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Re: ALF: why not DRSSTC?

Original poster: Greg Leyh <lod@xxxxxxxxxxx>

Hi Steve, All,

Not to stir the alphabet-soup further, but the ALF design approach is clearly and fundamentally different from both the OLTC and DRSSTC concepts. This isn't surprising since the ALF design is driven by unusual constraints that are largely unimportant to standard Tesla Coil designs. I'll attempt here to describe some of the key operating issues and differences in the design topologies as I see them.

RTC
Currently, the ALF uses what I've termed an RTC (Regenerative Tesla Coil) design approach, which I've outlined before briefly in a couple of posts:
http://www.pupman.com/listarchives/1998/january/msg00059.html
http://www.pupman.com/listarchives/2004/October/msg00146.html

The key feature of an RTC is its ability to *quickly* (in less than 2 cycles) return the secondary energy back to the primary after leader production in the arc channel drops off -- typically about 2-3 cycles into the envelope. Most standard coil designs do not have critical efficiency requirements, and can allow this unused energy to simply dissipate in the secondary. However in the ALF design, recovering this unused energy can represent over 1MW in AC power savings. The extra work required for an RTC design is clearly worth it in this case. The main disadvantage of the RTC is that it requires a more complicated charging system, since the returned energy will set the primary capacitor to some arbitrary voltage at the start of each charging cycle.

OLTC
The OLTC uses an elegant passive resonant charging technique to "Off-Line" charge the primary capacitor directly from the AC mains. The resonant charging swing in the OLTC depends on the fact that the primary capacitor voltage always starts at zero. Since the ALF will routinely start each charge cycle at some non-zero voltage, it cannot use the off-line charging method that the OLTC enjoys, as this would produce incomplete and erratic charging cycles. Instead, the ALF will require some form of active DC command charging, most likely a polyphase buck regulator array, between the 12kV prime power source and the primary cap banks.

The transformerless, off-line connection of the OLTC requires that the primary circuit float at the common-mode voltage of the AC mains. By contrast, the ALF primary circuit must be referenced symmetrically around earth ground to minimize the need for corona control hardware and to ease the application of controls and diagnostics. The DC command charging system effectively isolates the center-grounded ALF primary circuit from the AC mains.

The ALF 1:12 prototype is actually more distant from the off-line OLTC concept than the ALF, in that it uses 4 paralleled power transformers to isolate the primary drive from the AC mains. Tsk, transformers... yes, I know. This prototype unit will also operate as an RTC to test out basic concepts and determine overall effectiveness.

DRSSTC
The DRSSTC, as I understand it, employs a full H-bridge of switching elements to drive the primary of a Tesla Coil in a series resonant configuration. Secondary feedback controls the operating frequency in addition to having a tuned primary, hence the term 'double resonant'.
The quasi-CW nature of the DRSSTC results in lower stored primary energy and RMS currents, easing the stresses on the primary capacitor and IGBTs.
IMO there would be several key design challenges in extending the DRSSTC to >1MW power levels:
A) Managing the fast commutation dynamics between the IGBTs and back-diodes, since zero-voltage switching is not possible in this configuration.
B) Effectively recovering unused secondary energy, since the quasi-CW nature of the DRSSTC requires many cycles to transfer energy between coils. Typical DRSSTC envelopes appear to require about 10 cycles.
C) Managing circuit reliability and total parts cost, with the larger number of IGBTs that a full DRSSTC H-bridge requires.

Of these, the slower energy transfer characteristic would likely be the biggest obstacle. Since the Fo for ALF is unusually low (~6kHz) there is not sufficient time to operate in a quasi-CW DRSSTC mode, as 10 cycles of 6kHz would use up most of the time available between pulses at 300PPS. Ideally, we would like to spend at least 80% of that time charging, to minimize impulse loading on the generators.
To maximize charging time, the RTC design strives to transfer energy between coils at much higher impulse -- typically in less than two cycles. This impulsive transfer requires the primary capacitor to store the full shot energy, and support higher RMS currents than in a DRSSTC design. In fact at PPS rates over 200Hz, the DC command charging circuit will start its normal charging cycle *before* the energy transfer from the secondary has fully completed.

Regarding IGBT's, 1200 volts is not a magical number. The 4500V and 6500V IGBTs are just as easy to use, although the 6500V parts have somewhat longer turn-off tails. The best performing silicon I've seen by far is the Mitsubishi 4500V PT. Higher voltage operation can be readily attained by stacking IGBT's in series, with little degradation in dynamic performance. Here's some performance data for a homebrew 12kV IGBT stack, consisting of five Mitsubishi 4500V IGBTs in series:
http://www-group.slac.stanford.edu/esd/ILCMarx/12kVDatasheet.gif
The prototype looks like this:
http://www-group.slac.stanford.edu/esd/ILCMarx/12kVassemInset.jpg
Typical switching waveforms at 12kV, 180A:
http://www-group.slac.stanford.edu/esd/ILCMarx/12kV50kHzBurstTest.gif

To summarize, the ALF design does not fall into either the OLTC or the DRSSTC category, by any stretch of the terminology. The parameter space for resonant transformer designs is vast, where any one design topology occupies only a relatively small area.

-GL


Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hello all,

This message is particularly aimed at Greg Leyh, but I would like comments from others as well.

As far as i know (and i might be wrong) Greg is currently working on a scale model of his ALF towers. This prototype uses the OLTC topology to drive the Tesla resonator. Since silicon appears to be the weapon of choice already, I'm curious as to why not DRSSTC instead of OLTC?
It seems (at least on our hobbyist level) that the DRSSTC can outperform an OLTC for similar amount of silicon used. The DRSSTC also does not have the difficulties that the OLTC intruduces as far as primary coils are concerned (many OLTCs are just 1 or 2 turn primaries). The DRSSTC also does not have to store the entire bang energy in the tank cap (another benefit)

One possible issue i could see is this: 1200V devices will only get you so far until you are looking at using single turn primaries and giant tank capacitors (resembling the OLTC, but this is even more problem for OLTCs as they scale up as well). So you might be forced to look at 1700V or 3300V devices. But I'm aware that these devices also have their limitations (they are slower and have greater losses, but i think these are not much to overcome). Ive heard that the real problem is from cosmic rays causing the devices to turn on or avalanche (what is the exact mechanism?) when you don't want them to.
But, wouldn't this also be a problem with using higher voltage silicon in the OLTC?

So for each problem I see with scaling a DRSSTC to ALF size, it seems an OLTC would have the same problems. As I (and others) see it, the DRSSTC is overall a better topology. So to summarize: why OLTC over DRSSTC? I'm guessing Greg has thought about this more than i have, so i would really like to hear his response.

Thanks,

Steve Ward