RE: The OLTC II lives!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

Hi all,

In response to your questions:

K.C. asked if I had tried it without a breakout point. I have, and it
doesn't even break out :( But so far I haven't been able to achieve the full
bang energy I designed for, due to the crowbar triggering early. I hope to
get it to the point where it will break out without one, or failing that, to
use a smooth breakout "bump" rather than a point.

Dave Sharpe was asking what blew my IGBT. The short answer is, I don't
really know. It was an Infineon BUP314 rated at 1200V 42A. It failed with
collector, gate, and emitter all shorted together. If you look at the
schematic of the short circuit protection unit

http://www.scopeboy-dot-com/tesla/scpigbtprint.gif
view along with
http://www.scopeboy-dot-com/tesla/systemfull.gif

you can see that when the IGBT turns off, its Vce should be clamped at the
DC link capacitor voltage, which is never more than 650V. Also, in bench
tests, shorting the DC output with an SCR, it has successfully worked every
time. It only goes kablooie when plugged into the Tesla coil.

I _think_ it is getting killed by not having an undervoltage lockout. When
the control circuit power is turned off, the DC link caps stay charged, the
gate drive to the protection IGBT dies away slowly, and the main IGBTs
switch on. So basically the poor protection IGBT is getting the DC link caps
dumped through it while the gate is underdriven. Locking out the gate drive
would solve this, but so would getting rid of the IGBT :)

Terry mentioned John Freau's sparklength equation. Unfortunately 48" at 2kW
is well below par, John's equation predicts 76". But of course his equation
assumes 120bps operation, so 2kW would be a bang energy of 16.7 Joules,
whereas I was only using 4.5. We already know that power in the form of bang
energy is much more effective for spark growth than power in the form of
bps. I would like to think that if I ran my OLTC at a lower bps, it would do
better in terms of spark length v. power. I have run it at 2bps, and got 28"
streamers for a power input of about 10 watts, which is a factor of 5 over
the "Freau limit" :)

In the light of these results I've been trying to think how Freau's equation
could be adapted for coils of different or varying bps. How about
L=A*sqrt(E) + B*sqrt(P) where E is bang energy, P is input power, and A, B
are empirical constants ;)

But maybe there are other factors at work, like the low resonant frequency
reducing streamer currents, or the high self-capacitance of the secondary
compared to a thinner coil with fewer turns, which would eat my output
voltage. I haven't had the chance to explore all this stuff yet.

Steve C.