Re: sstc driver help

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Jan,

On 30 Aug 00, at 12:06, Tesla list wrote:

> Original poster: "Jan Florian Wagner" <jwagner-at-cc.hut.fi> 
> 
> Hi,
> 
> (I hope this is not too far off from TCs)
> >TC power supplies are perfectly on-topic - Terry<
> 
> Dreaming of a compact TC...
> ...I'm in the process of building a small, modular, directly mains fed
> 500W SMPS power supply to drive a TC secondary, 100..300kHz adjustable.
> Got some stuff for the SMPS already assembled, but I've a few questions
> yet. 

What is its job - to charge a primary capacitor or directly 
drive the coil?
 
> Maybe someone experienced with HV SMPS' could help?
> 
> The ferrite xfrm is about ready, should give 3..5kV, but I've now started
> wondering about insulation (a bit late maybe... ;). I've used LD-PE
> plastic bags and transparency sheets as insulation between primary and
> secondary, and also between each secondary layer (turns, one layer PE,
> one wire led across, one layer of transp sheet, next turns). Plus,
> poly-urethane varnish. 

Typically I would use teflon tape. Also I wouldn't consider 
any wire that wasn't thermally tough. I'd never use self-
fluxing wire. Not saying there is anything wrong with poly 
which is a good dielectric - I just like to bulletproof my 
designs.

> Does that sound ok?
> 
> Running the xfrm at about 1/50 of critical B-saturation, lots of primary
> turns, big 5cm x 5cm x 2cm core of 3C85 material. Two non-connected 8 turn
> primaries, one tapped 120..200 turn secondary.

What is the exact topology of your supply? Forward? Flyback? 
Pushpull? 1/2 bridge? etc. etc.  To run at that level of B you 
are robbing yourself of efficiency in copper losses I would 
say although 8 turn primaries doesn't sound like much. 

> Designed the xfmr for push-pull, but with my lack of experience and those
> symmetry/saturation problems I dropped that idea.
> 
> I'm using a CA3825 PWM chip (with dead-time expansion), but couldn't find
> any FET buffer chip so looks like I can use only two FETs safely (?).
> 
> Sooo... what is the "best" way to hook up that xfrm? 
> What would put minimal strain on the switch MOSFETs? 
> (They are 500V 8A, mains is 220V 50Hz.)
> 
> 
> Other thing, I was wondring with voltage feedback from the secondary...
> How is that done? Divider network with a small well insulated transformer?
> Or just 1-2 feedback turns on the xfmr, no messing around with secondary
> HV?

If you measure using a feedback winding, it must be *well 
coupled* to the winding whose behaviour you are trying to 
measure. 

> 
> And the last but main question, how do you go about designing the FET
> snubber network? 
> Use a series cap on the fet to cancel out xfmr reactance? That sounds
> wrong somehow - but how do you do it otherwise?
> Any ready formulas?

You have to work out how much energy the network has to get 
rid of (unused energy, magnetizing energy) and then use a 
steering diode (non-symmetrical design) and snubber cap and
size it for the maximum voltage rise you want to allow using 
the L.I^2 = C.V^2 relationship. The resistor must be sized 
based on a CR time constant so it empties the cap within a 
desiraed time and must be appropriately rated for the power it 
has to dissipate.

Regards,
malcolm
   
> (By the way I've never built a SMPS before but've tried to gather as much
> knowledge as possible for starters. If you happen to know any good web
> site on SMPS, I'd be very much interested.)
> 
> Many thanks in advance for any help!
> 
> thanks,
> Jan
> 
> --
> *************************************************
>  Jan Florian Wagner
>  http://www.hut.fi/~jwagner
> 
> 
>