Re: [TCML] SSTC full bridge control system question

[Steve Ward <steve.ward@xxxxxxxxx>]

Forgot to mention, the idea of designing (simulating) to get *more* peak
volts out assuming no load, does not make sense.  Streamer loading
effectively clamps the secondary volts, which is essentially reflected to
the primary, which should *reduce* the primary current.  So trying to get
1500kV in simulation will need a lot more primary amps than what it takes to
get 750kV in real life.

Steve

On Sun, May 2, 2010 at 2:21 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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>
>
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