How would *you* design a capacitor ripple current tester? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: March 10, 2021, 08:44:30 pm
content: Hi everyone,First off, I'm not 100% serious about starting this project, but I've been kicking around the idea for a while. And, for the sake of brainstorming, thought I'd ask here. Chroma (and other manufacturers) already make electrolytic cap ripple current testers, if you have $10,000 to spare, but in the event that you don't have that kind of money...I'm wondering what approach you might take to DIY a reasonable ripple current tester for electrolytic caps. The goal is to verify datasheet endurance specifications, i.e. 10,000 hours @ 105C, full rated ripple current, rated voltage, etc. Forget the temperature aspect for now...My first guess:Power op-amp (TO-220 type) with some current control loop based on 100KHz sine wave oscillatorAdd DC voltage bias...how?Generally, the RMS current range requirement would be 0 - 6A or 7A, if possible. I have yet to see a polymer cap with a ripple current rating much above 6A.Thanks,Tim

author: Gyro
author_rank: Super Contributor
timestamp: March 10, 2021, 09:09:47 pm
content: Quote from: TimNJ on March 10, 2021, 08:44:30 pmYou could always AC couple the output of the oscillator.... actually, joking aside, maybe you could if you used one of those beefy high ripple polypropylenefoilcapacitors (Epcos/TDK?)EDIT:https://www.tdk-electronics.tdk.com/en/1641622/products/product-catalog/film-capacitors/power-capacitors

author: floobydust
author_rank: Super Contributor
timestamp: March 10, 2021, 09:18:57 pm
content: I would use a decent audio power amplifier, DC-coupled and derated for 100kHz use. There are designs that are tough and fast enough.

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2021, 09:23:35 pm
content: I'm probably being thick but what's the problem with applying DC bias? Cant you simply apply it to the other side of the capacitor, fed through a resistor from a variable bias supply, with a massive parallel array*of decoupling caps to provide the return path to ground for the current injected by the power OPAMP.* The reason for a massive decoupling array is to split up the ripple current between enough capacitors to avoid fatigue testing the test jig rather than the DUT.

author: TimNJ
author_rank: Super Contributor
timestamp: March 10, 2021, 09:37:21 pm
content: Quote from: Ian.M on March 10, 2021, 09:23:35 pmDon't worry, 100% chance that it's me who's being thick.I barely even sketched this down on paper. My original thought was that some how you'd have control loops in conflict with each other. But uhh..yeah, probably just a resistor from the DC bias source small enough to counteract the leakage current of the cap.

author: TimNJ
author_rank: Super Contributor
timestamp: March 10, 2021, 09:42:14 pm
content: Quote from: Gyro on March 10, 2021, 09:09:47 pmHaha, thanks! I feel like you got it right the first time? A foil capacitor can also be a polypropylene capacitor, as far as I know. Foil should have the lowest ESR/ESL, best ripple performance. Metalized film, where they spray one or both sides of the dielectric (poly) film is cheaper and more widely used, I think. But, thinner "plates".

author: TimNJ
author_rank: Super Contributor
timestamp: March 10, 2021, 09:49:01 pm
content: Quote from: floobydust on March 10, 2021, 09:18:57 pmAny recommendations? I checked the usual suspects like TDA1875, LM3886, and so on, but no characterization above 20KHz. I'm not sure what to expect.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 10, 2021, 09:52:10 pm
content: Hi,I would take a buck regulator controller that operates in the forced continuous mode. The LTC3891 is a good example. I would set the output voltage to be 1/2 of the input voltage. The value of the inductor determines the ripple current.The ripple current will be triangular, not sine, but this really doesn't matter. Because of the symmetry, there is no second harmonic. The third harmonic is 1/9 th. the amplitude of the fundamental.The bias voltage is equal to the output voltage.Only the losses need to be supplied. Most of the energy circulates.Here is a quick model and the FFT.Regards,Jay_Diddy_B3891.asc(3.28 kB - downloaded 66 times.)

author: TimNJ
author_rank: Super Contributor
timestamp: March 10, 2021, 10:30:27 pm
content: Quote from: Jay_Diddy_B on March 10, 2021, 09:52:10 pmThanks for the recommendation. This is probably the most "logical" option in the sense that the capacitor under test is...literally just being used in a switch-mode power supply.The question is: Can you adjust the pk-to-pk ripple current simply by adjusting the current sense feedback signal, perhaps by programmable gain amplifier? I'm pretty confident you'd need to physically adjust the inductance value to change the ripple current, which is obviously not so convenient.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 11, 2021, 12:19:15 am
content: TimNJ,you can trim the ripple current in the capacitor by adjusting the 24V input.New modelI haven't stabilized the control loop properlyJay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 11, 2021, 12:30:56 am
content: Hi,You can add a blocking capacitor and a bias supply if you want to:I have stabilized the loop a little better.Jay_Diddy_B

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 11, 2021, 02:00:19 am
content: If you're OK testing capacitors in pairs, connect one side directly and the other side to a transformer. Then add a H bridge to drive the transformer and a resistor to connect a bias supply. Easy to scale up to very high currents and voltages.

author: Gyro
author_rank: Super Contributor
timestamp: March 11, 2021, 10:33:58 am
content: Quote from: TimNJ on March 10, 2021, 09:42:14 pmHa, yes. The reason I struck through 'foil' is that Polypropylene foil only tend to available at low capacitance values (or are presumably massive in large values). I think most of the ones that I linked are film types, but at a random pick they were good for around 70A 'continuous' at 320-350uF. Clearly all films aren't as as fragile as X caps! The linked ones are the upper end of the range (M6 terminals), but I'm sure I've seen smaller heavy duty Epcos snubber ones with mounting tabs.

author: Berni
author_rank: Super Contributor
timestamp: March 11, 2021, 11:17:40 am
content: I would also just make a switchmode buck converter with a really under speced inductor that would cause a large ripple current on the capacitor trying to filter it. The amount of ripple current only limited by how beefy of a switchmode chip you get, since the inductor can easily be hugely overspecced by making it a air core inductor made from thick wire.If the aim is for testing non polarized capacitors with no bias on a sinewave then i would build a ZVS oscillator to make a LC circuit making this capacitor oscillate with a nice continuous sine wave, again choosing a huge overspecced inductor with the correct inductance to create the desired ripple current. Since most of the current is recirculating between the inductor and capacitor means that very large currents in many 10s of amps is easily achievable without any powerful switching components. These are the sort of ripple currents that make even large beefy film capacitors get rather toasty.

author: TimNJ
author_rank: Super Contributor
timestamp: March 11, 2021, 09:36:23 pm
content: Quote from: Jay_Diddy_B on March 11, 2021, 12:30:56 amThanks. This is probably conceptually straight forward to implement for my mind.But, still, to get a range of even 1A - 5A RMS ripple current, the adjustable input voltage range needs to have a similar range, something like 12 - 54V range, let's say. So I need some flyback/forward based power supply with a good adjustable output, or a fixed high-ish voltage supply with an intermediate buck after it. Now it's getting more complicated.The blocking capacitor, I assume, should be high ripple current film/foil type.

author: TimNJ
author_rank: Super Contributor
timestamp: March 11, 2021, 09:38:52 pm
content: Quote from: NiHaoMike on March 11, 2021, 02:00:19 amI'm failing to visualize the setup. Any similar circuit on google images you can point me to?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 11, 2021, 10:25:38 pm
content: Quote from: TimNJ on March 11, 2021, 09:36:23 pmRemember you only need to supply the losses, so the power required isn't very high.You only need the bias supply and the blocking capacitor if the voltage you need is not equal to the output voltage.You can do course steps by inductor selection and fine steps with the supply voltage.IMHO, it is heat that ages the electrolytic capacitors.Heating effect is (ripple current)2x ESRThe bias voltage will have very little effect.Jay_Diddy_B

author: TimNJ
author_rank: Super Contributor
timestamp: March 11, 2021, 10:38:38 pm
content: Quote from: Jay_Diddy_B on March 11, 2021, 10:25:38 pmTrue true. Probably a 10 or 20W rated power supply would be plenty of overhead. The only other issue is maintaining good stability over a wide range of input voltage and a wide range of DUT capacitance and associated ESRs. Usually the control loops are tuned around a given capacitance and ESR. Now we are asking to deal with virtually any capacitance and virtually any ESR.

author: ogden
author_rank: Super Contributor
timestamp: March 11, 2021, 11:21:44 pm
content: I would take programmable DC supply, programmabe DC load, three DUT capacitors, two powerful & fast enough MOSFET switches and make programmable charge transfer switcher. Middle capacitor would be subject to positive/negative  pulses and actual subject of the test. Supply and load side capacitors - more or less sacrificial, to not stress supply or load. Current would be measured on middle cap. Such setup seems to me more flexible than buck converter which I find as great idea indeed.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 11, 2021, 11:53:35 pm
content: Quote from: TimNJ on March 11, 2021, 09:38:52 pmTake two capacitors and connect the negatives together. Now take the positives and connect them to the secondary of the transformer. The transformer is used to lower the voltage but increase the current so that the power electronics will be a lot easier to design. That takes care of pushing AC through the capacitors, now for the DC bias, you connect the negative side of the bias supply to the negatives of the capacitors and the positive side to one of the capacitor positives through a resistor, the resistor sized such that there would be negligible voltage drop across it once the capacitors charge yet be very large compared to the ESR. Something like 1k will work great for most setups.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2021, 12:23:59 am
content: Hah, I breadboarded one just the other day, to test a resonant circuit. Half bridge into series resonant tank, returned to ground through a cap divider. The supply bypass and C divider are all 1000uF 25V caps (supply is 12V) and the half bridge is a IR2101 into a pair of IRFZ34N, plus an inverter to complement the high side drive, and a pair of RCD networks to skew the rising edges (setting dead time). Signal source from the function generator -- about as simple as could be.Driving an inductor in bursts, I've got peaks up to 25A, not too shabby for breadboard work. (Measured with a current transformer, of course.) The transistors aren't too happy up there, and neither are the capacitors; I started with the divider being a pair of middling 220uF 50V types, which got rather hot, rather quickly, their ESR also limiting my output current. So, I moved to the larger, low ESR types. Down at a more modest 5-10A RMS, everything is cool enough for continuous operation.Anyway, the resonant tank is a good way to get nearly sinusoidal current. The Q isn't very high for the inductor I'm testing, so the peak isn't very sharp, easy enough to track by hand. To design something general-ish in purpose, you'll need to start with a range of ESRs, with the worst case minimum setting how much base ESR your circuit needs (to limit maximum Q and current) and the maximum setting what resonant impedance (Zr = sqrt(L/C)) you need, and whether you'll need multiple L and C to select from, to cover that range, given available supply voltage and whatnot.In the other tests above, triangular or square current isn't a bad thing, by itself. ESR should be relatively insensitive to harmonics, as far as I know, and the RMS value of all harmonics in a square wave only amounts to, what was it, 18% of the total? And of a triangle, even less.Tim

author: Berni
author_rank: Super Contributor
timestamp: March 12, 2021, 07:11:25 am
content: The issue with a series resonant circuit is that your mosfets need to carry all the current.The ZVS oscillator makes use of a parallel LC cirucit (Here formed by C1 and TR1 ) that keeps the high current nicely contained:http://www.kiblerelectronics.com/bob/app_notes/note11/note11.htmlThis means the mosfets only have to carry the current needed to keep the circuit going by covering the losses. Its a pretty popular circuit for cheep DIY induction heaters due to the ability of small mosfets being able to handle a lot of power (they switch at zero crossing so they are very efficient). The massive recirculating currents in the LC circuit is what makes it nice for a induction heater since the strong field the coil ends up making is sure to create hefty losses in the workpiece you want to heat. But the C side of the LC cirucit also has heavy work to do, typically its made from many foil capacitors in parallel and even those get toasty.You could repurpose the same circuit by placing your device under test capacitor in series with the LC tank capacitor, forcing all the circulating current to go trough it. Its also AC coupled so you could inject a DC bias onto the tested capacitor via a resistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2021, 01:08:21 pm
content: Indeed; what we're really doing is matching a resistance to a source. ESR tends to be low relative to typical amplifier ratings, therefore some matching is desirable. Can be simply a transformer, can be parallel resonant, LLC, whatever.And as you note, series resonant, and the series-L above, doesn't do us any favors as the total (reactive and real) current goes through the transistors. The only thing you get, is some control over output current by adjusting frequency, rather than varying supply voltage. So, it may be better than direct drive from a linear power amp, assuming the waveforms are an acceptable compromise; but it's not a whole lot.Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 12, 2021, 01:57:06 pm
content: Hi group,Here is an LTspice model of a ZVS design optimized for ripple current testing.I will let you run the model and measure the waveforms.Regards,Jay_Diddy_BZVS.asc(3.25 kB - downloaded 155 times.)

author: Berni
author_rank: Super Contributor
timestamp: March 13, 2021, 09:32:41 am
content: Quote from: Jay_Diddy_B on March 12, 2021, 01:57:06 pmNice touch on the Q1 gate supply undervoltage protection. Most people wire that directly to the supply and this makes the power MOSFETs prone to exploding if the circuits power supply is not brought up fast enugh to get it into oscillation. This causes both mosets to turn on, huge currents to flow and BANG.Lovely cirucit.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 13, 2021, 02:37:28 pm
content: Quote from: Berni on March 13, 2021, 09:32:41 amNice comment, you raise a good point. Unfortunately, the way I have used Q1 is as a simple LDO, to set the maximum gate voltage on the MOSFETs. It isnotan under-voltage lockout , UVLO.I will think about designing an UVLO circuit and report back ...Best regards,Jay_Diddy_B

author: Mazo
author_rank: Regular Contributor
timestamp: March 13, 2021, 02:56:03 pm
content: A single transistor and a few passives easily adds UVLO,4.something volts though adjustment of values is a user's exercise.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 13, 2021, 04:57:55 pm
content: Hi,Here is my version of the regulator with UVLO:The output turn on when the input is around 9V and off when the input drops to 7V. This will prevent the MOSFETs being starved of Gate voltage, as identified by Berni.The circuit uses a green LED Vfwd as the reference for the UVLO circuit.I have attached the model.Regards,Jay_Diddy_B6V8 reg with UVLO.asc(2.11 kB - downloaded 73 times.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2021, 05:08:56 pm
content: Maintained-high gate drive would be preferable; with a disable, there should at least be TVSs from GND to each drain -- the pull-up inductors will fly back when gate drive is removed. Which is the most common failure mode when people try out this topology -- it works great under load, but shutdown is a little different.Tim

author: Berni
author_rank: Super Contributor
timestamp: March 13, 2021, 07:29:41 pm
content: Ah yeah, i didn't look at it closely enough.Yeah you don't really need to regulate the gate drive voltage for these things. You want to use TVS diodes on the gates along with some series resistors to the feedback diodes to make sure the sensitive gate cant get blown. If you apply 30V of supply then a 15V TVS diode will nicely clamp it to that.So you probably want to make that transistor there only do the job of undervoltage protection. You can do that with a PNP and voltage divider. Once voltage gets high enough the divider pulls down on the base and lets current into the gate drive section. Also, have a resistor to ground to sink that voltage back to 0V when it turns off. Once the circuit is oscillating the parasitics of the diodes can sometimes provide enough backwards leakage to continue supplying the very high impedance gates, so it is sometimes necessary to pull that down to 0V to actually stop it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2021, 03:12:37 am
content: I actually prefer the pass regulator, as it's somewhat less wasteful of supply power and doesn't load the gates -- remember, putting capacitance on a gate is likely to make an oscillator. So you need to add ferrite beads or stopper resistors too. Slightly more components.Tim

author: Berni
author_rank: Super Contributor
timestamp: March 14, 2021, 08:52:16 am
content: Quote from: T3sl4co1l on March 14, 2021, 03:12:37 amNot really all that wasteful in this case since the pullup resistors can be pretty high in value (like 10K) and the circuit still runs fine. The cost of an extra 2 resistors and 2 zenners is not that high both in component cost and board area, while making sure any transients during startups or shutdowns or breaks in the LC circuit don't result in damage to the sensitive transistor gates.Sure it slows things down a little bit, but you can fix that by adding a small value capacitor across the diode to serve as feedforward. But not that you would need to anyway since this circuit doesn't require fast switching of transistors. As the name suggests Zero Voltage Switching means that the transistors are switched on and off when the voltage across them crosses through zero, so the mosfet is not burning any power while transitioning through its linear region, as such it doesn't matter even if the switching time is 10 times longer, the mosfets still don't burn much power at all. This is the big appeal behind this circuit as almost all of the losses to heat happen in the resistance of the LC circuit. This also makes it one of the lowest noise switchmode converter out there due to all the signals being nice smooth sinewaves.

author: TimNJ
author_rank: Super Contributor
timestamp: March 14, 2021, 02:35:56 pm
content: I appreciate all the great ideas. At the very least, there is some information here for anyone who's ever thought about validating ripple current/endurance ratings. Whether I get around to breadboarding this in the next few months, I don't know.FYI, the motivation behind this is as follows: At work, our sourcing department is always looking for more ~economical~ sources local to our factory in China. All of the electrolytic cap vendors we use appear legit-enough, i.e. they have nice websites, with enough information on process and quality, to instill some baseline confidence. But, still, without a proven track record, they're still just words on a webpage. So, I was thinking about stress testing some caps and see how they do.Of course, this does not fully address quality control and consistency over time. But, better than nothing.I like the ZVS circuit a lot. I have done some work with ZVS topologies, but never thought to to apply a resonant circuit in this application. Although, seems obvious now!

author: Ian.M
author_rank: Super Contributor
timestamp: March 14, 2021, 03:29:35 pm
content: The only major argument against the ZVS approach is its going to be difficult to instrument if you want a realtime readout of capacitance and ESR over the duration of the test.OTOH its relatively cheap to implement so if you simply want to do accelerated aging tests in a heated environment chamber under high ripple current stress simultaneously on a significant number of sample capacitors, its probably the way to go.

author: TimNJ
author_rank: Super Contributor
timestamp: March 14, 2021, 03:49:34 pm
content: Quote from: Ian.M on March 14, 2021, 03:29:35 pmI'd probably just pick a set interval to pull the caps out of the chamber and hand test them. For a 10,000 hour rated cap, you'd need to run the test for over a year (continuously), so once a month? Sure, maybe you lose some "resolution" on when the cap starts to go bad, if it does, but ~10% accuracy on endurance rating is probably okay.

author: ogden
author_rank: Super Contributor
timestamp: March 14, 2021, 04:26:17 pm
content: Quote from: TimNJ on March 14, 2021, 03:49:34 pmLifetime is estimated, not actually tested. Degradation curves of ESR, capacitance and electrolyte weight are known. It is enough to test for fraction of nominal lifetime to know - DUT in the end will meet specs or not.

author: TimNJ
author_rank: Super Contributor
timestamp: March 14, 2021, 05:02:07 pm
content: Quote from: ogden on March 14, 2021, 04:26:17 pmInteresting. I could see why some no-name capacitor manufacturer might just take this route, but if you're a big name, why wouldn't you just run the test and confirm the manufacturing process/materials are in line with the prediction? The mechanism is that well understood?Regardless of whether it's tested or not...it should still meet the spec!

author: ogden
author_rank: Super Contributor
timestamp: March 14, 2021, 05:25:45 pm
content: Quote from: TimNJ on March 14, 2021, 05:02:07 pmIf your engineering cycle is 10000 hours, you definitely can't be big name manufacturer by any stretch of imagination. further reading, one of many search hits with long enough reference list:https://passive-components.eu/aluminium-electrolytic-capacitors-accelerated-lifetime-modeling/

author: TimNJ
author_rank: Super Contributor
timestamp: March 14, 2021, 05:39:16 pm
content: Quote from: ogden on March 14, 2021, 05:25:45 pmDid you misunderstand what I was trying to say? Not end-product engineering cycle...I'm talking about the capacitor manufacturer's with stated endurance specifications.I'd be surprise that Nippon Chemi-Con, Nichicon, et. al., would still not verify endurance specs empirically.

author: ogden
author_rank: Super Contributor
timestamp: March 14, 2021, 06:23:29 pm
content: Quote from: TimNJ on March 14, 2021, 05:39:16 pmYes indeed they do endurance tests  for samples, projected endurance tests. Do you really believe that products having 10 years life expectancy are actually tested for 10 years before release?

author: TimNJ
author_rank: Super Contributor
timestamp: March 14, 2021, 07:38:32 pm
content: Quote from: ogden on March 14, 2021, 06:23:29 pmOf course you can't reasonably test for 10 years, henceforth HALT testing, reliability calculations and predictions, etc. I know these are common practices across many industries especially aerospace and automotive, to name two. I do not know how (a typical) electrolytic cap manufacturer's R&D works internally...but, validating 10,000 hours in a test lab doesn't seem completely unreasonable. Capacitor manufacturers might release a new series every...few years, maybe?And certainly it would ridiculous to test every single capacitor in every series for the full rated endurance, but perhaps a couple different can-sizes would not be totally crazy.But, from what you're saying, seems most manufacturers may just test to 30-50% of the expected full rated endurance, and then predict the actual endurance based on curve fitting to the actual data-points. From the link you attached, though, there is some deviation from the model at end-of-life. Granted, in the provided example the EoL performance is actually better than the model predicts, but I wonder if that's always the case.Bringing it back to the original point of this thread, frankly I don't care if a 10,000 hour cap really is a 7,000 or 8,000 hour cap. Usually the end design has sufficient overhead to account for this. But, if an advertised 10,000 hour cap is really a 2,000 hour cap, then that's probably going to be an issue. The point of running these tests would be to at least get a feel for the quality of some of these lesser known manufacturers.

author: Berni
author_rank: Super Contributor
timestamp: March 15, 2021, 06:35:15 am
content: Yeah this approach seams perfectly reasonable to me.I'm guessing cap manufacturers do run long term tests on caps just to make sure they do last, but obviously it takes too long to do that before a new capacitor series is released.But im sure you could weed out the crappy capacitors fairly quickly by running them significantly past the spec. For example running a 85°C rated cap at 100% rated ripple current in a 110°C oven. Or running a capacitor at room temp but exposing it to 400% the rated ripple current. Running them at a bias of 120% of what is rated etc.. Keep turning it up until you can get a brand name capacitor to fail in a week or two, then put the cheep chinese caps trough the same torture and see how long they live, if they die in a day or sooner then they are probably underpromissing on some spec. Just make sure you have a decent sample size of like 5 to 10 capacitors in tests since there is sure to be some manufacturing variance.Sure this is penny pinching but if you do high volume production i can see the motivation for it. Not all chinese parts are crap, they are capable of producing quality products too, but its difficult to know what the good ones are because the chinese love to slap imaginary made up specs on things.One example is when we wanted cheep 500A shunts from china. Someone found some and bought samples, the specs looked nice, similar to what a shunt from Murata would have. It looked decent, but i didn't quite trust it, the guy who bought it said "Don't worry it says its 20ppm/°C 0.5%" so i went on my own initiative to actually characterize it. This was actually a bit involved since it means accurately measuring 10s of microOhms, but the results ware in and it was actually something like 1% 2000ppm/°C. Since this was a difficult measurement i doubted myself a bit and bought a genuine Murata brand shunt to test too and i measured it slightly out of spec for tempco (Fault of my test setup likely). So yeah that chinese shunt was indeed garbage, but you wouldn't know unless you test it. We went to find a different chinese manufacturer, they made similar claims, but they passed the test just fine. So yeah, always doubt chinese specs, they could be correct specs or just guesswork.