When the primary MOSFET turns on in DCM operation, its current starts from 0A and ramps up to a peak value each PWM cycle. Therefore, to reduce gate drive losses and increase overall efficiency, it is desirable to turn the MOSFET on relatively slowly when its current (and losses) are low. On the other hand, when the current ramps up to its peak the MOSFET must be turned off quickly to limit its losses, which also helps increase efficiency. R14 is the gate driver resistor controlling the turn-on time of the MOSFET (Q5). The optional PNP pull-down transistor (Q6) is used to turn the MOSFET off as quickly as possible, when the MOSFET is far away from the controller gate driver pin.

The selection of R14 resistor value must be done in conjunction with EMI compliance testing and efficiency testing. Using a larger resistor value for R14 slows down the turn-on of the MOSFET. A 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, efficiency was measured for a range of values for R14. Efficiency peaked with a value of 39‑Ω for R14.