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Re: OL-DRSSTC - 12

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Dmitry,

I just picked the caps out of the cap list since they were rated for "very high ripple". Maybe that meant "super high ESR" too :o))) I will have to go back and look at the specs in detail to see if I can get better ones for this. I was very surprised at how quickly they fell out under current. The ring frequency was 2.5 MHz. So I guess there is 14nH of inductance in there somewhere...

The ESR and buss ringing thing seems like a serious issue in general. Steve and I just tried different caps till we hit upon ones that worked well. But maybe there needs to be more "science" in all that... Seems like many small poly caps are best to fight resonance, but I needed big caps too to carry the load since the elctrolytics would not. I am using two 4.7uF metalized poly caps now from the original OLTC project that seems to work well (and they also fit perfectly):

http://hot-streamer.com/temp/OL-DRSSTC-2005-10-14-004.gif

Cheers,

        Terry


At 11:30 AM 10/15/2005, you wrote: 
> Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

> Hi All,

> Today I tracked down the voltage droop problem.

> When the IGBTs turn on, the voltage across the caps drops to the 10
> volt IGBT rail.  The electrolytics contribute NO real current to the
> oscillation!!!

and if they should do it? :-)
could you give such parameters of your electrolytics like esr & esl,
and their Fres?