Electronic – Do i need a resistor between the mcu and a mosfet

microcontrollermosfet

This post here: 2N7000 connection diagram

the gate is (very) high impedance, so no resistor is required to limit the current, as a resistor would be needed in series with the base of a BJT

But i also heard on many other place that we should put a resistor between mcu and gate, to limit the current. What is the recommended thing to do, does mcu can directly drive mosfet?

If i need a resistor to limit the current, from which capacitance shall i calculate it?

For example, if i look at a 2n7000 datasheet: http://www.fairchildsemi.com/ds/2N/2N7000.pdf
There is 3 different capacitance, input, output and reverse capacitance, no gate capacitance.

Also, when i look on digikey, i can either sort result by gate charge or by input capacitance, both given in coulomb for a given Vgs, what is the difference between input capacitance and gate charge? Which one shall i use to calculate the resistor? If like the 2n7000, there is no gate capacitance…

Best Answer

Your quoted paragraph is correct: the gate is high impedance and a resistor is not required for small MOSFETs.

If it is switching frequently, then a small resistor (tens to hundreds of ohms) is suggested to suppress ringing and reduce emitted electromagnetic interference, especially if the MOSFET is on the end of a long wire or PCB track.

You may want to have a pullup or pulldown resistor to provide a default (or or off) when the MCU is not programmed, or held in reset.

(Large power MOSFETs are a little different, and may need a special gate driver to achieve good results).