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MOSFET gate resistor

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jshonw:
I want to control a resistive type dc heater with a N-channel mosfet from a microcontroller using PWM, but I am not sure if a resistor at the gate is necessary?  The rate at which I am driving the mosfet will be slow at maybe about 20hz from 0-100 duty cycle.  If one is necessary, how do you select a value?

Psi:
Yeah, you should always have one.
In this case it's more to protect the mcu though.

So if your mcu port is 30mA max,  5v / 0.03A =  166R,  so say 180R or no less than that.
Note, this wont protect the mcu from a failed fet that has shorted drain to base if you have a drain voltage above 5V.
It's more to stop the fet trying to pull more than 30mA in normal switching operations.

If your driving a fets base pin from a mcu you want to select a fet with logic level gate drive.
That way you can be sure your mcu will be able to turn the fet fully on.

jshonw:
Thanks, I am going try that.  I am using a logic level fet, so it should fully turn on from the micro. 

Short Circuit:
However, this might give you a dissipation problem, if the currents are high;
Assuming a 1nF gate capacitance, this 180E resistor results in a linear operation of the mosfet for about 0.7uSec on each edge. This does not sound like much, but often this linear switching is the main contributor of total mosfet power dissipation.

With the mosfet directly to the CPU, this high valued resistor does not really matter because the current sourcing capability of the uC is limited anyway.
All that the resistor adds is a current limit to a more precise value (which is usefull from EMC perspective), and said cpu pin protection of course.

When currents increase, this dissipation can be a huge problem, so that's where mosfet drivers come in. These devices are just simple push/pull drivers, but with serious current capabilities, often like 1amp or more. With such drivers, you're usually dealing with gate resistors much lower in value, often between a few and 10 ohms.

So if the mosfet gets hot, you should consider a part with much lower gate capacitance or an external mosfet driver. Lower capacitance usually goes hand in hand with higher on-resistance, so that's not very helpfull usually, although there's an optimum between switching losses and conduction losses. Also the better the capacitance vs resistance ratio, the more expensive the mosfet gets.

Psi:
yeah

You can also get logic opto isolators with push/pull high current outputs (1A)
They can be useful to both isolate your fet from the micro and provide high current drive for the fet, all one package.

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