Overcurrent spike in NFET will cause it to fail within 10 years?

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treez

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Hello,

We are using the BSS138PW NFET to switch off an IC by pulling a pin low. (it does this once per day for 10 years plus) . In so doing, the NFET suddenly discharges a 10uF ceramic capacitor, as in the schematic attached.

The current spike in the FET (during the discharge) goes up to 4A and lasts 15us.
The BSS138PW datasheet on page 2 states that 1.2A is the maximum current allowed, and states that even this is only allowed for 10us maximum.

Do you think we are going to see this FET fail within the ten year warrantee period?

Schematic, waveform and LTspice simulation are attached.

BSS138PW datasheet
https://assets.nexperia.com/documents/data-sheet/BSS138PW.pdf
 

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  • Waveform _Current spike in FET.jpg
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  • Schematic _Current spike in NFET.jpg
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The simulation results don't seem to fit typical BSS138 Id versus Vgs characteristics. But it's at least possible to exceed specified Id pulse current maximum.

Looking at the SOA diagram, I won't expect actual device failure. But you are not safely keeping the maximum ratings and should better place a current limiting resistor.
 
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Hi,

SOA diagram shows 1.2A only. For everything above you are responsible.
If it is likely or not to cause a fail....who knows?

I assume you talk about units that are already in use. Or are you stiil in development/production phase?

Klaus

Added: where does the gate drive signal come from?
Microcontroller? --> software solution?
 
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where does the gate drive signal come from?
Microcontroller? --> software solution?
Click to expand...

Thanks yes thats right, from a micro with a 3V rail.
 

Hi,

* try to use the pullup feature of the I/O instead of hard HIGH
* or try to use it PWM style

Klaus
 

Looks to me like you are in one of the many "gray
areas".

That pulsed current rating has some backing use
model assumptions. These might include a case
temperature, a repetition rate, etc. If the rating
for pulsed current is made at 150C and you are
at room temp or even 70C, "probably" no problem.
But rating at room temp gives a better "marketing
number" and derating is the ugly surprise, sometimes.

Since your sourcing network impedance is high and
your shunted voltage low, a 3 ohm drain resistor
could keep you safely within spec limits and yet have
almost no effect on operation (tau of 30uS, do you
care given that the circuit is fairly crude?).
 

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