RE: 48kW DRSSTC and RELIABILITY

["Tesla list" <tesla@xxxxxxxxxx>]

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

Steve,

We know the coil is not reliable enough because we are finding it still
failing through testing.  We presently have (5) mock up systems which
are being subjected to a number of different tests and conditions.

We have already "brainstormed" as many conditions as possible which may
present a failure, analyzed and measured them, and gone forth with
designing (or attempting to design) a system to withstand those
conditions.

For example, one test would be to run the full-bridge at full power,
then throw a direct ground to the primary.  Another test would be to
intentionally let the secondary power arc to the base of the unit and
control electronics.  Yet another could be running without a toroid, or
lack of break-out point.  The list goes on and on . . .

In my opinion, unless a coil can be designed to withstand all these
conditions, it is not worthy for commercial sale.  It has to be
bombproof or at least close to it . . .

And again, derating guidelines we follow apply to the highest stress
conditions we are likely to see during these "accidental" and "extreme"
operational modes, not normal operating conditions.

Anyways, thanks for the inputs.  I myself am not doing too much on this,
but i have several techs working for me doing a lot of the hands-on
work, building, and testing.

Dan





OK, now the philosophical question: How do you know the coil won't be
reliable enough? Did you build 100 identical coils and run them for
100,000 hours and take note of when each one failed? I'll bet you
didn't. You probably took the manufacturer's MTBF figures (or the
ones that the military publish) and combined them using the mil spec
equations.

But if you're running your IGBTs over current, the manufacturer's
implicit warranty that the device will last for a reasonable time is
void, and so presumably is the mil spec reliability calculation. You
may not even know you are running them out of spec under certain
streamer loading conditions, unless you use active current limiting.
(even then, if the active current limiting is unreliable, you still
may not know.)

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.

For me, overrunning the IGBTs turned up as a major weak point, for
the reason I explained above. So I fitted a current limiter and set
it to the peak current rating of the IGBTs I was using. I bench
tested the current limit extensively until I was confident it worked
and would always kick in when needed.

To keep the impressive spark performance we all expect from a DRSSTC
now, I just used one size bigger IGBTs than usual. The output o f the
coil ended up limited by tracking down the inside of the secondary
former, rather than the current limit.

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. 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.

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

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. 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.

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