I realized that I don't have much clue when it comes to bulk capacitance - Page 1
Forum section: electronics / projects

author: shadewind
author_rank: Frequent Contributor
timestamp: October 15, 2011, 07:30:06 pm
content: Is there a rule of thumb to use to determine how much bulk capacitance is needed for a board? From what I can gather, bulk capacitance is used both to smooth out poor quality power supplies and also to provide a low impedance source for sudden changes in load.A few examples:On one board I have, I'm using the LM2738XMY which switches at 1.6 MHz which allows the use of ceramics. So I have 10 µF at the input. Should I have added more bulk capacitance? Or is the capacitance enough? When you see electrolytics used, they usually have a rather high capacitance but from what I can tell, this is mostly because high capacitance often means low ESR. There aren't many sudden load changes on this board though I suppose the power supply powering it could be crappy?I'm building a driver board for RGB LED strips which is going to switch up to about 5A using MOSFETs. Should I add some bulk capacitance to the anode terminal so that there charge available at low impedance?This post may sound a bit confused but it's just that I started thinking about this now and realized I don't really know what I'm doing.

author: armandas
author_rank: Frequent Contributor
timestamp: October 15, 2011, 08:06:05 pm
content: Quote from: shadewind on October 15, 2011, 07:30:06 pmIs there a rule of thumb for how long a piece of string should be?The capacitance required depends on how much current "backup" you need and how often. Bigger capacitors can supply more current, but take longer to recharge. If you want to do the calculations, you can make use of the following formula:I = C(dv/dt).Once you begin to deal with high frequencies, you have to start considering other factors, such as inductance.At any point, you should find some application notes and read through them.Quote from: shadewind on October 15, 2011, 07:30:06 pmAbsolutely not.

author: IanB
author_rank: Super Contributor
timestamp: October 15, 2011, 08:11:12 pm
content: Quote from: shadewind on October 15, 2011, 07:30:06 pmMost application notes say that electrolytic capacitors have to be derated at high frequencies, so you need a value at least ten (or one hundred) times larger than a tantalum part for the same duty.

author: SiBurning
author_rank: Contributor
timestamp: October 15, 2011, 08:18:38 pm
content: It is a bit complicated at high frequency and high load current.A simpler form of the equation armandas gave isC = I / V * twhere I = dc load current, V = acceptable ripple voltage, t =~ (dis)charge rate, probably 2 * the switching frequency, but could be 4*, with some additional factor for non-sinusoidal waves. Keep in mind that the ac current (peak or instantaneous) is much higher than the dc load current.With high load current, ripple current is just as important. The capabilities of the physical part are frequency & temperature dependent, so you need to be careful, and can't rely on a single Ir and ESR figure.Just to add to what armandas said (again)--or, really, put it in terms dummies like me can understand easier--you use larger values of capacitance to get more charge stored, but the slower speed of charge/discharge also smooths the voltage by slowing the changes in ripple voltage. Note in the equation how ripple voltage is inversely proportional to capacitance.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 15, 2011, 09:37:59 pm
content: Quote from: armandas on October 15, 2011, 08:06:05 pmBy looking at at the data sheet for a typical low ESR cap (http://www.panasonic.com/industrial/components/pdf/pic_fk_series.pdf), it is clear that there is a relationship between the ESR and the capacitance. So it seems to me that the reason to choose a high value capacitor for a switcher isn't so much the capacitance itself but that higher capacitance of results in lower ESR which is the dominant factor in filter stages for switchers, for example. But maybe that's not what you mean by "Absolutely not."?Quote from: SiBurning on October 15, 2011, 08:18:38 pmYes, I know the theory, that's not the problem. I don't know how to apply this in real life. I don't know how bad/good a typical wall wart voltage rail is and so on.Regarding ripple voltage, in switchers, the ESR is usually the dominant term unless you're switching at low frequencies.

author: jackbob
author_rank: Regular Contributor
timestamp: October 16, 2011, 07:26:12 am
content: Uh a 5 amp wall wart did I read that right? You can't power those strips with a wall wart ( at least any wall wart I have ever seen). If you have a smps you want to use ...great for this project. Based on our conversation from your other topic I don't know whats running 1.6MHz but if your just looking to cut out ripple in a power supply for your led strips with the mosfet dimmers then go wild with electrolytics the more the better. I'm not sure I understand exactly what it is your trying to do, but for the led driver discussed in the other topic put in as many electrolytics as you want on the input power. You can do all these crazy formulas, but hey for something simple like this just put a fair amount of caps and you will be fine. Hope this helps,Jacob.

author: armandas
author_rank: Frequent Contributor
timestamp: October 16, 2011, 07:29:11 am
content: Quote from: shadewind on October 15, 2011, 09:37:59 pmI wanted to point out that large capacitance does not necessarily mean low ESR. You can get a 1F capacitor with an ESR of tens of ohms.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 16, 2011, 11:14:14 am
content: Quote from: armandas on October 16, 2011, 07:29:11 amRight. What I meant was that the reason National recommends certain 330 uF (or there about) electrolytics in the data sheet for the LM2675 is not because that much capacitance is needed (because it isn't, at least not for the purposes of input or output filtering), it's because the low capacitance range of most low ESR series caps do not have sufficiently low ESR. This is the reason, for example, that the LM22674 that I recently used for a project works fine with a single 47 uF ceramic for the output cap, the ESR is the dominant term and a ceramic has very little of it.For reasons of board space and because electrolytics generally have poor characteristics, I try to avoid them in favor of ceramics. But I'm wondering if this is wise. Even the higher capacitance is not needed for the switcher filtering itself, I'm wondering how important it is to have that extra capacitance.

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2011, 05:16:43 pm
content: Quote from: shadewind on October 16, 2011, 11:14:14 amI think you are fixating too much on ESR, which is only one factor among many.The reason large value electrolytics are specified, as I mentioned above, is because they do not actually have their label value at high frequencies. That 330 uF capacitor might not be 330 uF at all at the switching frequency, it might only be 30 uF, or even 3uF. This is because the spiral wound construction makes them look like a coil with inductance; inductance that increasingly cancels out the capacitance as the frequency rises. Therefore when using an electrolytic in a high frequency application you have to oversize the capacitor to get the result you need.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 16, 2011, 07:07:38 pm
content: Quote from: IanB on October 16, 2011, 05:16:43 pmThe reason I fixate on the ESR is that it IS (together with ESL for higher frequencies) the most contributing factor to output voltage ripple inswitcher output filteringas soon as you get up to the frequencies used in most modern switchers. Look at the datasheet for the LM22674 (http://www.national.com/ds/LM/LM22674.pdf), for example. On page 9 in the section about output capacitors, there's a formula and assuming a frequency of 500 kHz and a capacitance of just 33 µF and an ESR of 50 m? (pretty common value in electrolytics), the impact of the ESR is 6.6 times greater than the impact of the capacitance on the output voltage ripple.The inductance you are talking about is called ESL, Equivalent Series Inductance and it doesn't reduce the capacitance. The capacitance does not vary with frequency. What you are talking about is the impedance. The impedance changes with frequency according to the formula: ESR + 1/(w * C) + w * ESL

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 16, 2011, 07:41:18 pm
content: One has to be careful using rules of thumb with electrolytics as they don't always do what you'd expect, depending on both chemistry and construction. In general I've found that if one scans down the datasheet to find an acceptable ripple current rating, the ESR will obviously be low enough and the capacitance, regardless of what it might be, will be plenty high enough. Of course that only works for switching frequencies, not LF design.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 16, 2011, 07:48:30 pm
content: Quote from: Conrad Hoffman on October 16, 2011, 07:41:18 pmYes exactly. But if you're using ceramics for the filtering (47 µF for example, and they always have low enough ESR), would you need some additional bulk for handling load transients et.c?

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2011, 09:10:09 pm
content: Quote from: shadewind on October 16, 2011, 07:07:38 pmI am aware it is the equivalent series inductance.Why do you say the inductance does not reduce the capacitance? Inductance and capacitance are electrical inverses of each other (one produces lag, one produces lead on the phase angle). If you combine inductance and capacitance in appropriate measure the lag and lead will cancel out to produce a zero phase angle, and the combination will behave like a pure resistance. This is how power factor correction works in AC circuits.When looking at electrolytic capacitors it is true that theintrinsiccapacitance does not depend on frequency. However theeffectivecapacitance of the entire circuit element does decrease with increasing frequency. If you use an electrolytic capacitor in a high frequency application, or for input bypassing, you must use a higher value part than calculated to compensate for this effect.Note I am not saying ESR is irrelevant, simply that it is not the only factor in selection of electrolytic vs other kinds of capacitor. With electrolytic capacitors the ESR actually decreases with increasing frequency.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 16, 2011, 11:31:43 pm
content: Quote from: IanB on October 16, 2011, 09:10:09 pmQuote from: IanB on October 16, 2011, 09:10:09 pmThis is exactly the reason why I am saying this. Capacitance is an intrinsic value. The effect of the capacitance with respect to a certain frequency is called impedance.Quote from: IanB on October 16, 2011, 09:10:09 pmBut you can only compensate so much. The inductance is in series with the capacitance so if you have an infinite capacitance (which means the impedance of the capacitance is 0), you're still going to be stuck with the inductance. But you are certainly right for cases where the inductance and/or frequency is not too large.

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2011, 11:58:14 pm
content: When a physical capacitor is used in a circuit, the only capacitance that matters is the effective capacitance as seen by the circuit. If you take a certain electrolytic capacitor and measure its capacitance at 1 MHz, say, then that measured capacitance is the capacitance you have to work with. It might be quite different from the number printed on the can, and it is not ESR that explains the difference.Impedance is a notional quantity invented to express the resistance to the flow of AC current in response to an AC voltage by analogy with DC resistance. We can observe, for example, that electrolytic capacitors have a higher impedance at high frequencies than the notional capacitance value would indicate, which is why you have to use a bigger value. This is in spite of the fact that the ESR of an electrolytic capacitordecreasesat higher frequencies.All of this comes back to the question in your very first post. Large value electrolytics are sometimes used because they do not always have the capacitance you would expect from the number on the can. A 0.1 uF tantalum might do the same job as a 10 uF electrolytic when used for bypassing.

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 12:20:13 am
content: Quote from: shadewind on October 16, 2011, 07:07:38 pmMy reading is they chose those particular caps purely for the ripple current and voltage handling.Reading over those spec sheets you mentioned gives a better context to your questions. Maybe I can give a more focused response. I don't see anything in the spec sheets that suggests larger capacitor values are used for low ESR. They're needed to suppress transient (i.e. ripple) current, which in turn could cause instability.Low ESR will result in less voltage drop across the capacitor. This is important for an output capacitor if you need a precise voltage, but is less important for an input capacitor where you typically use an overvoltage that drops across the chip or regulator. But it's the capacitance that provides a stable voltage in the face of changing load or unstable input from an unregulated source. In all of the spec sheets you refer to, this voltage instability is reflected back to the stability of the chip, for both the input and output capacitors.In addition, there's stray inductance in the capacitors that causes instability in one or more of the chips. This is the main reason for using low ESR capacitors. I'm referring to a tuned LCR filter. (Reading this over later, I'm not quite sure this is the best or right answer. As suggested below by IanB, you want low input impedance to prevent oscillations, which might be a broader issue than what I'm referring to.) The LM22674 suggests that a higher ESR capacitor be placed in parallel because a higher resistance will (1) drop a larger voltage, thereby providing a more stable voltage to the chip by suppressing transients, and (2) change the frequency of the LCR filter created by the capacitor & inductor to be oustide the working frequency of the chip.My reading is that these are separate issues. If ESR decreases as capacitance increases for a give line of capacitors, so does the current handling, but you should take these as besides the theoretical point, even if they do have a practical impact on part selection.As an aside, the LM2675 spec sheet warns about the potential for ringing when using ceramic caps for input bypassing.In terms of practical advice, you should probably forget about ESR being the factor for now and try to follow what people are saying. If ESR looks important, it's because the capacitance you've chosen reduces the transient effects to that point, but there may be other things not accounted for in some of those equations. Look again at the t in the equation and connect that with inrush or transient current--the ripple current is... well, slew rate is a good way of looking at it, and it's ginormous at high frequencies. I'd focus on the current, with the voltage drop (from ESR and loss of charge in the capacitor) being effects. Sorry, I'm more gestalt technician than math head, but everything the others have mentioned seems like a different way of saying the same thing--maybe someone can translate, or point out any errors I'm making here. It seems to me that you're trying to build a stable, precision voltage output while relying on a capacitor, which isn't going to work out well without seriously over-specifying the capacitance and probably using many of them in parallel, but this aspect is just an aside.For an LED driver, you probably don't have too many transients to worry about (and the data sheet says so), so you only have to worry about on-off or blinking spikes. Does it matter if the voltage isn't completely stable at those points?In terms of picking capacitor values for purposes of smoothing, you want to select values that can handle the instantaneous load current (as well as any other variables in current), and then consider ESR and other things for the secondary effects. One of these is bypassing to suppress high frequency noise, but I suspect that's mostly a consideration for audio into lower RF frequencies, and it comes out naturally as a factor in the ripple current for high frequencies. Instead, the secondary considerations include the LCR pass filter from stray inductance, or in some of your cases, because there is a separate inductor already.

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2011, 01:31:46 am
content: Stability is indeed the key.A regulator circuit will be most stable when it has a very low impedance voltage source on the input side. This low impedance is to prevent undesirable oscillations in the circuit due to feedback from the output. These oscillations could be damped (ringing) or undamped (permanent oscillation). Both are bad.For a physical analogy, consider someone balancing on a tightrope. We can visualize that the tightrope walker is most stable when the rope is held very taut. If the rope is slack the walker will get very wobbly and may fall off. One can think of the input voltage to a regulator like the rope. If the input voltage moves all over the place in response to changing loads the whole regulator is going to get very wobbly.Interestingly, the source impedance must be low not only at the switching frequency, but at much higher frequencies as well. This is because feedback loops can happen at any frequency where the gain is high enough. As a result of this, the input bypassing on many circuits is often made of different capacitors in parallel: high value electrolytics for low impedance at low frequencies, and low value ceramics for low impedance at high frequencies. One could argue that the electrolytics alone should be able to do the job, but due to the fall off in capacitance at high frequencies they don't suffice and the parallel ceramic (or other) capacitor makes up for the difference.

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 01:46:18 am
content: Quote from: IanB on October 17, 2011, 01:31:46 amSeems there's less difference than I suspect between audio and high frequency.Might mean having to suppress the 50-60 Hz line and who knows what else if using a cheap wall wart.Incidentally, another reason to over-specify the capacitors is to keep the relative change in charge small to stay in a more or less linear part of the capacitor's operation. This is one reason you see crazy values in high end audio amps. Of course, they also tend to be part of the output circuit and directly across the low impedance speaker load, so low ESR is a second and maybe even more important factor, not to mention needing a large amount of charge to begin with. One of my amps can put out 30A at +/-28VDC, albeit for short periods of time, probably 2-100mS, though it can only do 85W (1.5A?) in rms terms. They also tend to have multiple values to have different time constants helping out with the different frequencies over the 10 decade audio range. I mention these things because some of them might be a factor in some of your designs.

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 02:52:08 am
content: Since you seem to know all of the relevant factors, I wonder if what you're really asking is how to prioritize the various factors and/or determine which are relevant.Certainly, you need to consider the ESR/ESL, as well as current handling, and anything else you think is important. From all of that, you have to pick a part that meets all of the requirements. Or as close as you can get. Any particular application needs to be dealt with separately--there's no one answer fits all. You might also not get it all right in the first try.Trade-offs? Many right answers? Black art? Over-engineering? I bet you get 3 answers for everyone that responds.By the way, you're not far off talking about ESR. The limiting factor for a cap is heat. For a capacitor to handle higher current, it either needs better heat dissipation or lower internal resistance. Hence, there is typically a relationship between ESR and current handling. It's not clear to me whether this relationship is intrinsic to the materials or a design issue. Looking over specs for various lines of capacitors, the relationship within any particular line seems to be not quite linear with respect to capacitance, with various jumps at different voltage ratings and capacitance values, suggesting slight changes in design for various ranges of values. Much larger differences appear between different lines of capacitors. One thing's for sure. This is an interesting subject, and I'm learning from it. I only hope what I've learned has some relation to reality.

author: jahonen
author_rank: Super Contributor
timestamp: October 17, 2011, 09:09:48 am
content: My understanding is currently that it does not matter which phase angle the power supply impedance has at particular frequency, as long it has low magnitude. Fortunately, since otherwise the high-speed stuff would become completely unmanageable.Every capacitor will look inductive beyond the self resonant frequency. That happens at surprisingly low frequency, 100n 0603 capacitor, it is just over 10 MHz (very low by the modern digital logic standards). But still, the impedance is quite low beyond that point which makes it usable bypass for even higher frequencies. Here are some measurements I performed a while ago (the measurement breaks up when approaching 1 GHz):It looks like that just paralleling relatively large-valued caps of same value gets the best overall response, systematically lowering the impedance across the whole spectrum, despite what numerous appnotes describe. I wouldn't blindly count on the series resonance points since parallel resonant frequencies might make the final result worse, unless the current spectrum is accurately known beforehand.Regards,Janne

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 09:40:57 am
content: Interesting chart. Is that all inductance after the trough? The graph shows pretty clearly what happens with different values of capacitance. The humps in the graph of 4 different values is interesting. Still, it's hard to figure out the exact effects of different values because your chart only uses different values for the smallest capacitors, so we can't use the chart to compare against the single values. I'd like to see, for example, 40u, 400n, and 1n in parallel and compare with the single values. Not asking, just saying. My gear's limited to a few MHz.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 17, 2011, 11:27:03 am
content: Quote from: SiBurning on October 17, 2011, 12:20:13 amAgain, for switching regulator output filtering where the frequency is much higher than when you're filtering mains you usually don't need a lot of capacitance. At this point, the ESR is the most important limiting value and voltage ripple depends almost entirely on the ESR. At these frequencies, it's not so much a question of how much charge a capacitor can store at the given voltage but rather how much charge can be accumulated in the when the output current from the inductor peaks. The amount of charge depends in the current going into the cap and this is limited by ESR.This is the main reason reason for the recommendations in NSC data sheets. Of course, you need to meet all the other criteria such as voltage rating and RMS current rating but improving voltage rating usually means decreasing ESR and thus increasing current capability. The reason that the capacitance values for the recommended capacitors in the data sheet for the LM2675 vary so widely is that (as far as I can see) when the ESR, voltage rating and RMS current meet the requirements, that's the capacitance you usually end up with, not because you need that much capacitance.Of course, all this assumes the output current is constant. When the load suddenly changes, the inductance in the power supply line and other factors limit how fast the switcher can respond to the change in demand. This is what I'm wondering about. In these cases, ESR is not as important, or so I've understood anyway.My philosophy with regards to decoupling devices is to use 0.1 uF caps for every power pin if possible unless the data sheet has other recommendations. This works well for me.But if you have sudden load changes you might need more bulk capacitance at the output and the input. I your power supply line is crappy, you might need more capacitance at the input. This is where I don't know what to do. I know the theory, I just don't know much capacitance you need depending on the circumstances. Sure, I could litter with electrolytics but I'd rather not since it's expensive and bulky. Any approximation formulas are appreciated.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 17, 2011, 11:36:35 am
content: Quote from: IanB on October 17, 2011, 01:31:46 amBut for stability issues, you usually have frequency compensation as well which reduces the gain below unity well below the switching frequency. If this compensation is internal to the IC, it imposes certain restrictions on the external components, though.

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 12:53:48 pm
content: "Of course, all this assumes the output current is constant." The output current of what? The regulator or the capacitor?"When the load suddenly changes" the first thing you'll need to do is increase the current supply.The ripple current rating for a capacitor is the (somewhat averaged) instantaneous current it can source, not just the more narrowly defined ripple caused by rectified ac or equivalents. Assuming the capacitor can source that current when the supply can't, the voltage drop will, of course, be the current supplied by the capacitor multiplied by its ESR. (Not really, but that's a different question.) Forget about the charging cycle. The whole point of these capacitors is to provide an alternate source of current when the source itself can't provide it. When the current demands are light, you can model it as a battery providing a steady voltage, but that's really a simplification that models the top of the discharge curve as a straight line with no voltage loss. At heavy loads, Ir is the important factor, as stated in those data sheets. They do start off saying low ESL is important, but in choosing a capacitor, they explicitly mention ripple current and voltage rating as the deciding factors.The difficulty is in figuring out what the Ir is going to be. It's easy enough for a clean rectified sine wave going in to the cap. Not so easy for variable or transient loads, or more complex waves and noise on either side. I think you use a factor of 4 or 8 for the square and/or triangle waves in a switching supply. I think you're aware of the equations to convert from that to acceptable ripple voltage.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 17, 2011, 02:19:44 pm
content: Quote from: SiBurning on October 17, 2011, 12:53:48 pmI meant the load current, that is the output current of the whole switching module.Quote from: SiBurning on October 17, 2011, 12:53:48 pmI don't really understand what you mean by this. Let's say that the load is at a constant 0.5 A and load suddenly increases to 1A, that's a load transient.Quote from: SiBurning on October 17, 2011, 12:53:48 pmOf course you have to meet hard specifications such as voltage rating and RMS current rating, otherwise your capacitor will break, age prematurely or simply not function properly. But RMS current rating and voltage rating does not contribute directly to the quality of the output. They are simply requirements. But good values for these parameter most of the time means low ESR as well.My whole point from the beginning was that capacitance itself is not a deciding factor for output filtering caps in buck regulators and similar devices. You do not need much capacitance at these frequencies. The deciding factor is the ESR or for very high frequencies, ESL. I'm quoting a Texas Instruments application note (http://www.ti.com/lit/an/slva057/slva057.pdf) on buck converters in general:QuoteAnother quote from an AVX app note (http://www.avx.com/docs/techinfo/highfreq.pdf):QuoteOf course, if you have large load transients, you might need more bulk capacitance but for constant loads, the capacitance is not the deciding factor in choosing a capacitance.Unless you're using ceramics of course. In ceramics, the ESR is mostly negligible and the ESL is very low. You suddenly don't need to use much capacitance to get good output. And _that's_ where my problem lies. If you are using capacitors where the ESR is low regardless of the size or the voltage rating, maybe you need to add capacitance to handle load transients and poor input voltage? How much do you need? When I'm asking for rules of thumb, what I mean is not "use three on mondays but four on fridays", I mean approximation formulas. et.c.

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 03:09:29 pm
content: All of the references you provided have the equations for minimum C, not one of which involves ESR.You say C isn't important, but what happens if you don't have enough of it? Same for Ir? The voltage drop is going to be a lot worse than what you're getting from ESR.You need to meet all of the criteria, ESR being only one of them.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 17, 2011, 05:00:04 pm
content: Quote from: SiBurning on October 17, 2011, 03:09:29 pmLM22674 lists the equation for the output voltage ripple which is a function of the inductance, ESR, frequency and capacitance.LM2738 lists the exact same equation.LM2675 doesn't list any technical details on choosing an output capacitor, it just has a table with recommended parts.The TI and AVX app note I quote above supports what I'm saying as well.Quote from: SiBurning on October 17, 2011, 03:09:29 pmI never said C is not important. I was saying that you do not need a lot of capacitance for output filtering. By the time you've met all the other requirements, you usually have more than enough capacitance. What happens if you don't have enough depends on how much you're lacking.Quote from: SiBurning on October 17, 2011, 03:09:29 pmThen you're going to have heat problems which breaks or reduces the lifetime of the capacitor.Quote from: SiBurning on October 17, 2011, 03:09:29 pmDo you mean the voltage drop from the lack in capacitance or current capability? Whether lack of capacitance is a bigger problem than ESR depends on how much you're lacking. Heat generation in the ESR is what mainly causes the limits on current.Quote from: SiBurning on October 17, 2011, 03:09:29 pmOf course. My point is that voltage ripple being too high in a switcher design is rarely caused by lack of capacitance. ESR usually to blame.

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 10:14:15 pm
content: Quote from: shadewind on October 17, 2011, 05:00:04 pmThere are also equations for C in some of those app notes & data sheets.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 18, 2011, 12:08:29 am
content: Quote from: SiBurning on October 17, 2011, 10:14:15 pmThere are two different things that impose limits on capacitance. Many switchers have a fixed internal control loop compensation which assumes certain things about the external components. These assumptions place limits on the capacitance and you need to keep within these limits to ensure control loop stability, that is, to avoid oscillations.The other thing is simply how much voltage ripple you can tolerate. If we assume an ideal capacitor with no ESR and ESL and only consider the effect of capacitance on the output voltage ripple, TI has a formula in their app note (as you say above) which gives the minimum capacitance for a given voltage ripple:C = IL / (8 * fs * Vo)Let's say our switcher has an average load current of 1A. A good tradeoff for component size vs. ripple current (IL) is to the IL at about 30% of the average load current. 500 kHz is a pretty normal frequency for switchers these days so let's use that. A maximum ripple voltage of 5 mV should be more than enough for most any purpose. Let's calculate the minimum capacitance based on these values:C = 0.3 / (8 * 500e+3 * 0.005) = 1.5e-5 = 15 µFThat's not a lot of capacitance needed!Let's pick a real electrolytic capacitor with a value as close to this as possible:http://industrial.panasonic.com/www-data/pdf/ABA0000/ABA0000CE114.pdfWe pick the 35 V, 22 µF one. The ESR is 0.36 ohms. We can calculate (or at least approximate) the voltage ripple using the following formula:V = IL * ( ESR + 1 / (8 * f * C) )We can rewrite this as:V = IL*ESR + IL / (8 * f * C)There are two linear terms, one which depends on the ESR and another which depends on the capacitance. The capacitance (or lack thereof depending on how you see it) term gives this much voltage ripple:Vc = IL / (8 * f * C) = 0.3 / (8 * 500e+3 * 22e-6) = 3.4 mV...and the ESR term gives this much voltage ripple:Vesr = IL * ESR = 0.3 * 0.36 = 108 mVThe ESR term is greater than the capacitance term by a factor of approximately 32! It's certainly not what you would expect though.Edit: It seems that the forum software cannot handle characters such as delta signs and ohm signs so you'll have to excuse the rather incorrect notation

author: SiBurning
author_rank: Contributor
timestamp: October 18, 2011, 01:33:42 am
content: That's exactly what I expect after you've reduced the ripple current due to capacitance. Capacitance is selected based on a tolerable ripple current. If you're not happy with the impedance loss in the capacitor, choose alower... err...higher tolerable ripple voltage or find a cap or caps with lower impedance. (And don't forget to derate the cap for high frequency--something I have to leave to others to advise.)Using capacitance is fine and good for the steady state. For dynamics, the Ir handling is also important. Just be aware that you want an Ir handling value for the capacitor based on what the circuit would do without the capacitor because this value specifies the limits to the smoothing & sourcing properties of the capacitor. It's not the same as the resulting in-circuit ripple current.What's the question again?

author: SiBurning
author_rank: Contributor
timestamp: October 18, 2011, 03:34:39 am
content: It's kind of hard to figure out your perspective. A little glimmer of light shone through while cooking dinner, but not enough to formulate it clearly. It seems like you've fixed the 3 most important things (V, Ir, C)--or put another way, that those are known problems--and are now saying that the next one on the list (ESR) is the deciding factor.That doesn't explain why you're still asking about bulk capacitance. You've more or less solved that, and the math shows that it's no longer an issue. Still, I only see this way of thinking as a viable position for the output capacitor, and only on the basis that the output of the regulator--and therefore, the input to the capacitor--is perfectly encompassed by that equation. It should be known, since it is a voltage regulator, after all. But you don't have this perfect information for the input side, and maybe not in the other circuits.In this context, everything you've been saying suddenly makes sense. Hoping that helps clear up some confusion.The chart jahonen posted seems very relevant to me at this point.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 18, 2011, 12:29:04 pm
content: The key here is to differentiate between switcher output filtering and general bulk capacitance. Though you only need 15 uF to filter the ripple voltage, you still need more capacitance to handle load transients due the fact that when the load suddenly changes, the voltage will dip due to the inductance along power supply chain - the current cannot change that fast. That's why you add more bulk capacitance, to handle load transients. The more capacitance, the lesser dip in voltage.But I don't know how much to add! There has to be some approximation formula or rule if thumb based on expected load transients?So as you say, there's been some misunderstanding. I have my output filtering figured out but I am a bit fuzzy when it comes to figuring out how to handle load transients. So far, it's been mostly guessing.

author: SiBurning
author_rank: Contributor
timestamp: October 18, 2011, 02:36:48 pm
content: I'd refer to the difference as light vs heavy load current. At least in terms of what the capacitor's doing. For transient purposes, I certainly don't think of the filter cap as part of the power supply in the heavy load case, but rather as a secondary source of current. I always model this in terms of current demands, i.e. a capacitor keeps a stable voltage by supplying needed current when (the rest of) the supply can't. (Maybe my reply #23 will make more sense in that context.) You need to figure what the worst case current demand will be, or rather, the worst case you care to deal with.There's no simple formula or rule of thumb. Mostly guessing sums up the process pretty well. If you want to do better, You need to figure out what expected load transients you want to handle, and pick a set of caps that can handle the current burst. Basically, what everyone's been saying all along.Ask some questions... When the load changes to what? When you power on a computer, all the devices start up at the same time, causing a burst in current demand. A speaker, for physical reasons, might suddenly present a 0.5-0.25 ohm load at some frequencies. (The main current demand in audio comes from the signal, not from impedance changes, but it makes little difference except that the relative stability of load impedance--a false simplification--explains why power supplies are generally unregulated.) A blinking light might require a short spike of current every time it turns on. Do you have some similar situations?The difference between high load and low load can be thought of as an additional ripple current on the cap since it may have to supply the current deficit. This is why the Ir specification for a capacitor is so important, possibly more important than the capacitance.I generally make 3 main decisions in specifying capacitors.ripple current-- You can choose capacitors where the peak instantaneous current is less than the rated ripple current handling, or figure out the rms (of any recurrent peaks and the average of random peaks) and select accordingly. Which to choose, or where in between, depends more on voodoo than reality, but boils down to a typical case of tight tolerance vs over-speccing, but any heat will cause some... let's just call it nonlinear behavior.capacitance-- Select capacitance based on how far along the discharge curve you want to be after a transient, and how fast you need to charge & discharge. You obviously can't flatten the cap at any point, and probably want to stay in the linear portion of the discharge curve, say 5% for some applications and more for others? Another, admittedly controversial, point is just how fast can any particular cap source current, and do you need various sizes to supply current at different speeds? There's a (indefinite?) relation between this speed and the selection of ripple current handling.ESR & leakage current--An audio power amp might suddenly face a 0.5-0.25 ohm load, so it might be wise to have a low ESR so as not to dissipate too much power in the caps. There's not much sense in constantly wasting power & losing current through the cap. On the other hand, higher ESR and some leakage can actually stabilize things in other places.If you want to think of load as purely inductive--I tend to think of load more in terms of current demands--and worry about how a drop in load causes more of a voltage drop internal to the supply (with the filter cap modeled as internal), then the simple answer is to make sure the supply impedance is much lower than the heaviest load. Of course, that's really the whole job of the chip or electronics in the supply.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 18, 2011, 03:28:27 pm
content: I believe one test commonly done with switchers is to attach an AC coupled signal generator, often with an added power transistor and limit resistor, to the output, and hit it with a square wave or pulse. How the switcher handles the transient current draw is indicative of how well the feedback and compensation has been set up. This same test should tell you if the bulk capacitance is sufficient, since they all work together. Other than that, IMHO, there are too many unknowns to bother calculating it.

author: shadewind
author_rank: Frequent Contributor
timestamp: October 18, 2011, 09:38:18 pm
content: I should probably add that all my calculations and discussions regarding switchers have been about buck switching regulators. Much of it does not apply to, for example, boost converters where the output of the inductor is switched as opposed to the input as is the case in a buck regulator.Quote from: SiBurning on October 18, 2011, 02:36:48 pmFor a buck regulator in continuous mode, the ripple current doesn't change with the load, it is only dependent on the inductance (assuming fixed duty cycle). At low loads, you enter discontinuous mode but that's another topic. Because of this, it doesn't matter if your load current is 0.5 A or 1A, the ripple current the output capacitance has to handle is constant. What does matter is when the suddenly changes, that's when you have voltage dips or voltage spikes.Quote from: Conrad Hoffman on October 18, 2011, 03:28:27 pmYes, this sounds like a very good idea, simply measure it. But when you're making something as a hobbyist, it's a bit of a pain to first make a board and find out that the capacitance didn't cut it and then have to make another board. Of course, if you're using parts that have standard footprints (mainly ceramics and leaded electrolytics) you can always use parts with more capacitance unless you have to actually add more of them.I suppose the only thing you can do is add what you believe to be enough and then a little more and measure until you develop a feel for it?