I am unable to get the P-FET turn off completely. The circuit is intended to turn off voltage supply to a module, by controlling the PFET via an MCU (picture is attached).
When I supply a gate voltage of 0V from the MCU, the output at the drain is 3.3V. However, when I supply 3.3V to the gate, my output at the drain terminal falls to 2.49V instead of 0V.
I initially connected a 47K Ohm resistor and there wasn't any noticeable change. However, by lowering the resistor value to around 10 Ohms the output at the drain dropped to 0.55V.
Any reason why the resistor value matters so much?
There is always some small leak current when a fet is off, but you should have measured close to zero volt with the 47k resistor.
Did you take ESD precautions when handling/soldering (hot air) the fet.
Leo..
Sounds like you blew the gate oxide handling the device - most MOSFETs are extremely static sensitive as they have no protection devices built in. Touching a MOSFET just by the gate pin when the other pins are
grounded is likely burn it out - the human body picks up dozens of volts of mains pickup which exceeds the
gate-source voltage spec. Keep MOSFETs in their anti-static packaging until they are safely in the circuit.
Wawa:
There is always some small leak current when a fet is off, but you should have measured close to zero volt with the 47k resistor.
Did you take ESD precautions when handling/soldering (hot air) the fet.
Leo..
I always wear an ESD strap and carry the board around in anti-static bags. I believe I have taken ESD precautions when I handled the board.
MarkT:
Sounds like you blew the gate oxide handling the device - most MOSFETs are extremely static sensitive as they have no protection devices built in. Touching a MOSFET just by the gate pin when the other pins are
grounded is likely burn it out - the human body picks up dozens of volts of mains pickup which exceeds the
gate-source voltage spec. Keep MOSFETs in their anti-static packaging until they are safely in the circuit.
When I remove the 2.4 Ohm resistor, the P-FET works just fine. However, when I try to drive the load, it can only partially turn off. The whole thing has been really weird.
When I remove the 2.4 Ohm resistor, the P-FET works just fine. However, when I try to drive the load, it can only partially turn off. The whole thing has been really weird.
Are you phantom powering the BLE device through the data pins? Perhaps a R10K from drain to ground would help
I wondered why you had a R in series with the supply. Thought you were trying to use it as a inrush current limiter of sorts.
tinman13kup:
Are you phantom powering the BLE device through the data pins? Perhaps a R10K from drain to ground would help
I wondered why you had a R in series with the supply. Thought you were trying to use it as a inrush current limiter of sorts.
Finally, I have figured out the issue. I was powering the module via the UART lines from the MCU. When I shut down the UART lines, everything seems to be working. I had never heard of 'phantom powering' before!
The 2.4 Ohm resistor is just to limit the current. I could probably do away with it.
