High current pcb options  - Page 1
Forum section: electronics / general

author: MadScientist
author_rank: Frequent Contributor
timestamp: December 20, 2020, 12:09:46 am
content: I’m looking to build some Allegromicro Hall effect current sensors typically 50 or 100A , for simplicity and cost I’m using the standard Chinese services.These are 1oz copperClearly it’s not practical to use a pure pcb trace at this copper weight.I’m wondering what options I have for small volume of units ( 10)Thinking of soldering on solid copperAny suggestions or optionsDave

author: bob91343
author_rank: Super Contributor
timestamp: December 20, 2020, 12:27:47 am
content: It's not too clear what you are asking and what you are doing. However, in my experience, high current through a PCB trace is asking for trouble. At those currents you probably should use bus bar. The size depends on your allowable voltage drop, considering the length.The most important parts are the interfaces between the various high current parts. When you do stuff like this it often pays to draw a current distribution path, a la beginning dc circuits. You know, equal squares and such.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2020, 03:28:57 am
content: Okay but 2oz copper is only another ten bucks? The sensors already cost as much, I know this yearsuckedbut that just sounds like poor planning..?Anyway -- sure, you can always solder heavy wire onto the pins. I mean, after all, who needs PCBs, back in the day you might've just stuffed everything into a perfboard and wired component leads together by hand. Tedious (not to mention an EMI nightmare, unless you happened to have the luxurious ground-plane perfboard, so nice!), but hey it works. Well, doing up everything else on a PCB sure makes routing easier, then you only have to beef up the current paths.If you have some copper foil/tape/sheet, you can definitely stick that down and solder it in. I'm rather fond of making prototypes carving up copper-clad board into (simplified) pads and traces, and just soldering on whatever I need. High current traces, just make them very wide, and if that's still not enough, slap down some thick wires or sheets. Same for thermal dissipation, can get lots of watts out of SMTs, done right.Tim

author: bob91343
author_rank: Super Contributor
timestamp: December 20, 2020, 06:46:06 am
content: It won't work. With so much current, you have to carefully direct flow so as to avoid hot spots. Once you have a hot spot you have disaster.I have worked with PC traces carrying only around 1 Ampere and it was terrible. Hot spots caused overheating, spontaneous desoldering, and finally smoke.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2020, 08:45:27 am
content: Quote from: bob91343 on December 20, 2020, 06:46:06 amCan't tell if trolling or satire.Tim

author: xavier60
author_rank: Super Contributor
timestamp: December 20, 2020, 12:29:23 pm
content: I'm doing an induction motor inverter for 20 amps on 2oz PCB using wide traces on both layers where possible and thick single core copper wire where I can't make the traces wide enough.I found this useful.https://www.trance-cat.com/electrical-circuit-calculators/en/pcb-trace-resistance-calculator.php

author: jmelson
author_rank: Super Contributor
timestamp: December 20, 2020, 05:51:18 pm
content: Quote from: xavier60 on December 20, 2020, 12:29:23 pmRight, I make servo amps that are rated up to 20 A peak. I use 2 Oz copper and 1/4" wide traces, and have NEVER had any issue with that.Now, 100 A is a different story.Jon

author: xavier60
author_rank: Super Contributor
timestamp: December 20, 2020, 07:17:07 pm
content: Quote from: jmelson on December 20, 2020, 05:51:18 pmThinking about it, going from 20 amps to 100 amps, the trace resistance would have to be decreased by a factor of 25 to maintain the same I² R heating. So going from 2oz to 13oz, the traces would need to be about an 1" wide if I have done the sums right.EDIT: Larger traces would dissipate more heat, so there would be no need to decrease the resistance by the whole factor of 25.

author: MadScientist
author_rank: Frequent Contributor
timestamp: December 22, 2020, 10:54:21 pm
content: ThanksI think I’ll go with 10mm wide 2.5mm think brass busbars and solder the leads of the allegro micro directly to the brass busbars , I’ll use a pcb to anchor the busbars and tap the busbar for M8 bolts , my only concern would be the resistance of the solder joint from the IC leads to the bus bar

author: Siwastaja
author_rank: Super Contributor
timestamp: December 23, 2020, 07:33:49 am
content: Again, don't just try to make traces "wide". Try to make them SHORT!A puny 10mm wide track on 1oz copper carries 100A fine, given it's also just 10mm long. The resistance is 0.5mOhm and power dissipated 5W. OK, that's quite a lot of dissipation so at 100A, you'd really like to go for 2oz but that's easily ordered and doesn't cost your arm and leg.The problems start when you lay out connectors "conveniently" on one side of the board, then MOSFETs on the opposite side, and so on. Now you have a 100mm distance to bridge, and your 10mm wide track needs to become 100mm wide to keep the same 0.5mOhm resistance!Due to the reality of components, you always have necks in traces, say, for example, the source leg of a D2PAK MOSFET is some 1mm wide, and you can only fit a track some 5mm wide from there. So what to do? Gradually increase the track width to finally make some monster 100mm wide track? Or, maybe just put all your effort trying to place the next part really close so that the whole track is only that millimeter long "neck" part.With optimal placement, I think you are good to some 60-70A with 1oz copper and to some 100A with 2oz copper without special tricks.Great thing about trying to minimize distances: even when you don't 100% succeed in that, at least the number of problematic long traces is down, so you have less work to do. What to do to fix them? First, expose openings to solder mask and paste mask. If you reflow the boards anyway, you get some "free" solder. Expect some 20-30% reduction in resistance. Not enough? Expose a large number of small paste mask openings along the track, cut some sheet copper (up to 0.3mm goes easily with kitchen scissors) to the shape of trace, reflow. You can make U shape bend to the copper sheet, increasing both total cross-sectional area and double as a heatsink. But this is all manual work, not really good for production.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2020, 08:12:23 am
content: Another remark on ampacity: as with facility wiring, PCB trace width tables are calculated assuming pessimistic conditions (low temp rise, in still air), and long lengths (so that heat dissipation through ends of the traces is negligible).More food for thought: note that inner planes increase ampacity significantly. The heat-spreading value of the inner plane allows a single trace (if maybe not a whole bunch of them together) to carry, say, twice the current or thereabouts, for the same peak temp rise. Check which standard you're using: IPC-2221 is an old and unrepresentative method, IPC-2152 is at least representative of typical layouts on contemporary fabs and materials.Anyway, neither of these need be true here -- a very short trace has lots of heatsinking "meat" on it (pads/pins), so that heat flow might be lengthwise (parallel to current flow), more than it is cross-wise (laterally, into the laminate -- as dominates the heat flow for thin traces -- or thru-plane, into inner plane layers, or dissipated from the surface).Also, consider jumpers -- besides doing it by hand, there's these things for example,https://www.vishay.com/docs/30192/wsl-9.pdfhttps://www.digikey.com/en/products/detail/harwin-inc/S1621-46/3924771which if placed directly beside critical component pads, well there you go, who needs resistance? Basically a solder fillet (or not, depending on how close you want to place the pads, and whether it's automated assembly and reflow soldering, or wave, or by hand) between solid copper and solid copper!Also may be relevant, this ongoing thread is doing similar things:https://www.eevblog.com/forum/projects/questions-on-designing-fwdrev-12v-dc-motor-controller-for-80a-typ-(200a-peak)/Tim

author: MadScientist
author_rank: Frequent Contributor
timestamp: December 24, 2020, 08:26:30 pm
content: Thanks guys