Re: [TCML] SSTC full bridge control system question

[Michael Twieg <mdt24@xxxxxxxx>]

I mentioned in my second post that I knew that 1-2MV was an unloaded
specification, and I expected the practical voltage to be much lower
due to streaming.  The unloaded voltage is, to me, a hypothetical
figure of merit that makes simulating easier (since my spark gap
models make the simulations freak out).  In reality I think we'd be
happy to break 700KV, or 9 foot streamers.

Regards,
-Mike

On Sun, May 2, 2010 at 11:41 PM, Steve Ward <steve.ward@xxxxxxxxx> wrote:
> 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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