MOSFET failure rates to MIL-HDBK-217F

[Tom Faloon]

MOSFET failure rates to MIL-HDBK-217F

Re failure rate predictions for MOSFETS.
I am using MIL-HDBK-217F (Section 6.4)

My main problem is with "Pi A" calculations.

What is meant by Rated Output power Pr ? Does it mean rated volts X rated
current ? Or what ?
e.g if calculations are for a MOSFET rated at 100V, 60A, to switch a 24V,
30A load, (ON/OFF), then How do I calculate "PI A" ?
If I use rated Volts X rated current it gives a very poor result.

e.g. Lp = Lb X Pt X Pa X Pq X P e


Given
Lb = 0.012,
Pt = 4.2 (Tj = 110 Deg)
Pa = 10 (for Pr = 100V X 60A = 6KW)
Pq = 8 (Plastic)
Pe = 9 (GM)

Lp = 36.3 This is a poor result for just one component ! It only need a
few of these on a PCB for their failure rates to dominate the calculations!

Is my Pa value calculated correctly? or just plain wrong!


Many Thanks,


Tom Faloon