Re: Recent s.s.t.c work

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>



> If anyone has some ideas as to how to maintain zero-current switching
> in a simple feedback-system, I'd like to hear them!

I explored a whole bunch of different ideas, but I kept coming back 
to the PLL. It doesn't give perfect zero current switching under all 
conditions but it gets close most of the time.

Whether it gets closer on average than a simple feedback driver does, 
under normal DRSSTC duty, is still an open question. But the new PLL 
driver has had a good track record so far and I have enough 
confidence in it now that I can think about turning it loose on some 
big IGBT bricks.


The reasons why I stuck with the PLL:

It seems more resistant to noise than an ordinary feedback system. It 
can reject feedback signals that would make an ordinary driver gallop 
off at crazy frequencies. This is a double edged sword however as it 
also makes it slower to respond to sudden changes in the resonator 
such as a ground arc. A plain feedback driver tracks these instantly, 
but the PLL takes a couple of cycles to catch up.

It allows you to add as much deadtime as you like for slow IGBTs, 
while maintaining ZCS (it can "look into the future" to compensate 
for the deadtime)

It lets you access an extra tuning that plain feedback drivers with 
primary current feedback can't use. I thought this was just a gimmick 
at first but I found that I get the biggest sparks from this tuning.

I thought that these advantages were enough to make up for the extra 
complexity and somewhat fiddly tune-up procedure. My driver has five 
pots to twiddle and the other Steve's had just one adjustment last 
time I looked.


BTW- I think I might have signed up for that "Scientists Called 
Steve" thing already? I'd better check.

Steve Conner
http://www.scopeboy.com/