Chuck, Your email brought up a point that I hadn't considered: i.e. variability of ESR causing unequal division of ripple current, thus overloading one capacitor.
I can see the reasoning for using the equalizing resistors (ensuring that the ripple currents balance across the paralleled capacitors). I want to add that adding the equalizing resistor would theoretically increase the ripple voltage on the output; since one component of ripple voltage is the ripple current multiplied by the ESR of the output capacitors. I'm not sure if we have the ripple voltage headroom in our design to use the equalizing resistors. >From reading the reponses; it seems like tantalum output caps on switching >supplies are TROUBLE! We may need to look at other options such as "OS-CON" >type capacitors or some of your other suggestions regarding larger caps with >fewer in parallel. Thanks, Chris > -----Original Message----- > From: Chuck Mullett [SMTP:[email protected]] > Sent: Wednesday, July 31, 2002 10:09 AM > To: [email protected] > Subject: tantalum cap reliability > > > Hi all, > > Paralleling low-esr capacitors is a little scairy. With 8 caps in > parallel and each allotted its maximum ripple current, one capacitor, > theoretically, could conduct 8 times its allowable rating! This, of > course, is the theoretical limit---all other caps would have to be > "open." > > That's a ridiculous example, but perhaps you get my drift. With two in > parallel, the worst case is twice the current. Esr is not well > controlled; it's unusual to see it bounded both with a "max." and a > "min." With lot-to-lot variations, and with 8 in parallel, I wouldn't > be a bit surprised to see one getting two or three times its rated > current. The more in parallel, the worse it gets. > > Your choices: 1. Use the largest available caps to parallel, thus > reducing the total number. 2. Design with 2x to 4x margin of esr. 3. > Place small "equalizing" resistors in series with each to "swamp out" > the variation in esr. > > Cheers, > > Chuck > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
