Small caps in parallel with large caps: what's the point? - Page 1
Forum section: electronics / projects

author: SteveLy
author_rank: Regular Contributor
timestamp: October 26, 2015, 12:51:18 am
content: What's the purpose of the small 0.1uF caps in parallel with the much larger 3300uF, 100uF and 10uF caps in the power supply circuit pictured below? (from Scullcom Hobby Electronics #25 "Build a Function Generator" video.) Naive circuit theory would tell us there is none: those small values added to the large values are insignificant. But I'm sure there are practical reasons I'm unaware of.

author: c4757p
author_rank: Super Contributor
timestamp: October 26, 2015, 12:54:06 am
content: Parasitic series resistance and inductance (due to the construction of the capacitor) prevent the big capacitor from being effective at high frequencies, so smaller (and better at high frequency) capacitors are added to provide effective capacitance over a wide range of frequency.

author: helius
author_rank: Super Contributor
timestamp: October 26, 2015, 12:55:01 am
content: The small value caps typically have lower ESR and filter ripple at higher frequencies, if they are physically placed close to the load, or in this case, close to the regulator. Capacitors are not ideal and their impedance is frequency dependent.

author: SteveLy
author_rank: Regular Contributor
timestamp: October 26, 2015, 01:00:28 am
content: Wow, thanks for the lightning fast answers! Sounds like something that would make a fun and instructive experiment to investigate on the breadboard.

author: tggzzz
author_rank: Super Contributor
timestamp: October 26, 2015, 01:08:27 am
content: Quote from: SteveLy on October 26, 2015, 12:51:18 amThe 3300µF 100µF and 10µF components aren't capacitors. They can be modelled as a collection of ideal capacitors, ideal resistors and ideal inductors. The resistors and inductors are often called "parasitics", and the "extra" ideal capacitors and ideal resistors model the dielectric soakage.Quote from: SteveLy on October 26, 2015, 01:00:28 amThe spring-leaf breadboards beloved of amateurs are actually the devil's invention - you spend more time debugging the breadboard than debugging your circuit. Plus the parasitics are awful.

author: Bud
author_rank: Super Contributor
timestamp: October 26, 2015, 01:12:28 am
content: Someone has done it alreadyhttp://www.wetterlin.org/sam/BypassTests.pdf

author: ovnr
author_rank: Frequent Contributor
timestamp: October 26, 2015, 01:20:07 am
content: Quote from: SteveLy on October 26, 2015, 01:00:28 am... please don't. Breadboards are horrible from a performance standpoint, with significant contact resistance, inductance, and inter-row capacitance.Just take a piece of un-etched copper clad board (in this case, preferably single-sided I suppose), cut a slot across it, then solder on your test connectors. Then you can go nuts soldering all kinds of weird and wonderful caps on while observing the impedance vs frequency characteristics of your system.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: October 26, 2015, 01:40:44 am
content: I would argue that, perhaps, the 0.1uF capacitors in this circuit is to make the regulators stable? Or, perhaps, the circuit designer have added them in because it has always been done that way? Surely, the inductance of the wires (or traces) to the load is 1-2 orders of magnitude larger than the mounting inductance (and ESR) of the 0.1 uF capacitors shown in the picture?Normally, one places the decoupling capacitors right next to the load such that the loop inductance of the connection between the decoupling capacitors and the actual load is minimized. This allows minimal voltage drop when the load current dI/dt is high (when the load draws current 'quickly' from the decoupling capacitors). If there is a large loop inductance (long path between capacitance supplying the charges and the actual load, including the ground return path) then this inductance will cause a voltage drop, causing ripple in the supply line at the actual load pins although the regulator and bulk capacitor voltage is completely stable. Remember, voltage drop due to inductance is proportional to the change in current, i.e. when the load current changes quickly (FPGAs and large clock networks that drive thousands of flip-flops come to mind). ESR of the decoupling capacitors should be low because there will otherwise be an additional IR-drop.This field of study is called Power Integrity and there is lots of information about this on the Internet:https://www.google.com/?gws_rd=ssl#q=power+integrity+decoupling+capacitor+designThis is an interesting subject that I plan to make a video blog about soon...Edit: Perhaps, though, the regulators would be able to regulate better if having low ESR filter caps? I'm not in tune with the finer details of linear regulators when there is ripple on the input so perhaps someone could elaborate. Somewhere in the back of my mind, something tells me that the regulation is not perfect and that high-frequency ripple on the input will transfer to the output to a certain degree. This page elaborates:https://en.wikipedia.org/wiki/Power_supply_rejection_ratio

author: digsys
author_rank: Unknown
timestamp: October 26, 2015, 01:49:47 am
content: Quote from: SteveLyOddly enough, it's mostly harder to make something unstable, than it is to make it stable, and you'll definitely need an oscilloscope.There are SO many factors involved, many that are difficult to "see". But you can try the hit+miss method and see for yourself.Make leads longer, change load from 0 - 110% etc etc

author: SteveLy
author_rank: Regular Contributor
timestamp: October 26, 2015, 03:04:19 am
content: Well I'm sure for playing around at modest (few MHz) frequencies and to get a feel for what's going on, a small breadboard would be fine. Capacitance of breadboard track pairs is of the order of a pF, so when we're looking at 100 and 0.1uF components, it's neither here nor there. Ditto inductance especially for a small board. I'd be more concerned about the scope probes interfering with the measurements, if anything.... and just because something's been done before is no reason not to do it again and see and learn for oneself. Most things hobbyists do has been done before.Thanks for all the great responses btw!

author: digsys
author_rank: Unknown
timestamp: October 26, 2015, 03:59:28 am
content: Quote from: SteveLyAbsolutely 100%. BUT just note that it may be difficult to make something unstable, and prove why, and where you stand in the "tunnel of death" (refer regulatortheory- as an example)https://www.google.com.au/?gfe_rd=cr&ei=voMtVt_mMcvu8weHnIEY&gws_rd=ssl#q=regulator+tunnel+of+deathActually, set up an LDO without any caps on Input or output, and you should see a 100KHz - 1MHz switching output, especially on load.

author: SteveLy
author_rank: Regular Contributor
timestamp: October 26, 2015, 09:22:42 am
content: ^^^Re instabilityRe LDO: I had to look that acronym up.btw. something like this would make for a great theory vs practice vs better theory in practice tutorial video. Seemingly redundant circuit components on schematics have often left me baffled.

author: tggzzz
author_rank: Super Contributor
timestamp: October 26, 2015, 10:18:40 am
content: Quote from: SteveLy on October 26, 2015, 03:04:19 amJust because you are "playing around at modest (few MHz) frequencies" doesn't mean the circuit is! Remember the old adage "amplifiers oscillate, oscillators won't".

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 26, 2015, 10:58:53 am
content: Quote from: helius on October 26, 2015, 12:55:01 amerrrm i dont think all small caps are made to have small ESR. i came about to know this after trying to choose caps of low ESR and looking into their PDF, apparently each manufacturer series ESR can vary greatly from another. when i say small, i meant below 0.1 ohm.

author: daqq
author_rank: Super Contributor
timestamp: October 26, 2015, 11:03:15 am
content: QuoteEvery component has parasitic values that are frequency, temperature, power and other things dependent.A simple resistor has a series inductance, self capacitance and a resistance. An inductor has a series resistance and capacitance between windings and the outside world. A capacitor has a self discharge rate, series resistance, lead inductance... A particularly precise model of a part can have even more parameters. Just read this article about resistors to get a picture:http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdfEven a wire or a trace on a PCB is not an ideal connection - as far as high frequencies are concerned, a few cm of copper PCB trace can be a very nice RF resonant circuit.See:http://www.xilinx.com/support/documentation/application_notes/xapp623.pdffor a more complex decoupling system.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: October 26, 2015, 11:12:40 am
content: Quote from: daqq on October 26, 2015, 11:03:15 am+1 so true, when i first encountered the term ESR, i thought it is a fixed value, but after reading more into it, it is not (google article from manufacturer murata about ESR)

author: tautech
author_rank: Super Contributor
timestamp: October 26, 2015, 11:35:57 am
content: Quote from: c4757p on October 26, 2015, 12:54:06 amThis ^^^

author: fivefish
author_rank: Frequent Contributor
timestamp: October 26, 2015, 02:38:56 pm
content: So you can visualize it..."when one capacitor’s frequency response is rolling off, another is becoming significant, thereby maintaining a low ac impedance over many decades of frequency."

author: Bassman59
author_rank: Super Contributor
timestamp: October 26, 2015, 04:32:51 pm
content: Quote from: fivefish on October 26, 2015, 02:38:56 pmIt is left as an exercise for the reader to determine whether your decoupling circuitry needs to be effective up in the megahertz range. For high-speed logic and fast converters, yes, you would assume so. For audio circuits? Not so much.

author: tggzzz
author_rank: Super Contributor
timestamp: October 26, 2015, 04:37:30 pm
content: Quote from: Bassman59 on October 26, 2015, 04:32:51 pmDon't be too certain of that! Poor layout and other factors can cause oscillations much higher than thesignal'sbandwidth. Read Jim William's famous AN47, and much of Bob Pease's writings.

author: Bassman59
author_rank: Super Contributor
timestamp: October 26, 2015, 04:52:56 pm
content: Quote from: tggzzz on October 26, 2015, 04:37:30 pmWell, that's indeed true -- if your audio circuit was built with 5532s and TL072s, and you changed them all to a newer, higher-bandwidth part, you need to ensure that there is sufficient high-frequency bypassing at the chips. otherwise, those new parts will oscillate. This trips up a lot of the people who swap out chips in a product for "Better" parts (because, you know, the NE5532 is TOTAL CRAP and DESTROYS AUDIO*), without understanding what they're doing.-a* sarcasm

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2015, 05:13:44 pm
content: Ne5532 is quite capable of being a good HF oscillator, just remove the decoupling from the negative side, and add around 30cm of solid core 0.5mm wire connecting it to the power supply, most will oscillate at around 1MHz. Especially nice on a mixing deck where this was coupled into the output wiring which radiated it, especially after the little 100pFoutput filter capacitors accidentally were broken on one lead. What pirate transmitter?

author: SteveLy
author_rank: Regular Contributor
timestamp: October 27, 2015, 01:53:46 am
content: From Gauss' law one can calculate that pairs of PCB tracks (or wires) have a capacitance of about 30pF/m. That is valid as long as they are ~5-10x closer to each other than they are long.@fivefish: the impedances of capacitors in parallel don't combine in the way you suggest. A small cap in parallel with a large cap is effectively still a large cap. Folks talking about inherent series resistance and inductance have the right idea.

author: fivefish
author_rank: Frequent Contributor
timestamp: October 27, 2015, 03:18:50 am
content: QuoteWhy don't you tell that to engineers at Analog Devices (analog.com)http://www.analog.com/library/analogDialogue/archives/39-09/layout.html

author: SteveLy
author_rank: Regular Contributor
timestamp: October 27, 2015, 03:56:16 am
content: So the graph is poorly annotated. Where the arrows are pointing is not where the action is: rather it's in the bottom right corner, where caps stop being caps and behave more like inductors.

author: tggzzz
author_rank: Super Contributor
timestamp: October 27, 2015, 07:53:40 am
content: Quote from: SteveLy on October 27, 2015, 01:53:46 amPlus an inductance of ~1000nH/m. And the conductor's shape is rather important too at some frequencies.QuoteYou can put a capacitor in a spice simulation and simulate its response. It is a shame that no shop stocks such a capacitor. OTOH, you can think of that as an opportunity - all you have to do is manufacture one and you will become as rich as Croesus!To put it more simply, if you look at the graph provided by fivefish and do the measurement at 1GHz, the "capacitors" are all inductors

author: daqq
author_rank: Super Contributor
timestamp: October 27, 2015, 07:58:31 am
content: QuoteDepends on the surroundings. I don't know the exact number, formula, but using Saturn PCB calculator (Great tool by the way!) you get around 0.52 pF/cm and 6nH/cm with a 0.5mm trace width and a plane present 1.6mm under the trace on an FR4 substrate. The tool also gives you crosstalk calculations, impedances etc...

author: jdraughn
author_rank: Regular Contributor
timestamp: October 27, 2015, 09:45:42 am
content: QuoteI have had the bad habit of buying the real cheap breadboards and the first few years I was into electronics I had various problems with the poor contacts of a breadboard, but I have gotten in the habit of doubling all power connections with second holes and most of my projects are pretty rock solid now. I can pick them up and move them around without spurious resets, and my analog to digital readings are more consistent.It's just a small band-aid for some of the issues that crop up using breadboards, but it does help.

author: fivefish
author_rank: Frequent Contributor
timestamp: October 27, 2015, 11:06:39 am
content: Here's a clearer graph.and characteristics curve of different type of caps.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 28, 2015, 04:59:39 pm
content: For all of those trying to calculate trace capacitance and inductance, remember that often the goal of putting capacitors in a circuit is to create a dominant pole so you don't have to worry about those things. This is the case with the linear regulator circuits which are the most common source of the big capacitor paralleled with a small one circuits. All of our circuit analysis tools are approximations that are only good under certain conditions. Therefore successful designers put a lot of effort into making sure those conditions apply. Those who don't do this spend a lot of time tinkering their circuits into submission, and even more time testing to make sure that they stay well behaved under variations in components and operating conditions.That is not to say that using trace inductance and capacitance is wrong, just remember that those tools also have limitations and if not well thought out can lead to tinkering and weird field problems.

author: tautech
author_rank: Super Contributor
timestamp: October 28, 2015, 07:19:48 pm
content: Quote from: CatalinaWOW on October 28, 2015, 04:59:39 pmVery well put and including an excellent signature for some member.

author: SteveLy
author_rank: Regular Contributor
timestamp: October 29, 2015, 02:33:41 pm
content: Quote from: fivefish on October 27, 2015, 11:06:39 amQuote from: daqq on October 27, 2015, 07:58:31 amGood point! In the end one needs to measure things or have things measured. Not because anything is wrong with the theory but because the world is a complicated affair.

author: P_Doped
author_rank: Contributor
timestamp: October 30, 2015, 10:21:30 pm
content: Quote from: fivefish on October 26, 2015, 02:38:56 pmI don't like this author's approach. As you can see, the impedance improvement from having all those caps is present at high frequencies but not substantial. This issue exists because the author likely used caps from the same family/package and ended up with the same lead inductance (notice how all the cap impedances at high frequency are on the same asymptote). The improvement possible at high frequency really comes when you take advantage of the fact that a small capacitor can be squeezed into a small package. Small package <--> small lead inductance. A better representative graph of the effect you'd like (pardon the quality of the scan) is attached. Notice the large improvement at higher frequency.

author: fivefish
author_rank: Frequent Contributor
timestamp: October 31, 2015, 02:38:51 am
content: Quote from: P_Doped on October 30, 2015, 10:21:30 pmThere's a way to reach the author.Why don't you tell that to engineers at Analog Devices (analog.com)http://www.analog.com/library/analogDialogue/archives/39-09/layout.html

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 31, 2015, 02:44:41 am
content: Quote from: fivefish on October 27, 2015, 11:06:39 amPlease provide the source.

author: fivefish
author_rank: Frequent Contributor
timestamp: October 31, 2015, 03:58:14 am
content: Impedance/ESR characteristics of capshttp://www.murata.com/en-global/products/emiconfun/capacitor/2013/02/14/en-20130214-p1And here's an 86 page PDF all about PSU noise suppression and decoupling capshttp://www.murata.com/~/media/webrenewal/support/library/catalog/products/emc/emifil/c39e.ashx

author: Votality
author_rank: Contributor
timestamp: October 31, 2015, 04:18:51 am
content: Quote from: c4757p on October 26, 2015, 12:54:06 amYep thats what they taught me at uni

author: P_Doped
author_rank: Contributor
timestamp: October 31, 2015, 04:56:14 am
content: Quote from: fivefish on October 31, 2015, 02:38:51 amLooking at the rest of the App Note, the author does say to put the smallest capacitor closest to the pin, etc. I think that they understand how mixing different magnitude (and physical size) capacitors will improve the impedance. He just chose a poor graph to illustrate the point he was trying to make. If I see him, I'll ask him to revise it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2015, 07:34:54 am
content: BTW, many caps in parallel typically performs worse than a simple circuit. The constraint is to have a low power supply impedance; the worst case impedance of your supply network is the greatest offender, not the individual ESR or reactance of the caps.Namely, high impedances occur at parallel resonances between caps connected in parallel. This is especially pronounced for long, thin traces between caps, but also beware of resonances with power pours and long traces.The best solution is notminimalimpedance, butoptimalimpedance. (Duh?) Bulk caps such as electrolytic or tantalum (or large ceramic, with an ESR resistor added externally) can be used to dampen the network. They are not, in fact, being used for bulk energy storage. Not at those frequencies (>> 1MHz), only damping. (At lower frequencies, their lower reactancewillbe more important, such as for power supply filtering.)For damping purposes, the bulk capacitance should be at least 2.5 times the total bypass on the supply, and ESR about equal to sqrt(Lstray / Cbyp).Tim

author: tggzzz
author_rank: Super Contributor
timestamp: October 31, 2015, 07:49:28 am
content: Quote from: P_Doped on October 30, 2015, 10:21:30 pmI have 0603 capacitors ranging from 3.9pF to 10µF, and you can probably get larger ones. The series inductance will be near 1nH, giving a theoretical SRF of ~1.5MHz to ~3GHz

author: Siwastaja
author_rank: Super Contributor
timestamp: October 31, 2015, 08:15:53 am
content: A good, simple real-world example on trace inductance and necessity of several level of power supply capacitors is a MOSFET gate driver. This is because they require very high currents in very short pulses.Try connecting a gate driver to a large power MOSFET and run it from a lab supply, without a small cap close to the driver, and it will not perform just poorly; it won't probably performat all, and you'll be able to see overvoltage spikes that can destroy the device.What happens is this:Gate driver tries to pumpseveral amperesto the MOSFET gate (which is like a capacitor that needs to be charged quickly). But even a few inches of power supply wire has enough inductance to slow down the current rise, as inductors do. This is seen as a voltage drop; there is voltage over the inductor (the wire). A 12-volt supply may easily drop down to just a few volts, measured by an oscilloscope. The gate driver chip may then enter the Under-Voltage Lockout mode and stop operating. At this point, current suddenly stops, but there is energy stored in the inductance (the wire), and now it is suddenly released, which shows up as an overvoltage spike. It works like a boost converter; the chip "almost shorts" the inductance and when it releases, you have inductive kickback voltage.Resistance works similarly (voltage drop) but without the overvoltage kickback effect. From stability viewpoint, resistance offers benefits by damping resonances by dissipating power instead ofstoring and releasingit, but nevertheless it also causesundervoltagetransients. Resistance is easier to minimize by just making the trace a big wider, and extremely easy to calculate, but the inductance is the real big problem.The only solution is to minimize the inductance (and secondarily, resistance) between the power supply and the chip. But the only way to do it is to have the supply physically close to the chip, because the inductance is solely caused by physical distance (the wire length). For this reason, you need to redefine "power supply"; only a small SMD part near to the chip can do well enough. It will be your "local" power supply during peaks. As the average current is small (and spikes are averaged out by the local supply), the wire with inductance and resistance can now maintain the energy level in the small cap very well.MOSFET gate driver is a good example because it's so simple; you can directly see how much capacitance you need, as the chip just basically dumps energy from one cap to another - from the supply capacitor to the MOSFET gate. MOSFET total gate equivalent capacitance can be approximated from datasheet values, and it is often stated as a rule of thumb that the supply capacitor should be 100 times more so that it doesn't discharge too much during the pulse, which would cause voltage drop - and associated overvoltage peak.

author: tautech
author_rank: Super Contributor
timestamp: October 31, 2015, 08:46:27 am
content: Quote from: Siwastaja on October 31, 2015, 08:15:53 amI was told to alwayshammerthe gate, thats the reasons why. ^^^

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2015, 10:50:21 am
content: A simpler model might be:Once the gate driver switches "on", the VDD and OUT pins are connected together with a resistor (Rds(on) of the driver).This, in turn, connects to the gate, which can be modeled as a series RLC, where R is sometimes given in the datasheet (R_G, gate spreading resistance), plus the driver's equivalent resistance, and whatever external resistance is used. L is also sometimes given, but is better estimated from layout (including trace length up to the driver), and C is the equivalent gate capacitance (use Ceq = Qg(tot) / Vgs(on) -- don't use Ciss, the small-signal, zero-bias parameter!).The gate also returns to ground, via the source, through whatever impedance the ground path has. This should always be ground plane, if at all possible, so we'll ignore it for now.Because it's one series loop, any VDD inductance (between main bypass and the chip pin) adds in series with the gate inductance, so we can simplify the circuit further. Likewise, the total bypass capacitance acts in series with the gate capacitance, so it should be fairly large in comparison (if Ceq ~= 10nF, Cbyp >= 100nF would be reasonable, so that the series total Cloop is >= 90.9nF, not much different), but needn't be massive (like 1uF+), unless the gate is as well (e.g., large IGBT modules).Now that we have all the parameters of this RLC circuit, we can put some braining into it.When R > sqrt(L/C), it's overdamped, and the transient will resemble an RC charging curve.When R < sqrt(L/C), it's underdamped, and excess ripple (and oscillation!) will be a great concern.The time constant is given by approximately sqrt(L*C) or R*C, whichever is larger. The 10-90% rise is about 2.2 time constants.These simple rules can be derived from the (also still fairly simple) second order differential equation that corresponds to the RLC circuit. On the time scale we're talking (rise time below 10ns, say), there are plenty of other wrinkles that contribute to the overall waveform, but this slower contribution will still be present, so it's still a fair approximation.Now, since everything's in a series circuit, you might think, those RLC components could be anywhere in that series chain, right? That would be correct. It does not matter. As long as the driver chip's supply does not dip down too low for its own function (usually below 5V, but higher power devices may have a UVLO at a more reasonable 9V or more),it doesn't matter how much supply ripple is at the chip.Indeed, you can see theintentionally added supply inductanceat work in this waveform:This was necessary because the simple yet fast circuit exhibits shoot-through in the pre-driver. To achieve the desired switching speed without burning transistors, some added impedance saves on switching loss, while barely compromising speed.Dead-bug construction. A discrete CMOS inverter (N and P MOS, gates together), with supply impedance (something like 220nH || 51 ohm), is amplified by an emitter follower, for a cool ~3A peak output current and 12ns rise time. Maximum switching speed about 3MHz.Tim