RE: 48kW DRSSTC and RELIABILITY

["Tesla list" <tesla@xxxxxxxxxx>]

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

Actually, let me correct myself.  When I speak of reliability, i'm
talking about creating a coil that can operate, without failure, through
most anticipated accidents, such as accidentlly grounding the primary,
secondary-to-primary arcing, power arcs to electronics, power arcs to
power cord, and so forth .. .

When I say i am derating components, i am assuming stresses on
components that would be present during these mishap or accidental
conditions, not normal operating conditions.

Basically, we have so far made many measurements of what type of
transients are present on the control electronics, power electronics,
etc... when arcing occurs, etc... and are designing the system to work
through these conditions.

Dan







In general, the probability of a single part failing is minor
compared to true design flaws, unintended operation, accidents,
external causes....  Many times I have "wished" that a system's
failures were just a matter of random component failures...  But most
real system failures don't follow that simple of logic.

Military deratings and calculated MTBFs tend to be low by a factor of
5.  Often they do not take "modern" components or non-military
environments into account.  Thus, if one goes "just by the numbers"
many perfectly happy commercial products will look terrible to some
military specs.  To make a long story short, stay within the rated
specs, and the parts won't die for no good reason...  There are
millions of complex SMPS systems out there that do just fine...  Our
little DRSSTCs are basically the same from a plain Jane electronics
standpoint.

However, we have two big problems.

Operating environment. - I don't mean aircraft or sea launched
missile things...  I mean Joe in has back yard in the rain, Sally at
the Halloween party, Gus at the school show, .......  Goodness only
knows what odd things they will do wrong to cause a failure.  Running
without the secondary, running without ground, running off one or
"three" phases when they should be running off two...  Someone really
WILL try to cook food on the primary...  Those are the things that
will kill a commercial DRSSTC.  Having a chip randomly fail is just
not going to be a problem.  It's all those goofballs out there that
are going to run it all wrong...

The second thing is the high voltage and pure "newness" of such
systems.  The basic electronics is plain, but these power supplies
have big sparks and big voltages that are "abnormal" and really not
well understood.  The effects of the sparks on the electronics and
all the sneaky ways that they can do damage is pretty unknown and the
learning curve could be pretty messy.  Suppose an arc to the case
induces a -2 volt spike somewhere that blows an IC that
blows....  "Finding" that hidden problem could be a nightmare!!!

So, IMHO, trying to derate the components to improve reliability
might be a waste of time other than checking to be sure they are not
run outside their specs (often MTBF studies are really "just" to
check for that only).  I would worry more about FMEA (Failure Mode
Cause/Effects Analysis) where you brain storm everything that can go
wrong or be done wrong to an otherwise happy system to make it
fail.  Then work to protect against those situations.

Second, LOTs of good hard testing!!!  The usual verification tests to
make sure it runs in hot and cold weather, low and high line voltage,
vibration, etc. to catch true design flaws....  And then some field
testing where "normal user" types get to try and break it by doing
normal and stupid things to it...   They can't run it in driving rain
or anything obviously stupid, but what happens if the control cable
gets jerked out will it is running (that will happen a lot) sort of
stuff.

Unfortunately, getting all that done is a pretty tough task!!!  Then
there might be EMC, CSA, FCC and other compliance issues...  A lot of
that can be worked around since DRSSTCs are pretty specialized "not
really" commercial and "sort of experimental" things...   But even
then, you really need to check to be sure that they will not attract
any "special attention" from the rules folks...

Then there is sales, marketing, and all that...  But that is not
"special".

At least that is MHO on it all...  If you want more humble opinions,
I am free ;-)))

Cheers,

         Terry


At 07:20 AM 12/4/2005, you wrote:
>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/