12. PPTC Selection & Failures cases

Question.4

Tony designs a 24V DC motor driver. The motor draws 4A during normal running.
He places a PPTC (I_hold = 6A, I_trip = 12A, Vmax = 50v) on the 24V input for overcurrent protection.
During a wiring short, the PPTC trips after approximately 4 seconds — but the low-side MOSFET is already destroyed.
The PPTC rating was correct. Why did the MOSFET still fail?

PPTC voltage rating was insufficient for 24V
PPTC I_hold should have been lower than motor running current
PPTC trips too slow — MOSFET fails in microseconds, PPTC takes seconds
PPTC was installed on the wrong rail

Restart quiz!