Offline isolated flyback track/pad clearances on PCB? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: August 09, 2023, 07:09:10 pm
content: Hi,Would you agree, if doing an offline isolated flyback of 90-265vac, then the minimum number of net class's that you can get away with , when laying out the PCB, is ten?The net class's are necessary, ayk, so that you can get the correct clearancing between the various nets. Some nets will obviously be in 2 or more of the net class's.(If you like , Take a 84/19 transformer, 20W and 24vout and 65khz, or therabouts.)

author: Gyro
author_rank: Super Contributor
timestamp: August 09, 2023, 08:02:49 pm
content: So one day you troll us again (the Cheap Chinese EV thread) and a couple of days later you're back again asking for technical help on something you should already know to do your job, according to your career history?I don't think that's the way it works.

author: ebastler
author_rank: Super Contributor
timestamp: August 09, 2023, 08:27:38 pm
content: Quote from: Faringdon on August 09, 2023, 07:09:10 pmWhere did you pull the number "ten" from?And what EDA system do you use which lets you assign a net to multiple net classes?Edit: Ah, seems that Altium lets you do that, since it uses net classes for other grouping beyond physical attributes of the trace. What happnes if the net classes defne conflicting physical properties? Largest trace width, clearance etc. wins?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2023, 08:38:38 pm
content: Shrug, I mean I have a design running currently with 16 clearance rules, which only roughly captures the creepage and clearance required in the design spec.As you know, Altium doesn't understand creepage so that has to be handled by manual inspection (at this stage, anyway), and several other clearances I've handled by inspection as well.Driving those rules, I have six net classes, and somewhat unrelated, three pad classes. (Plenty of component classes, but I never use those anyway.)Tim

author: tom66
author_rank: Super Contributor
timestamp: August 09, 2023, 09:14:10 pm
content: Where does the requirement for using any number of net classes come from? There's no requirement to use net classes in Altium, you can set traces to any width you want and you can use polys for large power planes... so the minimum number is zero and the maximum number could be 50, if you wanted to be really anal about your power track widths, for instance.

author: PlainName
author_rank: Super Contributor
timestamp: August 09, 2023, 09:58:30 pm
content: QuoteThe classes would be used to specify the clearance (and probably width, but that's not the important factor here, I think).

author: tom66
author_rank: Super Contributor
timestamp: August 09, 2023, 10:26:22 pm
content: Quote from: PlainName on August 09, 2023, 09:58:30 pmPoint is, you could do the clearances manually. There's no requirement to set that up, it's just going to make your life so much easier if you do.

author: Faringdon
author_rank: Super Contributor
timestamp: August 11, 2023, 07:23:52 pm
content: QuoteThanks, the attached diagram.I am sure all would agree that at least these net class's (12 shown here) are absolutely needed in order to properly get the right clearancing/creepaging between the various nets.Without the shown class's, the job just can't be done properly.....your layout will either be too big, or you will violate clearance rules.Though good luck in finding what EN62368 or IPC2221 says about these clearances, because none of the regulations define all the spacing between all the shown nets here.So we are on our own.....no guidance.The only guidance you have is that clearance/creepage from Line to Neutral, upstream of the fuse, should be at least 2.5mm.Anything else...take a guess.....though if its not the right guess, then some lawyer could call in a total product recall for you, since its open to interpretation.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 18, 2023, 12:53:56 pm
content: It would be pretty wild if Intertek/TUV/UL asked how many constraint classes you used in designing the PCB.Quote from: Faringdon on August 11, 2023, 07:23:52 pmSafety standards don't exist to do your designs for you.QuoteTypically I think one would have their design reviewed and certified by a notified body before selling or marketing a device, making this unlikely.