On Mon, 29 Jul 2002 15:41:32 -0400, "Chris Maxwell" <chris.maxw...@nettest.com> wrote:
> One of my colleagues is testing a new design. He has designed a > buck-boost switching converter which has tantalum output capacitors. > We have looked at his design and gone through the calculations. His > output current is 4 A maximum. His output voltage is 12 VDC His > caclulated ripple current is 800 mA. He needed a 120 mV ripple > voltage, so he put 8 each of 68 uF, 20 V tantalum capacitors (with > 150 mOhm ESR) in parallel on the output. Each cap is rated for > approximately 800mA of ripple current. > He has seen two failures of these capacitors during initial testing > and demonstrations. Meanwhile, many initial units run fine. From > what I can gather, he hasn't violated any design rules. He has 20V > rated caps on a 12V circuit. He has a ripple current rating of 8 X > 800mA (8 caps in parallel). > > It is tempting to just increase the voltage rating to 25V or > 35V....but why? Even if he does, how do you prove that the problem > is fixed. It would take months of testing the new capacitors to get > the history that we have on the existing design. > > I guess what I'm looking for is some tantalum capacitor wisdom... > Should we just go ahead and use 25 V or 35V caps in this 12 V > application? Are tantalum caps that flaky? What is the possibility > that we don't have a design problem; but just a couple of bad > capacitors? Is there some piece of knowledge out there that would > help us tell the difference? Tantalum capacitors tend to short-circuited and can cause fire when used in low impedance circuit. I think many capacitor manufacturers suggest not to use their tantalum capacitors in low impedance circuit. Well, changing 25V to 35V will decrease the failure rate, but it may not enough. Low ESR aluminum electrolytic capacitors are available these days, and I think they are easier to use in such case. Yet another possible choice may organic-semiconductor capacitors, such as OS-CON from Sanyo. (http://www.sanyo.com/industrial/components/oscon_home.html) Regards, Tom -------------------------------------------------- Tomonori Sato <vef00...@nifty.ne.jp> URL: http://member.nifty.ne.jp/tsato/ ------------------------------------------- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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"
