Re: El Supremo

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Ken,

I was looking at this all afternoon!! :-))))

At 09:09 AM 9/12/2002 -0700, you wrote:
>Terry Fritz's groundbreaking work with the OLTC has given me pause to
>think about my own design and how it might be improved.  Also constantly
>in my mind is an appreciation of the ratio of my MOSFET failures to his
>IGBT failures: it's infinity, not to put too fine a point on it, since
>dozens upon dozens have failed for me and none have failed for him.

The big factor there is that I am switching at only 120 Hz.  No high speed
switching stuff to worry about at all.  

>
>Terry's 1-turn-primary design is what I had had in mind in the first
>place, but my MOSFETs would not drive 1 turn and IGBTs back-then seemed
>too slow.  But now, I see a way to combine Terry's notion and mine.  I
>call it, perhaps with some want of humility, El Supremo.  With Terry's
>indulgence you can find its conceptual schematic at 
>Http://hot-streamer-dot-com/temp/tch2.gif.  See also the accompanying
>http://hot-streamer-dot-com/temp/tch2drv.  

The second URL really should have been this but I fixed it so most browsers
would still work.

http://hot-streamer-dot-com/temp/tch2drv.gif

>
>Referring to TCH2, all IGBT "1"s conduct during one half-cycle and then
>all "2"s conduct during the alternate half-cycle.  If you follow the
>current paths you will find that the four quadrants of capacitors become
>connected in a "daisy chain" around the primary loop in alternating
>polarities, thus establishing the requisite alternating magnetic field
>perpendicular to the image.  
>Each set of IGBTs is driven by a crossover-controlling circuit like that
>of TCH2DRV.  

The big thing "I" see here is that with only a minor rearrangement, you
have 2400 volts instead of 600 driving the secondary in sort of a Marx
configuration.  That allows higher frequencies (far less secondary loss),
and lower loss in the primary!!!  Just what my OLTC needs!!!! :-))))

"I" don't see the great need to switch two circuits at the Fo frequency for
my OLTC type of coil.

>
>Note that, during each brief interval between half cycles when no IGBTs
>are conducting, the "inductive kick" of the primary loop will become
>clamped, regardless of its polarity, by the "fast diodes", as coupled via
>the loop capacitors.  No transistor will ever see any voltage greater
>than twice a capacitor voltage.

That does work very well!!  The IGBT reverse diodes work perfectly!

>
>Now here is where a major difference with my present design comes in:
>
>The fundamental difference between sstc's and spark-gap tc's is that the
>former generates its spark by application of a constant-amplitude burst
>of excitation whereas the latter generates an exponentially-diminishing
>amplitude of excitation.  This means that by far the greatest amount of
>instantaneous energy going into the spark is delivered, in a spark-gap
>system, during the first few cycles of excitation.  It does not escape
>notice that, given the same mains input-power, a spark-gap coil will
>produce significantly longer sparks than a sstc--even given the
>relatively large loss of power in the spark gap.  So, it is very likely
>to be concluded that what's wanted is to cram as much power as possible
>into those first few cycles.

Remember the "peak power"!  A sstc runs 100% of the time at 1X the power.
A disruptive coil runs 5% of the time at 20X the power.  20Kv at 500 amps
is 1,000,000 watts of power into the streamer!  You be it "jumps" ;-))  Do
note the remarkable streamer length increases using the "staccato mode" in
CW coils.  A very interesting affect that is due to electrostatics but may
be very important to us and streamer length.  We are nearing the point of
having very good control over such unexplored factors.

>
>So what I propose with TCH2 is that the capacitances be markedly smaller
>that what I have in my present design.  In fact, they are to be small
>enough to give the desired rate of exponential decline in voltage during
>each spark event.  Note that the current in all the capacitors is
>unifirectional, not ac and that the primary is still untuned.  The
>capacitor voltages just decline during the spark event, they do not
>change polarity.  And since the IGBTs are to be driven from an external
>signal source, that source can either be tuned at will to match the
>secondary's Fr or it may readily be derived directly as a function of the
>secondary's return-current, as I do presently, to make the system
>instantaneously self-tuned to the secondary's Fr.

Self tuning and frequency control is really nice.  But the logistics of
making it work scare "me".  I'll stick with the plain simple 120BPS tuned
primary ;-))

>
>Most importantly, this scheme acts to completely separate the function of
>frequency-determination from the function of spark-energy storage. 
>Change the frequency?  Just do it: the energy available to create a spark
>won't change.  Increase or decrease the primary's time-constant?  Just do
>it: the frequency won't change. 
>
>And notice one more interesting thing:  I show the capacitors as being
>connected to the primary buses all along their lengths.  I think that it
>doesn't matter where they are connected since a) their current is
>unidirectional and b) the current in each of the buses is unidirectional.
> There exists the same length and locus of conductor regardless of where
>a capacitor is connected.  The advantage is that a large number of small
>capacitors may be closely attached to the conductors, both maximizing
>their overall current-carrying capability and minimizing their lead
>inductances and the overall ESR.  Also, it's a handy place to put
>them--all around the periphery of the primary.  

I worry a little about using caps as a primary inductor element directly.
Not sure what kinds of odd electrostatics goes on there.  Streamer hits to
a primary cap (optimistic ;-)) would be a bad thing.

>I envision two
>1/2"-diameter copper pipes for each segment, spaced vertically perhaps
>1/4" apart and with the capacitors' leads soldered to their outer
>peripheries.  Easy to assemble, easy to change the capacitors.
>
>I show 4 primary segments in TCH2 but could be any even quantity.  

There is a practical limit.  Probably more based on how much voltage/power
an IGBT can handle and what kinds of faults it may see.  Perhaps a physical
limit too.  Just have to work the numbers...

>Also,
>the capacitors could be charged in various ways, for example from
>current-regulating supplies as in my present design or resonantly as in
>Terry's OLTC design.

They can be "one" supply :-)))  Think Marx generator.  Simple resistors can
isolate the section during firing since say 5 ohms is trivial to a 0.01 ohm
primary loop.  All the current will ignore the 5 ohms...

>
>Current-sharing amongst the paralleled IGBTs should not be a problem:  

Watch out for that one!!  Just a little inductance or a late turn on and
boom!  This is especially true when running them at 3X their peak rating
:o)))  Try to think about using the cap impedances to force current sharing
like the OLTC.  That works great!!

>A
>very small emitter resistor developing perhaps 0.2V at peak emitter
>current should suffice to steer current away from the stronger
>transistors into the paralleled weaker ones.  

Yips!!  No!!  That 0.2 ohms is an enormous power drain for a disruptive
coil!  Think of what it does to the primary Q.  If you have say 0.5 ohms of
primary resistance in a 20Arms primary circuit that is 200 watts of real
power!!  I would never consider adding any resistance in the primary! 

>Such a resistor might well
>consist merely of the lead connecting each IGBT's emitter to the common
>point.  

Watch inductances!  13nH is small until you have 200 amps at 100kHz.  Then
you have 1.6 volts (reactive) which is enough to cause problems.  That is
just what goes on "inside" the IGBT case let along what we solder to it next...

>Further, avalanche-breakdown should never be a problem since no
>transistor can see any voltage greater than twice a capacitor voltage
>under any condition.

TVS transorbes are wonderful things too just to be really sure!!

>
>And finally, this is a low-voltage design.  I remember, when I was a kid,
>that the president of the ARRL was killed by his ham apparatus' high
>voltage.  That's always stuck in my mind.

Low voltage is wonderful.  All kinds of conventional tricks can be used
that are not available to us at 20kV!!  However, if we get higher (2400V)
firing voltages, things get a little more complex.

>
>It seems to me that this idea has great potential.  I hope I will
>personally have the energy to look further into it & build it and I urge
>others to contemplate doing so.  Or else...to contemplate telling me that
>it won't work.

Such a system for the OLTC looks very promising!!!!  Switching at only
120BPS still but using your four section system could solve many problems!!!

Cheers,

	Terry


>
>Ken Herrick
>