Electronics > Beginners
I keep killing mosfets - what I do wrong?
PMA:
I have medium sized PCB that have three identical driver circuits:
When I assembled the board, one of the circuits worked, one was constantly on and was constantly off. Then I changed Q3 for both and one of channels started to work. However, I cannot get third channel to work, it is constantly on. Channels seems to work reliable once they work first time.
Checked so far:
-I haven't goofed with resistors, they are correct and there are no short circuits
-D32 works as expected
-Control signal from micro is ok, I can wiggle it low/high. By default it is low.
-BTS555 works as expected - if I remove Q3 it stays off
-When I install new Q3 and measure the circuit, it gives identical reading than two other circuits, but when I switch power on, Q3 pulls low (enough).
Do I just have bad luck with ESD/components or is there something fundamentally wrong with the circuit?
Edit:
datasheets for major components
Q3: https://assets.nexperia.com/documents/data-sheet/2N7002P.pdf
U11: https://media.digikey.com/pdf/Data%20Sheets/Infineon%20PDFs/BTS555.pdf
D32: http://www.vishay.com/docs/87787/v20dl45bp.pdf
Solved
Ground of the soldering iron was in different potential than ground of the PCB.
magic:
You drive Q3 with 5V divided by 6 due to R83/R89. That's less than 1V, not really a lot.
The first guess is that most of 2n7002s have insufficient gate threshold voltage to turn on in such circuit. Remove R89. Perhaps reduce R83 value if Q3 takes too long to switch after a command from the MCU.
If BTS555 doesn't have an internal pullup resistor on IN, an external resistor should probably be connected. Presumably to VBB, but I don't know what this chip is so check the datasheet.
PMA:
R83 is only 100 ohms, so there is practically 5V on the gate. R83 is there to protect output of the micro.
Pin 2 of the BTS555 is pulled to VBB internally.
oPossum:
Try testing the 2N7000 before assembling the board to determine if you have bad parts or they are being damaged later.
PMA:
I kind of know how to test them, but just to avoid mistakes by being overly self-confident: What is the best/correct way to test mosfets (I don't have speciality test gear available, only scope and good quality multimeter)?
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