Electronics > Beginners

I did I kill my mosfet ?

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safarir:
Hi,

I from Quebec, Canada soo my english is very not that good.

I was working on pwm on a fpga development board when suddenly, I was unable to stop my circuit. After verification, I discover that my mosfet now have a 1.3ohm resistance when the gate is at 0v.

The circuit was the following:
12v-FAN-Mosfet-Ground
with a 1n914 freewheel diode across the fan and a 470uF cap connect between the FAN-Mosfet and the Ground.

My oscilloscope was connect at the Fan-Mosfet connection and I never detect a peak more that 13v. The max amp I see is 1amp when the fan was stuck.

The mosfet datasheet:
http://www.aosmd.com/pdfs/datasheet/AO3400.pdf

I am just trying to figure what I did wrong since I don't want to kill the others mosfet...

Zero999:
Yes, it sounds like the MOSFET is dead, the gate should not be connected to the source.

Just one question: did you take the MOSFET out of the circuit or at least disconnect the gate before testing it? If not, it's possible that the gate output was connected to either rail and the meter was just measuring the resistance of the gate output.

safarir:
Hi Hero999,

No, I didn't remove the mosfet from the board. I only connect the gate to the ground and mesure the resistance on the Drain-Source. The orders mosfet gave me 1.5mohm and this one gave me 1.6 ohm...

I just did some manual test to see what happend at the drain (replaced by a switch) when I open the circuit. I have a love of difficulty to explain what I see, I have join the image in attachement.

safarir:
There is a order image, I forget the diode in the last one...

The 2 image are without the capacitor, I think I probably remove it when the diode died

safarir:
I add a other 1n914 to simulate the body diode and I got something more similar to what I saw

The second Image is the same thing but with the 470uF capacitor

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