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PCB trace "style" between AC (mains) and DC circuits

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iroc86:
Circuits designed for AC operation at mains voltages often have a different "style" of PCB trace compared to low-voltage DC circuits. See the photos below. I see this appearance in chargers, power supplies, inverters, etc. The traces are usually chunky and cover a lot of board area, almost like a fill. I've encountered this style enough times to presume that it's not just engineers' preference.

I doubt the reason is for current handling, as many of these supplies operate at relatively low output (<1 A). I figure it might have something to do with minimizing temperature rise across the tracks. Or, perhaps the intent is to keep as much copper on the board as possible for etching and mass production, since there isn't a DC ground plane to fill in the voids. Any ideas?






SeanB:
Probably there to both minimise etching time, and to ensure that, even with a very poor process control ( likely if the PCB is being made by a lot of labour, and not in an automated controlled bath) they will have enough trace left to both solder to, and also a wide enough trace so that shipping vibration will not peel them off the board. All 3 look like they are made with SRBP, and thus made cheaply, so likely in a building without any temperature control, other than a fan for the workers in hot weather, and a coat in winter. Massive variation in etch time, and they probably etch till the outer board is fully clear, then give it a few minutes more in the tank with the bubbler on, to get the inner ones, thus overetching the outer ones. Then wash, clean with some scouring agent ( could be a pad, river sand or even silty mud, you never know), rinse, dry and screen on the solder mask, and then either they have an old NC drill to do the holes, or there is a drill press, a stack of boards, and a mask, and somebody doing it by eye, so you want a good amount of pad around the traces in case the drill is a little skew.

Gyro:
The primary side is all about maximising track spacing (creepage and clearance), while the secondary side is about providing as much copper as possible to minimise the loop area between secondary windings, rectifiers and capacitors. Large copper area also assist with heat dissipation for the secondary side rectifiers (and capacitors, as they tend to be more stressed in SMPSs).

I mentioned loop area, it is critical that the current loop around the transformer primary, switching device, reservoir capacitor and snubber is kept as small as possible (consistent with maintaining track spacing) to minimise EMC emissions.

Zero999:
Talking of creepage, the first photograph doesn't have sufficient creepage distances. This isn't a problem if the DC side is treated with the same precautions, at the mains, otherwise it's a no no and shouldn't be used.

Gyro:
Agreed, that first one is awful, no consideration of safety clearances whatsoever, typical 'no clue' cheapo design. I can't even identify the primary-secondary demarcation. Bin it!

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