Figure Of Merit: A Refresher Course

“Figure of merit” (FOM) is a way of evaluating FETs. It accounts for both their conduction losses and their switching losses. Commonly, it’s calculated as on-resistance (R_(DS)ON) times gate charge (Q_G). Q_G is the charge that must be brought to the gate of the MOSFET to turn it fully on. Design-wise, it’s hard to reduce both at the same time, which makes their product a good basis for comparison.

Of course, it’s only possible to make that comparison under a standard set of conditions. That means both the gate-source voltage (V_GS) that’s delivering the charge and drain-source voltage (V_DS) that’s being burned up in R(DS). (It also means not just when the channel is fully on, but while R(DS) is ramping up and down, too.) Complicating things, R_(DS)ON varies a little with drain current, so when comparing switching transistors, the operational value of ID ought to be specified as well.

Sometimes, you will see a slightly different figure of merit: FOMSW, which is the product of R_(DS)ON and Q. It’s a characteristic called switching charge, which is a little shorter than Q_G.