Re: SSTC idea - DRSSTC ?

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Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 10:16 18/03/03 -0700, you wrote:
>Original poster: "chris swinson by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <exxos-at-cps-games.co.uk>
>
>
>DRSSTC ?? Whats that all about, is there anything online which explains the
>idea behind it ? It sounds interesting for sure!

The DRSSTC (Dual-Resonant Solid State Tesla Coil) is a clever idea by Jimmy 
Hynes. I'm not sure if he ever wrote it up on the web. His intention was to 
make an SSTC that could compete with spark-gap coils in terms of streamer 
length. This was the point of the OLTC too.

http://hot-streamer-dot-com/chunkyboy86

The DRSSTC is like an ordinary SSTC, but the primary circuit is tuned with 
a series capacitor. So, instead of getting a constant output, you get the 
notched envelope waveform that you see with a spark-gap coil, but 
backwards: it grows bigger with time instead of decaying. The control 
circuit fires the inverter in a burst that lasts exactly one beat of the 
envelope.

Because it's running in very short bursts, you can really push the 
circuitry to the limit, so primary currents of hundreds of amps are 
expected. In this respect, it's similar to the OLTC, but the control 
requirements are more challenging. The OLTC just turns all its transistors 
on, waits till the first notch, and then turns them off on a current zero. 
The DRSSTC driver has to fire out a pulse of RF at just the right frequency.

The drive circuits are trickier too because you are driving a full bridge 
of IGBTs at hundreds of kHz. Plus, nobody actually knows if IGBTs can stand 
that much current in a high-frequency inverter, even though it is resonant 
with zero-voltage/current switching. IGBTs are pretty slow compared to 
MOSFETs. The switching losses might be massive. The drive frequency would 
be critical too: passing that much current, if you wandered off the 
zero-voltage/current switching instant, you'd be scraping the remains of 
your IGBTs off the ceiling. Therefore you would probably need a 
feedback-type circuit. And, being pulsed, it would have to start instantly 
and reliably, which can be a problem with today's feedback SSTCs.

On the other hand, the bang size might well be bigger for a given set of 
transistors/supply voltage than in the OLTC. Then again it might not be: 
Heating in the IGBTs is what limits the bang size in both OLTC and DRSSTC. 
The DRSSTC will probably have higher losses. I'm betting on the OLTC, which 
is why I built one.

Either way, the DRSSTC is serious cutting-edge stuff and I wish its 
designer the best of luck. If it were me, I'd be inclined to build a small 
prototype with MOSFETs that ran off 36 volts or thereabouts.

P.S. The SSTC built by Vladimir Mazzilli might be a good basis for a big 
DRSSTC. It already has the resonant primary, high-powered IGBTs, and 
feedback driver (in fact the whole thing is a power oscillator) and all you 
would need to do is rig up an interrupter circuit with first-notch pulse 
width, change the gate zeners to give 30 volts gate drive, and crank the 
power supply voltage up to something suicidal like 500 volts. You may need 
to reduce the value of the DC link inductor too. I don't know if this would 
affect the circuit in other ways.

Steve C.