Not only by changing the gate resistors is the efficiency affected, but also the drain to source peak voltage during turn off as well as the EMI behavior of the converter. This is a very iterative procedure. The gate resistor can be change as much as the efficiency is good enough, the drain-to-source voltage spike during turn off do not exceeds 80% of the MOSFET breakdown voltage during normal operation (according to IPC-9592) and the EMI performance does not exceed the Class A or B (according to the specified application case)* limits of conducted emissions.
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