RE: SSTC problems1 (no its not a PlasmaSonic ;-))

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

Original poster: sebastian gaeta <sgtporky-at-prodigy-dot-net> 

Hi David,

I built the full bridge Feedback coil at:
<http://dave.golfbuddys-dot-com/hvguy/>http://dave.golfbuddys-dot-com/hvguy/

My coil needs a lot of improvement though. If I had it to do over again, 
(and I AM doing it over again!) I would use the same driver that they use 
for their 10 KW design, even for the P460s because it is more rugged, and 
the parts are more available. I saw a scope picture of someones gate 
waveform using that design, and it even appears to be a faster switcher 
than the one I am using. If you look at my picture, the gate drive 
transformer is not in a good place because the back fets are too far away 
from it. My new one will have the GDT in the middle of the heatsinks and as 
close to each fet as possoble. I also put 10 ohm resistors in series with 
each gate with a schottky across each one to speed up the fall times.

I rewired my bridge rectifier to do half wave instead of full wave because 
my mosfets get too hot. And half of them get hotter than the others so my 
duty cyle stinks too. I played around some more last night by powering the 
H bridge with 12VDC, and probing around with a scope. You need something 
like 100 volts on the antenna to give steep enough rise and fall times to 
create a decent enough square wave with your 1N34 'chopping diodes'. If the 
feedback voltage is too small, the duty cycle is uneven, and one set of 
mosfets work harder than the other set. Just something to keep in mind if 
the same thing is happening to anyone else. Rewiring the bridge was the 
last thing I did before I went to bed last night, so I haven't even tried 
it yet with half wave rectification. This should force things to run cooler 
even if my circuit is messed up!

A properly switching MOSFET should never even get warm because it should 
never be dissapating power. When the current is max. the voltage should be 
0 and when the the voltage is max there is no current! Power dissapation = 
0 = I am doing something wrong! The "beast09" does not look like a typical 
MOSFET switching waveform though! If you look hard and long enough, you can 
almost make out a square wave buried in there!

If you want to look at my coil, it's at: 
<http://community.webshots-dot-com/album/113763981zJBnik>http://community.webshots-dot-com/album/113763981zJBnik
The Mosfets are on the light colored rectangle heatsinks and the 
freewheeling, and mosfet body diode disabling diodes are on the small black 
individual heatsinks. I'll bet they get really hot, but the fans cool every 
thing off before I can get my fingers down in there in time to feel them.

Sue

Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "David Trimmell"

Hi Sue, do you have a schematic of your SSTC design? I am still trying
to decide what kind of driver circuit I want to use for my first
self-designed Half Bridge. I am ready to populate my second PlasmaSonic
board from Dan, but also want to try for some Solid State Sparks (as
apposed to audio)! But being a newbie to FETs makes for slow progress,
but Richie's website (http://www.richieburnett.co.uk/tesla.shtml) sure
is a great wealth of information! It sure is great to see the natural
evolution of Tesla's technology.

Regards,

David Trimmell