Does a MOSFET need a gate resistor?

[DamoRC]

Does a MOSFET need a gate resistor in the same way that a BJT needs a base resistor? If yes, what are the key datasheet parameters to consider in calculating the value for such a resistor?

Thanks in advance.

 

[Speakerguy]

A gate resistor is recommended in most applications, but not for the same purpose as with a BJT.

A gate resistor limits the instantaneous current that is drawn when the FET is turned on. If you are driving a FET directly from a low-current device (microcontroller or logic gate) then gate resistors are recommended. Anywhere from 5 to 100 ohms is fine. They also can be viewed as slew-rate limiting devices for the gate signal, or as devices to eliminate ringing at the gate.

If you are driving the FET from something like a dedicated half bridge driver or similar then they can be eliminated, the drivers are usually meant to be directly connected to the FET.

 

[mvs sarma]

I feel a gate terminator is also needed and many SMPS designs have it.
While in Microcontroller based designs the port pin responsible driving a mosfet gate would also discharge it when the pin is made LOW.

 

[dknguyen]

It's mainly just used to slow down how fast the MOSFET turns on and off in order to reduce ringing, voltage spikes, and interference (ie. if the gate driver can turn on the MOSFET faster than needed.)

 

[DamoRC]

As always - thanks.

 

[John Martin]

I feel a gate terminator is also needed and many SMPS designs have it.
While in Microcontroller based designs the port pin responsible driving a mosfet gate would also discharge it when the pin is made LOW.

What is a "gate Terminator"?

 

[jpanhalt]

What is a "gate Terminator"?

I believe the answer is in mvs sarma's second sentence. He uses the term to mean a way to turn off the gate. If you simply charge a mosfet gate, then completely disconnect it, the mosfet will stay on for a long time. As he points out, with microcontrollers, discharge is often through the pin used to control the gate. Not all gate drives work like that, so you will often see a resistor from gate to ground that ensures the gate is turned off (i.e., terminated).

BTW, this is a very old thread. If you have other questions about mosfets, I suggest starting a new thread.

John

 

[ChrisP58]

I usually put a resister across the gate and source to prevent glitches at power up. Most microcontroller pins are hi-z until the startup code gets around to initializing them to be outputs.

A resister is cheap insurance to reduce what might happen when a fast rising voltage on the drain turns on an unterminated gate because of the parasitic gate-drain capacitance.

 

[jpanhalt]

And sometimes, you will see a "speed up" diode back to the driver to facilitate turn off. In high power drives, that can help avoid "shoot through."

John

 

[Mike odom]

You DO need them if you drive multiple MOSFets with the same signal, something about mismatched capacitance on the individual gates.

 

[jpanhalt]

I agree that they are needed on each gate of parallel mosfets for turn on. I thought we were talking about a turn-off resistor.

John

 

[Mike odom]

I agree that they are needed on each gate of parallel mosfets for turn on. I thought we were talking about a turn-off resistor.

John

Actually, the name of the thread is... "Does a MOSFET need a gate resistor?