Capacitor's Worst Enemy - Page 1
Forum section: electronics / projects

author: hgg
author_rank: Frequent Contributor
timestamp: April 29, 2014, 09:01:57 am
content: Hi,From my short experience, I've seen that the the most common failing component is the capacitor.Its like the plague of electronics.What do you think is the worst 'enemy' of capacitors?1) Heat ?I have never seen something like this on a board:http://www.aavid.com/product-group/capacitors2) High Voltage ?What is considered as a safe upper limit near the rated one?3) High Current?Why capacitors are not rated for current?4) Frequency ?Can frequencies near its resonant point cause damage?5) Transient Spikes?6) Vibration ?Thank you.George.

author: tautech
author_rank: Super Contributor
timestamp: April 29, 2014, 09:20:57 am
content: I think failures are just a symptom of today's throwaway society.It's all about price and cost cutting. Component manufacturers and designers all do it."They don't make em like they used to"My good retired mate a Dr. of EE gave me good advice from the the "old days":Do not subject any component, physically, electrically or temp wise past 62% of their max ratings.Where did he come by this you may ask, as did I.He said that was the required guideline/rule when he worked for a Govt department maintaining transmission stations.Just design for good old fashioned RELIABILITY

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2014, 09:41:25 am
content: Referring to electrolytics specifically?1. Keep 'em cool, don't place near hot resistors or heatsinks, use within or below ratings2. As a rule, I use the rated voltage as the "surge" rating, so, for example, a 12V linear power supply might need >15V before the regulator, which might be 15-18VDC from a rectified 12.6VAC transformer, which might go up to 20-22V under high line conditions: use a 25VDC capacitor. Or 120VAC rectified mains, 160VDC nominal, use a 200V capacitor. A PFC converter with 400V output, use a 450V cap.3. Because current into ESR makes heat. Keep ripple current down, too. If you need more, use more caps of smaller values, or consider paralleling with film caps to carry some of the HF ripple.4. Electrolytics aren't really resonant. A 220uF 200V "snap in" style might be 0.2 ohms from a few kHz up to a MHz or so.5. Tolerant. Voltage higher than rated* draws excess leakage current, which as far as I know, is more of a thermal issue: overvoltaged caps explode from the electrolyte boiling.*Meaning, not so much the nameplate voltage rating, but rather, the voltage the dielectric can withstand before reforming (high leakage, which decreases over time at a given applied voltage). This very slowly drops over time (particularly in storage for decades), and rises when "exercised" to a given voltage.Probably at very high voltages, the dielectric itself can break down, but with the relatively high ESR of the electrolyte, can a point on the surface of the dielectric even reach that voltage? Maybe that's a non sequitur, and leakage/reforming *is* the breakdown process per se (it just happens to be "self healing" in a much more inert, if even beneficial, way, as compared to, say, self-healing metallized film caps).6. Should be pretty good? The guts are firmly inside the can. The cap overall is pretty heavy to be supported on a pair of leads, so secure it to the board with goop, or reinforce it otherwise (straps, glued-on supports..). Some of the bigger types (computer grade sized) come with studs on the bottom, which can also be used for heatsinking.So all together, #1 is the big problem. #3 and #5 feed into the same problem, more or less, and the rest aren't any different compared to any other component.Tim

author: ConKbot
author_rank: Super Contributor
timestamp: April 29, 2014, 10:34:44 am
content: Plain and simple heatIn too hot of an environment is simple... too much heat. Too much voltage will cause leakage current, which causes power dissipation, aka heat. Too much current causes too much losses because of the loss tangent/ESR, which produces heat. There is nothing inherently wrong with running a frequency higher than the resonant frequency of a cap though it, just that its impedance wont keep dropping with frequency the way you'd expect an ideal capacitor too. But if something goes on with the circuit and it makes it draw too much current at that resonance, then its plain simple overcurrent.As far as transients, as T3sl4co1l said, electrolytic can take some transients due to their self healing nature. Other caps not so much, dont let tantalums or ceramics take any surge over their rated voltage, ever.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 29, 2014, 10:37:35 am
content: Because current into ESR makes heat. Keep ripple current down, too. If you need more, use more caps of smaller values, orconsider paralleling with film caps to carry some of the HF ripple.I tried that in a 1.5kW PSU with active PFC boost converter. The 1uF 630V film capacitor I added in parallel made no difference at all to the electrolytic capacitor temperature. I expected to see some temp reduction but it wasn't measurable.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 29, 2014, 10:42:03 am
content: Also, NEVER use 85 deg caps when you are going to have heat issues. Use 105 deg or even the specialised 125 deg caps. They are unfortunately much more expensive.Very often the electrolytic capacitor is placed near a heatsink or power transformer that raises the capacitor temp unnecessarily. Every 11 deg C reduction in cap temp will double the life.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 29, 2014, 11:49:55 am
content: AcHmedYes, it was CCM operation, so you are quite correct in saying that the main ripple was at 100Hz. The idea was to steer the HF ripple current to the film cap but you are correct in saying that the main ripple current would be 100Hz and not 100Khz. That is probably why it made no difference. I'll have to check my notebook.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 29, 2014, 01:26:13 pm
content: Biggest problem I see is badly implemented "smart" fans. Usually the fans are too "lazy" or they cycle on and off instead of being variable speed like they should be. (The latter is more of a problem for the solder joints.)

author: hgg
author_rank: Frequent Contributor
timestamp: April 29, 2014, 05:22:35 pm
content: Quotetautech I agree 100% with the above!That is why I am asking, in order to find out the best way to protect them.The <62% sounds like a good advice.T3sl4co1l thank you for the detailed explanation!QuoteConKbot I meant if it safe to run them close to their resonant frequency.Resonance is a powerful thing.QuoteUseful info.So it looks like that the main enemy of capacitors isheat.The other factors are also important but just because they will create heat as well.Then why voltage regulators get all the attention and poor caps blow like firecrackers?But seriously, shouldn't capacitor thermal design be a part of proper circuit design as it iswith voltage regulators? It there any way to calculate the temperature that a capacitor willreach in a circuit? I don't see anything like that in datasheets. Only hours of operation.Thank you for the useful advice!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2014, 08:40:37 pm
content: Quote from: Richard Head on April 29, 2014, 10:37:35 amObviously, you need to use enough.1uF is ~0.2 ohms (guessing ballpark on the ESR of the caps in your example here... probably an overestimate) up at 800kHz! So that'll take the edge(s) off, but do very little for the fundamental (which I might guess was in the 50-200kHz range..?). You'd need more like 10-30uF to see some effect, at least without putting filter chokes between them (which makes other problems: you don't want the film caps resonating, either).Hmm, 10uF against, say, 470uF (and 0.1 ohm ESR) would still be reasonably well damped at 160kHz and 0.1uH -- about a ferrite bead's worth (note: one that doesn't saturate... one or a few turns on a small powdered iron toroid would cover this). Probably more like 50-70% of the fundamental ripple (at 100-200kHz) should go in the film cap then, and essentially all the harmonics. The electrolytic still has to deal with the 120Hz ripple, of course.Note that CCM PFC delivers nice and spiky peaks to the filter cap, and if this was followed up by a forward, flyback, buck or whatever converter, those draw peaky currents as well. Polyphase converters would help there (2 and 3 channel PFC controllers are pretty common, and buck controllers for low voltages sport many more outputs: my desktop motherboard has an 8 phase converter on it, for whatever reason!).Tim

author: dannyf
author_rank: Super Contributor
timestamp: April 29, 2014, 08:57:32 pm
content: Different capacitors have different failure mode. Tantalums for example die quickly from reverse polarity / high voltage; electrolytics die quickly from ripple or high voltage / temperature. Film capacitors from high voltage; supercapacitors are very sensitive to high voltage; air/ceramic/plastic are generally very hardy.Tough to nail down to one cause for all of them.

author: violet
author_rank: Newbie
timestamp: April 29, 2014, 10:15:10 pm
content: Definately down to production costs. Just do a quick search for any general capacitor from a part supplier and you'll get hundreds if not thousands to choose from, all ranging from pennies to pounds. If one company can produce a component super cheap why can't another? If you buy a cheap screwdriver and an expensive one with a tungsten carbine tip that's been rigorously tested, which is gonna work best for you? the secret is don't cheap out..As an example of how this works, Sometimes you may notice that one brand of flat tv looks the same as another (literally exactly the same case and everything, one with a cheap brand name and one with an expensive brand name). The pcb's etc... are usually the same too, the difference is the components inside them. The better brand name has better (branded) components producing a much tighter tolerance and better user experience and longer term of service whereas the cheaper brand operates pretty much the same but isn't as reliable...This is true, don't be naive

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 29, 2014, 11:19:45 pm
content: Electrolytics can be very reliable if you start with good quality and keep the heat under control I was just servicing some old GR test equipment with a bunch of Sprague axial caps. The unit was probably built in the '60s. I was trying to improve the performance (noise floor) and replaced a bunch of caps with new quality modern caps. Every cap I removed and measured was still doing its job and had low dissipation factor. Some were lower than the new ones! DC leakage was still good. The values had usually risen by about 15%. When I was done with my (unwise) "improvements" the performance wasn't improved one bit.Heat is the enemy and one design error I see now and then is a voltage regulator or power resistor with a heavy short copper trace right to a capacitor. The heat is conducted very well through the copper trace, and the cap runs 5-10 degrees hotter than it otherwise would. In the case of a power resistor, a bit of extra resistance won't hurt, so a narrower trace, a longer trace, or even an otherwise unnecessary s-curve in the trace would make the cap last twice as long.There's a good reason not to block ventilation holes!

author: gxti
author_rank: Frequent Contributor
timestamp: April 30, 2014, 03:11:27 am
content: Quote from: hgg on April 29, 2014, 05:22:35 pmSelf-resonant frequency of a capacitor is not like a bridge that shakes when the wind blows just right. It's not physically moving. That's simply the frequency at which the impedance reaches its lowest point, and above that parasitic inductance takes over and the impedance increases with frequency.When a capacitor is being used as a pass element for a single frequency, e.g. a DC block for an antenna, it's common to choose a part with a self-resonant frequency just above the target frequency in order to get the lowest possible impedance. And conversely, an inductor as part of a bias tee would have a self-resonant frequency near the target in order to have the highest possible impedance. The precise value of the part doesn't even really matter.

author: pickle9000
author_rank: Super Contributor
timestamp: April 30, 2014, 03:17:09 am
content: Heat is by far the biggest killer

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2014, 07:36:10 am
content: Age & mice!

author: David Hess
author_rank: Super Contributor
timestamp: May 02, 2014, 12:53:44 am
content: Heat.If the temperature is too high, it dies.If the ripple current is too high, then the temperature will be too high, and it dies.If the ESR is too high, then the temperature will be too high, and it dies.

author: hgg
author_rank: Frequent Contributor
timestamp: May 02, 2014, 04:42:35 am
content: Any way to calculate the heat that a capacitor will reach ??

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2014, 05:36:22 am
content: Yes.

author: hgg
author_rank: Frequent Contributor
timestamp: May 02, 2014, 05:54:46 am
content: Something like that ?http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx

author: silent
author_rank: Contributor
timestamp: May 02, 2014, 05:17:00 pm
content: Design and rating is one thing, but the quality is also very important.I have designed a small device for my company with DC/DC converter (12V to 5V, up to 3A). We have assembled about 300 of these in past 2 years. Some day, I run out of caps (some well-known brand, Daewoo I believe, 105'C) so I have picked some caps from the nearest store (they had only chinese shit, also 105'C). Guess what happened about 1 year later.

author: David Hess
author_rank: Super Contributor
timestamp: May 02, 2014, 05:36:57 pm
content: Quote from: hgg on May 02, 2014, 04:42:35 amIf you know the ESR, RMS ripple current, and the capacitors thermal resistance to ambient, then you can calculate the temperature rise but the RMS ripple current and ripple current rating specification should be all that you need to calculate the operating lifetime.

author: Smokey
author_rank: Super Contributor
timestamp: May 02, 2014, 07:22:03 pm
content: capacitor's worst enemy.........The Widlar-izer!

author: dannyf
author_rank: Super Contributor
timestamp: May 02, 2014, 07:24:38 pm
content: QuoteDepending on where the capacitor is used, heat can be a life saver for some capacitors.

author: electronics man
author_rank: Frequent Contributor
timestamp: May 02, 2014, 07:41:00 pm
content: I think smokey is right a hammer is by far a capacitors worst enemy