Capacitors for the output of a buck - Page 1
Forum section: electronics / projects

author: Northy
author_rank: Regular Contributor
timestamp: September 14, 2020, 09:20:51 pm
content: Hi all,What are your preferred capacitors to use on the output of a buck convertor and why? This would be for a long life/high reliability design.Ceramics tend to have a terrible DC voltage de-rating so you need lots that take up a lot of space.Electrolytics dry out over time and degrade performance.Tants are not that well know for their reliability.I'd like to hear your thoughts.Thanks,G

author: Etesla
author_rank: Regular Contributor
timestamp: September 14, 2020, 09:28:15 pm
content: Aluminum Polymer caps. To my knowledge they have longer lifetimes, better temperature specs, and the big one is a much lower ESR, all compared to classic electrolytic caps. They don't have the DC derating problem that ceramics have. The downsides are that they have lower voltage ratings comparitively and are generally more expensive. My go to is an aluminum polymer cap of the right size with some ceramics in parallel to bypass any really HF stuff depending on the circuit it's powering.

author: Bud
author_rank: Super Contributor
timestamp: September 14, 2020, 10:47:30 pm
content: The correct answer is : check the converter IC's datasheet. If there is no specification for the type of cap , only then you can apply other considerations.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2020, 11:02:38 pm
content: Polymers have about the same lifetime as electrolytics. Whereas electrolytics' electrolyte diffuses out through the seal, polymers' electrolyte is contaminated by ambient moisture diffusing in through the seal. Both are accelerated by elevated temperature, and for both (as far as I know) the ESR rises and capacitance decreases as the part fails. Parts are available in a range of temperature and lifetime ratings.Ceramics are the best option for reliability. Choose lower K types, such as X7R, which age less than high K types like Z5U. Place them away from stress raisers like mounting screws and board edges, and use leadframes, potting, brackets or etc. if extreme vibration is expected. Using redundant series pairs can also improve reliability (allowing one capacitor to fail shorted without failure). In short, the main failure mode is cracking due to stress. The dielectric doesn't degrade as far as I know, but does age (capacitance drops with log(time)), and as already observed, there's an aggressive C(V) curve that demands relatively large ratings when real capacity is needed.Avionics are very high reliability, and also have somewhat unusual requirements like hold-up time of some hundreds of milliseconds. They achieve this by stuffing a board full of beefy ceramics. Or of course at those budgets, they can afford wet slug tantalums, too.Don't neglect other factors in reliability, like voltage:- Polymers may fail shorted, and not far above their rated voltage. (Apparently some seemingly exhibit self-healing. YMMV.)- Electrolytics tend to fail gracefully, reforming when exposed to gradually elevated voltages; when applied suddenly, the generation of gas and heat can lead to... accelerated release of contents.- Ceramics typically break down at many times their rated voltage, but fail shorted and turn into molten balls of slag if allowed to carry fault current. (Use a fuse, duh.)- Tantalums break down less reliably. They aren't supposed to break down until rated voltage, but stress (especially from soldering cycles) can cause cracks in the dielectric. These cracks can self-heal, but the self-healing breakdown can draw large gulps of current from a stiff supply. Tantalums are safest when derated generously (2-3x) and used on current-limited supplies, or for signal purposes (e.g. they're great for large timing capacitors, more stable than electrolytics). When used at high voltages (above or beyond ratings, or even below if a particularly unlucky crack occurs), and allowed to draw excessive currents, they turn into molten balls of slag...not just from fault current, but because of chemical reaction as they are essentially made of thermite! (Typical construction being sintered tantalum (a reactive metal) impregnated with MnO2 (a useful oxidizer; used here as electrolyte).)Polymers, electrolytics, and especially tantalums are sensitive to reverse bias (again with electrolytics being the most tolerant). Transient overvoltage may cause failure, with electrolytics being the most robust*. In any case, don't expect any help with clamping voltage surges themselves, beyond what the bulk value and ESR offers; use a TVS to ensure peak voltage is clamped reasonably.*Indeed, electrolytics are robust enough that they are used in long-life surge protectors. These are basically a series inductor (air cored) to increase transient mains impedance, then a bridge rectifier into a bulk capacitor to clamp the line voltage near nominal peak voltage. A bleeder resistor keeps the capacitor discharged just enough to track the peak voltage. These require two assumptions: 1. the surge amplitude, and 2. its duration, have known maximums.Tim

author: BrianHG
author_rank: Super Contributor
timestamp: September 14, 2020, 11:04:28 pm
content: LOL, an hour before the OP's post I was gathering the same info for another beginner's thread.Long story short, ESR of regular aluminum electrolytics VS MLCC at various frequencies:DC voltage derating curves of MLCC capacitors:Even though the chart is for 4.7uf caps, you get the Idea.For 10uf MLCC caps, the sweet price spot and availability for 5v are X7R 1206 16v/25v caps.For the least DC loss VS capacitance possible, X7R 1210 25v caps.Don't go over 22uf with MLCC. Price and availability becomes counterproductive compare to just placing 2x 10uf MLCC caps unless you are building in quantity with proper scheduling.I still use 1000uf electrolytics in parallel, however, the MLCC caps are closest to the device generating the switching noise.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2020, 12:27:08 am
content: Quote from: Northy on September 14, 2020, 09:20:51 pmTantalum *output* capacitors are not subject to uncontrolled surge currents on charging so the common failure mechanism does not apply with proper current limiting. Reliability can be further improved by using non-epoxy axial leaded parts but 2/3rds voltage derating of epoxy parts should be sufficient and is less expensive. Note that polymer tantalum parts are not subject to the surge failure mechanism at all.I have to defer to T3sl4co1l's comment about polymer aluminum electrolytic and polymer tantalum capacitors. I have the same understanding that water damages the polymer electrode over time but their operating life and reliability seem to be outstanding so this is a promising option.I am less sanguine about ceramic capacitors but not because of their voltage coefficient of capacitance. They can fail short with a result similar to solid tantalum capacitors and are more easily subject to mechanical strain.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2020, 01:52:07 am
content: Yup. To put still more nuance on that -- tantalum might be fine for the output of a small PoL converter, but contraindicated for local bypassing of a large supply -- even if the local use of that supply (e.g., a board in a card cage) is individually fused. I mean, the fuse will protect the system as a whole, as it is supposed to, but it won't help you with sheer uptime, you need to fix the underlying problem to help with that.(Apparently, tantalum self-healing surges have affected uptime in telecom hardware; replacing them with quiet types eliminated random resets.)Or there's internally fused tantalum, which disconnect themselves on failure. But then your bypassing degrades over time, maybe that's acceptable maybe not. (To that I have to say: why did you waste so many extra capacitors in the first place when you could've had the same performance in a fraction of the parts? Typical response being: it's just bypassing, why give it a second thought? It's a pretty shit response, but alas a very, very common one!)As to polymer vs. electrolytic, I've definitely had more electrolytics fail, but they're far, far older, much cheaper, and used in far, far greater quantity. It's both selection and confirmation bias. You abuse anything, it fails; or cheap out on suspect components, or just get a bad batch. Such is the reality of cheap consumer hardware. Picked judiciously, they last as long as each other, as far as I know.For example, the polymers on my PC motherboard and GPU are, oh heck when did I get this mobo, it's over 10 years now isn't it... They don't run burning hot, so the expected lifetime will be many times longer than rated. 10 years of nearly continuous uptime is nothing to sneeze at. Same with electrolytics, in fact I keep a laptop (2008 vintage) idling all the time, and its power adapter has TEAPO branded caps inside it (which I know matter-of-factly, having serviced it before -- because the output cord was frayed and shorting out). Those caps run even hotter I'm pretty sure; wouldn't be surprised if it's making a lot more ripple voltage these days, but it stillworks.And that's the real trick to getting long life on a budget.Keep those caps cool,electrolytic and polymer.The other types I think will handle high temperatures with greater reliability, do check the manufacturer of course -- I think tantalums are similarly rated for life or reliability at voltage and temp, maybe they pro-rate more steeply with reduced ratings, I'm not sure?Tim

author: jmw
author_rank: Frequent Contributor
timestamp: September 15, 2020, 04:48:37 am
content: What about film caps? They're comparable in available capacitance range to type II ceramics, no DC bias effect, and have self-healing properties. They're large, but OP didn't mention board space as a constraint.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2020, 08:31:00 am
content: Eh, ceramics are available in larger values, at very low voltages (under 50V).The energy storage is great at voltage.They may be unsuitable for some applications, as, proportional to size, they have higher ESL. Alternatives include many smaller films in parallel, or a few larger films or electrolytics in parallel with smaller ceramics, etc.Tim

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2020, 10:03:30 am
content: If I were designing one for maximum lifetime and durability, I would use good quality film capacitors for a first stage to take most of the spikey-blast that comes from the converter pulses, then a series inductor to aluminum polymers or high quality conventional electrolytics. This way you're not relying on the electrolytic to always maintain extremely low ESR and not smashing them with high current pulses throughout their lifetime, which will extend the working life significantly in most cases.

author: Northy
author_rank: Regular Contributor
timestamp: September 16, 2020, 09:45:07 am
content: Thank you all for your responses, I've been reading them but not replied until now due to feeling a little under the weather.I've got a board I've designed and built to test a few different power supplies and topologies, so I'll have a play around with the capacitors to see what happens.Reliability: I'm not designing aircraft or anything, but I'd really like these power supplies to work reliably for years to come.Board space: Not usually a problem, but ideally I don't want 10's of ceramics on the output of regulators. For example, one part I've got on the board has 47uF on the output on the evaluation board. The 10uF 25V ceramics I have de-rate to 2.9uF @ 17V (output of this supply) - so I'd need 16 of them!I am also going to play around with the switching frequency of the parts, see how high I dare run them and how hot they get at the current needed - that obviously helps the output capacitor requirements.I think my starting point I'm going to test is going to be this:1 x 100nF ceramic and as many 10uF 25V ceramics on the output as needed (within reason), a ferrite bead (which I may replace with an inductor also to try), then a 10uF 25V ceramic then a decent general purpose SM Panasonic 100uF Electrolytic.Thanks,G

author: drussell
author_rank: Super Contributor
timestamp: September 16, 2020, 10:35:38 am
content: Quote from: Northy on September 16, 2020, 09:45:07 amIf you're going to the trouble to use multiple ceramics, youreally shouldalso try plastic film capacitors. That makes more sense as they're almost as indestructible as ceramic and no retarded DC de-rating factor....I hope you're feeling better.

author: Northy
author_rank: Regular Contributor
timestamp: September 16, 2020, 12:54:57 pm
content: Quote from: drussell on September 16, 2020, 10:35:38 amYes thank youBy plastic film capacitors do you mean these:https://uk.farnell.com/w/c/passive-components/capacitors/film-capacitors/general-purpose-dc-film-capacitors?capacitor-terminals=smd&range=inc-in-stock|exc-delivery-surchargeThanks,G

author: Siwastaja
author_rank: Super Contributor
timestamp: September 16, 2020, 01:50:23 pm
content: For <60V, almost always ceramics. If it seems like they are getting expensive or large, that's usually a sign that you need to go for higher switching frequency. Old school 100kHz converters tend to require a lot of C and often use some combination of small MLCC and larger aluminium electrolytic.Just ignore all controllers that require significant ESR for stability, problem solved.I avoid package sizes bigger than 0805. 0805 seems to give good reliability against cracking, good availability of parts and sizes, and high enough (actual) capacitance. For 63V rating, I tend to go for 1206, though.Remember that you don't need to derate the voltage.Always look up the DC bias curves. I like to calculate $/µF_actual for certain options. Sometimes one rated for lower capacitance ends up being a better choice.Forget the nonsense regarding paralleling different values of MLCCs. Use one part number. Parallel to get more C as necessary. For low stray L, it's usually better to parallel say four 0805 part than to use a massive 1210 part, depends on your actual layout though. Sometimes you can fit a large package to an optimum spot, sometimes using smaller parts helps you get the closest cap significantly closer.

author: wraper
author_rank: Unknown
timestamp: September 16, 2020, 02:00:07 pm
content: Quote from: T3sl4co1l on September 14, 2020, 11:02:38 pmPolymers have an order of magnitude higher ripple current rating. And them failing is a very rare occurrence, even if they were made by some 3rd tier manufacturer.QuoteYou don't need large capacitance on output of buck converter. What you need is low ESR and high ripple current rating. And electrolitycs are way more expensive for the same ripple current rating unless it's a high voltage capacitor. It's not like you would use the same capacitance for any type of capacitor.QuoteOnly if you talk about general purpose stuff. Electrolytics are basically extinct in computer hardware (high ripple current) other than in power supplies. And even there they are largely replaced.

author: Simon
author_rank: Unknown
timestamp: September 16, 2020, 02:05:19 pm
content: Funnily enough i was about to start a thread. What about ESR? I am using the LT8614 and it recommends X7R or X5R ceramics on the output. I am going to run at 1MHz but that just happens to be the resonant frequency of any 10-47µF ceramic. Are they OK to use?

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 16, 2020, 02:10:59 pm
content: Quote from: Simon on September 16, 2020, 02:05:19 pmI don't know surely but I guess I would use parallel connection of caps with several times different rated capacity to they could dampen each other. For example 10 uF in parallel with 47 uF, etc.

author: Simon
author_rank: Unknown
timestamp: September 16, 2020, 02:12:50 pm
content: Well yes there can be several values in parallel but there is not much between them in frequency response. I would also have a polymer cap.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 16, 2020, 02:14:19 pm
content: Quote from: T3sl4co1l on September 14, 2020, 11:02:38 pm- the same lifetime but much lower ESR and much higher rated ripple current. So they over-perform usual electrolytics by 10 times or more (on lifetime).

author: Simon
author_rank: Unknown
timestamp: September 16, 2020, 02:16:37 pm
content: Quote from: Vovk_Z on September 16, 2020, 02:14:19 pmA polymer with better specs than a plain electrolytic but same lifetime will probably last longer, lower ESR, lower stress, longer life.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2020, 06:50:08 pm
content: Quote from: Vovk_Z on September 16, 2020, 02:14:19 pmQuote from: Simon on September 16, 2020, 02:16:37 pmYes, for each type, operated at their own ratings, I think you will find both do what they say on the tin.Quote from: Simon on September 16, 2020, 02:05:19 pmSeries resonance is meaningless; it's already about half what's in the datasheet just by putting it in a circuit (i.e., with trace and via lengths), and orders of magnitude less than that once you put a filter choke in series with it.Even if the reactance isn't capacitive, you still get an inductor divider from switching node to output to ground, and the ratio of choke to ESL gives the attenuation. Can always put another LC after it to clean up whatever trash is leftover.Tim

author: Northy
author_rank: Regular Contributor
timestamp: September 17, 2020, 08:32:48 am
content: Plastic Polymer capacitors, am I looking at the right thing here:https://www.digikey.co.uk/products/en/capacitors/film-capacitors/62?FV=mu10%C2%B5F%7C2049%2C-8%7C62%2C69%7C409393&quantity=0&ColumnSort=1000011&page=1&pageSize=25Thanks,G

author: wraper
author_rank: Unknown
timestamp: September 17, 2020, 08:51:38 am
content: Quote from: Northy on September 17, 2020, 08:32:48 amThese are SMD film capacitorshttps://www.digikey.co.uk/products/en/capacitors/aluminum-polymer-capacitors/69

author: Simon
author_rank: Unknown
timestamp: September 17, 2020, 09:29:44 am
content: I don't actually bother to segregate capacitors anymore. I list all electrolytics together with their specs by their side and pick based on spec.

author: Northy
author_rank: Regular Contributor
timestamp: September 17, 2020, 02:24:26 pm
content: Quote from: drussell on September 16, 2020, 10:35:38 amdrussel suggested I looked at plastic film capacitors?G

author: wraper
author_rank: Unknown
timestamp: September 17, 2020, 02:48:50 pm
content: Quote from: Northy on September 17, 2020, 02:24:26 pmOh my. Using them for buck output is very expensive and lame. Yes, no need to take into account capacitance dependence on voltage, but at what PCB space and price. You can simply take MLCC of 4x higher capacitance, still it will take so much less PCB space and will be so much cheaper. Also I suggest not using SMD film capacitors at all. They are very unforgiving to soldering and washing process.

author: Simon
author_rank: Unknown
timestamp: September 17, 2020, 02:51:43 pm
content: I use wurth's red expert to pick caps as I can see a list of how much capacitance is lost for a given voltage and just go down the list until I find one that is not awful, strangely a 10V cas was better at 5V than a 16V one in some cases, there really is no rule so anyone that actually gives you the data is worth using.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 17, 2020, 04:14:23 pm
content: Obviously suggesting film caps for extra low voltage buck converter output means anyone suggesting this has never designed or seen such converter. It's completely ridiculous and impractical.Youusuallyneed some ten uF at least, if not more. Easy to achieve with ceramics; equivalent film caps are massive. Seeing OP has a problem with the "space" MLCCs take, suggesting something that takes at least ten times more (taking the MLCC DC bias into account) is a non-option.For high voltages, several hundreds volts and above, film caps may make sense, though.MLCC DC bias effect only affects applications where exact capacitance is required. Buck output is not such a case. Why not just look at the data and multiply the nominal capacitance by the inverse of the capacitance loss percentage? Say you calculated you need 10uF, and a 10uF cap datasheet shows 35% of C left at your DC bias, just buy three caps and you are done.IMHO energy density is quite good, capacitors usually tend to end up taking a bit less space than the inductor. For miniatyrization, bring the f_sw up! If still having trouble with size, stack them in 3D, be creative.

author: drussell
author_rank: Super Contributor
timestamp: September 17, 2020, 04:36:18 pm
content: Quote from: Northy on September 17, 2020, 02:24:26 pmI have no idea about surface mount films. I would expect they would be good for things like coupling an audio signal but doubt they have any ripple current capability and be suitable for high pulse, high Dv/Dt applications.I was talking about the good old giant through-hole monsters. Due to construction necessities, they are physically relatively large.If your application requires it to be tiny size and SMD-only you're going to limit your options if you want extreme longevity / durability.

author: Simon
author_rank: Unknown
timestamp: September 17, 2020, 06:36:08 pm
content: Last time I used a film cap was for a 555 timer circuit, it was 10's of nF (maybe 100) yet was the size of several µF in ceramic format.

author: Northy
author_rank: Regular Contributor
timestamp: September 17, 2020, 06:41:43 pm
content: Quote from: drussell on September 17, 2020, 04:36:18 pmAh, ok, sorry, my misunderstanding.Yes this will be machine build SM.Regards,Graham

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2020, 10:40:40 pm
content: Films are fine where the voltages are high, typically industrial converters. For example, the first power project I did, I just used a handful of films on the inverter boards, nothing else; power was 3-phase FWB'd, which delivers uninterrupted power with modest ripple, no need for electrolytics.You can't get films under 50V and over a few uF, not in any reasonable price anyway.SMT films are fine, but they are:- More expensive- Potentially leaky (not a problem for power, but annoying for high impedance signals)- Hard or risky to solder (follow the reflow cycle in the datasheet, avoid hand soldering if at all possible)- Again, still not very big, nor in low voltagesPPS has a fairly high melting point but even it will melt under the tip of an iron. You can very easily smoosh the layers by applying too much heat for too long. Interestingly enough, smooshing isn't necessarily a problem -- the metallization is flexible too -- but it's very easy to let it smoosh too far and short out.True fact, during testing of those inverter boards, I ran enough ripple current through them (DC link caps) to heat them up enough to melt their contents, which swelled the body, cracking the epoxy underfill and extruding shiny metallized-PP goo towards the board. Some of the capacitors so affected were still perfectly operational afterwards(!), but the tests ended of course when one or two erupted in sparks and smoke. Understandably, 650V is rather aggressive when it comes to "self healing".Tim

author: cdev
author_rank: Super Contributor
timestamp: November 16, 2020, 02:03:33 pm
content: Quote from: Northy on September 16, 2020, 12:54:57 pmI'm getting blocked from seeing that web page by a cryptic message, that I suspect is coming from a content delivery network.Can you think of a good alternative discussion of the subject, since I have no intention of buying them from Farnell?I have a lot of film capacitors, and its been my experience also that they are very reliable