Electronic Design

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 (QG). QG 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 (VGS) that’s delivering the charge and drain-source voltage (VDS) 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 QG.

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