#257: Power Supply Decoupling & Filtering: why we use multiple caps in different locations

in today's video we're going to take a brief look at some of the practical aspects of power supply decoupling and filtering but we'll learn is that the size and placement of the various capacitors on a circuit board is largely driven by these various roles of that these capacitors perform large value capacitors like these big electrolytic capacitors essentially form kind of charge storage reservoirs and do the bulk of the filtering for a large relatively low frequency current demands of the rest of the board while these smaller surface mount capacitors are placed very close to various active devices form a different role they also provide a local charge storage but not nearly as much charge as the bulk electrolytic capacitance but more important what they provide is a return path for high-speed signal currents so the high speed signal currents that might be flowing out of a pin are going to flow back through the ground plane and ultimately have to get back to the power supply and they typically get back through these locally placed higher frequency decoupling caps I'm going to attempt to illustrate this on a more simplified board now here's the simplified mock-up board that I'm going to use to illustrate the various roles that these capacitors play I've got 3.3 volt power supply coming in running along a power supply rail which could essentially be a trace on a circuit board running up to in this case a an inverter chip that's running as an oscillator and then driving a 50 ohm transmission line into a termination here and that transmission line is sitting over a ground plane now right now there is no power supply filtering on this at all which is never a good idea but this is where we're going to start improving the output of the pulse that we're generating here and this is going to generate about a 3 volt pulse so result in about a 60 milliamp current draw when the pulse is on and much lower current draw when the when the pulse is off we also prove the power supply pin right at our active device we can see the effective filter ring right at the power supply pin okay so the first thing we'll do is take a look at the effect of the bolt capacitance on the low frequency content of the noise of the power supply in order to see that I'm going to zoom in a little bit on that power supply better way to do that is to set up an offset that's equal to the power supply voltage and that will allow me to set up a better vertical scale say maybe a hundred milli volt of division or 50 milli volt of division to actually see that noise a little bit better so we can actually see that when we're driving at the transmission line we're pulling an extra 60 milliamps through the supply not too close a little bit of a dip in the supply voltage and that it releases back up again so if we place this volt capacitor right near the power entry point to this board we'd see we've collapsed that low frequency noise down to just about nothing maybe a tenth of a division if I move that both decoupling pep cap closer to my active device it's basically about the same is marginally better it's basically about the same and the reason for that is that this low frequency content really is very insensitive to this placement because the as far as the low frequencies are concerned all of these points are virtually identical there are simply at the ground plane there's no change in impedance between those various points so typically you'll have these both decoupling caps right at the power supply or the power entry point into the board because the placement isn't that critical so let's actually solder this in place and then take a look at the effect of the high frequency component okay with our bolt capacitor in place let's change our vertical scale here so I can now see the extent of the high frequency noise associated with the edges of our waveform and we'll take a look at the placement of the high frequency decoupling cap and how the place the location of this on the board will affect how well it works in reducing that noise okay let's start off by placing this one-and-a-half nano farad capacitor right near the power entry point you can see a relatively small reduction in that high frequency noise I pull the cap off see it grew about two-tenths of a division there and then reduced again and now let's place that same capacitor very close to the chip and we can see a very dramatic change in that high-frequency noise so this is essentially why we've got these capacitors located very very close to the devices to get rid of that high frequency noise locally okay so we've got the both capacitors in place here now let's talk about how the roles of these two capacitors differ as I mentioned earlier the large bulk capacitance is taking care of the large low-frequency current demands and all the power supply and helping to stiffen the power supply rail in that case but the high frequency cap here is kind of forming a little bit of a different role it is still kind of a charged reservoir but it also is a high frequency current path let me explain what I mean by that the active circuitry in here is creating a square wave that's coming out here with some very fast edges and those edges contain obviously high frequency components now we've all heard the adage that current will take the path of least resistance when we talk about current or at high frequencies you have to modify that statement a little bit and say that the current is going to take the path of least impedance which in this case really means least inductance and the amount of inductance in a current loop is proportional to the loop area so let's see what that means here I've got that power supply no current coming in to power up this chip that's creating the signal that's going out into this transmission line so high frequency currents are going down this transmission line through the terminations into the ground plane and they're going to go through the ground plane and ultimately return back to the power supply but remember we're talking about the high frequency currents here and we want to minimize Loop area and that means that the high frequency currents are actually going to go into the ground plane and follow a path underneath the transmission line because that will represent the least amount of Loop area it's not going to just go straight across this board because that means that the current would have a big loop and a lot of inductance so the high frequency currents are going to follow a path underneath underneath the transmission line back towards the chip so how does it get back essentially to the power supply well actually gets back through this cap we had this cap located up over here the power entry point that means the current kind of came back through here because that's where the high-frequency currents were flowing but then had to kind of work its way all the way back around and come through here so adding a lot more loop area by placing the high-frequency decoupling caps close to the chip we provide a path those high-frequency currents to get back to the power supply minimizing the inductance and minimizing the amount of noise generated so this is really a very big part of the reason why if you look at any of the active devices and ICS on any circuit board you will see local decoupling caps right here right here you know those will actually remove I removed off of the board right there there's plenty on the underneath side of this boards associated with this chip and these are providing that you know local storage and high frequency return path or any of the high frequency currents that are demanded by these particular chips and this is essentially what we modeled here on my little mocked up board okay so I hope you learned a little something about the different roles that the different power supply decoupling and filtering capacitors play on a circuit board and why some of them are placed close to our active devices and some of them are not if you like what you see please give me a thumbs up if you haven't subscribed already please do so and thanks again for watching

Channel: w2aew

Views: 97,591

Rating: 4.987886 out of 5

Keywords: W2AEW, Tek, Tektronix, power supply, decoupling, filtering, filter, electrolytic, capacitor, ceramic, low frequency, high frequency, image current, ground, inductance, ground plane, oscilloscope, probe, transmission line, noise

Id: 9EaTdc2mr34

Length: 8min 11sec (491 seconds)

Published: Tue Apr 11 2017

Reddit Comments

[u/PrincessTyphoon]

Self promotion is fine when the content is excellent! Thankyou!

[u/hatperigee]

As a visual learner, it really helps to see the impact of the different caps & their placements to the high and low freq noise on your o-scope. Nice video!

[u/NoTurnBackNow]

You are the best, thank you!

[u/V1ld0r_]

Fantastic! Keep up this "educational series" you have oging on. It's a must watch :D

Off-topic: Is that a miniature o-scope you got there? I want one!!!

[u/healthybaconjuice]

w2aew is a beast. I have a list of his videos to translate.

[u/Johnathan_e]

One of the best electronics tutorials- by far!

[u/DerpyDan]

Keep up the great work, really appreciated!!!

[u/safetyinnone]

I saw your channel a couple weeks ago, I think looking for videos on oscillators. Anyway, just wanted to let you know I the content you piece together! Keep up the awesome work!!

[u/torndarkness]

Great video, thank you for sharing your insight and information. Side question: what kind of scope do you have from your video? (Not the tiny one!)