Re: [TCML] SSTC full bridge control system question

[Michael Twieg <mdt24@xxxxxxxx>]

I do see some slight beating in the output voltage and primary
current, but it's very negligible compared to the rising envelope of
the primary current.  Definitely no noticeable peaks or notches.  And
after 7 cycles (about 500us) the secondary voltage only reaches about
200KV with the primary current peaking at 1.2KA.  This is with my coil
parameters adjusted to meet your condition where primary LC equals
secondary LC.  My primary L is 56uH, primary C is 0.715uF, secondary
L=270mH, secondary C=148pF, and K=0.14.

And I mistyped once in the last email:  we are doing a full bridge.
There's definitely no way we'd get so much power out of just a half.

Regards,
Mike

On Sun, May 2, 2010 at 3:21 PM, Steve Ward <steve.ward@xxxxxxxxx> wrote:
> If you tune primary L*C = secondary L*C, then the primary and secondary
> currents should be a sinusoid enveloped by another lower frequency sine wave
> (aka, "beating").  You should be able to run the primary for one "beat"
> cycle, which for you should be about 1/.15 (~7) cycles long.  This should
> the best secondary peak voltage vs primary voltage/current.  When the
> secondary is peaking, the primary is notching, because the energy has been
> transferred from the primary to the secondary over those 7 cycles.
>
> I did not realize you are attempting to get this much power from a
> half-bridge, that will be very hard without a lot of current, so 1.6KA seems
> like a reasonable current to get that much secondary energy.  There really
> isnt any way around this, if you work through the physics, you are going to
> be required to store a significant amount of energy in the primary before
> the secondary can use it up, and this means high peak primary currents.
> Streamers prevent you from building up energy over a really long time in a
> high impedance primary (so I is less because L is bigger), so if you want
> fast streamer energy delivery (which makes for efficient streamer growth)
> then you need high peak primary currents.  Tuning should only make a small
> improvement in spark length vs primary current, once you get it close of
> course.
>
> If you can go full-bridge drive, id suggest it because it sounds like you
> are really after much more power than a practical half-bridge will support.
>
> What silicon are you working with for the main switches?
>
> Steve
>
> On Sat, May 1, 2010 at 10:25 PM, Michael Twieg <mdt24@xxxxxxxx> wrote:
>
>> 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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