Thank you so much for taking a look at this switch and posting your findings!
A few (very technical) thoughts…
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The continuous current rating of 169A in the datasheet is a theoretical number and can’t be used here. Everyone uses the front page current rating (it’s incredibly convenient) but it’s listed for a case temp of 25°C and once current flows in the FET the case temp is no longer 25°C, no matter what kind of heat sink you have. The datasheet also mentions that the current rating is limited by the max junction temp rating and that will be reached very quickly at high current levels.
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Calculating the power each FET must dissipate at this switch’s continuous 150A rating (37.5A/FET) means that each FET creates more than 14W of heat! That would bring the FET temp to over 600°C, probably way over as I am being extremely generous with my calculations here.
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The wire-to-board and bus-bar-to-board connections each create heat too. The four wire connections could add about 3W more heat at 150A. The bus bars would add at least another 1W. That means the board is creating about 60W of heat at 150A…it would not survive.
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We need to use the on-state resistance and thermal resistance numbers to calculate how much current each FET can handle before reaching its limit of 175°C. If each FET was alone and on a square inch of 2oz copper that would be about 15A. But they are on tiny copper pads and each FET heats up its neighbor. That lowers the current rating considerably. Being on an aluminum PCB brings it back up a bit so let’s call it 10A per FET…about 40A total.
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Adding up all the FET+connection+bus bar heating numbers for running at 40A continuous gives us about 7.5W of heat for that switch. In my opinion, based on my experience, that sounds about right for a normal board that size (with all the necessary compromises made to keep it that small). Using an aluminum PCB, thermal pad, and an aluminum case helps to raise the current rating some though. I can’t say by how much. A lot depends on the desired life span for the FETs. Some companies are willing to run them at their max temp since it doesn’t happen often during actual use. Other companies, for increased reliability, would lower the current rating so the FETs never exceed about 80% of their temp rating. For very high reliability designs the rating might be halved.
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The board traces are the equivalent of a wire much smaller than 12AWG. This combined with the 12AWG wiring and all that component and connection heat means that the 150A rating is absolutely preposterous. But since esk8 doesn’t (typically) run continuously at high current levels this doesn’t have to be a big concern…unless, like me, you hate seeing these kind of bs ratings games being played.
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Regarding the voltage ratings…the board layout looks to be pretty low inductance. As long as the FET driver was set up properly I don’t think there will be a lot of extra board-created voltage spiking (need to use a scope to know for sure) but since they say 80V max then I would stick to that or lower as there are spikes everywhere in an esk8 setup.
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It appears that this switch uses the FETs themselves for the pre-charge instead of a separate resistor (with its own smaller on/off FET) to slowly let the current through to the ESC. This is perfectly acceptable for an AS switch but when used this way there is a certain FET operating characteristic that can cause big problems and can easily lead to FET overheating. This can lead to FET failure if the switch manufacturer does not take it into account. I have met power electronics product designers who were not aware of this operating characteristic so it’s certainly a possible failure mechanism.
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If the aluminum PCB is not grounded then it just becomes a big antenna, possibly making any voltage spiking worse and/or interfering with operation of the switch because of electrical noise. I do not know if they ground the PCB aluminum or not.
Thanks again for posting that info and the photos! I always enjoy teardowns.