Choosing Decoupling Capacitors - Page 1
Forum section: electronics / projects

author: paolo
author_rank: Contributor
timestamp: December 03, 2012, 08:20:43 pm
content: Well, I've read plenty on this. Maybe I should read some more. But some things are either whizzing over my noob head, contradicting each other, or just not explained plainly enough.So, the background. I have a little project, with two seperate pos rails, and a common ground. One rail has high current needs, the other much lower.In my case, the capacitance values are the easy bit. There's a manufacturer demo design and the values they have in that seem to work fine for the bits I've tested so far (using plain old 'wet; eletrolytics) - using a scope to look at the effect.Aside from the workaday 0.1uf on each rail, the low power rail has a 10uf, the high power rail a 330uf.Next thing then, ripple current. Would I be right in thinking I need that to cope with the biggest transition in current draw? On the high current rail, it could - at the extreme - go from zero to 720ma in a single edge. Does that mean I need that (plus some headroom) for the ripple current rating, or is it something else entirely?(I have some more questions about voltage, ESR and choice of dialectric, but maybe deal with one parameter at a time)

author: SeanB
author_rank: Super Contributor
timestamp: December 03, 2012, 08:26:45 pm
content: Basically yes, though you need to look at ESR as to how much the voltage at the capacitor terminals will drop, along with the tracks which will add some inductance.

author: paolo
author_rank: Contributor
timestamp: December 03, 2012, 09:03:26 pm
content: Quote from: SeanB on December 03, 2012, 08:26:45 pmCool, thanks. (I'm going to come back to ESR in a bit. I'm very vague on what I need there.)So, next up, voltage. Spec for the highest amplitude of ripple, or highest absolute voltage? (I don't think the latter is very much more significant than the former in my case; additionally I've got a some suggested cap voltages from the reference design, and the scope seems to confirm they are within range of either definition).

author: SeanB
author_rank: Super Contributor
timestamp: December 04, 2012, 04:41:27 am
content: Highest voltage seen at the point, add 10% then use the next higher voltage. That will survive for the rated life of the chosen unit. If you want it to last longer use a higher voltage rated unit. e.g. for a 12V rail you can use a 16V unit, though it is better life wise to use a 25V or a 35V unit that fits the available space. A simple rule of thumb, it will need changing if you are at high temperature or at high currents.

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 04, 2012, 07:36:00 am
content: And if you use ceramics (they have really low ESR), pay attention to the dielectric used. Some types have highly reduced capacitance at higher bias voltages and temperatures. Avoid stuff like Z5U and Y5V. For use in switching converters, X5R and X7R are commonly recommended.

author: tom66
author_rank: Super Contributor
timestamp: December 04, 2012, 10:20:34 am
content: Generally the only advantage nowadays with going with higher voltage caps is lower ESR and higher ripple current at the same capacitance rating due to a bigger can.A 16V cap can withstand 16V continuously for example.http://www.chemi-con.com/u7002/life.php(see "(a) Operating Voltage")For decoupling ripple current is usually only transient so you can go with a capacitor with a smaller ripple current. The ripple current is just an rms measurement.

author: T4P
author_rank: Super Contributor
timestamp: December 04, 2012, 10:29:47 am
content: Quote from: tom66 on December 04, 2012, 10:20:34 amCode:However in certain applications or misapplications, the applied voltage can be detrimental to the life of an aluminum electrolytic capacitor

author: paolo
author_rank: Contributor
timestamp: December 04, 2012, 11:10:36 pm
content: Quote from: SeanB on December 04, 2012, 04:41:27 amAs it happens, my scope is showing 21v peaks on the (high power) 3.8v rail fed from a bench supply. That's during switching of 48 x 30ma LEDs. Much more ferocious than I expected, and it explains the 35v cap used in the design I'm following. They spec a 330uf cap, and although I don't have any to hand, a 1000uf damps it down massively.So, all good so far, and thanks for the simple explanation.(fyi... I'm not working in high heat. Ambient will be 20 degrees max; albeit with almost no airflow around the board and absolutely no option for a fan).Next then, ESR.A few things spin off from this: cost and size. If I choose the lowest ESR, with clear headroom for ripple current and of course voltage, things get expensive *and* bulky.Lower ESR means less heat, and (by association) less power loss, yes? Anything else? And I assume this all going to be factored by just how hard I'm driving things, how much ripple I'm generating.So would I be right in saying that I need to test, in order to figure out the *necessary* ESR? In my case, I'm not bothered about losing energy. I am however concerned about heat. So for an optimal design (not overengineered), I'd test under load and measure temperature? Or is there more to it than that?

author: SeanB
author_rank: Super Contributor
timestamp: December 05, 2012, 04:46:33 am
content: Correct, that is basically part of the design process. ESR is a compromise on how much heating in the cap along with how much ripple that you are happy to live with. Often you will find the typical specs are a lot better, but best design practice is to use the worst case values as the start values, and then any part will work.

author: paolo
author_rank: Contributor
timestamp: December 05, 2012, 10:14:44 pm
content: Brilliant - this is really helpful in demystifying the whole subject.So a final question:Behaviour of different dialectics in failure?I've seen videos of exploding cans, mini inferno tantalums......how do different dialectics behave in failure? Are any more/less likely to fail outside of overload, or ageing?(I stumbled across a product the other day that had an internal fuse - very cool, but incredibly specialist I expect)