A MOSFET with total gate charge Qg = 50 nC is driven with a 10 V gate voltage at a switching frequency of 100 kHz. Roughly how much power is needed to drive the gate?

Answer: A. 50 mW

Qg is the charge needed to charge the gate once, and the drive power is P = Qg × VGS × f = 50 nC × 10 V × 100 kHz = 50 mW. The average gate current is Qg × f = 5 mA. This power is dissipated mainly in the driver IC and gate resistor. Because the gate input capacitance varies greatly with voltage (the Miller effect), Qg, which directly gives the charge required, is more convenient for drive-circuit design than a capacitance value. For example, driving at 1 A, the charging time can be estimated at about 50 ns.