I am trying to figure out the purpose of the gate resistor in this circuit:

enter image description here

Pictured are the UCC28700 flyback controller and the STD3NK80Z-1 MOSFET.

My understanding is that the resistor(s) serve two main purposes:

  1. to limit the in-rush current that is caused by the parasitic input capacitance
  2. to dampen the ringing on the Vgs signal when the drive pin pulls the signal low

At 14V, 10Ohms is not going to bottleneck the already current-limited DRV pin (datasheet says it is automatically current-limited to 25mA). My conclusion is that the resistor in the pictured schematic is to dampen the ringing, is that correct?


1 Answer 1


#1 isn't exactly the goal. It would be to limit rise and fall times to reduce EMI.

#2 is to dampen ringing on both rise and fall with the driver. But in addition, to dampening ringing between parallel MOSFETs which means one per MOSFET since a ringing loop exists between them. Sharing a gate resistor won't prevent parallel MOSFETs ringing amongst themselves.

  • \$\begingroup\$ So what does it look like when we need one resistor value to control the rise/fall time of the signal, and a different resistor value to manage the damping? \$\endgroup\$ Dec 7, 2020 at 2:16
  • \$\begingroup\$ @RGBEngineer You pick the resistor value that deals covers both. The larger of the two. \$\endgroup\$
    – DKNguyen
    Dec 7, 2020 at 2:32

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