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I need to design some traces or polygons for high current (15A) on a 4 layer Lithium Battery Management System board, but I am unsure about their sizing.

I am following a reference design from TI, from which the screenshots of the PCB below are taken. The board is a Lithium Battery Management Solution (https://www.ti.com/tool/TIDA-00982).
My question is general ,but I use this board as an example.

enter image description here

The red marked polygons go to a Lithium Battery positive terminal with 15A continuous current. The narrowest side of the polygons is 3.5mm on top and second inner layer, 2mm on the bottom layer (no connection on first inner layer).
I'm trying to understand how they calculated the sizes of those polygons.

I can order only 0.5 oz inner layers and 1 oz for external, so I probably need to increase the size of those polygons.

I thought: the polygon on the top layer is fixed due to other components, but I can increase the polygon on the second inner layer and bottom one (connected through multiple vias).
The problem is how much?

If you have two mirrored traces between the same points, on two different layers, the current will be halved. What if you have different polygon sizes and on different layers (externa/internal)?

Should I calculate like below?

  • current per polygon: 15A / 3 polygons = 5A per polygon
  • top layer polygon size (fixed): 3.3mm --> use Trace Width Calc with 5A, 40C temp rise, 1oz copper --> 1.2mm trace width for external layer-->top layer is ok
  • inner layer 2 polygon size: use Trace Width Calc with 5A, 40C temp rise-->6.2mm trace width for internal layer enter image description here
  • bottom layer polygon size: use Trace Width Calc with 5A, 40C temp rise, 1oz copper --> 1.2mm trace width for external layer.

Now that I have the width of the polygons, what about the length? Does the length not matter for heat dissipation? Is length decided by the requirement (in this board) “short low impedance connections for power connection”?

Thanks for your help

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2 Answers 2

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You used a tool for calculating track width for current. As far as i can judge, the tool is made to calculate how wide a track has to be for support a current of a certain amperage. This is for tracks that only transport current, think of it like a cable. Usually the large polygons are not only for the current transport itself, the also serve as a heat spreader to take heat away from the device.

You can use this Calculator to find out appropriate values for your parts

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  • \$\begingroup\$ Thanks sgt_johnny and M lab for your help, but I still don't understand the amount of current flowing through each polygon: if there are 3 polygons of different sizes, does 1/3 of the current flow through each of them (assuming enough vias are placed)? Is the logic used in my calculation corrrect? I used 4pcb.com/trace-width-calculator.html (IPC-2221 ) in my question as tool. \$\endgroup\$
    – haku15
    Commented Jun 15, 2020 at 23:20
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There are 2-factor of this.

  1. Trace width that calculate by current ambient temp. max pcb temp and heat transfer. you can use online calculator for that
  2. Via current. size and number of via will increase polygon size.

Additional size that current not passing through will help to dissipate heat just like heat sink. (maybe for via in your case)

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