[TCML] SSTC full bridge control system question

[Michael Twieg <mdt24@xxxxxxxx>]

Thanks for the quick reply Steve,
I understand that 1-2MV is unrealistic.  I'm using that as a goal for
unloaded output (no streamers), so obviously in reality a spark will
break and compress the voltage to much lower levels.  But in my
simulations I'm not even able to get to 400KV before the primary
current hits 1.6kA, and this is without any load on the topload.
We're working with a coupling coefficient of about 0.15, and our
resonances fall around 24KHz and 34KHz.
I definitely get what you're saying about simulation being dubious. At
this point the coil is still being built and I want to be sure the
controller is good to go when it's finished, so simulation is all I
have to go on.  The coil will have a tapable primary (btw I mistakenly
said our coil is a two coil system-it's actually double resonant, my
mistake), and we'll be able to change its elevation to some extent in
order to change our coupling.  So we'll have some control over our
system, but I don't think any combination of parameters will allow me
to operate at the secondary resonance.
I'm not sure I understand what you mean by notching in the primary
current.  Are you referring to the minima in primary impedance (as
seen by the full bridge) that occur at the two resonance peaks?  I've
noticed that a zero-crossing converter like yours seems to always
resonate at the frequency with the lower primary impedance, and this
is part of my concern.  If it were possible to operate at the other,
higher impedance peak, I think it would be possible to get at least as
much voltage on the topload with much lower currents from the half
bridge.  That's what LTspice is showing me, anyways.

-Mike
---
Adjust the primary tuning for best energy transfer to the secondary.  This
can be somewhat pointless to do in simulation, other than just getting a
feel for how a 4th order resonance behaves to various inputs. The reason
modeling it is almost pointless is that no one has a really good model of
the streamer load (i have some models that are closer than others ive seen)
which really determines largely the behavior of things.  Also, 1MV is a HUGE
tesla coil output and likely unrealistic.  My big coils are about 700kV peak
i estimate from base current and other simulations, and i need about 1600Apk
primary current to get there.

Anyway, im not sure if your analysis is right.  The double peak comes from
the mutual inductance between the coils, where the M either adds or
subtracts from the apparent resonant inductance, which means there are 2
peaks, one just below Fres and one above.  Increasing the coupling (hence
more M) causes these peaks to move further apart as the adding/canceling of
the M term is more dramatic.  Generally, the most *efficient* tuning method
is to tune the primary to the secondary which results in primary current
notches.  The primary current notch is indicative of complete energy
transfer to the secondary (secondary I and V should be peak during primary
I/V minimum).  The transfer time is essentially controlled by the coupling
coefficient, where it should take 1/k cycles for the transfer to take place
(so k=0.1 needs 10 cycles, k = 0.125 needs 8 cycles etc..).  This places
some upper bound on the energy you can deliver to the spark within one
energy transfer cycle, for small coils it often ends up you cant get enough
energy transferred within 8 cycles or so.  So the other trick is to tune the
primary to excite just one of the resonant "poles" which means the primary
current should not notch, and the currents/voltages grow consistently over
time (until the streamer starts to consume all of the energy in the
system).  For my "transient" systems i find tuning to the lower pole
frequency works well because streamers tend to detune in that direction
anyway, which seems to make the system happy.  Tuning the primary really low
can allow you to effectively increase the "bang" energy to really large
amounts, allowing you to make really long sparks, provided your silicon and
capacitors can stand it.

Steve
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