RE: 48kW DRSSTC and RELIABILITY

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Mccauley, Daniel H" <daniel.h.mccauley@xxxxxxxx>

Reliability is a very complex issue.  In the design and test of our
commercial DRSSTC system, MiniBrute system, we are finding is
exceptionally difficult to get a unit to have enough performane vs. cost
(bang for the buck), yet at the same time be reliable enough to sell as
a commercial unit without it becoming a warranty nightmare.

We had originally designed our system to follow standard NAVMAT
deratings on all components,  and found that this does provide a good
factor of reliability, but at the same time, reduces performance by more
than 50%.

The trade-off is do we sell a coil that will outperform any other coil
in the world for the size, power input, but will likely be plagued by
failures / reliability problems, or design one very conservatively, that
doesn't put out much arc (would be considered non-spectacular by most
amateurs)

We have almost come to the point to abandon the entire project
altogether as a good performance vs. cost vs. reliability is difficult
to obtain.

The biggest factor in the reliability front of the DRSSTC which makes it
much different than other systems, is that there are MANY sources of
single point failures which degrades reliaiblity considerably.  With a
conventional tesla coil, you might only considered the capacitor as the
single-point failure mechanism or the weakpoint in such a system, and
such a system can easily be made bombproof.  Not so with a DRSSTC.  And
this is ESPECIALLY true for high power DRSSTCs which have many complex
subcircuits including AC/DC frontends, some with active PFC, high power
gate drive circuits, complex fault detection, etc...

Dan



>Anyway, commercial product (not 833C
>compliant) is allowed to fail and will someday. Electron tunneling will

>put small holes in silicon no mater what you do (run at spec), and
>someday a path to metal will happen.

If you ever run your DRSSTC long enough for this to happen, I'll eat
my hat, and any other items of clothing I might have on at the time.
Assuming I haven't died of old age that is. I'm no reliability
expert, but as far as I understand, if the device is run within its
ratings, systematic failures like this should not happen for >100,000
hours.

There isn't much work done on reliability of IGBTs that are pushed
outside their ratings (well not in the public domain anyway- maybe
you know some "trade secrets" that I don't) but I'm willing to bet
that that 100,000 hour figure drops like a stone as soon as you go
outside the datasheet peak current. I don't know why it would, but I
just have that feeling. After all, if the devices were capable of
taking higher peak currents with good reliability, then the
manufacturer would print that bigger number on his spec sheet to make
the device look better.

So I reckon it all comes down to the same tradeoff as Terry Fritz
explored with MMCs. He discovered that you can run the capacitors
outside of their AC voltage rating. It shortens their life
drastically, often to less than 100 hours. But that is still plenty
enough for hobby Tesla coil use. We discovered that you can run IGBTs
outside their peak current ratings, but the life implications are
nothing more than a "Bad feeling" just now. That is in opposition to
the case with MMC caps, where there is even an equation to tell you
how long they will last at a given overvoltage. If someone could
figure out a similar equation for overcurrented IGBTs, it would be
worth a NoPig Prize ;-)

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