Vgs(th) = Vt is defined as the threshold of leakage before it becomes a switch.. V
GS = V
DS, I
D = 250μA. ( The Id leakage levels is unique for each FET and depends on OEM test method and RdsOn)
You much choose a FET with Vgs(th) max < 50% of 3.3V aka "logic level" FET.
If you plan to switch off quickly , you need to know the flyback voltage does not exceed the Vds max or add a 100 mA diode to Vdd to clamp it.
if you want to control speed, then you need to add a 50 mV current sense resistor and regulate the current while adding a heat sink equivalent to the fan power rating. Ideally mount the FET or heatsink where the fan air velocity is highest.
This could also be low cost NPN 1.5A 1 Ohm switch
or for a few cents more a 3A 0.4 ohm NPN switch
https://www.digikey.ca/en/products/detail/stmicroelectronics/BD241C/2827178
But there are cheaper choices in low Vt Nch FETs .
My Rule of Thumb
- for Vt (aka Vgs(th) )
- Use at least Vgs= 2x Vt max for low threshold types
- Use at least Vgs = 2.5 x Vt max for standard enhancement mode FETS with Vt = 2 to 4V.
- Use much more I max than you need otherwise it must have a a good heatsink.
such as ;
RdsOn (Max) @ Id, Vgs = 90mOhm @ 3.6A, 4.5V
Vgs(th) (Max) @ Id = 1V @ 250µA |
OPTIONS
If you didn't want to change FET or layout the only solution would be to cut and jumper in a CB switch or CE switch and invert code.
Common Base NPN can raise the voltage (non-inverting) with some method to limit Vgs< 20V ( R divider or zener)
But you are not alone . A high percentage of students forget the threshold effect and the quadratic equation for Vgs-Vt vs Id and that all old FETs were conventional threshold Vt= 2 to 4V. I'll let you find your notes.