Bypass caps - any advantage in using electrolytic or bigger MLCC packages? - Page 1
Forum section: electronics / beginners

author: Saimoun
author_rank: Frequent Contributor
timestamp: August 09, 2020, 08:59:20 am
content: HiJust wondering about bypass caps - I recently realised it is possible to get somereallysmall SMD multi-layer ceramic caps (MLCC) as high as 220uF!I found for example a 22uF in a 0402 package: the Murata GRM155R60J226ME11D (+/-20%, +6.3V, X5R).And even the cost is not that different - the 22uF Murata costs 7 cents in quantity, a cheap electrolytic 22uF would be around 1 or 2 cents in quantity.And if we go down to 10uF the MLCC are actually cheaper than their electrolytic equivalent.So, speaking only about 22uF and below bypass caps, I was wondering:1) Is there any reason to use an electrolytic cap rather than a MLCC?2) Is there any reason to use a 1206 MLCC instead of a 0402 (other than hand-soldering)?Thank you!

author: mariush
author_rank: Super Contributor
timestamp: August 09, 2020, 10:42:37 am
content: Capacitors of different types and footprints filter different frequencies.A 0402 capacitor may behave differently than a 0805 ... also x5r can have significant different specs compared to x7r or cog/npo variety.For decoupling, may not matter much, but for bulk storage of energy it will make a difference.Also, ceramic capacitors will lose capacitance depending on voltage they're subjected to. Seehttps://pdfserv.maximintegrated.com/en/an/TUT5527.pdf- "Temperature and Voltage Variation of CeramicCapacitors, or Why Your 4.7μF Capacitor Becomes a 0.33μF Capacitor"Your 22uF 6.3v ceramic capacitor won't hold 22uF worth of capacitance if your microcontroller or whatever works on 5v or even 3.3v - those ceramic capacitors are probably meant to be used with 0.8-1.2v at most, think mobile phone chips, fpgas, other stuff like that.For that 22uF Murata capacitor, see bottom left graph on first page of datasheet:https://static6.arrow.com/aropdfconversion/70237f41b402bb8324b39cbd0e80a84326501d78/grm155r60j226me11electricalcharacteristicsdata.pdf-80% change with 5v... your capacitor is now 5uF or something like that.And... some circuits require capacitance with some minimum ESR otherwise the circuit may oscillate or behave in weird ways.For example a lot of 1117 linear regulators require a capacitor on the output with an ESR between 0.1 ohm and 1 ohm - ceramic capacitors almost always have less than 0.1 ohm ESR ... you can in theory work around this by placing a 0.1-0.5 ohm in series with the capacitor.Some datasheets for this chip and others like it may not even mention this "limitation" but they'll say or show in schematics "a 10uF tantalum or 22uF electrolytic capacitor will be enough" ... assuming you'll get the clue that most tantalum capacitors have esr above 0.1 ohm, and that modern 22uF..100uF electrolytic capacitors will have ESR below 1 ohm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2020, 11:35:25 am
content: Smaller packages have less stray inductance. Inductance is proportional to length (ultimately, this is because the speed of light is finite).If you don't need low inductance, you can use bigger parts.To some extent, you can use more in parallel (when laid out correctly) to compensate for length; but this takes up even more space than a smaller part, so is impossible in many places.Yes, as mentioned, beware the C(V) curve, as well as C(T), and C(t) (that is, aging).Anything worse than X7R grade is not a great idea. X5R is passable, just with a narrower temperature range. The really sloppy types, Z5U and such, can vary by more than decade over their operating range.C0G is essentially an ideal dielectric by the way, an excellent alternative to film caps. In small values they're fine (indeed, you wouldn't want a 100pF X7R anyway); but they're more costly even at modest values (~nF). They're hard to make in large values (very few >= 0.1uF). Well worth it, when you need it! (Your main alternative is stacked PPS film caps, which are also expensive, and can suffer from leakage current because the open stack is impossible to wash residues out of.)Electrolytics aren't competitive with small ceramic bypasses; they're bulky, and the ESR is just too high. In modest values (10s uF), they're nice for dampening power supplies. Downside: they age quickly (1000s of hours at rated temperature, and worse near or beyond rated voltage), and the ESR is unreliable -- it increases with age, and at low temperature. Available in large values.Tantalums are a good alternative, offering stable ESR. Downside: pricey, conflict minerals, and a chance at incendiary ignition(!) when used with low impedance supplies subject to surge/inrush.Preferred I would say, is a nice bulky ceramic with an external resistor in series. ESR doesn't have to be internal, though it can have lower ESL when internal.Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 09, 2020, 03:59:40 pm
content: As pointed out above, with the very nonlinear dielectrics (Y and Z), the manufacturer's voltage spec is a safe non-smoking value, not a good working voltage, since the capacitance decrease with applied voltage is so dramatic.

author: tooki
author_rank: Super Contributor
timestamp: August 09, 2020, 05:03:43 pm
content: Quote from: TimFox on August 09, 2020, 03:59:40 pmQuoted for emphasis. This is critical knowledge.I’ll add that C0G are the kind of MLCC’s that have quite linear characteristics, more or less like what we envision an ideal capacitor as. But they’re much larger (and thus much more expensive) than the nonlinear types.

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2020, 11:28:52 pm
content: Quote from: simonlasnier on August 09, 2020, 08:59:20 amIn some applications, the higher ESR of an electrolytic capacitor is an advantage.QuoteThe physically larger ceramic capacitors usually have a better voltage coefficient of capacitance. They should also have a higher ripple current rating but that might only matter in an RF power application.

author: mariush
author_rank: Super Contributor
timestamp: August 10, 2020, 04:41:33 am
content: Besides the price, other problems you have to worry when it comes to big size ceramic caps is microphonics - they can be more easily affected by vibrations and noise.It also matters how you place the components on board so they won't crack if the board flexes or if you place them near edges/corners/near connectorsBig ceramic components are sometimes used to increase separation between high voltage and low voltage sections, or to provide more mass for thermals ... you may see cutouts in the circuit board under a 1206 part or something like that.For example, you'll often see 2-3 200v+ ceramic resistors in series used to charge up a offline switcher ic / control ic / switching regulator before it sends the first pulses through the transformer which can then take over by sending some power to the ic through an auxiliary winding.

author: Saimoun
author_rank: Frequent Contributor
timestamp: August 10, 2020, 10:10:05 am
content: Wow thank you all for the replies, this is really great!!Actually I did watch Dave's video on the subject (https://youtu.be/2MQyQUkwmMk) but I guess I forgot a bit!! Also he did not mention the impact of the size but someone wrote it in the comments. Anyways, it is pretty clear nowI am still a bit confused about the ESR:I thought electrolytic capacitors had much larger ESR (like a few ohms), but looking a few I found mainly values between 5 and 80 mOhms... Did I get that right?And as I understood the ESR is always frequency dependant, so when let's say an LDO specifies a "max ESR" value for the input or output cap, at what frequency do they mean? (it seems the ESR goes up to a few 100Ohms at very low frequencies / DC). And if it is a SMPS I guess the ESR matters at the switching frequency?And last question - if I am using a MLCC as a coupling capacitor to "move" an AC signal from a 2.5V DC bias to a 0V DC bias (acting with a resistor as a High Pass Filter basically), I guess that means the the capacitor actually has a DC bias of 2.5V - then that will affect its capacitance the same way, correct?Thank you

author: tooki
author_rank: Super Contributor
timestamp: August 10, 2020, 10:25:57 am
content: Take a close look at a capacitor data sheet. It’ll answer a lot of your questions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2020, 10:51:46 am
content: Quote from: simonlasnier on August 10, 2020, 10:10:05 amThey are what they are; bigger caps are basically just smaller caps in parallel... So you should expect e.g. a 1000uF of 50mohm, 100uF of 500mohm, 10uF of 5 ohm, etc. Varies with construction, formulation and voltage rating, so, there's plenty of overlap.I think if you plotted ESR vs. capacitance for all parts on the market, you'd see a scatterplot with a lot of noise, showing the underlying trend (ESR ~ 1/C), but that within any given band of values (like 100-200uF) you'd still have a pretty fair range of ESRs on offer.Not that most electrolytics are rated in impedance or ESR, and some aren't even rated in ripple current (from which you might make a crude guess about their ESR). I'm assuming you'd measure them (this would be quite an endeavor, obviously).The same is true of Ta and polymer types, again with Ta ESRs being stable, and polymers generally being lower (10s mohms even for small values ~10s uF). But the available spread in both is overlapping, so if you need a capacitor with say 0.1 ohm ESR, you can get it from any of the three types, at least in the nominal case.QuoteYes, indeed! It would be great if they documented that, unfortunately they don't!ESR is most important around the crossover frequency, typically in the 0.1-1MHz range. That's an okay assumption to make, I think, but YMMV, and test it if you need to be sure!This also gives you a frequency to work with in terms of further improving bypassing. You can put low-ESR caps in parallel, as long as the ESR is still dominant in the critical range. For example, if 3 ohm ESR is required at 100kHz, you can use up to, say, 0.16uF in parallel. The 3 ohm ESR then needs to be in series with a capacitance of 1uF or more, again to be dominant 3 ohms ESR at 100kHz. This will give an impedance curve that's very high at DC, dropping along a line of 1.16uF, until leveling out around 30kHz and 3 ohms, then dropping again by 300kHz, along a line of 0.16uF. The impedance has a shelf, a flat spot at 30-300kHz, and is generally capacitive elsewhere, including a very low impedance at very high frequencies.This is yet another thing that LDO datasheets don't discuss, but is a necessity for powering digital ICs!QuoteExactly. And at frequencies where its voltage is changing -- at and below the transition frequency -- the change in capacitance affects the filter itself. Low-frequency components affect the attenuation of high-frequency components: it generates mixing and distortion products.Now, a coupling capacitor is usually intended to have a cutoff well below the signal bandwidth, so this doesn't have to be a problem. Those frequencies simply aren't applied!Be mindful of this when using them for filtering. Prefer C0G or film types. If you have to use a type 2 dielectric, pick X7R or better (higher numbers and lower letters are preferred), and prefer higher voltage ratings.For example, in a phono amp: you might want to use film, or tantalum or even electrolytic capacitors, in a passive or active highpass filter, to remove "rumble" and other undesirable low frequencies < 20Hz. Then the remaining amplifier can use coupling with relatively poor quality capacitors and low cutoff frequencies.But then, a phono amp probably can't use type 2 ceramics due to microphonics -- induced voltages on the order of 10mV are common here, and indeed you can see this in commercial oscilloscopes (AC coupled mode) as well as audio amps (and RF amps, if the passband extends low enough: ultrasonic vibrations can be seen). Or even just control loops (keep this in mind for low noise power supplies!).And ESR, doesn't usually matter for coupling. The signal impedance is usually high, 100s of ohms to 10s of kohms, so an ohm here or there has negligible effect, and only amounts to slight attenuation. Conversely, when signal impedance is low (say you need to remove DC offset to a loudspeaker?), it may matter. (Indeed, a power amp delivers real power, so you might want a low ESR just so it doesn't heat up -- the lower attenuation is merely a bonus!)Tim

author: Saimoun
author_rank: Frequent Contributor
timestamp: August 14, 2020, 02:47:36 pm
content: Tooki -> these last few months it is pretty much what I do, going from datasheet to datasheet... ha haBut this info in particular is rarely in the datasheets, I found a few graphs on the manufacturers websites thoughT3sl4co1l -> Really nice and informative reply!! Nice to know about all of this!! I actually have an audio design which has very very low requirements in terms of audio quality (simply playing a "click" sound), but uses a very simple first order LPF with a 100nF cap, so I was thinking just using one more of the 0402 MLCC that I use for decoupling on the board. But then the danger is microphonics as I understood!Anyways thanks for the answer! I did not know what the crossover frequency of an LDO was - but I found a nice article about LDO's and bypass caps from TI which explains it (among other things):https://www.ti.com/lit/an/snva167a/snva167a.pdf