Figure of merit: gate charge (Q_g) and R_DS(on)

Total gate charge (Q_g) is the amount of charge that needs to be supplied to the gate to turn on (drive) the MOSFET, for some specified conditions. A small value of Q_g is highly desirable in high-switching frequency applications, since it directly impacts on the driving losses.

The gate-to-drain charge Q_gd represents the part of gate charge associated with the Miller plateau extension, required to complete the drain voltage transition. For the same driving circuit, a lower Q_gd means faster voltage transients, hence lower switching losses. This is of utmost importance in high-switching frequency, hard-switched SMPS, where switching losses play a significant role.

Focusing on devices with same R_DS(on) of 2.7 mOhm and in SuperSO8 package, the new OptiMOS™ 6 100 V, shows a reduction of 35 percent in total gate charge compared to OptiMOS™ 5 while at the same time, gate-to-drain charge, is 45 percent lower, with correspondent benefits in switching losses.

The MOSFET “figure of merit” (FOM) is a performance indicator of a technology which accounts for both conduction and switching losses. FOM is calculated as on-resistance (R_DS(on)) times total gate charge (Q_g) and is usually expressed in mΩ x nC.

Why is the Figure of Merit an important parameter for technology evaluation?

R_DS(on) is a measure of the conduction losses while, on the other hand, total gate charge Q_g impacts on both the driving losses and part of switching losses. In order to minimize the total losses, both R_DS(on) and Q_g need to be minimized.

For every technology, the FOM = R_DS(on) x Q_g is a given number: it is not possible to improve R_DS(on) without impacting the charges, unless FOM is improved as well.

OptiMOS™ 6 100V shows significant improvement up to 30% in FOM compared to OptiMOS™ 5 100V. In other words, for the same R_DS(on), OptiMOS™ 6 100V  would show 30% lower Q_g compared to OptiMOS™ 5 100V.

Similarly to the FOM, the gate-to-drain charge figure-of-merit FOM_gd is a performance indicator of a technology which takes in consideration both conduction and switching losses. FOM_gd is calculated as on-resistance (R_DS(on)) times gate-to-drain charge (Q_gd) and is usually expressed in mΩ x nC. 

Why is the FOM_gd an important parameter for technology evaluation?

R_DS(on) is a measure of the conduction losses while, gate-to-drain charge Q_gd impacts on switching losses (especially during turn-off). In order to minimize the total losses, both R_DS(on) and Q_gd need to be minimized.

OptiMOS™ 6 100V shows significant improvement up to 42% in FOM_gd compared to OptiMOS™ 5 100V. In other words, for the same R_DS(on), OptiMOS™ 6 100V  would show 42% lower Q_gd compared to OptiMOS™ 5 100V.