Re: A double resonance solid state Tesla coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jan Wagner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

On Sat, 28 Dec 2002, Tesla list wrote:
 > Original poster: "jimmy hynes by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>
 > I've posted a schematic and some simulated waveforms at
 > http://www.hot-streamer-dot-com/temp/drsstc.doc

I hope the mosfets/igbts and diode backtabs are insulated from the
aluminium heatsink, right? ;)

 > I think there's a clear advantages to having a resonant primary circuit in
 > addition to the 'coupled' secondary resonance. The SSTC is efficient for
 > 'continuous wave' energy transfer, but to achieve the quick secondary
 > voltage rise required for streamers, a very low primary inductance is
 > required. This results in very large magnetizing currents with low power
 > factor. Soft switching loss is low in most designs, but when you are trying
 > to get quick rise times, there is so much magnetising current that you
 > don't switch at a zero crossing anymore. The conduction loss is also large
 > because of all the extra current, so most SSTC designs are stuck with a
 > soft rise.

Yes... basically you can get rid of the magnetizing current going
"nowhere" by using a series capacitor as you did.

The downside of making the primary capacitor and shunted inductor resonant
at the resonant freq of the TC secondary is that you'll get a huge current
circulating in the pri cap and inductor.

I just did a model copied from your simulation model, and with a 1V
sine drive voltage (yup, just one volt!), it ended up with an rms current
of 17A. The peak was around 69.45kHz. Re-running with a 230V sine drive
voltage, the circulating current was as high as 2800A.

Unloaded, the system is low-Q (i.e. high reflected TC secondary
load/streamers impedance). With the streamer load, impedance goes down
and Q goes way up. You'll have to be careful in designing the pri<->sec
reflected impedance, so that you still end up with a low-Q loaded circuit
and more current into the secondary than circulating in the primary. This
is just from a quick-and-dirty simulation. It may or may not be a
problem in a real running SSTC... these Q considerations are from a
"slightly" different circuit, so it just could be it isn't a problem here
(?)

But in any case, you'll have to make the primary side capacitor large
enough (not capacitance wise, of course) for it to handle the higher
currents involved in the double resonant scheme. After that, i'd
guess it will work very well  :o)

good luck!

cheers,
   - Jan

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