0805 cap across rectified mains? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: May 08, 2023, 11:10:01 am
content: Hi,We have an 0805 cap (1kV, 4n7, X7R) pushed in tight to the FET switching current loop of a LNK3209 HV buck converter......its downstream of the fuse, and downstream of a filter inductor (as well as downstream of the half wave rectifier) .......so its not across line and neutral so to speak.So will this be OK?The distance between its pads is 1mm.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: May 08, 2023, 12:29:45 pm
content: Components either directly, or indirectly, connected to the mains, live in a pretty hostile environment. You will be responsible for failures that may occur in the future.I’ve had to modify many failed mains input components from well regarded equipment manufacturers that either shorted and blew a fuse, or in a few cases let the smoke out! Some even failed on PAT testing…Mains input capacitors have to be class X or Y rated, I’m not sure how far “downstream” that requirement applies. Refer to IEC 60384 (or later version). This paper by TDK might also be useful, it suggests rating capacitors well in excess of 1KV to allow for surges in the mains due to switching transients, lightning strikes, etc.https://www.tdk-electronics.tdk.com/download/530772/73cd72e2e0c87913fbca16a2c9b597bb/pdf-emigeneralstandard.pdf

author: David Aurora
author_rank: Frequent Contributor
timestamp: May 08, 2023, 12:57:33 pm
content: Fuck me, how do I still open these posts without checking who posted them?

author: tszaboo
author_rank: Super Contributor
timestamp: May 08, 2023, 01:09:04 pm
content: Quote from: David Aurora on May 08, 2023, 12:57:33 pmI don't know, but if he doesn't have the two dozen new thread every week it's been a bad week.For sure we should charge a consulting fee for this already.

author: jmelson
author_rank: Super Contributor
timestamp: May 09, 2023, 03:23:57 pm
content: 1mm gap with 600+ V DC across it? You gotta be kidding! With the slightest bit of residual solder flux it will flash over right away, and when there is a mains transient, it will flash over for sure. I think you HAVE to go with a much larger cap body.Jon

author: floobydust
author_rank: Super Contributor
timestamp: May 09, 2023, 05:32:46 pm
content: I stepped into this turd here in the other threadClearance between conductors in primary side of offline SMPS?The safety standards are 2mm for 400VDC (no transients) in PD2 on FR-4. Did not get an answer for my question:Quote from: floobydust on May 09, 2023, 02:59:00 amOddly enough, I got a chinese PSU to repair from the bowels of guangdong where the 340V DC bus ceramic cap shorted. Hint: it was bigger than 0805I'll look if I have pictures.

author: coppercone2
author_rank: Super Contributor
timestamp: May 11, 2023, 09:13:39 pm
content: Quote from: David Aurora on May 08, 2023, 12:57:33 pmyep caught myself 3 posts downI was like who can it be. oh I know

author: Faringdon
author_rank: Super Contributor
timestamp: May 13, 2023, 09:38:02 am
content: QuoteThanks, as discussed, this 0805 cap is in parallel with two 22uF, 400V electrolytics......so if voltage goes >450V, those lytics are dead anyway.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 13, 2023, 09:53:16 am
content: Better use 0402, 16V ones are rated enough for 400V, they just lie

author: Faringdon
author_rank: Super Contributor
timestamp: May 13, 2023, 11:38:33 am
content: 0805 caps rated for 1kV are freely available...so what are they used for?In your opinion, what applications are the 1kV 0805 caps allowable to be used for?

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: May 13, 2023, 04:10:37 pm
content: See this link for design guidelines. It’s pretty straightforward but nowhere are they trying to miniaturise by using 0805 or similar small capacitors.https://www.vicorpower.com/documents/applications_manual/08filter_autoranging.pdf

author: Faringdon
author_rank: Super Contributor
timestamp: May 13, 2023, 04:57:09 pm
content: Thanks, with that FARM., they dont spec what small value caps they would use.....0805 is a good value for a ceram cap, since 1206 and above are contenders for cracking damage when board is flexed/ dropped in production / thrown into packing after being soldered up.Also of note....most low power offline flyback smps, often have a drain voltage which goes up to some 700V....so the diode in the RCD clamp has 700V across it......and they are usually SMA size.....and the recomended footprint for SMA has just 1.5mm between the pads....not 2mm...SMA footprinthttps://www.diodes.com/assets/Package-Files/SMA.pdf

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 13, 2023, 05:27:16 pm
content: Quote from: Faringdon on May 13, 2023, 11:38:33 amYou'd at least want conformal coating or potting for that distance at that kind of voltage- creepage in air would be marginal at best

author: Faringdon
author_rank: Super Contributor
timestamp: May 13, 2023, 05:58:14 pm
content: QuoteThanks,And may i ask what are the thoughts on the case in hand here?....whereby the 0805 cap is across two 22uF, 400V lytic caps.(In other words, if voltage goes above 450V, the lytics are dead anyway...and it doesnt matter if the 0805 gap flashes over..)

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 13, 2023, 06:02:20 pm
content: Quote from: Faringdon on May 13, 2023, 05:58:14 pmI'd probably use a 1206 or two 0805s in series to improve creepage distance

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 13, 2023, 06:28:45 pm
content: Enough is enough.There're X and Y rating capacitors exactly for this, firefighters have work enough already.

author: temperance
author_rank: Super Contributor
timestamp: May 13, 2023, 09:28:55 pm
content: Anyhow: IPC2221, not expensive and a time saver.I added Faringdon to my ignore list. But I'm still seeing his threads. Is there something I can do about that?

author: tooki
author_rank: Super Contributor
timestamp: May 17, 2023, 05:02:43 pm
content: Quote from: Faringdon on May 13, 2023, 11:38:33 amIn a board I assembled at work, it’s the bypass cap for the high voltage supply of silicon detector devices:https://sengerm.github.io/Chubut_2/doc/testing/index.htmlJust this week I finished converting that board from KiCad to Altium in order to get it really, really perfect for external assembly. (We don’t use KiCad normally and didn’t want to risk this as a guinea pig for pick and place with KiCad.)

author: Faringdon
author_rank: Super Contributor
timestamp: May 17, 2023, 06:51:48 pm
content: Hi,In an enclosed PSU, with pollution degree 1, two bare copper pads, separated by 0.1mm, with solder resist coated FR4 between them , will not even flash over if the voltage between those pads is 1 Mega Volt.EDIT>>>>>...Thanks to Floobydust below, i meant 1 Kilo Volt <<<<END OF EDITIf the voltage between the pads of that 0805 went up to 550V.....then the 0805 cap's own pads wouldnt flash over...but the 450V electrolytics that are across that 550v...would smoke and blow up.

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2023, 09:31:09 pm
content: Quote from: Faringdon on May 17, 2023, 06:51:48 pmDid you mean Maga Volt?

author: Faringdon
author_rank: Super Contributor
timestamp: May 18, 2023, 07:54:01 am
content: Thanks...woops, now edited...to 1kV.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2023, 08:12:46 am
content: Quote from: tszaboo on May 08, 2023, 01:09:04 pmThe other old traditional technique is to give answers than are superficially plausible but have amusing and obvious flaws. Effectively an intellectual April Fool's joke.But that won't work with treez/ocset/faringdon since hisquestionsand responses fulfill that criteria

author: ebastler
author_rank: Super Contributor
timestamp: May 18, 2023, 06:42:32 pm
content: Quote from: Faringdon on May 13, 2023, 05:58:14 pmJust keep asking until you find someone who tells you it's ok.

author: tggzzz
author_rank: Super Contributor
timestamp: May 19, 2023, 06:10:51 am
content: Quote from: ebastler on May 18, 2023, 06:42:32 pmNot a bad idea. If aliases were allowed on this site, I'd be tempted to create one solely for that purpose. It might even raise the SNR

author: Faringdon
author_rank: Super Contributor
timestamp: May 19, 2023, 05:34:57 pm
content: QuoteThanks, what would be great , would be someone to proove the statement wrong....that an 0805 1kv cap, across two 450V rated 'lytics is bad....Pollution degree 1, and FR4 with solder resist, its going to take >10kV to flash over between 0805 pads 1mm apart.

author: Gyro
author_rank: Super Contributor
timestamp: May 19, 2023, 06:03:07 pm
content: Why don't you putyourneck on the line? If you're so confident, then why even ask here? A professional design engineer doesn't need the approval or otherwise of the members of an electronics forum - I never did when I was at the workface.Take an proper engineering decision and be prepared to defend it - in court if necessary as it is safety related. The buck stops with you andonlyyou!

author: tggzzz
author_rank: Super Contributor
timestamp: May 19, 2023, 07:25:58 pm
content: Quote from: Gyro on May 19, 2023, 06:03:07 pmExactly.Group responsibility and democratic decisions are not relevant in such cases.Understanding and personal responsibility are what matters.But then quite a few people wonder whether the OP's questions are realistic or aspirational.

author: ebastler
author_rank: Super Contributor
timestamp: May 19, 2023, 07:43:36 pm
content: Quote from: tggzzz on May 19, 2023, 07:25:58 pmWell, but how about "Some anonymous guy on the internet told me"? Does that count?

author: temperance
author_rank: Super Contributor
timestamp: May 19, 2023, 07:47:50 pm
content: I looked at what's available in 0805 1KV. The largest one I could find is 4.7nF. The remaining capacitance at 320V is probably less than 60% of that and more like 70%.Placing a 4n7 - 70% in front of the switcher you mentioned isn't doing anything except creating high Q circuits which resonate with every bit of stray inductance around.There are two ways to solve an EMC problem: the wrong way is trying to stop things from radiating caused by ringing by adding more low impedance energy storing devices. In short, this question can be classified as nonsense.Vive la resistance. But don't overdo it.

author: tggzzz
author_rank: Super Contributor
timestamp: May 19, 2023, 08:40:51 pm
content: Quote from: ebastler on May 19, 2023, 07:43:36 pmYes, it does count.Not much for most of us, but apparently it satisfies treez/faringdon.

author: floobydust
author_rank: Super Contributor
timestamp: May 19, 2023, 10:14:28 pm
content: The mind will believe whatever it wants, and the Internet always has enough noise to find supporting baloney.He's bungling science and engineering.Paschen's lawgives the voltage values but one skilled in the art would know the safety standard's generous values for creepage and clearance are based on _____ .

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2023, 05:08:14 am
content: Quote from: floobydust on May 19, 2023, 10:14:28 pmThe OP has, apparently, convinced some people that he is skilled in the art.

author: Faringdon
author_rank: Super Contributor
timestamp: May 20, 2023, 04:37:04 pm
content: QuoteThanks, thats an intereting statement.So placing some small ceramic cap "in tight" to the power switching loop. so as to reduce the area of the power switching current loop...is a bad thing?Its very common to do it...often you see it in the form of an SMD ceramic in parallel to the leads of a radial electrolytic cap......again, its there to reduce the current loop area.But yes , i agree, there is going to be resonance with the stray L.So , this would raise interesting questions...1....in 4n7 is too low a value, what value would be OK?2....Is the concept of putting a small capacitor "In tight" to the power switching current loop of an SMPS, always a lame duck?.....obviously, being a small package, means it can advantageously be squeezed in nice and tight to the power switching current loop....thereby reducing the loop area.

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2023, 04:46:44 pm
content: Quote from: Faringdon on May 20, 2023, 04:37:04 pmYou can choose the capacitor based on what you want the resonant frequency to be, as per the standard equation.And as per the standard equation you can vary the amount of resonance by changing the resistance.Where did you say you got your degree?

author: Faringdon
author_rank: Super Contributor
timestamp: May 20, 2023, 08:01:04 pm
content: Thanks, as you know, RC circuits just upstream of power switch loops are not common.Neither is changing "inner loop" capacitor values common when trying to pass radiated EMC.Is there a suggestion that removing a small ceramic from just next to the power switch loop, would help pass radiated EMC?The loop in which this LC ringing would take place is likely to include the bus electrolytics and their ESR is likely to give the necessary damping to the ringing.This is worth another new post, a lot of people will be interested in this.

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2023, 09:44:24 pm
content: Quote from: Faringdon on May 20, 2023, 08:01:04 pm"... is likely to ..." is amateurish drivel.Look at the datasheet to find the ESR.Measure the ESR capacitance (including as a function of voltage and frequency).Then calculate using the bog-standard equations that I came across when I was in school (i.e. pre-university).Then simulate.Then measure the system to see how it compares with your understanding.Finally refine your understanding.That's howengineeringis done. Anything less is cargo-cult random meanderings.You do have a degree in a relevant subject, don't you?

author: Faringdon
author_rank: Super Contributor
timestamp: May 22, 2023, 06:09:54 pm
content: Thanks, though ayk, in this case, it is believed, that no calculation is necessary...a quick inspection of the situation tells us that no radiated EMC scan, or conducted EMC scan, for an SMPS, could be improved, by removing a low value ceramic capacitor that is placed up tight to the power switching loop.