Re: [TCML] DRSSTC CM600 Ringing

[Tyler <tyler.c.c@xxxxxxxxx>]

I'm running the gates off a powered gate driver with a 3 ohm turn on
resistance and 1 ohm turn off resistance. Schematic can be seen here:
http://ctyler.scripts.mit.edu/gdriver.jpg, fairly simple implementation with
an N/P fet half-bridge driven by a UCC37322, fed from an optocoupler.

Did some scope observations at the first transition and it does look like
the phase lead doesn't really work on the first phase, which is interesting
because simulation shows that it begins working effectively instantaneously.


I did also notice that as the voltage goes up, the ring amplitude proportion
goes down as Steve discussed, which is good. I haven't actually scoped it
above 150VDC so I'm not sure what it's doing at the full 750, don't want to
have my scope sitting under the coil with arcs so near by.

Anyways, everyone seems to think it's no issue so I turned it on and had no
problems, ran it right up to 750V on the bridge with 3kA peak currents
detected. Here's a video: http://www.youtube.com/watch?v=VRAOBwTtA4A

The top-load is awful and insufficient which I think is limiting my spark
length more than anything, which is next on my list to improve after finals
end this week.

Tyler


On Thu, May 12, 2011 at 1:25 PM, Finn Hammer <f-h@xxxx> wrote:

> Tyler,
>
> As others have noted, your waveforms don´t look all that bad. I have
> never been able to remove the overshoot on the first transition myself.
> My own unlearned explanation to this is, that the phase lead inductor
> doesn't work before it has seen a reversal, but that is probably
> nonsense.
>
> What are you driving the gates of the bricks with? I ask because if you
> whack them open really fast, then it is those rippely waveforms you'd
> expect to get.
> I have found that if I can open the gates with a nice swoop from the
> gatedriver, without any ripple or overshoot on _that_ waveform, then I
> also get calmly looking main bridge waveforms.
> That's why I use discrete gate drivers with constant current sourcing
> capability.
> As long as the overshoot remains lower than 1200V you should be allright,
> but I never liked any ringing, because i think it looks as if something
> is about to break, it's "un beautifull", that's why I built the prediktor
> in the first place.
>
> Cheers, Finn Hammer
>
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