Does capacitor lifetime include "simply powered" - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 06:36:15 pm
content: So if i have a device permanently powered although not in use (power shunted in and out of capacitors) does that count towards the capacitors life ? does it make a up a significant part of the capacitors life given that the temperature while off would be much lower (sub 50C) as opposed to working at say 80C? Basically if I am remaining powered all of the time would it be a good idea to have a fat main power off switch so that when the ting goes to sleep it actually removes power ?

author: SeanB
author_rank: Super Contributor
timestamp: November 12, 2017, 06:55:30 pm
content: Would contribute to lifetime, but as you say it is cooler, so the lifetime while cool will be better than the hot life. Depends on the application, if it is just charged to a static DC voltage with no ripple there will be almost no difference between powered or not.However, as this likely will experience some ripple from the supply side ( I assume it is in a 12/24V vehicle system from your previous work) there will be some ripple current in it, which will reduce lifetime slightly. Best is to choose a capacitor that has a longer lifetime at high temperature, or one with a higher max temperature ( really the same thing, you get the longer life with temperature derating) so that the extended lifetime at 50C is longer even with the ripple current in it.So, instead of specifying a capacitor with 1000h at 105c, you spec one with 1000h at 125C, or one with 8000h at 105C instead.

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 07:04:56 pm
content: Well thing is I am unsure of the exact under bonnet temperature but lets go with 80C, I am using capacitors rated for 25V 125C for 3000 hrs, I have no idea how well they de-rate used at 12V automotive in 80C, I am hoping for more like 6000 hours. My other problem is that the leakage current is 0.01*V*C which for mine works out at 27uA, I am aiming for less than 100uA and i am using 6 capacitors so It's sort of looking like i will need a switch anyway. I already put my control electronics to sleep with a P mosfet but with a few mA draw this is not a problem. Now I'm looking for up to 25A so a few p mosfet in parallel I suppose. For example:http://docs-europe.electrocomponents.com/webdocs/139f/0900766b8139f5fd.pdf3W of heat worse case, if I assume a doubling of Rdson at higher temperatures then put 2 in parallel.

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 07:07:45 pm
content: At the end of the day can I extend me capacitor life to 150'000 hours life while sat at 12V in 40C bearing in mind that it has a worse case of 1500-3000 hrs at 25V, 125C

author: Siwastaja
author_rank: Super Contributor
timestamp: November 12, 2017, 07:15:01 pm
content: Basically, the electrolytic capacitor life is defined by its internal temperature.Sometimes, "being powered" means that there is an upstream switch mode converter which applies ripple current through the capacitor all the time. This heats it up internally, shortening the life compared to what you'd expect only looking at the ambient temperature around the cap.On the other hand, if the cap is on an already smooth DC bus, it won't be a problem - unless the ambient is really hot.

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2017, 07:17:51 pm
content: The real life leakage current is likely to be much better, than that given on the data sheet.Leakage current goes down with time, after the capacitor has fully charged and is also lower at lower voltages.The leakage current also increases with the temperature, so if you find it's higher with high temperature capacitors, then it will be because it's specified at the maximum operating temperature.http://www.tadiranbatteries.de/pdf/applications/leakage-current-properties-of-modern-electrolytic-capacitors.pdfhttps://www.vishay.com/docs/25001/alucapsintroduction.pdf

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 07:20:00 pm
content: No engine running = stable battery voltage only.I'm actually veering towards a positive power switch as it means i can control power to the load as well. So say my switching mosfets that controls the power regulation to the load fail because the load is short circuited, I can still remove power with the positive side switch that will not have failed as they are simply on and not getting anywhere near as hot particularly with several hundred amps of capacity.

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 07:28:00 pm
content: hm:http://www.farnell.com/datasheets/1973300.pdf?_ga=2.175118944.397446062.1510340349-1136071739.1509783694&_gac=1.138202628.1510340349.EAIaIQobChMI0LKqmti01wIVVZ0bCh0-igWzEAQYASABEgL7UfD_BwEthat is saying the leakage current in uA is equal to 0.01*C*V so my 26.4*10^-6 is actually 26.4*10^-12, is that possible ? I'm still not sure on the capacitor life though

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 09:19:53 pm
content: I'm looking at this intelligent switch:https://www.infineon.com/dgdl/Infineon-BTS50025-1TAD-DS-v01_01-EN.pdf?fileId=5546d4625b62cd8a015baf36cff24613I am actually struggling to find the overload reporting details. The fault pin provides a constant current proportional to the current going through, but if an overload occurs then a set fault current is outputted. This is all supposed to be compatible with a digital input (I have run out of analogue inputs and won't use it for measurements anyway as the measurement can be +/-10A!). But it's not overly clear on what exactly happens with a fault, does it go high or low? I assume that I'd have to have a resistor in parallel with the resistance at a value that means for a valid current output range the voltage developed is low enough to not trigger a high on my micro input and the "fault" current will be that much higher that it will pass the high threshold on the micro. Or the fault current is 0 in which case no resistor required or a very high value one....