Normal electrolytic capacitor in place of low and super low ESR electrolytic cap - Page 1
Forum section: electronics / projects

author: samimi_electronics
author_rank: Newbie
timestamp: December 08, 2018, 07:58:06 pm
content: Hi,I'm working on designing a SMPS. I used the software PI Expert to get a base design based on what I need. I'm currently building the schematic presented by the software based on a TNY278PN TinySwitch-III. Anyhow, there are three filtering caps along one inductor for filtering the output after the rectifier (schematic attached), two of which (C6/C7) are specified super low ESR and one (C8) specified low ESR. The problem is here in Tehran, low ESR and super low ESR caps are not available. My question is that: Can these capacitors be replaced by general purpose electrolytic capacitors? Would it cause instabilities? Why are they specified so?I'm not proficient at SMPS design and this is my first attempt at building one.

author: Miyuki
author_rank: Frequent Contributor
timestamp: December 08, 2018, 08:21:23 pm
content: You will get huge ripple voltage (ESR x Transformer output peak current)And also this ESR will cause heating of capsCan be replaced with more general purpose ones or help with something like film cap

author: samimi_electronics
author_rank: Newbie
timestamp: December 08, 2018, 08:27:41 pm
content: Oh, that makes sense. Thanks.In other words, the low ESR caps are specified to reduce heat losses at the high frequencies and to increase working age of the capacitor. But I don't understand why would it cause a huge (bigger) ripple voltage compared to low ESR? (ESR x Transformer PK-Current)Why is it proportional to the transfo's peak output current?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2018, 08:36:39 pm
content: Quote from: samimi_electronics on December 08, 2018, 08:27:41 pmYes, that is the definition of Ohm's law, V = I*R.It is also exacerbated by the flyback topology, which has the transformer current going between zero and full, every cycle. (The improvement there would be a push-pull, usually forward converter, design, or a multiphase design.)Do you not have access to Chinese sources? Western suppliers are pretty obviously off limits, but you're still not exactly a deserted island, as far as I know!Film caps aren't very useful here on account of the large values required (>10uF?).General purpose electrolytics can still be used, but you need many more of them in parallel, maybe >5,000uF worth, and then the compensation has to change, and everything becomes a bulky mess.Tim

author: DaJMasta
author_rank: Super Contributor
timestamp: December 08, 2018, 08:51:06 pm
content: Haven't tried myself and don't know if it applies to this use case, but what about using standard electros for the replacement and then bypassing them with a lower ESR cap or two in parallel (maybe a small film and/or a ceramic)? It wouldn't really help with the low frequency ripple, but if this is an SMPS secondary side, the frequency would be pretty high for a 1500uF electro to have that low impedance, so the smaller value low impedance caps would help for filtering out the higher frequencies.

author: Miyuki
author_rank: Frequent Contributor
timestamp: December 08, 2018, 09:04:08 pm
content: Or 10uF; 10V MLCC cost cents per pieceMust be calculated

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 08, 2018, 10:27:31 pm
content: One often chooses SMPS output filter caps by desired ESR, knowing the value will be more than sufficient by the time you reach the desired ESR. You just go down the chart until you find the ESR you want, and whatever value you end up with is what it is, assuming it's larger than the target. Though impractical, I suspect you could parallel several standard caps to get low enough ESR, though the value might be quite large compared to what you'd have with low ESR caps. Using smaller low ESR ceramic caps might or might not work- you have to look at the reactance of the cap at the frequency of interest. If the reactance is too large, its low ESR won't help you much.

author: samimi_electronics
author_rank: Newbie
timestamp: December 24, 2018, 01:51:52 pm
content: Sorry for the late reply. Thanks to everyone for their insight on the topic. I will try to use normal Caps with the same the capacitance and see the results.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: December 24, 2018, 02:50:51 pm
content: Hi samimi_electronics, of course you can buy good caps on ASiDeisgner.com which is located in Iran

author: soldar
author_rank: Super Contributor
timestamp: December 24, 2018, 07:11:26 pm
content: Quote from: samimi_electronics on December 24, 2018, 01:51:52 pmBad idea. You can expect the life of those caps to be pretty short. Electrolytic caps failing due to ESR is the primary cause by far of power supply failures. It is a well-known thing and just ignoring it is setting yourself up for trouble.High ESR causes heat which causes higher ESR which causes more heat which causes ... you get the picture. Once the chain reaction gets started it is self-sustaining.As has already been said you can use many caps of smaller value in parallel, you can use greater sum total of microfarads, you can use bigger physical dimensioned caps as they tend to have lower ESR. Miniaturization has caused ESR values to increase substantially. Older caps had better values. Do all these things and you can get much lower ESR.

author: David Hess
author_rank: Super Contributor
timestamp: December 25, 2018, 02:58:11 am
content: It can be done but to attempt to get the same performance, lots of physically smaller capacitors will need to be used in parallel which may not be possible due to available space. You could also get good results using lots of solid tantalum capacitors in parallel because they are better at higher frequencies but this is more expensive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2018, 08:35:33 am
content: Had to do that a while ago, I had built another variant of my customizable flyback power supply, this one:but found I only had -- argh, whatever they are, I think 2200uF 35V 85C Samwha (generic type too I think?) in my junk bin. Tried it with two in parallel, initially (the ones by the black vertical choke). Got way too hot for comfort. I put two more in parallel, wired up on stilts, behind them (you can see how they stand taller than the others -- they aren't soldered flush into the PCB!), and that got the temperature more reasonable, though I wouldn't be surprised if the internal (peak core) temperature is still a bit uncomfortable.So in the end, it's way more capacitance than I expected to need, because I used what I had on hand (would've been fine if I felt like ordering parts of course, caps with adequate ratings are easy to find online over here). Consequently, the error amp compensation was way off, which took some adjustment, and I had to add extra aux supply capacitance to ensure startup (because, if there isn't enough hold-up on the primary side, the controller runs out of juice before the output has stabilized, and it sits there ticking away in hiccup mode).It also doesn't really help that the current limit seems to be more like 8A with the exact components in there. So the whole thing got much hotter than I expected. Mainly in the output caps, and the primary snubber. Which I initially had as UF5406, which desoldered itself. That was a fun "clink" sound when I noticed what it happened to... (the supply survived -- extra switching and probably avalanche loss too, didn't instantly explode the transistor, and I noticed the smell. The fallen diode also didn't short anything out, kind of miraculous.) Replaced that with a SiC schottky (the TO-220FP floating in front of the main switch -- yes, it really is tack-soldered onto pigtails there..), which runs ice cold all the time. That was also around the time I added the fan, to help ensure no more runaway recovery losses, at least until such time as the fan chokes up with dust...(By the way, this is basically a stock UC3842 offline isolated supply circuit; refer to the appnote for a schematic.)Tim

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 25, 2018, 10:22:56 pm
content: Quote from: soldar on December 24, 2018, 07:11:26 pmESR of aluminum electrolytics does not increase with increasing temperature--it decreases.See:https://www.avnet.com/wps/portal/abacus/resources/engineers-insight/article/understanding-esr-in-electrolytic-capacitorsunder the heading "Constructing the capacitor" is found this: "In aluminum electrolytics, ESR falls as temperature increases - its effects reduce as assemblies warm up."Here:https://en.wikipedia.org/wiki/Electrolytic_capacitorunder the heading "ESR and dissipation factor tan δ" is found: "For electrolytic capacitors generally the ESR decreases with increasing frequency and temperature."Here:http://www.cde.com/resources/catalogs/AEappGUIDE.pdfFigure 7 on page 6 shows how ESR decreases with increasing temperature.This paper:http://lipo.ece.wisc.edu/2002pubs/2002_35.pdfin figure 4 gives an example of ESR decreasing with increasing temperature.

author: wraper
author_rank: Unknown
timestamp: December 25, 2018, 10:27:36 pm
content: Quote from: Miyuki on December 08, 2018, 08:21:23 pmNope, it cannot. Film cap will barely take any ripple current on itself and general purpose electrolytic will die within a few weeks or months.

author: wilfred
author_rank: Super Contributor
timestamp: December 25, 2018, 11:10:17 pm
content: I got nearly 6 weeks before failure when I tried normal ESR electrolytics in my PVR's switching power supply. That was pretty much on all the time. The low ESRs I replaced them with have been going over two years now.

author: soldar
author_rank: Super Contributor
timestamp: December 25, 2018, 11:19:08 pm
content: Quote from: The Electrician on December 25, 2018, 10:22:56 pmWell, I might be wrong although I think there may be two different things to consider here. It may be that as long as the cap is working at a safe temperature the ESR does indeed decrease with temperature. That makes sense to me. What I understand is that if excessive current for a given ESR takes the cap to working at excessive temperature for which it is not rated or designed then the electrolite is damaged or dries up and the ESR increases as a result which leads to a runaway process. The cap is permanently damaged. I believe this, which is what I have been lead to believe, is compatible with ESR decreasing over the acceptable working range of temperatures.

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 26, 2018, 12:11:23 am
content: Quote from: soldar on December 25, 2018, 11:19:08 pmYou could have avoided confusing your readers by specifying with respect to which operating regime you were commenting.

author: soldar
author_rank: Super Contributor
timestamp: December 26, 2018, 12:26:28 am
content: Quote from: The Electrician on December 26, 2018, 12:11:23 amI do not see how any confusion can arise because what I was referring to is what is applicable to the case in hand. The OP will not benefit by using high ESR caps in place of the required low ESR caps. On the contrary.

author: BravoV
author_rank: Super Contributor
timestamp: December 26, 2018, 04:09:29 am
content: Quote from: The Electrician on December 25, 2018, 10:22:56 pmNot doubting about this, but, in practice & reality, it will be case by case basis, as I really wonder if the advantage of decreasing ESR can cope with the disadvantage of increasing ambient or surrounding temperature, if its placed near to heat source, like at PSU when the power usage increase, these heat sources like the semiconductors near to the cap tend to heat up significantly.As we aware, its the temperature that have significant effect on the cap's life, exponentially if I'm not mistaken.An example, the black cap that is near the heat sink, basically its constantly being toasted all the time if the air flow is bad, which is common in a cramped & packed PSU.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 26, 2018, 08:24:21 am
content: Note that ESR does not decrease much above room temperature. The admonition is mainly not to freeze the poor buggers.It is also an immediate change. Over a long time scale, high enough temperatures will dry it out faster, leading to increasing ESR even at room temperature.Tim

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 26, 2018, 12:05:41 pm
content: Quote from: soldar on December 26, 2018, 12:26:28 amPerhaps. But imagine how much respect for your communication skills those who are slow on the uptake would have, if you were to include a few words of explanation.

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 26, 2018, 12:17:45 pm
content: Quote from: T3sl4co1l on December 26, 2018, 08:24:21 amFigure 4 in this paper:http://lipo.ece.wisc.edu/2002pubs/2002_35.pdfshows a 105C ESR which is 70% of its 20C value.I suppose this decrease could be characterized as "not much", but I don't know much about the capacitor bank they used.I have found the decrease to be larger. I made some measurements of the ESR of a Nichicon 1000 uF, 25 V aluminum electrolytic. I measured the ESR at room temperature (17C) to be 105 m\$\Omega\$. I then heated the capacitor to 105C (the cap's rating) with a controllable temp hot air gun.The ESR was then measured with an LCR meter. The result is shown in this image:The capacitor was allowed to cool to room temp for about 1/2 hour, and measured again:This is a reduction in ESR of a little over a factor of 3. I don't think I would refer to this as "not much" of a reduction.I also measured an ELNA 1000 uF, 16 V capacitor, with the following result: Room temp ESR = 48.1 m\$\Omega\$, ESR at 105C = 15.5 m\$\Omega\$, again a reduction of about a factor of 3. I've made these measurements in the past, and this approximate 3 to 1 reduction in ESR is what I got then.I've always made these measurements on "modern" capacitors. I don't know how capacitors from 20 or more years ago would have measured. Maybe I can dig out some really old caps and measure them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 26, 2018, 01:04:49 pm
content: Negative capacitance..?Excuse me if I question the measurement slightlyTim

author: soldar
author_rank: Super Contributor
timestamp: December 26, 2018, 01:26:16 pm
content: Quote from: T3sl4co1l on December 26, 2018, 01:04:49 pmAlso known as "inductance".

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 26, 2018, 02:25:21 pm
content: Quote from: T3sl4co1l on December 26, 2018, 01:04:49 pm100 kHz, the measurement frequency, is above the self resonance frequency of the capacitor when it's hot, so the impedance of the capacitor is inductive. If the LCR meter is set to measure capacitance, it shows a negative capacitance when the impedance is inductive.When it's at room temperature, 100 kHz is below the SRF, so the impedance is capacitive, and the LCR meter shows a positive capacitance.Edit: Just for kicks, I left the capacitor in the freezer for an hour at -10C. The ESR then measured 583 m\$\Omega\$To show the change in the capacitors impedance when hot, here is a sweep of the phase angle and ESR from 100 Hz to 5 MHz. The ESR is the yellow curve, and the scale for ESR is logarithmic, with 1 millohm at the bottom and 1000 ohms at the top. The scale for phase angle (green curve) is linear, with -90 degrees at the bottom, +90 degrees at the top and the middle of the screen is zero degrees (purely resistive). There is a marker (marker A) at 100 Hz, and marker B at 101.7 kHz.Here is the sweep obtained when the capacitor is at room temperature. The phase angle starts out at nearly -90 degrees (the measured value is -83.679 degrees) at 100 Hz. At this frequency the capacitor''s impedance is quite capacitive, as we would expect. As the frequency increases, the phase leaves -90 degrees (capacitive) and approaches +90 degrees (inductive) at 5 MHz. The green phase curve crosses the middle of the screen (zero degrees, purely resistive) at about 250 kHz; this is the self resonance frequency (SRF). Since the LCR meter is set to display capacitance, at frequencies below the SRF the measured capacitance will be positive. At frequencies above the SRF the LCR meter will show a negative value for the capacitance. Since the SRF is above 100 kHz when the capacitor is at room temp, the capacitance measured at 100 kHz will show as a positive value.Here is the sweep obtained when the capacitor is at 105C. The green curve crosses the middle of the screen at about 80 kHz. With an SRF of 80 kHz, when the LCR meter is set to show capacitance, a measurement made at 100 kHz will show as a negative capacitance:Here is an image showing both sweeps superimposed:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2018, 12:36:53 am
content: Ah, nice. Thanks. Best part about sweeps, you can construct accurate RLC models for them -- not that it matters much given the variation of ESR!Tim