In lots of places, we use a simple N-FET switch to turn on/off 24V outputs. We use a 30V rated part and stay well under the power/current limits.
We've started to see recently however a couple of instances of LEDs turning on of their own accord when wired as below:
In a non-failed state, if the MCU pin goes high, the FET is turned on, the voltage at the Drain (LED's cathode) goes to 0V and the LED lights. In the failed state, even when the MCU output is 0V, the LED lights and the Drain voltage is ~0.15V. A high output still pulls the Drain to 0V, but it returns to 0.15V when the FET is switched back off.
I realise that when the switch is open the drain of the FET is floating rather than in a defined state, but how is the LED turning on when there is nowhere for current to flow to?
The datasheet for the part we are using does stipulate a "Zero Gate Voltage Drain Current" (no FET is an ideal switch) but it's measured in micro-amps which is orders of magnitude too small to light the LED. Using the datasheet we can also see that the part is being used within spec as we don't come close to the maximum voltages/currents.
Can anyone suggest what might be going on?