Chris,
I've had this same problem over the years with many man hours spent looking for why. We changed to 35 and 50 V parts to good effect in some cases. Other cases were not so easy. I like Cortland's idea the problem is charge current at turn-on. We had a 5 output 350 watt supply and there were definitely some heavy currents flowing at turn-on. In fact, that is when we saw most of the failures, turn it on and blow the caps clean across the test floor. Started a small fire once.
We incorporated soft start into our power supplies at some point in time and this problem seemed to go away too. Coincidence? Never did know for sure. Anyway, good luck!
Scott At 04:10 PM 7/29/02 -0400, Cortland Richmond wrote:
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: [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"
------------------------------------------- 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"
