Chris, The issue isn't voltage rating; low-ESR caps such as these are susceptible to excessive charging current at turn-on. At a former employer, we saw REALLY GOOD, expensive caps used on a computer's 5V bus exploding at turn-on, even ones rated at 50 volts. Replacing them with cheaper electrolytics (TEN volts!) took care of that. You might be able to alleviate turn-on stress by using a power-on monitor circuit to slow down the initial charge. But it'd be far cheaper to go to electrolytics.
Cortland >> 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. ------------------------------------------- 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"
