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I seem to need to pass more current through a PCB than is possible?

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David_

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Hello.

I am developing a circuit to pass a maximum of 40A but when I use a **broken link removed** I find that even if I where to allow the temperature of the trace to rise to 100°C then I cannot pass such currents.

But that calculator has to be defective since if I input a 10mm trace width, 1 oz trace thickness, 60°C temperature rise, 25°C ambient temperature and 30mm trace length it says that the maximum current is 31,8A and a temperature of 85°C.
But if I raise the temperature rise allowed to 75°C(instead of 60°C) then the results become 27,9A @ 100°C...!


That is simply impossible isn't it, assuming all other parameters to be the same how could allowing a higher temperature result in a lower current?

Although it is true that in my application the current isn't continuous but I have had this situation before and I don't know what to do.
I can't get 2 oz copper boards, so I ether have to use vias to connect both top and bottom layer(this is a 4 layer design) in parallel or use layer 1 and 2(top and the one under it) or cover the top layer trace with solder, or put a thick solid copper wire of 2mm diameter on top of the layer and then solder it to the layer.

Non of which is options I thought that I would need to use, but a 10mm wide trace is already almost unviable to fit onto the PCB because the pads aren't made to connect to such wide traces.

What would you do?

Keep in mind that one of these traces to be carrying max 40A is the switch node of a non-isolated DC-DC converter, and although I don't now I think it sounds as one of those big layout mistakes to route the switching node to both sides of a PCB?
30mm trace length is longer than it will be but according to that calculator the trace length doesn't impact the current carrying capacity.
But the traces to be carrying that current is the ones from the battery connections all the way to the output wich shouldn't be much more than a low number of cm, 10cm worst case.

Though I do know that this calculator as well as all other such calculators are only rough estimations and depending on which standard they use the take different things into account and the results of calculators using the older standard is supposedly much more inaccurate than ones using the newest.

Any advice would be greatly appreciated.

Regards
 

Hi,

No idea. Makes sense: lower current permits higher temp, and vice versa. Maybe the calculator takes into account glued on copper layer separating from board at that current and temp., maybe it factors in when pcb may burn or set on fire, presumably it'll incorporate some safety standard which sets limitations on what you can safely do on x type of pcb, like minimum distance between AC mains and DC and so on.

Probably not for 40A, but most of the traces for high dissipation I've noticed show a puzzlingly narrow trace from the IC pin(s) which opens out into a much wider trace.

Have you soldered wires from copper plane to copper plane? It's messy, and unreliable, and a pita to get the wires to stick. With your mega taser vaporiser circuit there, and in case the solder may melt at hot points, I suggest using screw/round terminals, nuts, bolts and spring washers.
 

Having the switching node on top and bottom isn't necessarily a problem but it depends on what's on your inner two layers. The biggest source of C which is the thing to avoid on this node is plane-to-plane capacitance. So it would be ideal for your switching node to occupy all 4 layers and thus not overlap with any other traces. This node should just come from the switch device to an inductor correct? Make that as short as possible.

It's reasonable to have a wide trace that necks down to get into a part. The narrower neck will generate a little heat but it can spread to the wider part of the trace.

Other options include copper busbars that can solder to the board or "power-bugs" which could mate with a wire properly.

Are you sure you don't have other PCB options like more layers? Typically outer layers can be plated to additional thickness.
 

Having worked in a company which manufactured very high current PSUs, a copper bussbar is the way to go.
Unfortunately, these bussbars are custom made.

For personal projects, what I've done is to lay out the high current carrying trace on the bottom of the board without a soldermask. Then I solder a copper foil on top of it.
Not pretty, but it works fine.
 

I would suggest to go with Bussbar .If Bussbar is not possible then design the PCB in 2 PCB and split the current path .We did the same approach in one of our design.
 

I can't say for sure yet as I have't gotten far with the board layout, but since the hole current path will need to carry the current, using all 4 traces isn't going to work, assumably sInce that would probably compromise the other signal traces need or make them very problematic to route since the power path would be in the way.

But I think I will route the power trace on the top or bottom and leave and remove the solder mask in order to enable some kind of additional copper to be added.

I think it sounds hard to solder but I like that copper foil idea, maybe I should look into how one applies solder mask at home, or simply some other viable paint. So that I can cover the added copper and maybe make it look nicer, but the main purpose would be to make it safer by not being conductive.

Whatever I end up reinforcing the high power trace with it would be good if it could be covered.
 

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