The gate lead resistor serves two purposes. 1) it supresses undesired oscillation, and 2) it limits the inrush current when the gate driver supplies current to the gate capacitance to charge it.

 

Like Alec said the purpose is to stop ringing and oscillation. But using to big of one also slows down the charging of the gate/source capacitance and slows turn on of the mosfet. It's also a good idea to put a resistor in the k ohm range between gate and source to keep the mosfet gate pulled down and turned off when its supposed to be off.

 

I use a pull-down resistor on the gate; I have seen a variety of values used from 1k to 100k, but don't really know what is optimum.

Is the ringing/oscillation you describe caused by the MOSFET or by the driver, and if it is the latter, is the resistor still required with the driver is a uC?

 

I use a pull-down resistor on the gate; I have seen a variety of values used from 1k to 100k, but don't really know what is optimum.

Is the ringing/oscillation you describe caused by the MOSFET or by the driver, and if it is the latter, is the resistor still required with the driver is a uC?

The only purpose of the pull down resistor is to make sure the gate doesn't float to dangerously high static levels if the driver is switched to a high impedence state or physically unplugged from the mosfet or a switch disconnects the gate from the circuit. There are some other switching mechanisms with diodes to the gate that allow a path to ground and must be smaller value resistors to speed turn-off speeds but, if diodes (or single transistor switching is not used, then fairly high resistor values can be used (100k).

A gate resistor also can be used to slow the mosfet switching speed to control (reduce) EMI for high current or mains-connected devices.

 

Is the ringing/oscillation you describe caused by the MOSFET or by the driver

Both. The gate has capacitance which can resonate with the inductance of the gate connecting wire/trace when the driver changes state.

is the resistor still required with the driver is a uC?

Yes.