How can I size my current carrying trace width for surge current? any standard or formula?
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\$\begingroup\$ It depends on what properties you’re interested in, resistance is fairly easy to work out based on cross section and length, and short-term heat-dumping capacity is based on the specific heat capacity of copper (assuming that there’s time for heat to dissipate through the track but not significantly into the PCB or air. \$\endgroup\$– FrogCommented May 22, 2023 at 10:36
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\$\begingroup\$ @Frog where can I find details of the method you mentioned? \$\endgroup\$– Arun Kumar ThawaitCommented May 22, 2023 at 10:44
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\$\begingroup\$ You can calculate trace resistance here allaboutcircuits.com/tools/trace-resistance-calculator the specific heat capacity of copper is 0.385 J/gC the density of copper is 8.96g/cm^3 So you can work out the instantaneous temperature rise (mass of copper/(0.385 * energy) which will be by far the worst case, as heat will dissipate quickly \$\endgroup\$– FrogCommented May 23, 2023 at 4:10
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2 Answers
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For a surge rating, you need to do an adiabatic calculation. This is where the heat generated in the copper simply raises its temperature, and has not yet had time to dissipate into the air or the board. This is also known as an I2t rating.
First choose a starting temperature, and a maximum temperature. The difference between them gives you the allowable temperature rise you can use to absorb the surge energy. The starting temperature might be your ambient, or your max operating temperature, or the elevated temperature remaining from your last surge. The maximum temperature might be the melting point (for fuse wire), or for PCB perhaps the delaminiation temperature, or the max temperature you might encounter when soldering. Or a lower temperature than those, depending on whether you are trying to meet any particular standards.
As you see, you need to start off with some assumptions.
Once you know the maximum temperature rise available to you, the rest is fairly easy. From the dimensions of the track, compute the volume, hence mass, hence heat capacity of the track copper in Joules/degree. From the dimensions, compute the resistance, hence energy dissipated with your surge duration and magnitude. Equate the two.
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A nice tool that can help with PCB design is Saturn PCB Toolkit. Besides providing the calculators has all the references to formulas as well.
