What are possible reasons for physically big ceramic capacitors on PCBs? - Page 1
Forum section: electronics / beginners

author: MMMarco
author_rank: Regular Contributor
timestamp: June 08, 2023, 05:10:35 pm
content: Every time I see Dave teardown a device and is showing the PCB I'm wondering why engineers sometime choose relatively big ceramic capacitors in their design - I think they look very cool but that obviously can't be the reason why.Voltage seems to be one property, but I don't think its the main one.Here a picture to show you what I mean:

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 08, 2023, 05:22:43 pm
content: High capacitance, low ESR and inductance, cheaper than similar values of aluminium polymer.

author: TimFox
author_rank: Super Contributor
timestamp: June 08, 2023, 05:53:30 pm
content: High-capacitance X5R ceramic capacitors are now a popular replacement for tantalum and aluminum electrolytics for > 1 uF.Like anything else, if you need a high capacitance value that's a lot of stuff inside the package, requiring a large package.On pages 3 and 4 of this AVX catalog, the preferred sizes are shaded.https://datasheets.kyocera-avx.com/cx5r-KGM.pdf

author: MMMarco
author_rank: Regular Contributor
timestamp: June 08, 2023, 06:00:45 pm
content: Quote from: mikeselectricstuff on June 08, 2023, 05:22:43 pmWhen you say values, do you mean the combination of low ESR, high capacitance and low inductance?I.e. it's cheaper to buy a ceramic capacitor with those properties than it is to buy an aluminium polymer version of it?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 08, 2023, 06:03:06 pm
content: Quote from: MMMarco on June 08, 2023, 06:00:45 pmgenerally yes

author: TimFox
author_rank: Super Contributor
timestamp: June 08, 2023, 06:11:00 pm
content: Things change, and you should consult catalogs to see which choice makes more sense for size and price.In my youth, there were no ceramic capacitors readily available above 1 uF, but they were readily available with 500 V and 1000 V voltage ratings.(Disc capacitors, before MLCC took over the capacitor market.)

author: iMo
author_rank: Super Contributor
timestamp: June 08, 2023, 07:48:01 pm
content: They use the larger sized MLCC capacitors because of their voltage ratings.An MLCC 1uF for 50V or 100V has to be physically larger than the one for 6.3V, for example..

author: TimFox
author_rank: Super Contributor
timestamp: June 08, 2023, 09:16:43 pm
content: Also, with non-linear dielectrics such as X5R, you need to use a higher voltage rating to avoid excess loss of capacitance at the working voltage.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 08, 2023, 09:21:14 pm
content: Quote from: TimFox on June 08, 2023, 09:16:43 pmI recall EEVBlog Nr. 626 where Dave talks about how ceramic capacitors can loose a great amount of their capacitance based - I think due to the piezo electric effect - on the voltage their biased at?I'm not entirely sure.Are you referring to that?

author: TimFox
author_rank: Super Contributor
timestamp: June 08, 2023, 09:44:59 pm
content: Quote from: MMMarco on June 08, 2023, 09:21:14 pmThe piezoelectric effect refers to generating a voltage due to mechanical stress.Except for C0G/NP0 which is a linear dielectric that obeys the capacitor lawQ=C V, with the capacitanceCa constant, independent of voltage, the other common dielectrics are non-linear, similar to an iron core in an inductor, hence the term "ferroelectric". A good capacitor datasheet will show the decrease in capacitance with voltage. The voltage rating for these non-linear capacitors is a safety rating, where the manufacturer guarantees they will not be damaged by applying the rated voltage. Y5V and Z5U dielectrics are usually worse than X7R in this non-linearity.Since the properties of the dielectric material are a function of the internal electric field (voltage gradient), a thicker slab of material will have less change in capacitance for a given voltage than will a thinner slab with the same nominal capacitance.See Fig 1 in this technical note from Kemet:https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=27df70038a62dabf4e5ec33983b54e683957a966To my mind, a possibly more relevant question is "Why must the other capacitors be so small?" (there are possible reasons).

author: MMMarco
author_rank: Regular Contributor
timestamp: June 08, 2023, 10:00:50 pm
content: Quote from: TimFox on June 08, 2023, 09:44:59 pmAh, so this reduction in capacitance has nothing to do with this effect then?Quote from: TimFox on June 08, 2023, 09:44:59 pmSo generally speaking, physically bigger ceramic capacitors (except C0G/NP0) are more stable due to their bigger dimension?Quote from: TimFox on June 08, 2023, 09:44:59 pmI was about to ask a similar question, so for capacitors, the higher the voltage rating, the better for stability?E.g. 10uF X7R 6.3V is less stable than 10uF X7R 50V ?When choosing capacitors I'm never really sure what to think about the voltage rating

author: Psi
author_rank: Super Contributor
timestamp: June 08, 2023, 10:03:41 pm
content: Usually if you can get away with using ceramics over an electrolytic its a good idea.Electrolytic fail and can break off the PCB under vibration.To get both high capacitance and high voltage ceramics are quite large. but there gets to a point where it's cheaper to use two smaller ceramics in parallel instead of buying a single larger one. eg, Usually 50V 10uF is super cheap but 50V 20uF is premium because they are not made in high volume. They also make smt packages that are many smaller caps attached together with metal framing. They are even more expensive.You also sometime see two ceramics in series as a protection measure from the failure mode of ceramic to a dead short.

author: Infraviolet
author_rank: Super Contributor
timestamp: June 08, 2023, 10:33:00 pm
content: If you want ceramics with a big capacitance they will be bulky, nobody can manufacture 100uF in something as small as 0603 or 0805. And there is a voltage rating matter too, cermaics of the same capacitance have to get bigger to handle higher voltages, a 10uF 0603 is rarely rated for more than 12V maximum whilst an 0805 10uF ceramic is often rated for 25V. And if you're tight on spac these big ceramics are still smaller than, or similarly sized to, equivalent electrolytics (and better on the height diemsnions often). I've used that sort of 100uF ceramic quite often.

author: Psi
author_rank: Super Contributor
timestamp: June 08, 2023, 10:53:39 pm
content: Quote from: Infraviolet on June 08, 2023, 10:33:00 pm100uF is available in 0805 no problem. But the max voltage you can get is 6.3V and the cost is high.https://www.digikey.co.nz/en/products/detail/murata-electronics/GRM21BR60J107ME15K/9887675220uF 4V 1204 is also something that exists and used a bit for 3.3V power rails in small high power DCDC converters.https://www.digikey.co.nz/en/products/detail/murata-electronics/GRM31CR60G227ME11K/16286007It's the combination of high voltage and high capacitance that requires physically big capacitors.

author: TimFox
author_rank: Super Contributor
timestamp: June 08, 2023, 11:44:47 pm
content: As an exercise for the reader, look up that 0805-package 100uF/6.3V X5R capacitor and see what the actual capacitance will be with, say, 5V across it.

author: aduinstat
author_rank: Contributor
timestamp: June 09, 2023, 12:27:03 am
content: ADI has a good article on temperature and voltage variation that is worth reading:https://www.analog.com/en/technical-articles/temperature-and-voltage-variation-ceramic-capacitor.htmlLarger packages are more stable over temperature and bias voltage.Another thing that's possible with larger packages is that the cap can be constructed so that the layers do not overlap so much, so a crack is less likely to cause a short. Don't rely on that unless it's in the datasheet though.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 09, 2023, 01:40:35 pm
content: Quote from: TimFox on June 08, 2023, 09:44:59 pmI think I figured it out by chance.In EEVBlog #859 Dave talks about bypassing capacitors and mentions that the smaller the package the lower the inductance.So if you want the lowest inductance you'll choose the smallest possible package.Quote from: TimFox on June 08, 2023, 11:44:47 pmThat's a good exercise! Will do and reply my findings once I'm doneThanks!

author: M0HZH
author_rank: Regular Contributor
timestamp: June 09, 2023, 03:02:39 pm
content: I use large SMD capacitors for high power handling.RF filter board rated 1500W @ 50ohm, up to 70MHz:

author: radiolistener
author_rank: Super Contributor
timestamp: June 09, 2023, 03:24:13 pm
content: capacitance, ESR and voltage...Ultra low ESR porcelain RF capacitors are even larger despite the fact that they have about 100-1000 pF

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2023, 03:48:25 pm
content: Note that the size of these capacitors is actually only their footprint (mounting surface and pads).The popular sizes (1206, 0805, 0603, 0402, etc.) are typically twice as long as wide: 0603 is nominally 0.060 inch long and 0.030 inch wide.If you check the detailed chart in my Kyocera/AVX link above, you will find several different values for height in available capacitors for a given footprint.The inductance of such units is very roughly proportional to the length, but only weakly dependent on the height (thickness).Increasing the height can increase the "slab" thickness of the dielectric layers, decreasing the voltage gradient for a given total voltage.When inductance is very important, the capacitance value is often relatively low and the good dielectrics (C0G) can be used: they have lower loss (reflected in ESR) and no change with voltage, compared with the higher-dielectric-constant ferroelectric materials (X7R, X5R, Z5U etc.) which are non-linear.I have used C0G capacitors with relatively high capacitance values (100 nF) for critical applications, but they are available only in quite large packages because of the low dielectric constant.These capacitors are more stable not because of their size, but because of their dielectric material, which requires a larger size for a given capacitance.The porcelain RF capacitors mentioned by radiolistener have extremely low loss: typicallyQ= 10,000, and they have well-specified resonant modes. The series inductance gives a series-resonant mode, but above that frequency there can be an important parallel-resonant mode.

author: EPAIII
author_rank: Super Contributor
timestamp: June 12, 2023, 01:15:58 am
content: Reliability or long life was a thought in my mind. In the past decade or two, there have been many, well publicized problems with electrolytics. In over 45 years of experience maintaining electronic equipment, I have replaced thousands of bad electrolytics. But I can count the number of ceramics that I needed to replace on the fingers on one hand - and not pass the half way point on that count. Even well made electrolytics go bad. But even cheaply made ceramics only very rarely do.This has to at least be in the back of the minds of designers.At least, it would be in my mind while designing any device.Quote from: Psi on June 08, 2023, 10:03:41 pm

author: Psi
author_rank: Super Contributor
timestamp: June 12, 2023, 11:50:29 am
content: Ceramics do fail quite a bit in high density consumer electronics like cellphones and laptops.Shorted ceramic caps are pretty common in iphonesHigh density compact designs push the edge of what's possible in order to make the device small, and running on the edge usually has higher failures.Also. Because phone/laptop makers deal in such high volumes they have bargaining power to negotiate the component price down a lot.This can lead to the component manufacturers cutting corners to meet the price point which leads to more failures.