protect gate mosfet pull down resistor
There is many reason to connect a "small" value resistor in series with the Mosfet gate, a resistor act as a limiter for the peak current into the MOSFET gate then the MOSFET will switch slower than if you don't put the resistor. This behaviour may be useful for EMI emission and to prevent parasitics high frequency oscillation (see later), but for hard switching application (like PWM DC-DC converter) this has a drawback because slow down the MOSFET commutation increase the commutation loss of the MOSFET (this will be proportional to the MOSFET commutation time).
But there is also another very important reason to add a small resistor and this is to tur off spurius oscillation that can arise between the gate input capacitors and the parasitics inductance of the driving stage (parasitics oscillation can turn off and on the MOSFET at high frequency with a lot of power loss and noise), from this last point of view you can search the best compromise from commutation time (MOSFET speed when used as a switch in hard-switch commutation circuit like PWM DC-DC converter) and the parasitic high frequency oscillation (basically you've to calculate the resistor value in order to keep the Q of the resonant circuit equal to 1 then sqrt(L/Cgs), for most application a 10÷15 Ohm shoud be enough to prevent oscillation).
Just a first reply, hope it help.
Cheers
F.F. a.k.a. Powermos