Nonetheless, inrush current aside, a 20V Tantalum is considered marginal for a 12V circuit if reliability is desired. A 60% derating factor was and is a typical max for reliability circuits, i.e a 30V min rated cap is recommended. Tantalums require additional derating than Al-electrolytics. Furthermore, switchers are notorious for destroying Tantalums due to the large and fast V-swings. I don't recall seeing too many Tantalums on switcher designs.
--- Cortland Richmond <72146....@compuserve.com> 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: > 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" ===== Best Regards Hans Mellberg Regulatory Compliance & EMC Design Services Consultant By the Pacific Coast next to Silicon Valley, Santa Cruz, CA, USA office:831-454-9450, cell:408-507-9694, fax:831-454-0755 __________________________________________________ Do You Yahoo!? Yahoo! Health - Feel better, live better http://health.yahoo.com ------------------------------------------- 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"
