Tantalum inrush current - Page 1
Forum section: electronics / projects

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 23, 2017, 01:23:20 pm
content: I've always heard that when you use Tantalum caps, you need to be careful with inrush current to keep from damaging them. There are also rules with derating the voltage, and apparently that also affects the inrush current limit. I'd like to know more about this, to start I have two questions:1) How are the voltage rating and inrush current limit related? If I'm feeding 14V into a 16V cap with a 20mR ESR, what would I need to limit the voltage to to keep it safe? How about if it's a 35V cap?2) I've heard that Tantalum polymer caps are more tolerant and require less voltage derating, is that true? How would that affect the inrush current limit for the two scenarios from #1?

author: dmills
author_rank: Super Contributor
timestamp: December 23, 2017, 05:11:38 pm
content: You really don't want to be running a 16V TANT at 14V, no matter what you do for inrush limiting!With a 16V part I would not be designing for more then about 8V maximum rail if using it as a supply cap (Something like a PLL loop filter I might be more relaxed about it, this more then power supplies is the proper use of the tant IMHO).Check the manufacturers datasheets and app note for derating and inrush limits, most of the worthwhile manufacturers have this stuff documented and it does tend to differ a little between vendors.One warning with very low ESR parts, watch for ringing due to supply inductance, there is often something to be said for ESR to kill the Q of the external supply lines.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2017, 07:48:16 pm
content: Quote from: suicidaleggroll on December 23, 2017, 01:23:20 pmSurge related failures in solid tantalum capacitors occur when at the fault site, the tantalum oxide dielectric layer cannot absorb enough oxygen converting to Ta2O5 as 2MnO2 converts to Mn2O3+O. Higher voltage parts have a thicker tantalum oxide dielectric so they can clear larger shorts without going into thermal runaway.With 1/2 voltage derating, surge related failures are practically zero and with a 2/3rd voltage derating, they are almost zero. Where voltage derating is not used, controlled series resistance or surge current limiting can be used instead.QuotePolymer tantalum capacitors use a different mechanism for self healing which is not subject to thermal runaway.

author: tszaboo
author_rank: Super Contributor
timestamp: December 23, 2017, 10:37:10 pm
content: 1) If you feed 14V, with 0 source impedance into a not charged cap with 20mOhm ESR, then the peak current is 700 Amps. Ohm's law. Also, de-rate that cap more.2) Polimer tantalums for new design are... Pointless IMHO. They make polimer aluminium caps in the same sizes, and all their parameters are better. Also, you dont get "whoopsie not on stock because african crime lords massacring everyone in the mines, lead time 99 weeks" with aluminium capacitors.

author: gnuarm
author_rank: Super Contributor
timestamp: November 03, 2022, 03:45:54 pm
content: I know this thread is old, but it's still relevant as long as the facts are updated.Quote from: tszaboo on December 23, 2017, 10:37:10 pmSure, but I've never seen a supply with 0 ohm source impedance at all currents.QuoteI checked on Digikey and the Aluminum - Polymer parts seem to be the same size as electrolytics, which is no where near the size of either solid tantalum or polymer tantalum. The tantalum I had been using is 7.3 x 4.3 x 4.3 mm. I can't find a polymer aluminum cap that is shorter than 6 mm, and 8 mm diameter. Am I doing something wrong? I can't use a 6 mm tall part on my board. 4.3 mm is about the limit.