RG is the gate driver resistor for the power switch, QA. The selection of this resistor value must be done in conjunction with EMI compliance testing and efficiency testing. Larger RG slows down the turn-on and turn-off of the MOSFET. Slower switching speed reduces EMI but also increases the switching loss. A tradeoff between switching loss and EMI performance must be carefully performed. For this design, 10 Ω was chosen as the gate driver resistor.