Riple current in Aluminium Electrolytic capacitors Vs Temperature Vs Duty cycle. - Page 1
Forum section: electronics / projects

author: raulcomander
author_rank: Newbie
timestamp: September 20, 2015, 08:45:01 pm
content: Hi thereI have been reading about aluminium caps here and there because I was worried about the ripple current in the caps for my application. The fact is that I am using it for an input filter for an H bridge that is making the caps to withstand a big ripple current.After reading this docs...http://www.nichicon.co.jp/english/products/pdf/aluminum.pdfhttp://www.cde.com/resources/catalogs/AEappGUIDE.pdfThis on is for tantalum, but can be interestinghttp://www.eetimes.com/author.asp?doc_id=1279791... I have understood that the ripple current limitation given in the datasheets is based on a thermal limitation at a given ambient temperature. So basically if an application is running for 0.3 sec and stop for 1.5sec a 1:5 ratio (This is what I called duty cycle in the title) and below maximum temperature, my thoughts are that it is safe to exceed this ratings without concerns of reducing the capacitor life.What are your thoughts?Similar analysis I have seen for mosfets, however never for caps, that is why I am asking for information about how much could it be pushed.Thanks!

author: tautech
author_rank: Super Contributor
timestamp: September 20, 2015, 09:01:52 pm
content: Welcome to the forum.IMHO it will depend on how far and wisely you wish to push component ratings and the reliability you require.In days gone by, it was good practice to not subject any component to more than 2/3 of any of its ratings, this ensured long working life providing quality and appropriate components were selected in the first instance.If you provide more detail better technical minds than mine should give you good advice.

author: Marco
author_rank: Super Contributor
timestamp: September 20, 2015, 10:24:55 pm
content: AFAICS heating is pretty much a bulk process without hotspots, so I'd agree. So doesRIFA.Quote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2015, 10:44:18 pm
content: Yes, the thermal averaging time will be seconds, at least for more than the smallest parts.FYI, the temperature rating is the internal hotspot temperature. You would hope it's something that could actually ever be measured, like ambient, or the hottest point on the body, or the lead or end temperature or something (give or take specific conditions), but no, they don't do that.Pretty closely, life goes up by double every 10C less than rating (I think that's the right ratio?), so if they're operated cooler in general (due to reduced ambient or easier average conditions), you can either run them harder for the same life, or get much better life for the same rating (or anything inbetween).Note that RMS current causes heating, and that RMS goes as sqrt(D), not D. So for 1A RMS, you can do 1A RMS sine wave, or 1A peak square wave. Or 3.16A peak at 10% duty*, or 10A at 1%.*Though still not exactly. This is closer though. To be exact: for a square wave current, in a typical application where you'd have this -- a switching converter for instance -- the cap of course sees zero average current, so it's +Ipk for duty cycle D, and -Ipk*D/(1-D) for duty cycle (1-D). The RMS then goes as D*(1-D) times the peak value. Remember finally that the peak capacitor current isn't exactly the peak switch or inductor current, because the load current is subtracted. Which should be about a (1-D) factor.Tim