EEVblog #742 - Why Electrolytic Capacitors Are Connected In Parallel

hi welcome to fundamentals Friday this one comes from a four oppose from a user named Laurie thank you very much Laurie for asking this very interesting question why do you parallel electrolytic capacitors you do its head out of a product you open up and you see multiple electrolytic capacitors in parallel it's quite common to actually find this thing in power supplies and all sorts of applications why do we do it there's actually quite a bit to it let's take a look now I will just stick to what Laurie asks here specifically about electrolytic capacitors I won't go into for example you often see you'll have a hundred nano farad in parallel with a ten nano farad impel in parallel with a one nano farad across bypass in say a modern FPGA for example that's a different thing entirely and I have covered that before so we'll just take a look at electrolytic capacitors generally in power supplies like this and other sort of big higher power applications now you might typically see these parallel electrolytic capacitors in say your standard linear power supply we've got an AC transformer full wave bridge rectifier and then you might see more than one sometimes you'll let me see one other times you might see you know two or three or five or even ten in some sort of our extreme example and also you'll see them in for example white DC to DC converters like this so I got DC and we got DC out whether or not it's a boost or a buck step up step down doesn't matter but you typically see at it you know generally a couple of electrolytic capacitors on the output you could also see one but it's a fairly common to see two or three why why not just use one well basically when you design a circuit or a product you're going to have a whole bunch of specs you're going to meet at least if you design it properly I mean you know you can't just go I'll just bug you 100 might cap you know she'll be right no worries but if you actually design it properly you could have a whole bunch of specifications for your electrolytic capacitors here some of them can include well the main one of course is generally going to be your capacitance value it could be a hundred microfarads for example could be a thousand ten thousand whatever you're going to have a maximum ESR value for example naught point one ohms that's very common in DC to DC converters for example for stability they need a either a minute minimum maximum sometimes like a window where the ESR you have to get their correct ESR which is the equivalent series resistance inside the capacitor because remember capacitors aren't ideal you have to deal with the real world where your typical capacitor like this actually has an ESR in here this is the equivalent series resistance in there it's kind of a certain resistance value it can depend on the frequency and all sorts of complicated things then you're actually going to have the capacitor itself the ideal capacitance in there and then you're going to have a little bit of lead inductors as well quite a complex little beast your practical capacitor there's no avoiding this stuff so you typically read your data sheet for your DC to DC converter and it might say you need a certain amount of capacitance for that particular load may be a either a minimum maximum or a window of the ESR value so you've got to meet that the other main thing is our size you know you don't have an infinite amount of room inside modern products sometimes you do you've got the luxury of you know space isn't a problem but with today's miniature modern electronics and everything else be it surface mount or through-hole here everything we're talking about is going to be applicable to both surface mount and through-hole electrolytic capacitors but yeah you might have to meet a valium envelope typically our height is a major requirement for capacitors and well it can go into you know you may be forced to use several different ones because of different high restrictions then you've got cost restrictions for example which we'll go into that may be a reason why and then you have operating life as well there's quite a few specs in here so let's go through all the reasons I think we'll find quite a few why you might want to parallel electrolytic capacitors instead of using just one so I've up with a list of nine different reasons why you might want to parallel capacitors and they're all going to intermix as you'll see there might be one specific reason or a combination of reasons that push you towards either using a single capacitor nothing wrong with using a single professor if it meets all your requirements or as the question asks why do we parallel them let's find out now some of them have been copied from the specs over here and we'll talk about them in more detail but there's at least double the number of reasons in here well let's take a quick look hopefully quick hmm now the most obvious one of course is your capacitance value take for example your linear power supply here you've got a full age wave bridge rectifier so we're getting a hundred Hertz typical full wave bridge as you should be familiar with standard building block circuit well we have to generally meet a ripple voltage requirement on here so if the black one there is the output of your bridge rectifier your capacitor is going to smooth that and depending upon your load is going to be dependent upon how much ripple you actually get there and it's basically dependent upon three things the your load current your capacitance and your frequency your frequency of course here in Australia 50 Hertz mains voltage your full wave read rectify it doubles it going to have a hundred Hertz for example so it's only dependent upon your load value and your capacitance or the ESR and stuff doesn't really come in - well it kind of does but let's not go there this is like a just a rough rule of thumb formula that you can use to calculate your ripple voltage so you might be using a 7805 voltage regulator drops out at seven volts for example so you don't want your minimum ripple current there to be under a certain value so you don't have a certain amount no capacitance you plug the numbers in and you figure out okay I need a thousand microfarads of capacitance for example do you be my required ripple current now of course you can just stick in one capacitor of a thousand microfarads and Bob's your uncle right you've done the job okay in this basic example that might work but aha there's a whole bunch of other stuff which could influence your decision to use more than one now the first one us might be physical size you might only have ten millimeters hide available but you need a thousand microfarads at say you know 15 volts or something like that so you look through the catalogs and data sheets and you find that well you know it's really quite difficult to get a cap that means that physical height requirement so you might have to go for a shorter cap so you might only be able to get say 330 micro farad 16 volt cap in the height you want and their diameter you want and everything else so hey bingo you have to use that might force you into using 3 330 Mike are capacitors Mike is short for micro farad capacitors in parallel instead of 1 1000 mike cap obvious so we've covered two things capacitance and physical size well what about cost well yield once again you look through your digi-key catalog or whatever who wherever you're sourcing your path from you might find that hey the 3 330 micro farad's at 16 volts might be cheaper than the equivalent one thousand microfarads at 16 volts for example so hey you might go just on cost reasons alone if space wasn't an issue or whatever that could be a driving factor who knows and another big one bomber reuse bomb is Bill of Materials you might find elsewhere in your design you're already using a 330 micro farad capacitor for example so well why specify another thousand microfarads in your Bill of Materials if you can get away with just reusing three of the ones you already use and if they're surface mount for example you can buy them all on one big-ass reel like this which is much more economical these are what to 20 micro farad 16 volts so if you're already using these in your design hey you get a bulk price and you only get to take up one of the feeders in your pick and place machine beautiful that can be reason enough to use multiple capacitors another one that might be important for specific products is product configuration let's first say for example you had an audio amplifier okay it means a big DC voltage rail with lots of by past caste because it's you know a hundred watts per channel or something like that well let's say you had two models that had one was 50 watts and the other was hundred watts well you're the amount of filter capacitance you're going to need is going to change depending on the model so you might layout your PCB to have the multiple footprints for the capacitors and you might only populate the number that you actually require you might have one Quebec or two in a particular design and then you might have four for one that has twice the power or something like that now here's a big one which has an impact on many different reasons here this is the ESR the equivalent series resistance as I show before the model of a capacitor is the ESR in series with the capacitance itself in series with a bit of lead inductance and there some other little niggly stuffing there we won't worry about like our leakage resistance and dissipation as well but what we concern with is this ESR here now if we take the case of a 100 micro farad capacitor that's got nought point 1 ohm zsr for example that is equivalent you can build that same capacitor up by putting two capacitors in parallel of 50 micro farad's each of course that's not like a preferred value said choose like 47 mic air near enough electrolytic capacitors are typically like plus minus 20 percent anyway so yeah near enough anyway these can have a point 2 ohm ESR which means that they can be smaller because the ESR is going to be pretty much determined by the physical size of the capacitor so the larger the capacitor for the same capacitance typically the lower the ESR so you can get away with two smaller capacitors in parallel of a higher ESR figure and that's equivalent because they act like parallel resistors so you just use your standard parallel resistor formula you get naught point one ohms and total they're exactly equivalent but apart from the size issue where this really matters and we get in a couple of things down here the ESR of course it's a resistor when you pass current through it in this case ripple current that we saw before especially in high-power sort of applications you can get you know a lot of current like amps or something like that you're going to get power dissipated in that equivalent series resistance inside the capacitor body itself that little aluminium cased electrolytic capacitor is going to heat up internally due to the power dissipated in the ESI in an ideal world in an ideal capacitor there would be no power dissipated but it doesn't work like that in the real world you're always going to get some form of power dissipated inside that capacitor and the reason that temperature rise matters is because aluminium electrolytic capacitors are filled with a dielectric liquid which essentially can dry up the higher temperature they get they have a finite life of done many videos on this you know the capacitor play for example bad capacitors typical failure mode inside you know at typically a high power product like a television or something like that gets quite hot in there use a lot of power you need a lot of capacitance and they all get hot internally and it can shorten their life and typically all you need to do to fix a product might be just to replace the capacitors because the dielectric has dried up in there they're going a finite life take a look at this data sheet for example of a typical Rubicon capacity here and you'll see that they can have anywhere from say 4,000 hours life up to say 10,000 hours the larger the voltage rating of the capacitor typically the longer the life the bigger the diameter the longer the life all that sort of stuff because they don't heat up as much internally the physically larger capacitors so this is going to make a huge difference to your product and you'll notice down in the table down in here you have different ripple current ratings for different size and values and voltages of capacitors and that ties into life up here where it tells you that it's going to have X you know thousand hours life at 105 degree C rating is a typical high temperature capacitor for example they typically come in two temperature grades either 85 degrees Celsius or 105 you can get slightly higher than that but they're the two major ones and it's going to have a rated life based on a certain temporary and that's all determined by you are ripple carried your ESR and everything else and of course a specific size of capacity is going to have a specific ability to be able to dissipate that internal heat and roughly it might be equivalent to the outside surface area of the capacitor for example you know just ignoring like internal construction and things like that so for a typical cylindrical radial capacitor like this we'll have an an area determined by the outside of the capacitor and then we'll have the area on the top as well and you know your typical formulas area of a circle is PI R squared and then the outside layer of the aluminium casing is going to be pi times the diameter times the length of the capacitor ie the height of the capacitance can add those together you've got a total surface area and for those really keen you could go into all the thermodynamics of it and how to actually attempt to calculate and model a temperature rise of the capacitor and has to do with do you have any airflow it over it is it radiated is it in a sealed enclosure how can it radiate to the air inside and then how does it radiate outside the case and our thermal arm calculations like this very ugly very complex but generally the more capacitors you have the greater the total surface area you're going to spread the ESR across remember in this case we had two separate capacitors of 0.2 ohms ESR each they're going to share the current they're going to share the power dissipation between them so you're going to end up with a typically a longer life than your our single capacitor with equivalent electrical specs so your longer life here can be a major major reason for having multiple capacitors in parallel might be the only reason to do it and you might have done the calculations to figure it all out or you might have gone well this product needs to be reliable its industrial it's in a hot environment for example so the temperature rise inside the capacitor of course is going to be above ambient so if you design your product to work up to 50 degrees Celsius then you've got to design margin on top of that for use in 85-degree caps you can calculate the ripple current the temperature rise inside or typically you might do empirical measurements actually put a thermocouple use a thermal camera actually get some temperature measurements on the prototypes for example is it good enough then you can calculate other capacitors going to be the Achilles heel in the reliability of your product or not and you might decide to spread it across five or even ten capacitors something like that because in this case they just all parallel up so if we wanted to go to the extreme and say use 10 capacitors we use 10 10 micro farad capacitors with one ohms ESR each could give us the equivalent capacitance here and in that case each of the 10 capacitors is only dissipating 1/10 of the power so depending on the you know the thermal performance of the thing the temperature rise is going to be very very small compared to a single capacitor so you lie you might have increased your lifetime by several orders of magnitude now another big thing is redundancy okay if you've got a single cap then you've got a single point a fire if that capacitor fails or you've just got you know you're not going to be able to fill to your power supply anymore your DC to DC converter is going to go crazy whatever your products going to muck up but if you had say two in parallel like this well then one if do to say manufacturing reasons you could have a defective cap or something like that then well the other one might be able to take over if you're designing sufficient engineering margin into there so you've added redundancy to your design just by adding an extra capacitor in parallel to take it out because capacitor fires heat up it's sort of like a snowballing type effect the more it heats up the more the dielectric dries out as the dielectric material inside dies out then the ESR goes up and the ESR goes up it's going to dissipate more power and it's a snowball anything and these capacitors can start to fail very quickly but if you have moldable capacitors in there you can add a bit of redundancy to your system just to ensure reliabilities you can see how it's all starting to come together all these many factors to determine reasons why you might or might not use multiple capacities in parallel but generally speaking it's a pretty good idea to do two if you're designing a reliable product now last and maybe least it depends on what sort of requirements you got are the peak current considerations now if you have one capacitor for example and one load let's say we just have one capacitor this is looking down onto our PCB we're talking about physical PCB implementation here so they're red just imagine these two capacitors don't exist you've got one capacity is driving the load then all of the current including peak currents has to flow through that particular trace and depends on you you know on really high power designs you can get a lot of loss in those PCB traces but if you went for three capacitors in parallel for example you wouldn't just wire them straight here you might have a separate trace going to the load like that so you share our current through each particular trace so you can have smaller traces less drop go in to to a specific star point over here at the load so you've got to have a prety a current flowing through this trace this one and this one which will be one-third the current you would have with only one capacitor and then you would get smaller voltage drops across there Ohm's law because we've got to have a certain PCB resistance depending on the thickness of your coppers typical 1 ounce copper for example you're going to have a voltage drop as small as it might be it can in big high-power systems this can be a real consideration so now you've split your current up three times like this you can have much smaller voltage drops and your loads going to be much much happier because you've used multiple capacitors in parallel so you might have physically implemented them as three separate capacitors like that but on your schematic you're just going to show them as three like that but typically if you good designer you'll have a note on your schematic like that saying explaining to the PCB layer person which might be you because you might forget explaining to lay it out in this sort of configuration because it's important and a different configuration yet again well you might dedicate one of your capacitors for example to another load out here just so that you don't have to share the same copper here because if if this load here is a pulse load for example and it's pulsing then you know physical layout if your PCB and your traces matter you don't want that tied on to here because if this load here you know takes a big gulp of current then you're going to get a voltage drop and a droop on your traces go into this load over here which may not like it so you might actually show them once again electrically all in parallel like that but physically going off to a different load but you know if you were once again you'd put a note there but if you're a good drawing your skin you're good at drawing schematics and join them cleanly then you put that capacitor over against the load on the specific part of the circuit but hey you might see them drawn like that and they physically might be right next to each other but you might actually layout the traces differently and it's not just the internal SRO the capacitor either that heats up the thing you've also got connection losses as well the leads on the capacitor for example they've got a certain amount of resistance your solder joints everything else that can propagate they're going to heat up at large currents for example even though resistances of the lead wires are very very small right in large high-power applications it's going to matter that's why some really really big caps don't have just leads pin stick and now they have gigantic lugs on them and big mounts that you know you bolt them down to the PCB and everything like that so you know it's a really high currents really big deal and you might have to use proper crimp cables to go up to them and real you know if you open really high-power amplifiers and things like that then I just have a couple of lead 'add radial capacitors stuck into the board it can be a real big deal so isn't those interconnects of course they have a certain amount of resistance I squared our losses they're going to heat up as well they can conduct the heat into the capacitor itself and that all contributes to the internal heating of the copper so it's not just the ESR but there's a whole bunch of other factors so when you split capacitors like this you're also sharing the interconnect the connection losses between the multiple capacitors and that can make or break your design so there you go I hope I've answered Lori's rather innocent question why do they put multiple electrolytic capacitors in parallel and I came up with easily came up with nine different reasons to actually do that as you saw they can all intermix together and you may only use one of these maybe you know a deal breaker right I've got to use multiple captain capacitors because of cost or because the BOM reuse or or might be some sort of you know weird ESI requirement it could be some big physical load requirement it could be longer life redundancy that's a very popular one in fact that's probably one of the most popular reasons as I said before if you kind of design a product and you want it to be reliable and you've got a capacitor in your power supply and that load is you know as a reasonable amount of load and you might check how much ripple current you get in and you know all that sort of sparked stuff especially on DC to DC converters for example where ripple peak ripple currents can be very high well you know one capacitor may not cut it what if you get a faulty one out of the batch not so good then hey put some extra capacitors in parallel for redundancy now it's actually quite hard to show a lot of this in practice on the bench but I've got one little bench example where I'm going to show you how we can actually spread the temperature rise among capacitors by having multiple ones in parallel let's check it out okay what I've got is a simple very board here with one capacitor and then ten capacitors and we'll see the difference between the heat rise in these things now one is a hundred microfarads fifty volts and the other is ten 10 microfarads at all so 50 volts and yes they're exactly the same art rubicon brand I'll link in the data sheets down below so they can have a look at it if you're keen now I've got this powered from an AC transformer over here it just allows me to select the voltage we've got a diode bridge rectifier therefore wave bridge rectifier I'm actually powering them both at once and you'll see why in a second and so I've got just another additional diet isolator there and just some probes on that so that we can probe the ripple voltage on this thing and I've got the outputs here so we've got a standard full wave bridge rectifier and then I've got two constant current dummy loads my do-it-yourself one which seen in a previous video and the BK precision one here and this'll allow us to set a constant current load so that we can get a nice big ripple value on both of these art capacitors so both are 100 microfarads except this one is made up with 10 and of course the total surface area here is going to be a hell of a lot more than the surface area here so yeah we should see a temperature difference and we're going to have a look at that with our flurry 8 infrared camera let's go now I don't currently have anything that powered on you can see that our temperature range is only small at 4 degrees so this difference you might be able to see in there some difference in the temperature on the rear capacitors over here and that watch this I haven't got it turned on but look at that I'm adding my hand in here this is actually reflection of my hand down off the metallic top seeing the aluminium tops on those capacitors and it's reflecting back so you've got to be careful when you're doing thermal measurements like this to make sure you're not actually getting heat reflected and you've got to set up the emissivity correctly and yeah and the bright aluminium tops on the capacitors isn't the best thing but anyway so we could like I paint the top of the capacitors are black or something like that if we wanted to anyway so you've just got to be careful to block out any reflected heat I'll show you that look I'll put you I'll put my hand here and you can see the heat and I'll put a book in between boom gone now please forgive me this is going to be really tricky to get in one shot but I wanted to power this up and like from cold and show you so you can see there's hardly any temperature differential in there at all you can't see much now if I apply power to the thing and then apply it so I'm going to feed in 15 volts AC here we go and I'm going to you can see that the bottom one you can see the diode heat up there now I'll cover that in a minute because that's going to affect our temperature range but you can see that I've got 200 milliamps on the single 100 micro farad capacitor that's the one down the bottom here this one down here I'm going to be careful not to touch anything otherwise yeah it's you can see it probably starting to warm up actually I should do the other one so I'll set the other one to 200 milliamps as well and bingo now we're drawing 200 milliamps from the top one and you can see that you can see look you see the hundred microfarad capacitor starting to warm up so what I'll do is I'll cover this I've got a actually that was probably bad I was touching it for too long damn yeah that posted note anyway I'm trying to cover up those diodes so it doesn't affect the maximum range of our reading there but you should be able to see after a while it's going to ramp up now the important thing to note here is that our conditions are identical because what do you want when you have a simple full wave bridge rectifier circuit like this you want to all you care about is the ripple current okay and you can see I've got channel ones and and channel two in there and then the same ripple current you can see that right in there I mean this is really extreme okay because I want to be able to heat these caps up this is not normal design you know that we're doing here you wouldn't have twenty thirteen point six volts peak-to-peak of ripple they're just that's insane with the lower voltage they're about 10 volts remember we're 5 volts per division there so they're both identical so we've got 10 capacitors a thousand microfarads as we saw in that a basic rule of thumb formula before the ripple voltage is just going to depended upon the frequency and the low current the low current is the same for both of them we've got basically the same hundred micro farad capacitance for both of them so we're getting the same ripple but you can really see that bottom one heating up compared to the top ones I'll get a closer shot on that we've only been like powered up for less than five minutes okay now I can replace that post-it note there with a big aluminium shield and here's my pointer coming in here you can see the scale changed dramatically there but we're not talking about a big temperature difference here I mean it's showing that the maximum that caps getting to is basically 27 degrees but it's a lot but that is like you know a good like five degrees more than the other caps which are all sitting around ambient you know and like sort of not far above ambient temperature just a degree or two so you can see the temperature spread between these things let alone they're like the core temperature inside because you're staking in into the the thermal properties of the capacitor and how can get its heat out and things like that it's like a semiconductor die they like the die temperature inside is always going to be hotter than the case temperature for example and then the heat sent less than the heat sink temperature gonna have losses along the way but you can see how that capacitor it's much much hotter and it's exactly the same ripple current exactly the same design conditions please excuse the the light popping in there but you can see that the capacitors at the top only like less than 25 degrees so it's not too far above the ambient in here but the 100 the single hundred mite capacitor down there is like 27 degrees it's a good few degrees warmer and if we leave it there for longer or we use higher currents and you know this is just designed to show the difference but I think that quite dramatically shows the difference how you can actually shorten the life of these capacitors because as I said it's a snowball effect and hey if you got in high ambient temperature this is just like free air here in the lab it'll be and be cooler again if I actually are turned on the aircon these top pastors might actually be more efficient because there's more surface area so when you've got air flow ever just start here in the lab with just the air con artist tiny little air flow can make a quite a significant difference to the temperature or if you're designed in your product in your products got proper thermal design with fans and everything else then having all that huge surface area on those 10 caps can be a hell of a lot different to this single 100 microfarad cap here now if you were sufficiently keen you could attempt to calculate the temperature rise of the capacitor and you could probably do this using the stefan-boltzmann law this is the thermal energy that is radiated per second per unit area so there's the basic equation which you can slightly rearrange to get your thermal energy radiated from a hotter object to a cooler object in this case would be ambient or whatever environment you've got so what we've got is the emissivity of the material here so you'd have to go look that up and then of course the capacitors but like wrapped in that plastic wrap and things like that and it gets very complicated but anyway the emissivity of your radiative material ie the capacitor multiplied by the stefan-boltzmann constant that's some weird ass funny number you learn in physics and multiplied by the total surface area and this is the big thing that makes the difference here between the having 10 caps in parallel with a massive surface area compared to just one capacitor for example and then you multiply that by the fourth power of the temperature of the capacitance minus the fourth power of the ambient temperature and of course you can rearrange that formula to get the fourth power under what to get the temperature of the capacitor after yeah after you know letting it that stabilized and all that sort of thing but it's not gonna be that easy but hey for all you nerds out there go for it see if you can do it better homework so there you go I hope you enjoy that rather lengthy look at why didn't do your parallel capacitors and that seemingly simple subject like that turn into like a half-hour epic sorry about that but yeah anyway there is a lot to it even like basic things like this in electronics you can go to town on these sorts of things and the reasons behind choosing certain things so anyway if you want to discuss it jump on over to the Eevee blog forum and as always if you liked it please give it a big thumbs up on YouTube and you can leave comments all over the place and yes stalk me on Twitter and all that sort of jazz catch you next time you

Channel: EEVblog

Views: 293,190

Rating: 4.961772 out of 5

Keywords: Electrolytic Capacitor, Electrolyte, voltage, capacitance, capacitor, why, how to, parallel, connected, thermla camera, flir, flir e4, flir e8, measurement, oscilloscope, full wave, bridge rectifier, dc-dc converter, half wave, pcb, design

Id: wwANKw36Mjw

Length: 34min 3sec (2043 seconds)

Published: Fri May 08 2015

Reddit Comments

[u/Jimbo_029]

I found this pretty interesting and useful, some of the reasons I hadn't thought of. I always like some more practical information once in a while since often lectures and textbooks can lean towards being more theoretical and maths based but don't about things like bill of materials or redundancy, which some might say are obvious but these things don't necessarily come to mind when designing.

Also I'm a big fan of the EEVblog, so if people like this I'll post more in the future, if not, I won't :)

[u/madscientistEE]

Honestly, aside from math, I've learned more from taking stuff apart, fixing things sans schematic and looking at stuff on the Internet than I have from university. EEVblog is awesome!

Post anything you find interesting.

[u/[deleted]]

This is a fantastic channel! I loved his video on Op-Amps, he explained things really well.

[u/BurningBushJr]

I love EEVblog. Thanks!