Capacitors on switching power supply vs linear power supply - Page 1
Forum section: electronics / general

author: Eldi4
author_rank: Contributor
timestamp: February 09, 2019, 07:59:10 am
content: Hello, simple and noobies questionBased on my experience, i have seen a lot of smps failure because of bulging caps. But on a linear power supply, i think i've never seen any bulging caps, or am i wrong?Why do capacitors tend to bulge on smps?, The reason is operating frequency which correlate to reactance? or anything else?,

author: mariush
author_rank: Super Contributor
timestamp: February 09, 2019, 08:13:42 am
content: Capacitors on linear power supplies are subjected to less "stress".You're dealing with 50/60 Hz x 2 ... versus 30kHz and higher on switching power supplies.Bigger bursts of current, potentially higher ripple...

author: Ian.M
author_rank: Super Contributor
timestamp: February 09, 2019, 08:21:36 am
content: Capacitor temperature rise is proportional to RMS_ripple_current2x ESR / Surface_area, (given identical ventilation and copper pad area).As you increase the frequency, the filter capacitance required for a particular pk-pk ripple voltage and load current decreases, and so does its can size and thus its surface area.Therefore the big reservoir capacitor operating at double line frequency on the input side DC bus rarely goes bad - lots of surface area, and not a lot of ripple current for its size, but the secondary side caps which have to handle ripple current at the switching frequency and the small cap that filters the control chip supply, from the auxiliary secondary on the primary side are notorious for going bad. Also the secondary side caps tend to be packed together a lot more so often are't well cooled. You'll also notice that if the supply has filter chokes on the secondary side with identical caps before and after the choke, the one before the choke almost always goes bad before the one after it on the same rail, because the one before handles most of the ripple current.Edit: Corrected equation in first paragraph. Note: As dissipation is proportional to the SQUARE of the ripple current, a small increase in the ripple current can cause a large increase in the dissipation. This is a big problem if the capacitor was underrated or the ambient temperature is higher than expected.

author: Eldi4
author_rank: Contributor
timestamp: February 09, 2019, 08:27:10 am
content: So, the main reason why capacitor is bulging (on any case) is temperature rise?, the higher the temperature the more likely it is bulging?, okay then it's clear, thanks!

author: mariush
author_rank: Super Contributor
timestamp: February 09, 2019, 08:39:03 am
content: No, they're bulging because the electrolyte in them is getting damaged and gasses are produced inside the capacitor which cause increased pressure and the tops bulge because they're designed that way, to be a pressure relief instead of capacitors blowing their bottoms or exploding.The electrolyte deteriorating and producing gas could be caused by excess currents, too high voltage, reverse voltage and other things... and just one of the reasons could be excessive temperature.Capacitors are rated for a certain temperature, like 105c or 85c and they have a lifetime rating, you can see in datasheets something like 4000h @ 105c ... that means something like "after 4000 hours at 105 degrees, the capacitor will still be within 70% of its technical specifications (esr, capacitance etc)For electrolytic capacitors, the general rule is that for every 10 degrees celsius less, you double the lifetime rating... so a 4000h @ 105c capacitor would have 8k hours at 95c, 16k hours at 85c and so on ... at a typical 40-50c ambient and operating temperature, you'd have 100k+ hours at ~50c ... that's 10y+ of life... so no, temperature alone would not cause capacitors to bulge under normal conditions.That generalization applies for electrolytic capacitors only, for polymer/solid capacitors the formula is different, the life increases way more with drop in operating temperature.

author: Ian.M
author_rank: Super Contributor
timestamp: February 09, 2019, 09:41:08 am
content: Mariush is correct that high temperatures under the rated maximum, are not the root cause of electrolytic capacitors bulging.OTOH, as is typical with wet chemistry*, all the factors that cause the electrolyte to decompose and produce gas are temperature dependent. The hotter an electrolytic capacitor is running, the less well it tolerates being close to its specified electrical limits. This is compounded by the increasing tendancy to design down to a price, which results in components typically being run far closer to their abs. max. ratings than was regarded as good practice several decades ago.* seehttps://en.wikipedia.org/wiki/Arrhenius_equation

author: soldar
author_rank: Super Contributor
timestamp: February 09, 2019, 10:41:47 am
content: It is easy for a capacitor to enter a spiral of destruction: it starts to deteriorate which increases ESR, which increases heat, which increases deterioration which increases ESR which increases temperature, which increases deterioration... you can already see the end.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 11, 2019, 01:21:18 am
content: This may be a case of two trends converging.Most switching supplies are relatively more recent, let's say in the last 20 years.Most linear supplies are somewhat older. I have supplies that are 40 to 50 years old and work fine.However, as switching supplies became more and more popular, there was also more and more pressure to reduce cost. This can lead to using parts with very little margin, which then get stressed to failure.One of the biggest causes and curses of switching power supplies is capacitor plague:https://en.wikipedia.org/wiki/Capacitor_plagueWhich quickly leads to the ESR, etc. failure modes listed earlier.

author: TheNewLab
author_rank: Frequent Contributor
timestamp: April 07, 2019, 03:48:10 am
content: don't know where to put this and do not want to open a new thread.Can a capacitor explode when the electrolyte has already left it?here's an old capacitor blown from a tube amplifier. just found on YouTube

author: helius
author_rank: Super Contributor
timestamp: April 07, 2019, 04:11:29 am
content: The electrolytics in tube amplifiers are exposed tohigh voltages(frequently 450V or above) and they can fail catastrophically as shown. In addition, the old "can capacitor" shown has no pressure vent! Together, these factors mean that pressure can build up not just from boiling the electrolyte, but actually burning the paper separator or the aluminum foils. The smoke before it fails is evidence that is what happened.So yes, in high-voltage caps with no pressure vent, explosions can happen even when all the electrolyte has dried up. It's stupid to even apply power to an old dried up cap since it can not possibly be in tolerance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2019, 06:36:53 am
content: Typically what happens is the electrolyte solvent slowly evaporates through the rubber seal, causing ESR to rise, in turn causing the internal temperature to rise. The capacitor may not vent dramatically in this case, because there's too little solvent left to boil suddenly, but still enough to make enough internal pressure to bulge the can. As soon as the bulge cracks, it leaks out that pressure, and the capacitor dries out very quickly. The dry capacitor has very little capacitance and very high ESR, and the circuit performs very poorly (very high ripple, at whatever frequencies are relevant).This can of course go on simultaneously with other degradation effects, corrosion of the metal (making hydrogen) and so on.Tim

author: tautech
author_rank: Super Contributor
timestamp: April 07, 2019, 07:30:07 am
content: Quote from: Eldi4 on February 09, 2019, 07:59:10 amMostly bean counters overriding engineers.SMPS require therightcaps and they cost money, only a little more but over 1000's of units the BOM starts adding up.If we were to build in the over-rating of components commonly done in previous decades things would last a lot longer. Working margins are today trimmed to the bone, sure much componentry has improved over some decades however the list of A brand cap manufacturers is pretty small so B brands are used but without significant operational headroom.There's nothing mystical about failing SMPS caps only the so called experience of those that build them.