DRSSTC thoughts...

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi All,

I have not been following the DRSSTC things much.  Been very busy for about 
the last year...  But after reading Dan's new book I have some 
questions/comments.


Dan – I see you used the model for streamer loading of 220K + 1pF/foot for 
a streamer load in your SPICE models.  I was just wondering how well that 
worked for you?  There is some concern that model is just “too simple” for 
some things, so I was curious if it worked well in this case?

I also note from Dan's book that he has “almost” as many “toys” as "I" do!! 
:o))


I see that super high current IGBTs are really in demand now ;-))  But 
monsters like the fabulous Powerex CM600HA-24H my be too heavily designed 
for “power” when what we need is “current”.  It can “officially” do 1200 
amps peak which is nice but I know we don't need the 4200 watts of power 
dissipation ;-))  So I wish to remind of the simple IR IRG4PH50UD.  A lowly 
24 amp rated TO247AC IGBT with diode.  But it uses a great big die in a 
small package thus it's 180 Amp peak rating ;-))  Note that some people 
have put a lot more current through them like this one happily eating 700+ 
amps :o))

http://hot-streamer-dot-com/temp/OLTC08-30-02.gif

http://hot-streamer-dot-com/temp/OLTC08-30-01.jpg

Above about 740 amps, the signal starts “flat topping” do to the fact that 
the gate structure simply cannot support more current no matter how high 
the gate voltage is.  BTW – I note a lot of folks no longer worry much 
about the maximum gate voltage specs :o))

Using big IGBTs might not be a problem if you can get them cheap.  Mouser 
has them for only $330 (new which is a great price!!)  But suppose we just 
can't find them “cheap” or want new IGBTs without spending $1300...  Then 
using a few of the little IRG4PH50UD IGBTs may start to be a good 
deal.  They cost $16 a “pop”.  If you get $1300 worth of them, you have 20 
IGBTs per leg at 3600 amps peak “rated” and 14000 amps “unrated”!!  So it 
may be useful to consider a “multi-mini IGBT” (MMIGBT ;-)) array in some 
cases.  Of course, we don't need 80 of them.  But maybe 12 would do for 
about $200.  Everything from the drive circuits back is just the same as 
usual...  One can typically find the peak current using straight forward 
calculations.  However, Dan's data shows the primary current can almost 
double during a ground strike!!  That is a critical bit of info!!  Do the 
models show that behavior too?

Of course, if off the self IGBTs just are not right, check this DigiKey 
part out ;-))

IRG4CC50UB-ND

600V 55A for a buck ;-))  I bet you can find a wire bonder on E-bay for a 
song (Yipps!! guess not 
(http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&category=45045&item=3842974611)... 
Just use the silver epoxy for the pad bonding too...)...  Silver filled 
epoxy for mounting, and your there baby!! :o)))


Also note that the primary cap strings could be distributed between the 
IGBTs to force current sharing like the OLTC does if that would help:

http://hot-streamer-dot-com/temp/OLTC8-10-01.gif

http://hot-streamer-dot-com/temp/OLTC8-10-02.gif

For driving IGBTs, don't forget the wonderful TLP250:

http://rocky.digikey-dot-com/WebLib/Toshiba/Web%20Data/TLP250.pdf

Too bad it probably does not have enough frequency in this case.  But it's 
real claim to fame was that it was optically coupled to give 2500 volts of 
insulation which could  eliminate the drive transformer!! =:O  One can play 
little zener/resistor/cap tricks from emitter to collector to find the 
power to run the hot side.  Hopefully, there may be a faster version 
lurking out there somewhere now.


Apparently, primary to secondary arcing is a  problem.  This is often due 
to incorrect primary to secondary tuning.  Since DRSSTCs are self tuning, 
there may need to be a little “tweaking” the that circuit to better 
match/lock the primary and secondary frequencies.


Dan states that “protection” circuits could use some work...  They will 
probably be pretty simple and easy.  “Figuring them out” is the hard part 
;-))  Transorbs and MOVs can do wonders!!


The SPICE models can be used to force all kinds of fun fault conditions 
too.  Primary to secondary arcs and such are just t timed switch 
away.  These studies tend to reassure one that such events usually are not 
too bad and can also find a few real doozies!


One may “possibly” be able to use 60Hz resonant primary charging for the 
DRSSTC like the OLTC used.  That would eliminate a lot of the DC charging 
and giant cap stuff.  But I do note that “I” seem to be the only one 
foolish to go that way ;-))  But I have not had any problems with it...


It would be neat if there were a fairly off-the-shelf system that could be 
hooked up to any already made coil to get rid of the HV stuff and simply 
convert it to DRSSTC operation right then and there.  I know a number of 
people have boards out now so maybe I am just all behind times ;-))


So it looks like the old NST class systems have a giant competitor 
now...  I thought the OLTC would do it, but that system runs into some 
issues that make it rather difficult.  Of course, Steve Conner took it to 
the MAX!!  Sorry I never go a nice write up for it done...  I am happy to 
see a few of it's tricks made it into the DRSSTC, but I think a few tricks 
may have gotten missed too ;-))  I too must think that the days of the 
spark gap coil are numbered.  Not because there “are” good SSTCs out there, 
but because the will soon simply be the “best” TCs...  Even “big” systems 
are coming to speed fast!!  The record for a point to point arc is 59 feet...


Maybe this all will be a bit of help, forget any parts that are obviously 
stupid :o))

Cheers,

	Terry