Hi,
May I ask what is the purpose of connecting RC circuit in series between 555 timer output and power mosfet gate? Between there is a diode and resistor connecting power mosfet gate to its source. May someone help to explain this? Thank you.

 

The capacitor helps to charge the gate of the MOS fast and reduce switching losses. The diode should be a zener diode to protect the gate from voltages greater than 15V (typically). The resistor in series with the gate is to maintain the gate charge and the resistor in parallel with the gate is to pull discharge the gate (faster than with out it) after the 555 goes low.

A diagram will make things clear.