SMC diodes in Three phase, Three diode, Mains rectifier bridge - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 22, 2024, 01:43:34 pm
content: Hi,Standards disallow use of SMC diodes in 240VAC Three-phase, Three-diode, Mains Rectifier bridge as the creepage/clearance from anode to cathode pad would not beenough for adequate clearance/creepage.So why have we opened a customer SMPS and find SMC diodes?Is there a clause in the regs? (EU/Aus/UK/US)

author: floobydust
author_rank: Super Contributor
timestamp: July 22, 2024, 05:30:37 pm
content: Are you not learning creepage and clearance and safety standards?It's annoying youcross-postand never reach any decision or answer on your questions, for years now.Literally spamming the Internet forums with questions on spacings and it looks like some inability to understand or learn the Art.https://www.edaboard.com/threads/offline-smps-isolation-creepage-and-clearance.408281/https://www.edaboard.com/threads/clearance-creepage-in-offline-isolated-flyback-smps.408020/https://www.edaboard.com/threads/offline-smps-clearance-from-earth-net.407787/https://www.edaboard.com/threads/clearance-from-pcb-to-earthed-enclosure.407961/https://www.edaboard.com/threads/what-clearance-distance-do-you-need-for-1500vdc-isolation.404886/I could tell you why the SMC can be acceptable, but you wouldn't understand and it's the old "yes but" you argue against and then vanish. It's all so tiring.

author: Faringdon
author_rank: Super Contributor
timestamp: July 23, 2024, 11:31:24 am
content: Thanks, as you know, the phases can be 746V apart.But this is all on the non-isolated "hot" side together, not on the "safe" side.These volt difference things come after the mains fuse and after the mains breaker.Flashover for 3kV is a distance of ~1mm or so in air. (humidity and dust to consider etc)So 0.33mm for 1kV.So realistically flashover isnt a danger for nets 746v apart and 1mm apart. -And on a dry PCB.Thanks, there are indeed many threads on this, many are mine...as such i am in the process of making a catalog of all these threads.There are often non concensus's among the kind respondants,,,particularly with regard to high voltage flash testing, and whether its to simulate an "accelerated life test"...or to just expose it to a high voltage which, by wayof a mains transient, it might get exposed to anyway.......it kind of makes one wonder though...supposing you have a 220uF cap after a mains rect bridge...how is any mains transient going to raise the voltage across thatcap more than a few 10's of volts.?There is also much difference about distance to earth enclosures from line conductors...because earth is the safety conductor anyway...ok, it may not get connected...but surely the earth conductivity test would find that out.I have been places where they track earth from primary side, where its 0.5mm from line conductor...then they track it round to isolated side, where its 0.3mm from other isolated tracks...and they ring up some standard name to you andsay its fine.

author: floobydust
author_rank: Super Contributor
timestamp: July 23, 2024, 06:55:11 pm
content: You had another thread-fest about TO-220 lead spacings, I think the Fullpack or maybe D2PAK as well. Did you conclude something?There's over a dozen details to know before venturing into the safety standards requirements for spacings. It seems you get stuck in a loop circling around things but are not considering all of them, or enough to sort it out.SMC diode footprint pads are4.4to4.7mmspacing. In some circumstances I'll make special footprints with increased pad spacing, that can help too.Mains has overvoltage transients present which commands increased spacing. Conformal coating and potting can reduce spacing requirements.Use of a surge-protector or MOV module on the input will also reduce the spacing requirements. It's likely what they did.The old breakdown number for air 3kV/mm from Paschen's Law depends on electrode config and shape, humidity, altitude and surely more.It doesn't include ozone buildup. So I wouldn't use it even as a ballpark in product design like you do, with your arbitrary div/10.