Re: 48kW DRSSTC and RELIABILITY

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hi Steve


> My approach to SSTC reliability was like Terry said: to brainstorm
> everything that could possibly go wrong, decide what out of those
> possibilities were actually likely to happen- as opposed to
> theoretical flights of fancy- and then add protection circuits against them.

I still only fear primary strikes with my current systems, but i think
you got it right with the shielded GDT and bypass caps on the DC buss
to ground.  I may perform some deliberately *bad* things to my coil
once i employ these 2 small features.  Could you please explain your
GDT construction? pictures would help too.


> The other major failure route I identified was bad drive. This
> happens when a ground arc makes the resonator jump to a funny mode,
> in combination with a naive feedback circuit that doesn't know it
> ought to reject the resulting crazy frequencies.

This only happens with secondary feedback... right?

 You end up toggling
> the IGBTs on and off way too fast, probably hard switching high
> currents too, and they explode from massive switching losses.

I know all about that ;-).

>
> Steve Ward fixed this with his "HF Protection" circuit as far as I
> know:

Well, yes it worked but i was never fully satisfied with the solution,
so i just used primary feedback and now its a moot point.  All that
occures during heavy ground strikes is a slight increase in primary
current, if it increases enough then my limiter catches it.  I put
this to the test at the RATCB thon by halving the spark length from my
green coil to only 3 feet and then cranking it up to full power.  The
limiter LED appeared to be lit continuously.  Picture:

http://www.tb3.com/tesla/ratcb2005/pages/IMG_4213.html

 I chose to use the PLL approach. This is my only quibble with
> Terry's DF-DRSSTC by the way: its feedback circuit has no way of
> rejecting bad feedback and ensuring clean switching.

I dont know what you mean about bad feedback anymore?!  Its dependent
on the primary current zero crossing, so unless somehow you short out
the primary and get a super high Fo on the tank, i dont see what would
ever be "bad" about the feedback.  But, with the DF-DRSSTC the
feedback is the actual gate drive, which could be weak and slow (since
its not cleaned up and buffered), so that could be bad.

>
> Finally, there was all the boring stuff like UVLO and soft-start and
> shutdown to ensure the IGBTs never get bad drive signals: they either
> get good drive or nothing. These techniques are common place in
> industrial SMPS and motor drives. There was also EMC and shielding,
> and I put a lot of thought and experimental work into laying out the
> PCB and wiring harness so that RF from ground arcs and flashovers
> wouldn't go where it wasn't wanted.
>
> So I now have a driver with no flaws "That I know of". The next step
> is to find the ones I don't know about.

Have you tried allowing primary strikes from the toroid?  If you need,
i can send you some spare IGBTs :-).

 For this I'm trying to use
> the open source development model. I released the circuit and PCB
> artwork in the public domain and encourage everyone else to play with
> it and help me find the bugs ;-) So far, the complexity has scared
> most people off, but I believe that every part is in there for a
> sound reason and have no plans to try simplifying it.
>
> Lastly, I should mention that I owe one to Steve Ward and Jimmy
> Hynes. They were kind enough to share their DRSSTC experimental
> results and ideas with me, so my design could build on all their
> findings too. This was the main reason I chose to make it an ope n design.

Cool... i guess i owe one to Jimmy as well as Justin and Aron (since
they started me on this SSTC fix...).

Steve Ward

>
> Steve Conner
> http://www.scopeboy.com/
>
>
>
>