Must have flat, stable input voltage when testing SMPS - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: December 18, 2022, 01:06:33 pm
content: Hi,We were testing a 90W Boost SMPS, 5vin, 24vout, 350khz.Vin was unstable when we went near max load , going from 2v5 to 5v5, oscillatory.Do you agree, one of the "seventh commandments" when bringing up any SMPS, is that you must supply it from a source with such low output resistance, that the vin stays flat and stable, even if the SMPS goes unstable?Do you agree, that this is an absolute law?.....it must not be violated....you must supply an SMPS being brought up with a rock solid vin.........if vin oscillates wildly like ours, then you must stop....get a source with low enough output impedance, then start again?...then, when you have the SMPS running stable, from that solid vin, you can then look into what vin source the SMPS needs to maintain stability and not start getting trouble with Middlebrook's theorem.The saying goes, "they who mess with Middlebrook, gets it with both barrels".....would you agree?

author: Gyro
author_rank: Super Contributor
timestamp: December 18, 2022, 02:29:06 pm
content: Well if the input is swinging from 2.5V to 5.5V then the SMPS isn't receiving 5V is it?I don't think commandments, laws, or your other gobbledygook come into it.P.S. What's you input voltage tolerance spec? Or did nobody bother with one of those.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 18, 2022, 02:48:37 pm
content: Rock solid 5V only exists in your dreams, and it is capable of destroying the whole universe with its unlimited energy.In real world, switch mode power supplies are tested using lab power supplies, which are CC-CV supplies. Maybe your SUT (Shit Under Test) exceeded the current limit that was set, which made the input supply drop the voltage, which made your SUT trigger its UVLO, which made the whole thing oscillate?

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2022, 02:54:24 pm
content: And yet another poorly conceived/defined question from treez/ocset/faringdon.For readers that aren't as "experienced" with SMPSs as the OP, SMPSs are inherently feedback control systems. Control systems can be more or less stable depending on the circuits at the input and at the output.Engineers - whopredictcircuits' performance - will take that into account.Hackers - who onlymeasurecircuits without understanding - will hope and too often declare success/failure on the results of one not-very-good experiment.Classic control theory, known for the best part of a century, can be used to understand the topic.Memo to self: try one of the new breed of "AI" chatbots, to see if their questions are better than some of the posters on this forum.

author: pcprogrammer
author_rank: Super Contributor
timestamp: December 18, 2022, 03:36:19 pm
content: Another observation in the treez/ocset/faringdon fairy tail is the speed with which the projects he/they (Royal) do(es).It is we made this, we need to make such, etc. Even when I was on the top of my game it took its time to finish a single project. So how can it be that every so many days there is another treez/ocset/faringdon project with problems.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: December 18, 2022, 09:17:46 pm
content: Hello Faringdon,(hope I'm not writing to ***mpsmoker again)Just power it from a hefty battery and carry on with the Good Life. No need to worry about drops, just keep in mind to use wires as big as you can.HTH.

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2022, 09:40:05 pm
content: Quote from: pardo-bsso on December 18, 2022, 09:17:46 pmJust make sure there isn't so much inductance in the leads that the lag causes oscillation

author: Faringdon
author_rank: Super Contributor
timestamp: December 19, 2022, 05:30:07 am
content: QuoteThanks, nice one...i believe now, thanks to yourself and others above, it is in fact the CPX400D bench supplys feedback loop fighting with the feedback loop of the boost smps we are testing.

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2022, 06:55:27 am
content: Quote from: Faringdon on December 18, 2022, 01:06:33 pmIC used ?Link to datasheet ?Was the SMPS PCB laid out as specified in the datasheet using components of the specified type ?Local decoupling as specified ?So somany things contribute to a stable SMPS design.

author: Faringdon
author_rank: Super Contributor
timestamp: December 19, 2022, 08:27:56 pm
content: Thanks, it was LTC3786https://www.analog.com/en/products/ltc3786.html

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2022, 08:52:12 pm
content: Quote from: Faringdon on December 19, 2022, 08:27:56 pmYeah well did you even properly read the datasheet to see this below the 24V output schematic on P 26:*WHEN VIN < 8V, MAXIMUM LOAD CURRENT AVAILABLE IS REDUCEDhttps://www.analog.com/media/en/technical-documentation/data-sheets/ltc3786.pdfAlso note the specified components list directly below the schematic.If you are not to take note of the design engineers requirements you will find it difficult to obtain expected performance.

author: jonpaul
author_rank: Super Contributor
timestamp: December 20, 2022, 02:03:23 pm
content: just another FTTS, Farrington Treeze time sinkthose of us who are active on the forum are happy to assist others, but not to waste our ever dwindling time.FARRINGTON PLEASE CONSIDER OTHERS, JUST TAKE A HOLIDAY AND STOP YOUR ENDLESS POSTS.J

author: Siwastaja
author_rank: Super Contributor
timestamp: December 20, 2022, 02:27:52 pm
content: Quote from: jonpaul on December 20, 2022, 02:03:23 pmThere is this ignore list feature, or you can simply choose not read and not to post. If you find your blood pressure rising to the point of having to SHOUT, maybe think about why you are participating in the first place?

author: pcprogrammer
author_rank: Super Contributor
timestamp: December 20, 2022, 03:02:19 pm
content: Quote from: Siwastaja on December 20, 2022, 02:27:52 pmFor the entertainment of itQuoteIs this wrong on purpose? The used member name is Faringdon

author: Faringdon
author_rank: Super Contributor
timestamp: December 20, 2022, 06:34:09 pm
content: QuoteThanks, but thankfully in our schem this doesnt apply....in any case, thank the goodness fairys that there is no relationship between current sense pin threshold and Vin.In the schem they show there, its probz because of the rdson of the fets..we used two BSC027N04LS in direct pllel, so should be ok.

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2022, 08:17:06 pm
content: Quote from: Faringdon on December 20, 2022, 06:34:09 pmSo you deliberately departed from the recommended design in the datasheet and then wonder why you have problems. What else did you change in your experimental design ?It seems you have issues following clear design guidance in datasheets, why ?

author: tggzzz
author_rank: Super Contributor
timestamp: December 20, 2022, 08:33:49 pm
content: Quote from: tautech on December 20, 2022, 08:17:06 pmStrictly speaking there should be no problemifyou understand how the device works and how the surrounding devices work with the device. Then, as with any design, you'll have to test the hell out of it.However, there's a lot of scope for the Dunning-Kruger syndrome to rear its head. I make no comment about the OP in any of his incarnations.

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2022, 08:38:26 pm
content: Quote from: tggzzz on December 20, 2022, 08:33:49 pmWell obviously there is a problem, why did you even post ?Are you stalking me ?

author: tggzzz
author_rank: Super Contributor
timestamp: December 20, 2022, 09:17:21 pm
content: Quote from: tautech on December 20, 2022, 08:38:26 pmQue? ? ? ? ?My comment is in the relevant context, which you chose to snip, of your stating "So you deliberately departed from the recommended design in the datasheet and then wonder why you have problems. "There is a problem in this specific case, but in general it is normal to depart from what's in a datasheet.

author: dmills
author_rank: Super Contributor
timestamp: December 20, 2022, 09:55:06 pm
content: Quote from: Siwastaja on December 18, 2022, 02:48:37 pmHad that happen with a buck/boost battery charger design, so it only did it sometimes (When the battery was a bit flat but not totally flat), much hilarity ensued.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 21, 2022, 08:29:09 am
content: Quote from: dmills on December 20, 2022, 09:55:06 pmYeah, that's quite usual with SMPS because they have this "negative input resistance". When voltage drops, current rises, so dU/dI < 0. With any sort of current limit (active or just battery ESR) plus UVLO, the thing becomes a slow oscillator, frequency defined by the SMPS start time (mostly defined by the soft start feature).

author: BradC
author_rank: Super Contributor
timestamp: December 21, 2022, 08:45:41 am
content: I traditionally bring up and SMPS with a dim bulb tester. In this case it sounds like there’s already one seated at the workbench, so it should come up just fine.

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2022, 09:49:16 am
content: Quote from: BradC on December 21, 2022, 08:45:41 amCoffee hits keyboard, damn you

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: December 21, 2022, 10:48:56 am
content: Quote from: tggzzz on December 18, 2022, 09:40:05 pmBut Everybody Knows That...(also, given the history, it may help the smps ha..)

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2022, 01:43:59 pm
content: Quote from: pardo-bsso on December 21, 2022, 10:48:56 amI rather like sijwasta's observation about negative resistance

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: December 21, 2022, 02:40:36 pm
content: Quote from: tggzzz on December 21, 2022, 01:43:59 pmMe too. It's also more consistent with reality.

author: temperance
author_rank: Super Contributor
timestamp: December 21, 2022, 04:43:40 pm
content: The negative resistance, or constant power if you will is one of the reasons to use an appropriate under voltage lockout.For any converter it's important to decouple long lines. Or isn't that just proper decoupling in general which is rule number one in any circuit.Now try feeding a discontinuous mode switching PFC by means of an isolation transformer. You might be surprised with the result. A mistake myself long ago because i didn't think about the energy stored in the leakage inductance in the isolation transformer which is then part of the boost converter.

author: Faringdon
author_rank: Super Contributor
timestamp: December 23, 2022, 06:31:14 pm
content: Thanks thats interesting, AYK, eg downhole converters often are not allowed more than a 1uF of input capacitance, and have an enormous long line inductance.They use special converters that can handle this...typically current input controlled converters with vout clamps.

author: temperance
author_rank: Super Contributor
timestamp: December 24, 2022, 04:19:35 pm
content: I think tactic being used in those downhole converters must be a combination of series resistors and clamps in combination with a power limited converter (probably flyback).