A
Andrew
- Jan 1, 1970
- 0
I have had a circuit working for some time now that has worked great.
A particular portion uses a 2N7000 FET to invert a 5V signal, to 12V
(inverted). The input is from a servo driver, outputting about 4.8V,
directly to the gate. At the drain is a 10k resistor connected to the
power supply, which is ~11V to ~14V depending on battery charge. At
the resistor/drain node, the output is fed as clock to some CMOS chips.
This clock feed has a 470pF filtering capacitor to ground also, if
that is significant. The source is connected directly to ground.
At any rate, this setup has worked great, but I have since done another
revision of the board to clean up a few areas that should reduce noise.
No changes were made to the schematic or even component placement,
just routing.
Within the first hour of operating this new board (the old boards, 4 of
them, have at least 40 hours each, probably alot more on some of them,
without any single problem), the transistor I described earlier failed.
When viewing the (usually 4.8V peak) signal that is being inverted, at
the point that it enters my board, it was about 100mV peak. The square
nature of the signal was in tact, but the voltage was VERY low, so low
that it would not trigger the FET.
I measured the resistance between the Gate and Source and found that it
was low - about 150 ohms +/- 10 ohms. I replaced it, and the circuit
worked great again, for about another 20 minutes, then, the same thing
happened!! Measured the resistance of the transistor (which, in a
"good" circuit measures in the many many mega ohm range) and it was
again, in the 150 ohm range. This explains the 100mV peak, anyway.
So, I switched to a different board (same new rev, just a different
physical board). During comparisons of the "bad" board and "good"
board, somehow the second board's transistor blew too! Same problem.
I replaced it, and decided to run it with a new servo driver board
also, and now, for 2 hours it has been perfectly fine, but needless to
say I am skeptical. I am trying to find a cause for the failure of
this FET and I can't find one.
So, that brings me to the point of this post. What are things that can
cause a FET to fail? My guesses are over current the drain to source,
or over voltage the gate to source. Those are my guesses anyway, but
both are pretty impractical as the drain source current is limited by
the 10k resistor (and at 12V, is way way way under the 800mA rating at
Vgs = 5V), and the Vgs can be up to 20V and is at around 5V. Is there
a way to tell what would cause the gate to source resistance to fail in
the way I described?
My guesses as to the cause of the failure are that a) there is a
problem with the first board I tested, or b) there was some problem,
unrelated to my board, from the input 0-4.8V servo driver board that
blew the transistor. The power supply to the servo driver board is 6V,
though, so I don't know how it would get above 20V, which is the max
rating for Vgs.
Anyway, how can you kill a FET, and which way of killing one (if any)
will result in a very low gate to source resistance?
Thanks in advance.
A particular portion uses a 2N7000 FET to invert a 5V signal, to 12V
(inverted). The input is from a servo driver, outputting about 4.8V,
directly to the gate. At the drain is a 10k resistor connected to the
power supply, which is ~11V to ~14V depending on battery charge. At
the resistor/drain node, the output is fed as clock to some CMOS chips.
This clock feed has a 470pF filtering capacitor to ground also, if
that is significant. The source is connected directly to ground.
At any rate, this setup has worked great, but I have since done another
revision of the board to clean up a few areas that should reduce noise.
No changes were made to the schematic or even component placement,
just routing.
Within the first hour of operating this new board (the old boards, 4 of
them, have at least 40 hours each, probably alot more on some of them,
without any single problem), the transistor I described earlier failed.
When viewing the (usually 4.8V peak) signal that is being inverted, at
the point that it enters my board, it was about 100mV peak. The square
nature of the signal was in tact, but the voltage was VERY low, so low
that it would not trigger the FET.
I measured the resistance between the Gate and Source and found that it
was low - about 150 ohms +/- 10 ohms. I replaced it, and the circuit
worked great again, for about another 20 minutes, then, the same thing
happened!! Measured the resistance of the transistor (which, in a
"good" circuit measures in the many many mega ohm range) and it was
again, in the 150 ohm range. This explains the 100mV peak, anyway.
So, I switched to a different board (same new rev, just a different
physical board). During comparisons of the "bad" board and "good"
board, somehow the second board's transistor blew too! Same problem.
I replaced it, and decided to run it with a new servo driver board
also, and now, for 2 hours it has been perfectly fine, but needless to
say I am skeptical. I am trying to find a cause for the failure of
this FET and I can't find one.
So, that brings me to the point of this post. What are things that can
cause a FET to fail? My guesses are over current the drain to source,
or over voltage the gate to source. Those are my guesses anyway, but
both are pretty impractical as the drain source current is limited by
the 10k resistor (and at 12V, is way way way under the 800mA rating at
Vgs = 5V), and the Vgs can be up to 20V and is at around 5V. Is there
a way to tell what would cause the gate to source resistance to fail in
the way I described?
My guesses as to the cause of the failure are that a) there is a
problem with the first board I tested, or b) there was some problem,
unrelated to my board, from the input 0-4.8V servo driver board that
blew the transistor. The power supply to the servo driver board is 6V,
though, so I don't know how it would get above 20V, which is the max
rating for Vgs.
Anyway, how can you kill a FET, and which way of killing one (if any)
will result in a very low gate to source resistance?
Thanks in advance.