Tantalum wet electrolytic capacitor not suitable for SMPS use? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: November 13, 2022, 10:44:18 am
content: Hi,Would you categorically say that the TWAE477x100 (470uF,100V) wet tantalum capacitor is of no use in SMPS designs with switching frequencies greater than 100khz?(even at 100khz its of virtually no use, and should not be selected?).TWAE477x100 capacitor datasheethttps://www.mouser.com/datasheet/2/40/TWA_E-890185.pdfFig 1.1.4b of this...(page 155)https://www.avx.com/docs/techinfo/WetTantalum.pdf...shows that its capacitance at 250kHz is about zero (!!!)....It makes you wonder what woudl happen if it were the only capacitor at the output of a 200kHz Boost PFC , say......that was also the inptu cap to a 250khz, 500W Full bridge.What would happen?...presumably large overvoltages due to the lack of capacitance at the input and output of those converters?And,Please advise...what is this capacitor actually for?(its datasheet says for "filter" use, but no more than that...nothing about at what frequency the filtering is for)

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2022, 11:03:26 am
content: The wet tantalums are a rather old technology. The early SMPS were lower in frequency, like 20 kHz (just outside the hearing range to avoid noise problems). There were also different types and some may have been better when it comes to ESR and ESL. Not sure if the measurements in the application note are actually accurate - it could also be an aritfact in the measurement (e.g. wire inductance), not really caring about the higher frequencies.Today they are largely replaced by other capacitors - maybe except for some special MIL and space use, where cost are less of an issue.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: November 13, 2022, 11:29:22 am
content: Quote from: Faringdon on November 13, 2022, 10:44:18 amI think it's quite suitable if all you need is a couple of uF. In this day of component shortages, if you can make it work then go for it.Wouldn't be my first choice though! Also.. please let me know what you put it in so I can avoid it

author: wraper
author_rank: Unknown
timestamp: November 13, 2022, 11:43:57 am
content: You can use them... but why? They have a decent ripple current rating, however ESR is so high they are not quite feasible for SMPS. Not to say their price should be an immediate showstopper to even consider them.

author: dmills
author_rank: Super Contributor
timestamp: November 13, 2022, 12:37:50 pm
content: I have used these, in the right place they are fine.In my case it was a preamp board that needed to be specified to 60,000ft pressure altitude (Quite funny as it was going in a underwater ROV for work deep under water, but transport to site was the concern), and the thing was going to wind up on the end of a LONG power and signal cable in use.I needed a hematic cap of reasonable value and reasonably high ESR that could sit across the DC input to damp the cable inductance, electrolytic wouldn't really do it due to pressure altitude limits (Turns out CDW do actually make hematic electrolytic caps, but you try getting them!), so hematic wet tant is was.The actual power supply board was running at well over 100kHz, but that was not what this cap was for, it was to deal with the issues caused by a 2km cable. Incidentally, ceramic caps were right out (Including in the power supply) because the ultrasonic acoustic output could interfere with sonar operation, lots of polypropylene film on that board.There are all sorts of really cool components out there for specific application niches, wet tant is one of them, you don't need it often (and it might be that it is really obsolete today) but for the right application, in a sufficiently tough environment, all the **weird** stuff has its place, wet tants, mercury relays, beryllium oxide thermal pads, weird ceramic substrates, some really funky pule power stuff, you don't use them if you can avoid on cost (And sometimes safety!) grounds, but sometimes they solve a problem.

author: Faringdon
author_rank: Super Contributor
timestamp: November 13, 2022, 02:12:40 pm
content: Thanks,Here , wet tant is used because we need high capacitance in small space, and we are not allowed to use Aluminium electrolytics. (due to lifetime concerns)However, from the above, concensus appears to agree with us that Wet Tant is simply not for 100kHz+ SMPS input/output caps?This has been layed out on a 500W SMPS....200Khz Boost PFC into 250khz Full bridge.For the DC link between PFC and FB, the only capactitance there is five of the above 100V, 470uF Tants in series (!!!)No other caps whatsoever on the DC link.Also, these 5 series wet tants are not even on the same board as the FB and PFC...they are on a daughter board connected to it by a pin header.I wish the datasheet gave the stray inductance of the part?It seems weird that capacitance goes down massively with frequency when ESR also gets less with frequency?

author: dmills
author_rank: Super Contributor
timestamp: November 13, 2022, 02:29:51 pm
content: So about 80uF at DC?Hope they have equalising resistors (and that the worst case ramp on power up or down is slow enough), you need to ensure both DC leakage equalisation AND that during startup of shutdown that the capacitances match closely enough to avoid either over volt or (worse) forcing one cap to become reverse biased.Series strings of polar caps can have some 'FUN' failure modes, especially when they are fucking tants.Pretty sure I would have done the lifetime sums on a premo grade electrolytic (or several in parallel), and a few good quality film parts to handle most of the HF ripple.

author: Faringdon
author_rank: Super Contributor
timestamp: November 13, 2022, 05:58:49 pm
content: QuoteThanks, yes thereaboutsQuoteThere is 180k across each of the five series Tant caps.But yes, i think we need a diode across each tant too.

author: David Hess
author_rank: Super Contributor
timestamp: November 14, 2022, 12:55:15 am
content: Tektronix used them in switching power supplies but that was between 20 and 60 kHz.

author: Faringdon
author_rank: Super Contributor
timestamp: November 14, 2022, 06:26:25 am
content: QuoteThanks,But were they axials?And axials in series?...And were they the only ones on the bus?.......they are in our case.These should have a warning label "not fur use as only capacitor in an SMPS", would you agree?

author: David Hess
author_rank: Super Contributor
timestamp: November 14, 2022, 10:57:12 am
content: Quote from: Faringdon on November 14, 2022, 06:26:25 amThey are axial parts. I do not know what you mean by in series. I do not know why Tektronix used them.QuoteI disagree.

author: SeanB
author_rank: Super Contributor
timestamp: November 14, 2022, 02:19:30 pm
content: If ESR is your concern, put a 1uF metallised film 1kV capacitor on the main board, or probably 3 330n 1kV in parallel, as those likely will be cheaper and easier to place, which will give the needed low ESR on the converter board. I have abused 10uF 25V wet slug tants before, that 10R ESR is really a limit, all parts will be much lower, most of the time they showed up on the LCR meter as perfect capacitors, with ESR under 1R, despite them actually having been sitting for close to 2 decades in uncontrolled storage, and I did experiment with a pack of 100 in seeing if reforming made any difference, which it did not.They all came in as either perfect, or with 1R ESR, or 10M leakage, though the leakage did go away with 1hour of reform. I abused a few using them to replace PC PSU output capacitors, as I needed it to work, but the output caps had done the blow top failure, and those 10uF 25V tants were to hand, so 4 in parallel went in, along with a 100uF capacitor that was guaranteed to not even exist at 20kHz, and it worked again. 5V rail went from having 2V of noise on it to almost zero.Those tants are exceedingly expensive, it might be a lot cheaper to use some 80-PEG124VG2220QL1 and put them in series parallel, to get your roughly 100uF of capacitance at over 400V, and they are at least one in stock at Mouser. Aluminium electrolytic yes, but will last at least 3 years at 100C, and you probably can also find with a bit of searching 125C ones as well.

author: Faringdon
author_rank: Super Contributor
timestamp: November 14, 2022, 05:41:35 pm
content: QuoteThanks, these ones have very little capacitance between 20khz and 60khz (TWAE477M100)Page 155, Fig 1.1.4b of this ....https://www.avx.com/docs/techinfo/WetTantalum.pdf...shows the way capacitance drops with frequency, i cant see these being of use with f(sw) > 1kHz.QuoteThanks, i am not surprised that you dont know what i mean,.......because what is before me here is total madness.We have a 500W SMPS....200Khz Boost PFC into 250khz Full bridge.For the DC link between PFC and FB, the only capactitance there is five of the above 100V, 470uF Axial Wet Tants in series (!!!)No other caps whatsoever on the DC link !!!Also, these 5 series wet tants are not even on the same board as the FB and PFC...they are on a daughter board connected to it by a pin header....i think you would agree we are looking at an SMPS designed by someone with "not of sound judgement"...or most likely, someone in a heck of a rush !