnell) data available.
150v 500va psu.
The secondary is a 4" pvc pipe, close wound with 0.4mm
enamelled wire to a hieght of 2' with a 1' diameter toriod
on top (2" foam pipe lagging wrapped with Aluminium 
tape).
(Farnell Components is a UK supplier)

That's about it.

I hope this month to finish this design. And then
start on a half bridge using rectified mains :)

Alan Sharp (UK)