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Short on VCC_MAIN but no heat

Hi,

I have a board which I am using for learning. It does not have any water damage. When I connected it to DCPS for the first time it showed current draw so I assumed it has a short. After checking VCC_MAIN line in diode mode it was confirmed that it has a full short (0000V drop).

After injecting 3.8V to VCC_MAIN line i noticed that the Q2300 was heating up. I removed it suspecting that it is to blame. But after checking the schematics I found out that it does not have path to ground therefore could not be the one to blame. Anyway I am still having short on VCC_MAIN but when injecting voltage to that line nothing heats up.

Then I tried to set current to 5.2A and increase the voltage gradually. After reaching 1.5V it also reaches 5.2A current limit and nothing gets hot. After waiting a bit I could fell that bottom of the board gets warm but I still could not locate individual component. I tried waiting longer but still nothing. I consumed almost all bottle of my freeze spray trying to locate what’s heating up.

Please note that I am using this board for learning purposes but it would be nice if I could still find the short. What would you do if you would have such a board on your desk? Thanks in advance.

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Looking for a heat signature can be tricky, especially when using a DCPS as the power source. Most bench power supplies offer current limiting protection. That means that when the current reaches the maximum set output, the PS will drop the voltage output in order to maintain the max current.

So when looking for heat, you are essentially hoping some component will give off this heat as a result of power dissipation. However, P=V*I (POWER equals VOLTAGE times CURRENT) through the component. So if your current is 2A but the voltage is essentially Zero (due to current limiting), then the power will be 0W...so no heat!

This is even harder for VCC_MAIN shorts as everything goes through the MOSFET so it’s hard to find the culprit. A thermal camera would probably be helpful here as it will be more sensitive to minor changes. The other thing to try is to probe the downstream circuits. These are all powered by the PMIC and VCC_MAIN so if one of them is shorted, it could be pulling down VCC_MAIN also.

Finally, look for visual clues like water damage (not in this case), evidence of prior repair or physical damage.

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Thank you Minho. I have read your reply at the other thread regarding similar issue. I agree that thermal camera would help in this situation but at the moment I could not invest in it.

It turned out to be Arc Driver. I started removing ICs and after removing Arc Driver the short was cleared. :)

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