Re: SSTC newbie

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

> Now, to your question. The transformer is for isolation, yes, but the
> turns ratio depends on your setup. If the output of your driver is
> zero - 10V, you will want at least a 1:2.4 turns ratio (ten turns on
> primary, 25 turns on each secondary) so you will step 10V up to
> around 24V. (it doesn't have to be exactly 1:2.4 ratio, but it made a
> good illustration).

I've tried to write up some design tips and examples here at
 http://www.hut.fi/~jwagner/tesla/SSTC/general-sstc-notes-gatedrv.htm

And be sure to read
 http://www-s.ti-dot-com/sc/techlit/slup169.pdf
starting on page 31 (or, maybe the whole paper, it is pretty good)

Toroidal cores are much better than EE or pot cores, as they will give
less leakage inductance. Usually they have a higher permeability too, so
the magnetizing inductance will be "sufficiently" large with only a few
turns. And, many are broadband xfmr cores. So dump that EE core...

Also, the xfmr output depends much on how good your transformer drive
circuit is...

Umm and yeah, as someone already pointed out, if you use a "single ended"
xfmr drive i.e. only one end of the gate drive xfmr is switched, you need
a turns ratio 1:2 pri:sec  - winding this is tricky, you should at least
use two primary wires so that the secondary winding(s) are at all points
well coupled to the primary. Otherwise => large leakage inductance, much
ringing and spikes, no clean output.

For more than 2 outputs, it's best to use 1 pri 2 sec pairs and then
parallel the primaries. T.ex. 4 outputs: 2x a combination of 1 pri 2 sec
twisted/weaved/interwound, and these two strings wound bifilarly(sp?)
aroudn the same toroid.

In the final prototyping stage, use a metal or carbon foil >0.5W resistor
(as in "not wire wound" ;-) of maybe 10 Ohms or so on each mosfet gate.
This keeps the voltage spikes down which are caused by the leakage
inductance.

Ok my _final_ tip: don't bother winding a full xfmr right from start!
First get two enameled copper wires twisted together and use this to find
out the minimum number of pri turns required. You should clip away the
"left over" wire when you reduce the turns count, otherwise with the
"extra" wire the output waveform may look worse than without.

Ok then, good luck!!

The hardest part of making a SSTC seems to be to build a good gate drive
xfmr and xfmr driver circuit, the rest is a piece of cake. Almost. ;o)
Personally I've spent much more hours making a good gate drive xfmr
(ok, I admit, _trying_ to make a good xfmr ;) compared to the time put
into making a half/full-bridge.

So don't despair if the gate drive doesn't work right from start! :-)
[there's a good reason for commercial gate drive xfmrs being so expensive]

cheers,

 - Jan

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