A technician replaces a shorted power MOSFET on an e-bike motor controller. The new MOSFET fails within seconds of power-up. The technician reaches for a third MOSFET. What does the section teach?

Select one answer.

• Nothing — a MOSFET that fails on power-up is simply a defective part, so a fresh one from a different batch will resolve the problem, and two bad parts in a row is bad luck rather than a sign of anything on the board. Parts fail randomly, so a quick second failure means the replacements themselves were faulty, making a third MOSFET the logical next step, since the board was working when the original shorted and the repeat failures reflect a run of weak components rather than a condition the board is imposing on each new part fitted into the same socket.
• Only the soldering — the replacements are failing because the rework overheated them, so fitting the third MOSFET at a lower temperature, without investigating the circuit, will let it survive.
• The MOSFET is a victim, not the culprit: a part that fails as fast as its predecessor is being killed by a condition still on the board — a driver fault, a downstream short, an overvoltage — so the technician stops replacing and does failure analysis, finding and correcting the cause before fitting another part, because replacement without the cause only resets the clock on the same failure.
• The rating — the original MOSFET was underrated, so fitting a higher-rated MOSFET, without finding why the part is stressed, is what makes the repair hold.