I was managing engineering for a medical device manufacturer. We had a problem with dying power FETs on a stepper motor driver in an IV pump. One of my engineers came running into my office saying "I found what's killing the FETs! There are 20 V spikes on the gates, vastly over the rating." I went to the lab with him. I found he had put a scope probe on the gate as expected; the scope ground was connected to circuit ground with a 36" clip lead. This created a large loop antenna which picked up the current spike from the drain current. I attached the scope ground directly to the nearest available ground on the board; the spikes disappeared. I said "keep hunting." Fast forward. . . . we returned several of the offending FETs to the manufacturer. They found out that the wire bond from the pin to the source of the FET die had failed due to inadequate pressure in the bonding process. They replaced all of the offending parts and re-calibrated their bonding equipment. Ground was important. ---- Dennis
Kendall:
I was managing engineering for a medical device manufacturer. We had a problem with dying power FETs on a stepper motor driver in an IV pump. One of my engineers came running into my office saying "I found what's killing the FETs! There are 20 V spikes on the gates, vastly over the rating." I went to the lab with him. I found he had put a scope probe on the gate as expected; the scope ground was connected to circuit ground with a 36" clip lead. This created a large loop antenna which picked up the current spike from the drain current. I attached the scope ground directly to the nearest available ground on the board; the spikes disappeared. I said "keep hunting." Fast forward. . . . we returned several of the offending FETs to the manufacturer. They found out that the wire bond from the pin to the source of the FET die had failed due to inadequate pressure in the bonding process. They replaced all of the offending parts and re-calibrated their bonding equipment. Ground was important. ---- Dennis