CMOS logic MOSFET gate resistor

Thread Starter

k1ng 1337

Joined Sep 11, 2020
101
Hi does a MOSFET require a gate resistor when driven by a CMOS nor gate? Specifically is there a danger to my components due to high instantaneous current during switching if I don't use one? I am using cd74hc02 NOR gate and a baby size logic level MOSFET (350ma), my switching frequency is less than 1hz and the load is an LED so noise is not a concern.

Thanks :)
 

dl324

Joined Mar 30, 2015
13,095
does a MOSFET require a gate resistor when driven by a CMOS nor gate?
HC gates are designed to drive capacitive loads. If the gate cap of the MOSFET is higher than what HC gates support, you'd need a gate driver. But no resistor in either case.

Resistors are more likely to be used when the driving circuit doesn't behave with capacitive loads.
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
101
HC gates are designed to drive capacitive loads. If the gate cap of the MOSFET is higher than what HC gates support, you'd need a gate driver. But no resistor in either case.

Resistors are more likely to be used when the driving circuit doesn't behave with capacitive loads.
What parameters do you look for when investigating compatibility?

My reasoning suggests that the peak inrush current will be very low due to the MOSFETs tiny capacitance but then again the MOSFET is not a simple device!
 

crutschow

Joined Mar 14, 2008
27,712
Typically CMOS gate outputs are tolerant of momentary short-circuits, so any momentary high output current due to a capacitance load should be no problem, particularly for a small MOSFET.

The continuous maximum output current rating of the cd74hc02 is 25mA so, if you wanted to be extra safe, you could put a 200 ohm resistor in series with the MOSFET gate.
That should have little effect on the switching characteristics of a small MOSFET.
 

AnalogKid

Joined Aug 1, 2013
9,353
The peak inrush current is very low for your part. But even if it isn't, a CMOS output stage has a relatively high equivalent output resistance. This protects the stage from brief transient currents.

The resistor-in-series-with-the-gate thing comes from noise control in switching power supplies. Big power MOSFETs have a large gate capacitance, and can draw a transient current of amperes. With a very fast gate voltage transition (for better efficiency), pcb trace or wiring inductance combines with this capacitance to form a resonant circuit that can ring at a surprisingly high frequency, like 10's or 100's of megahertz for a 250 kHz switcher. This produces both differential and common mode noise, both radiated and conducted - a grand slam noise problem that can kill certification. A resistor in the series circuit damps this oscillation.

For normal people doing normal things, none of that matters.

ak
 
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