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Re: ALF: why not DRSSTC?

Original poster: Greg Leyh <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?

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.


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. 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?

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.

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. 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
ALF will operate at roughly 6x the total power. It will be highly desirable to avoid large energy storage requirements.

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.

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.


-GL