I've got very limited experience with N-channel MOSFETs. I've got a circuit built that is using them to drive a fuel injector. Now that the circuit is working (!), my design/analysis is switching to safety. Is there a common failure mode where the drain and source could short together? Where the MOSFET is ON even though the gate is LOW? What might cause this mode of failure if it can occur?
For reference, I've selected the Fairchild FQP4N20L.
Where the MOSFET is ON even though the gate is LOW? What might cause this mode of failure if it can occur?
That is certainly a possiblity however remote. Generally it would be caused by a short circuit in the drain line (bypassing the normal drain load device) to Vcc. Over current or over heating could cause internal melting, causing a drain to source lead short (as well as a gate short to drain which could toast the Arduino).
A lot depends on the max current rating of the MOSFET. If it's way over rated Vs the short circuit current then it could just stay shorted, but if it's max rating is only somewhat over rated over the short circuit current value then it would most likely burn open after shorting.
One simple solution is to just fuse protect the voltage source that feeds the Drain's circuit such that is will protect the load and possibly prevent a meltdown of the MOSFET.
Lefty
Thanks. It sounds like I'm reasonably safe. The drain load (fuel injector) is a 12ohm solenoid operated at 12V. If there is a short that bypasses the load (as you describe), I don't care if the MOSFET or Arduino die. As long as the fuel injector doesn't get held ON and pour fuel into the engine continuously.
As long as the fuel injector doesn't get held ON and pour fuel into the engine continuously.
Then one other safety precaution you might consider is to install pull-down resistor from the gate to source terminals on each MOSFET, say 5-10k ohms. If the Arduino goes south, loses power, or stays in a reset mode then all its I/O pins go tri-state (high impedance) mode and can cause the MOSFET to stay on because of the gate capacitance.
Lefty
Yep, already done, thanks to a previous recommendation you made...
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1258324489/6
I guess I should go and update that thread with the circuit I ended up with.
Jesus...I was going to write up a post explaining MOSFET theory and potential applications. An hour and a half later, I realized I didn't even answer your question...
So, instead...
Here is a pretty good list of common causes of MOSFET failures:
Yeah I found that while I was looking around. It describes mostly why they fail rather than how. At the bottom the short-circuit failure mode described I think is drain to gate, not drain to source.
Thanks for posting it.
