Cant calculate stuff in the world of SMPS - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 13, 2023, 06:33:26 pm
content: Hi,There are so many things in SMPS design that we cannot calculate (not within reasonable commercial project expenditure and time).We rely on "rules of thumb", or just guessingPlease can we provide the list here.?I will start it with ..1...Finding the core temperature of a leaded radial electrolytic capacitor when you have measured its external case top temperature, and you know its ripple current, and its ripple current is above the nominal rated ripple current.This cant be calculated because there is no available figure for "thermal resistance from core to case of a particular radial electrolytic capacitor"And equations provided for some capacitors, are not applicable to those cases where ripple current is greater than rated ripple current.Supposing the cap experiences triple its rated ripple current , and such is "allowed" , because its in blown air and its case stays below 50degC, and only gets used at max power for some 20 mins per day, say..

author: Gyro
author_rank: Super Contributor
timestamp: April 13, 2023, 06:52:18 pm
content: Meanwhile, in theengineeringworld. We make use of the data provided my manufacturers. The manufacturers base their ratings on their own development data, they do not guess their figures. We base our calculations on their information and do not exceed their specified maximum values for relevant parameters - and in fact de-rate to achieve our reliability goals.We do not guess.P.S. Please don't double post, it wastes peoples' time and patience...https://www.eevblog.com/forum/renewable-energy/ripple-current-in-electrolytic-capacitor/msg4810859/#msg4810859

author: mzzj
author_rank: Super Contributor
timestamp: April 13, 2023, 07:07:35 pm
content: Dig deeper, watson:http://www.capxongroup.com/files/Lifetime%20Compendium_EN.pdfhttps://file.elecfans.com/web1/M00/20/FE/ooYBAFmk4daAcHSbAIJSorLC0EU785.pdfhttps://www.nichicon.co.jp/english/products/pdf/aluminum.pdf

author: jonpaul
author_rank: Super Contributor
timestamp: April 13, 2023, 08:43:03 pm
content: No need for such a question, see mfg spec. There is no thermal resistance for a lytics.Notice the at rated ambient and ripple current, the cap life is 2000 hrs defined as a % decrease in C or increase in ESRIf the usual commercial 85 C cap is marginal, just get 105 C or 135C rated cap.Expect much higher cost and much worse availability.ChiCom caps may be junk.J

author: Someone
author_rank: Super Contributor
timestamp: April 13, 2023, 10:39:47 pm
content: Quote from: Gyro on April 13, 2023, 06:52:18 pm... or at the extreme opposite end of the "profession" the OP, people actually can calculate these things and pretty much every aspect of switching regulators. The trick being to balance putting effort into practical experiments and theoretical models, those aspects go back and forth to support and refine each other.

author: jonpaul
author_rank: Super Contributor
timestamp: April 14, 2023, 01:47:26 am
content: Someone: I was refereeing to the specific issue raied byt the OP re calculations of SMPS lytics.We indeed use simulation as well as prototype of the ckt and magnetics.Especially Magnetics Designer and Micro-Cap, for simulation and design.j

author: Faringdon
author_rank: Super Contributor
timestamp: April 14, 2023, 05:26:05 am
content: QuoteThanks..Quote..And Thanks, but all the "rules" surrounding el caps concern operation at rated ripple current or less.....its common in eg products which dont get used much at high power to have the el caps with "x" ripple current rating, seeing "2x" (or more) ripple current when at max power....there is no way of calculating what their life is in that case.QuoteThanks, see page 6 , equation 1, ofhttp://www.capxongroup.com/files/Lifetime%20Compendium_EN.pdf...it speaks of operation at "rated ripple current"...thats no good for many applications, where the el cap is operated at say 2x rated ripple current or more.......on the basis that the product is only at max power for say 20 mins per day....and there's say, a bit of blown air.We all know you can pay a fortune for thermocouple equipped el caps.....but most co's simply couldnt afford the lead time or cost....and certainly not every time there was an el cap obselesence/nil stock replacement.

author: jonpaul
author_rank: Super Contributor
timestamp: April 14, 2023, 11:00:58 am
content: FTTS, as always noise reply.<<<<We all know you can pay a fortune for thermocouple equipped el caps..... simply couldnt afford the lead time or cost... el cap obselesence/nil stock replacement.>>WHAT TRASH!"WE" do not "ALL KNOW" ....pay a fortune for thermocouple equipped el caps..You can DIY mod caps.... #30 guage type K teflon or kynar insulated TC wire.Not a "fortune" just knowledge and work.If you exceed the ripple current the caps performance is impaired even for 2x at 20 min.get a larger cap or better rated cap..expect to pay more and wait longer.j

author: TimNJ
author_rank: Super Contributor
timestamp: April 14, 2023, 03:07:02 pm
content: I like this document:https://jianghai-america.com/uploads/technology/JIANGHAI_Elcap_Lifetime_-_Estimation_AAL.pdfYour assertions, are as usual, crazy...but to your question, the stated ripple current rating is just one data point on a family of curves which is shown below. I believe each curve represents a constant capacitor core temperature.This particular family of curves is for a large snap-in variety (CD297BB). They show a usable range up to ~2.5x ripple current rating, given proper cooling. If I'm thinking about this correctly, for any capacitor series, at IA/IR = 1.0, the curves will obey the typical Arrhenius relationship. But, where these curves intersect the Y-axis (i.e. maximum IA/IR) may differ depending on thermal characteristics of the actual cap(??). So, for a large snap-in variety (20-40mm diameter), I assume the temperature differential between core and case could be more than a small radial type running at the same IA/IR. This curve seems to imply that, for the CD297BB series, if running at 2.5x IA/IR, the internal temperature rise will be something like 60C (at the center of the winding, relative to case/outside ambient).In fact, now that I read through the above Capxon document, they provide the below multipliers based on diameter. So, if 2.5x ripple current gives 60C internal rise with a 30mm cap, then with a 10mm cap, the same IA/IR probably only gives about 45C internal rise. Then, if you want to use it at 3x ripple current rating, I think you would just multiply 45C * (3.0/2.5)^2 = 65C rise. Just a seat of the pants estimate.In any case, you should probably not exceed the category temperature rating of the cap (105, 125C, etc.), as this rating may be material related.As for calculating actual life, think I would just use a duty cycle approach. The wearout is integrating. So, if you calculate 1000 hours life @ 3x IA/IR condition, but 100,000 hours life during idle, and the peak condition has a duty cycle of 10%, then probably the actual life is like 0.1 * 1000 + 0.9 * 100,000 = 90,100 hours...or something like that.

author: jonpaul
author_rank: Super Contributor
timestamp: April 14, 2023, 03:13:35 pm
content: TimNJ! WOW homerun, forgot the Arrhenius equationhttps://en.wikipedia.org/wiki/Arrhenius_equationAll chemical processes will double in rate for every 10 dec C increase in temp!Thus cap life is doubled at ambient 10 C below rated and halves for every 10 dec C above!Jon

author: Faringdon
author_rank: Super Contributor
timestamp: April 14, 2023, 08:04:53 pm
content: Thanks,The capxon documenthttp://www.capxongroup.com/files/Lifetime%20Compendium_EN.pdf..declares on page 16 that ripple currents above 2.8 times the rated ripple current are "not permissible"The capxon and Jianghai documents are detailed...but who knows if their literature applies to other caps?...and what equation did they use to calculate that above IA/IR = 2.8 the caps cannot be used?There is no equation for thatAnd certainly even those charts cant be relied upon, since they only relate to the caps called out in that app note.We are actually using two 8x16mm 400v 10uF caps and are running double the rated ripple current in them...Capacitor 400AX10MEF:https://www.rubycon.co.jp/wp-content/uploads/catalog-aluminum/AX.pdfRubycon dont supply the detailed docs that capxon do.....to be honest, you cant rely on buying capxon because they may go obselete or nilstocked when you go into production.

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: April 14, 2023, 09:42:11 pm
content: No substitute for a good old smoke test. Does it blow up, catch on fire, or emit noxious gases when you turn it on? If not, then you probably did something sort of right

author: Faringdon
author_rank: Super Contributor
timestamp: April 15, 2023, 05:01:45 pm
content: On page 9 of the capxon document....http://www.capxongroup.com/files/Lifetime%20Compendium_EN.pdf...it says that for electro caps <100V rated, you should never go above IA/IR = 1.0..They offer no equation through which they calculated the IA/IR = 1 restriction.And you would never know if this applies to any other (non capxon) electro cap which is <100v rated.What ultimately gets electro's is seal failure allowing the inside electrolyte out - temp cycling to extremes makes this worse (lower life)....The thing is, you would never know, what kind of seal any particular electrolytic capacitor on digikey may have , in comparison to any other radial leaded electrolytic capacitor.The charts and equations in the capxon document apply to those particular capacitors with their particular seals......goodness knows if they apply to other electro caps which might have very different seals etc.

author: jonpaul
author_rank: Super Contributor
timestamp: April 15, 2023, 05:57:13 pm
content: >>.The thing is, you would never know, what kind of seal any particular electrolytic capacitor>>>Wrong again, certain 105 and 135 C rated lytics specifically have a specials seal (epoxy?) that is much more reliable that the usual crimp seal.We have a bunch o these for electronic ballast usej

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2023, 06:27:26 pm
content: Quote from: Jim from Chicago on April 14, 2023, 09:42:11 pmISTR there was a poster here who designed SMPSs which experienced a high failure rate in the field. He kept on asking people to validate his somewhat strange assertions about why they shouldn't be failing.Treez was the poster's moniker.I'm sure there is no connection with this thread and it's OP.

author: Faringdon
author_rank: Super Contributor
timestamp: April 16, 2023, 01:08:57 am
content: QuoteThanks, you confirm our worst nightmares about seals in some electro caps.Which type of seal does this electro cap we wonder.400AX10MEF (10uF, 400V)https://www.rubycon.co.jp/wp-content/uploads/catalog-aluminum/AX.pdfThen you wonder which lifetime equations apply to which capacitors with which kind of seal?..As the following says.https://www.cde.com/resources/technical-papers/AEappGuide.pdf...if the seals are too tight in wet elec caps, then the hydrogen (produced during normal operation) cant escape and pressure builds up in the cap