Calculating Gate Resistor for Logic Level MOSFET

Thread Starter

John Czerwinski

Joined Jun 19, 2017
29
For educational purposes, I would like to create digital logic circuits (inverter, NAND, NOR, etc) using discrete n-type and p-type logic level MOSFET in a CMOS configuration. Reading from this website and others, there's mention that a Gate Resistor is needed to protect the MOSFET gate from sudden in-rush of current to charge the gate capacitance and, also, to damper ringing on the Gate.

I plan on using the high-level schematics found below to build this circuits using +5v Vdd. What parameters on the datasheet should I use to calculate the Gate resistor? Also, where can I find the formula for this calculation?

https://www.allaboutcircuits.com/textbook/digital/chpt-3/cmos-gate-circuitry/

Thanks!
 

MrAl

Joined Jun 17, 2014
7,455
For educational purposes, I would like to create digital logic circuits (inverter, NAND, NOR, etc) using discrete n-type and p-type logic level MOSFET in a CMOS configuration. Reading from this website and others, there's mention that a Gate Resistor is needed to protect the MOSFET gate from sudden in-rush of current to charge the gate capacitance and, also, to damper ringing on the Gate.

I plan on using the high-level schematics found below to build this circuits using +5v Vdd. What parameters on the datasheet should I use to calculate the Gate resistor? Also, where can I find the formula for this calculation?

https://www.allaboutcircuits.com/textbook/digital/chpt-3/cmos-gate-circuitry/

Thanks!
Hello,

The gate resistor is there to provide a reasonably fast response while limiting the dv/dt of the drain voltage as well as di/dt of the source current. This helps to limit ringing and also more recently to limit radiated EMI. It can also be used in conjunction with a diode to force an asymmetrical gate drive to the mosfet.

It is not that easy to blow out the gate because the gate can usually take at least 1 amp peak as the surge is fast. In fact, one of the design goals for many circuits is to charge the gate charge capacitance quickly. Not only that, because of lead inductance and drive circuit output impedance it is hard to force that 1 amp into the gate in most cases. It takes a concerted effort to get that high of a surge for fast turn on or turn off.

So your attention should really go toward getting the mosfet to turn on quickly but not too quickly. A good starting value for the gate resistor is 10 ohms, then observe the drain voltage to see how fast it is turning on and if any ringing is noticed. You should also pay close attention to the source lead trace or wiring to keep it short as that helps a lot in reducing the ringing due to source inductance.
If you dont really need fast turn on then you can go higher on the gate resistor value. This will reduce EMI and keep things working smooth. You should also check the mosfet temperature if there is a heavy load.
 
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