Re: OL-DRSSTC - 12

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

At 12:14 PM 10/15/2005, you wrote:
> This is why "real" DRSSTCs use BIG low ESR/ESL electrolytics... well 
> at least mine do :-).  I believe that the energy storage caps should 
> store maybe 50X the bang energy if possible... that way if an IGBT 
> does fail, you get a nice BANG!

:o))  It is amazing how much voltage drop there is across a few 
inches of heavy wire too!!!  Big buss copper is a GOOD 
thing!!  Apparently, the electrolytic caps I am using ARE the 
"best"...  If I can get the caps right near the IGBTs on the same 
board, that will help a lot.  I am happy just different poly caps 
really seem to have fixed it!

> Of course, a "real" DRSSTC is not a simple as this machine you've 
> been developing.  There is always a compromise to be made.  I will 
> be interested to see what sort of performance you can get from this 
> machine... its a very neat little design, and hopefully it will 
> introduce more people to solid state coiling.

I am trying to keep it super simple and cheap at the expense of some 
performance.  The biggest limit is the 60BPS of this design.  I'll 
work on that later...  It would be super cool just to put it all on 
one PC board that people could play with easily.  Then, "everybody" 
could play with DRSSTCs.


> Ive been meaning to ask, how warm are those gate zeners 
> getting?  They are certainly clipping quite a bit of power, but i 
> suppose the low duty cycle saves them from bursting into flames :-).

The primary RMS current is about 50 amps at full crank, so the TVS 
RMS current should be 500mA.  At 24V, that is 12 watts.  Good thing 
they are rated for 1500 watts :o))))  The parallel resistors were 
supposed to help that out, but now at 1000 ohms they are not much 
use.  They are fan cooled and they handle 5W at a 75 degree lead 
temperature so they can take some real heat!!  The fan cooling will 
help a whole lot there.  But I do have to be careful.  TVSs are a 
bulk silicon device so they are not real temperature sensitive.  They 
happily melt solder and fall off of ceramics without real harm to themselves.

Today I am wondering if I should take the primary peak current down 
some.  Maybe go to a 75uF primary cap to take stress off 
things...  The "spare parts" will be here Monday so I can start 
blowing stuff up now ;-))

Cheers,

        Terry



> Steve