Re: ALF: why not DRSSTC?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

On 25 Sep 2005, at 15:44, Tesla list wrote:

> Original poster: Jimmy Hynes <jphynes@xxxxxxxxx>
>
> Hey,
>
>
> On 9/25/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> wrote: Original poster: Greg Leyh
> <<mailto:lod@xxxxxxxxxxx>lod@xxxxxxxxxxx>
>
> Hi Jimmy, Steve, Terry,
>
> For simplicity, allow me to respond to both of your comments together
> here.
>
> Part of the problem I have had in grasping the OLTC and DRSSTC
> concepts is that I haven't found a concise one-sentence description
> for either of them.  For instance, I had thought that OLTC addressed
> the techniques for 'Off-Line' operation of a Tesla Coil.  Does the
> term also refer to the general case where a single solid-state device
> replaces the spark-gap in a classic impulse-type Tesla Coil?

"Traditionally" the term Offline appears to refer to anything using
directly rectified mains as a power source and hence non-isolated.
Where transformation is required, the usual mains frequency
transformer is replaced by a SMPS and a much smaller transformer
running at high frequency (which also isolates the output - e.g.
computer power supply).

Malcolm

> Like Conner said, it only counts as an "OLTC" if it uses IGBTs to
> directly replace the spark gap. How you charge the capacitor between
> bursts is completely separate from how you transfer the energy to the
> secondary, but I guess it wouldn't really be "OL" if you use some
> fancy charger to get to 10kv or something. When I say OLTC, I just
> mean using IGBTs as a direct spark gap replacement.
>
> In my simplistic view of coil design space, there are two types of
> Tesla coils -- Impulsive and CW.  Impulsive TCs discharge the primary
> cap over a period that is small compared to the charge time.  CW coils
> provide a continuous-wave drive to the primary.  The basic design
> philosophies for impulsive and CW coils are fundamentally different.
>
> Adding solid-state switching devices doesn't change the basic
> operation of these coils.   Impulse TCs can use static sparkgaps,
> rotary sparkgaps, hard-discharge tubes, and solid-state devices.  CW
> TCs typically use electron tubes or solid-state devices.  Rather,
> solid-state devices offer dramatic improvements in efficiency,
> compactness and quenching accuracy.  Increased reliability and fault
> tolerance has yet to be proven, but the potential is certainly there.
>
> The OLTC is clearly an Impulse-type TC; it stores the full bang
> energy in the primary cap then transfers the energy to the secondary
> quickly, compared to the bang period.  The fundamental operation of
> the coil itself is the same as a classical sparkgap coil.  By
> contrast, the DRSSTC is not clearly an impulse-type nor a CW coil; it
> contains essential features of both designs.  Like CW systems, the
> DRSSTC uses feedback to control the freq and phase of the primary
> drive elements. However, the DRSSTC attempts to gain some of the
> pulsed-power benefits of an impulse-type coil by concentrating power
> delivery into shorter, more intense bursts, at a rep rate suitable for
> sustaining an arc channel.
>
> The DRSSTC is the first example I've seen of a coil design that
> explores quasi-CW operation in this fashion, although there might be
> similiar examples of pulsed CW TCs of which I'm unaware.  For this
> reason the DRSSTC might qualify as a *third type* of Tesla Coil, owing
> to its quasi-CW nature.  It appears that the full-wave bridge drive
> isn't essential to the quasi-CW operation, so I would view the
> full-wave aspect more as research into alternative primary drive
> techniques.
>
>
> Yeah, there's no clear distinction between where it stops being
> 'impulse', and starts being CW.
>
>
> Jimmy Hynes wrote:
>
>  >What's your trick for doing it "quickly"? The DRSSTC also sucks the
>  >energy back out of the secondary. If you don't think it's quick
>  >enough, we could probably implement the same sort of 'trick' you got
>  >to do it quickly.
>
> No tricks, other than higher coupling and accurate quenching.
>
>
> Ok. The same can be done with a DRSSTC.
>
>    Also,
> if one chooses a coupling value near one of the 'Magic-k' values, it's
> possible to achieve both zero-voltage and zero-current switching.
> However, if you transfer the energy that fast, is it still a DRSSTC?
> At what point does it become a standard impulse-type TC?
>
>
> It is possible to run near the "Magic-k" values, and "quench" at the
> second notch, letting the energy build up more than is possible in one
> beat.
>
> As long as you're running at least a cycle, it's operating
> differently than the "normal" type, but it can still be very much an
> impulse.
>
> If you use the detuning trick, the coil won't breakout at all for the
> first 20 cycles or so, and the streamers form only on the last couple
> cycles. From the streamers point of view, it is not CW like at all.
> This case is interesting because the inverter's peak power is
> relatively low due to the long ON time, but all the energy gets dumped
> in the streamers rather quickly.
>
>  >300PPS!?!? Why so high? Even much smaller coils find better
>  >efficiency at half that. Bigger coils seem to need less PPS (which
>  >would make sense with the square/cube law thing going on with the
>  >streamers), so a huge coil should need rather low repitition rate.
>  >With your concern with the RTC thing, that was a real suprise to me.
>
> Based on my personal experiences with the 40kW experimental coil and
> the 130kW Electrum.  The 40kW coil would display continued streamer
> growth up to ~350PPS.  Electrum seemed to top out at around 270PPS.  I
> certainly appreciate the squared/cubed argument for arc channel heat
> retention, and I would expect larger arc channels to require lower
> break rates.  However, it would seem an unnecessary risk to reduce the
> break rate capacity of the machine if it can be avoided.  The break
> rate can be reduced later, much easier than it can be increased.
>
>
> While I certainly agree that it's good do design for a higher PPS than
> you need, I still find 300PPS to be excessive.
>
> If you're goal is to make the biggest streamers possible, it might be
> easier to design for super high break rate instead of trying to jack
> the bang energy up, but if you're worried about efficiency at all,
> you're gonna keep the break rate down. By using a low breakrate, it's
> almost certain to have more of an efficiency benifit than the RTC
> could give.
>
> I am suprised that your coils showed an increase up to such a high
> break rate. Do you have any idea what the PPS/spark length curve
> looked like?
>
>  >With a DRSSTC, you could use the whole time to charge the
>  >capacitors, since its just a DC rail.
>
> This is true, although one could conceivably add a high capacity DC
> rail to the front end of any primary drive topology.
>
>
> Right.
>
>   The problem
> with adding a high energy storage DC rail is the potential for
> creating utter carnage in the event of a circuit fault. One example is
> a recent klystron modulator design, which employs full-wave IGBT
> bridges driving resonant step-up transformers.  Average power is ~1MW.
>  The DC rail is 2500V, and has enough rail capacity to supply one
> pulse.  Break rate is 60PPS.  Here's what happened as a result of
> shoot-through current in the H-bridge:
> <http://www-group.slac.stanford.edu/esd/HbridgeDebris.jpg>http://www-g
> roup.slac.stanford.edu/esd/HbridgeDebris.jpg ALF will operate at
> roughly 6x the total power.  It will be highly desirable to avoid
> large energy storage requirements.
>
>
> I see what you're saying, and agree that you wouldn't want the
> capacitor to hold much more than necessary, but that doesn't change
> the fact that you can be charging the capacitor nonstop. There may be
> a ton of ripple, but the capacitor isnt oscillating, it is just
> supplying DC power, so it can be charged without waiting for the bang
> to end.
>
> Steve Ward wrote:
>
>  >>C)  Managing circuit reliability and total parts cost, with the
>  >>larger number of IGBTs that a full DRSSTC H-bridge requires.  -GL >
>  >But, you are talking of using a 12kv supply, requiring seriesed
>  >switches, while you might obtain the same results from an H-bridge
>  >running at a lower voltage.
>
> This is true.  However the reason I would prefer using a single,
> series-connected switch array at twice the voltage is simply to
> mitigate the hazards of single-point failures.  In the standard
> single-switch design, all the IGBTs are turned on simoultaneously,
> avoiding the possibility of timing errors between opposing IGBT banks
> causing short-circuits.  Each IGBT in the stack can protect itself
> against overvoltage by turning back on momentarily.  Also, a series
> switch stack will tolerate failures of individual IGBTs, since they
> are intentionally designed to fail shorted.
>
>
> Well, in one case, you have to worry about timing to prevent short
> circuits, in the other, you have to worry about timing to prevent
> overvoltage. Just as IGBTs can turn on to prevent overvoltage, they
> can turn off to prevent short circuits.
>
> It's just a question of which would be easier to design. No one
> really has problems with short circuits unless one IGBT dies for some
> other reason, and an overcurrent protector would not be difficult to
> implement.
>
>  >To quote from your other post:
>  >
>  >>" I believe that it's not only possible, but essential to determine
>  the >>best topology at the beginning.  Simulations can accurately
>  model >>much of the complex behaviour exhibited by TC's, and good
>  >>physics-level models now exist for the HV IGBTs." > >Exactly, this
>  is the only true reason i asked this question to begin >with.  I
>  wanted to know if the DRSSTC topology (or something >similar) has
>  been considered.
>
> No, I have not yet considered a quasi-CW mode of operation or a
> full-wave primary drive scheme.  However, I am curious about the
> DRSSTC theory of operation and would like to run some simulation
> studies. Perhaps then I'll have some better questions to ask.
>
>
> Ok, run some simulations and get back to us ;-). I'm still convinced
> that it's the way to go :P
>
> -GL
>
>
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