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Epever Tracer 4210AN MPPT Charge Controller MOSFET Repair

Failed FETs
Failed FETs

Here’s some damage to a 1-week old Epever Tracer 4210AN MPPT Charge Controller, where some of the power FETs have decided they’ve had enough of this world. These are Alpha & Omega AON6512 N-Channel Enhancement devices, rated at 30V 150A. From probing around, these seem to be on the battery bus for output protection – they’re just used as power switches in this application. The controller did work in this state, but charging from the solar input was accompanied by a very strong burning PCB smell.
I’m not sure what caused the failure, but as they’re all in parallel, if a single device failed, then it’s likely that the remaining parts having to then compensate for the extra load put them under enough stress to cause a failure.

Cleaned PCB
Cleaned PCB

The hot air gun was used to get the old parts off the board, which had got hot enough to fully oxidise the solder on the thermal pad, along with causing a bit of damage to the PCB itself. I scrubbed the board with a fibreglass pencil to try & get all the Magic Smoke residue off, along with any oxide on the copper. There has been some flaking of the soldermask, but luckily only between connected pads, and not around the gate pads. There was some unfortunate collateral damage to the main fuses, with minor melting of the plastic case, but they’re still electrically intact.

Reflowed Replacements
Reflowed Replacements

Replacement MOSFETs were sourced from Farnell, in this case ON Semi NVMFS5C628N parts, rated at 60V 150A. Since these parts are in a DFN package, solder paste & hot air was used to reflow them back onto the cleaned pads, and then everything checked for short circuits.
The replacement FETs have slightly higher RdsOn resistance, but this shouldn’t be an issue.