I need some help choosing a MOSFET for the circuit that I described here, which I originally designed using a BJT but decided that FETs make more sense in this case.
The FET will be controlled by a PIC24 that sends a logic high or logic low to the FET. I know that FETs are voltage-controlled devices, but I am wondering if there is also a minimum current needed to switch the FET on?
If so, does the FET need to be biased so that the PIC24 can source enough current to turn on the FET?
I'm also not too familiar with biasing FETs, so I'm also curious about pre-internally-biased FETs but they're somewhat hard to find on Google. Could you recommend any other resources?
Best Answer
A FET's gate has nearly infinite resistance, but some parasitic capacitance. What this means is that there's 0 DC current draw when on or off, but some current is required to switch between the states. Larger, higher current FETs tend to have higher parasitic capacitances, and thus require more power to turn on or off.
The current required to switch is generally very low, and unless you're switching at high speed (hundreds of kilohertz and up) or your FET is very large, you'll be able to drive it directly from your microcontroller.
The important thing to consider when choosing a FET for this purpose is not biasing, but gate threshold voltage. Make sure the selected FET's threshold voltage is low enough your microcontroller can turn it fully on. Don't rely on the figure in the datasheet table, this is often quoted for very low currents. Instead, check out the gate voltage vs source/drain current graph, and make sure that at your microcontroller's logic high voltage, the FET will be able to conduct the desired amount of current.