OLTCs, DRSSTCs and untuned primary SSTCs

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Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Hi again,

I have been meaning to get this email out for a while now, so here it is.

  I did some more thinking about "M", and I realized my definition for M is 
also flawed.
It is possible to get a good M for really short, high current pulses. The 
voltage drop would stay around 5-6 volts, but more IGBTs are needed, so it 
isn't as good. If we are trying to find a number for (energy to secondary/ 
number of IGBTs), assuming heating is the limit,  then big coils would have 
an advantage, because the lower freq. and longer drive time reduce the I^2 
term. Because of difference in coil size, this wouldn't be a fair 
comparison of OLTCs, DRSSTCs, and untuned primary SSTCs. To "fix" that 
problem, I guess we could design coils on pspice that all have identical 
energy transfer. I "think" this could be our equation:
  energy/(rmscurrent*time^0.5)

I'll attempt to explain my derivation

E=energy transferred,  R=resistance,  I=current,  T=time,  N=number of IGBTs

First of all, since this isn't meant as guide as to how many IGBTs you 
need, proportional to is as good as equal to, so I use = for both.

First we start with M=E/N, since the limitation is the heat in the IGBT, it 
is the same as M=E/(I^2*T*R). Now we have the problem to figure out what 
"R" is. If  M=E/N, and R = 1/N then R*E = M. Therefore R = M/E. If I 
substitute that in, I get M=E/(I^2*T*(M/E)), or M^2=E^2/(I^2*T). Finally, I 
get to M=(E^2/(I^2*T))^0.5, which can be simplified to
E/(I*T^0.5)

Of course, you could still cheat by using a really long pulse, but I can't 
think of how to penalize for that. There isn't enough info about frequency, 
rise time, and envelope shape on streamers. "Normal" SSTCs have shown that 
shorter pulses are better, but it isn't clear how short it needs to be. 
Aside from losses, I dont think there will be a big difference if you take 
2 or 4 cycles to ring up. It should be an easy test with an OLTC. All that 
you would have to do is change k and slightly change the input voltage to 
account for losses. If you could do this test with your mini OLTC, I would 
be very interested in the results.

There is also a 0.5 factor for H-bridge coils. H-bridges have four times as 
many switches, but they are only sucking up heat half the time, so the 
multiplier is 1/2.

I have decided to use matlab to do a more accurate analysis of it, with 
more detailed IGBT models (forward drop, and resistance). It turns out that 
switching loss is pretty insignificant. If I use the datasheet's values for 
switching loss, and scale it up linearly with current, then even when hard 
switching, the loss is only 1.6%. I have been spending all of my time on 
matlab for a while so no progress on the DRSSTC. I have some plots for 
everyone, they're on my website
  <http://www.hot-streamer-dot-com/chunkyboy86>www.hot-streamer-dot-com/chunkyboy86.


Jimmy