adum 4221-gate resistance

 my question is regarding the selection of gate resistance R9,R10,R11.As in the datasheet of adum evaluation board it is mentioned that diode D2 is used to provide different turn and turn off time and in that case i have to tune R9,R10,R11.How i tune these values any reference from analog devices

  • Hello,

    The external series gate resistors are used primarily to control the slew rate of the external switch being driven. This is accomplished by limiting the effective drive current across the charging and discharging of the external switch gate. A larger resistance results in a slower turn on/off. The external series gate resistor also helps manage power dissipation, where a larger gate resistor will dissipate a larger ratio of the turn on/off power to outside the gate driver. The final thing the external series gate resistors do is to dampen the ringing that might exist between the parasitic inductance of the gate loop and the equivalent capacitance of the power switch.

    When choosing the external series gate resistor, most designs begin by either choosing a target peak current, or a specific rise/fall time for a switch. From there, the resistance can be found. Here is an article talking about peak current:

    Peak Current of Isolated Gate Drivers | Analog Devices

    Typical numbers for external series gate resistors are in the range of 2-10Ω, but can vary widely with different designs and requirements.