WEBENCH® Tools/UCC28740EVM-525: Some questions about schematic

Spoon,

R8 is used to control the turn-on and turn-off speed of the MOSFET Q1.

However, the UCC28740 internal gate drive is deliberately designed with current-limited turn-on (typ. 25 mA turn-on drive current). So even with R8 = 0, the turn-on speed is still well-controlled through the 25-mA DRV current source. I am not sure what you mean by “inrush”, but there will be a small leading-edge spike of current on the CS pin every time Q1 turns on, as the Coss gets discharged. But the current spike is limited because the limited 25-mA gate drive, and because the UCC28740 controller uses valley-switching to only turn-on at the valley of the DCM ringing on the drain of Q1, which minimises the voltage on Coss and hence the current spike.

The peak driving current out of DRV to charge the gate capacitance of Q1 is also inherently limited by the 25-mA turn-on current source.

At turn-off, the internal DRV pull-down is ~6-ohm typical, this ensures fats discharge of the gate capacitance and fast turn-off. The external resistor R8 can be sued to slow down turn-off. Slower turn-off may help improve HF EMI, but will make the eff worse. R8 = 0 should give the best eff.

Regardless of the value of R8, the internal driver dissipation will be the same, it only depends on the size the MOSFET gate charge and the switching frequency.

The pull-down Rgs resistor should not affect turn-on/off or power dissipation, as long as the value is not too small.

Pull-down Rgs resistor is usually only added to prevent false turn-on of Q1 if there is a board fault, e.g. missing or open R8.


I hope this helps to answer your questions.

Thanks,
Bernard