interactive
Four ways to switch a load with a MOSFET. Press ON and OFF on each, change the gate drive, and watch the node voltages, VGS, current and FET dissipation. The drive menus include the common mistakes, so you can see each one fail. Companion to the MOSFET Switch Primer.
Each FET is a level-1 (square-law) model. In the ohmic region I = K·(Vov·VDS − VDS²/2); in saturation I = K·Vov²/2, where Vov = |VGS| − |Vth|. The loop current is solved by bisection so that the FET current equals the load current.
Default parts: NMOS Vth = 2 V, RDS(on) ≈ 20 mΩ at 10 V. PMOS |Vth| = 2 V, RDS(on) ≈ 50 mΩ at −10 V (PMOS is worse for the same size). The "logic-level" NMOS has Vth = 1 V; the "standard" NMOS has Vth = 3 V, the high end of a 2–4 V spread.
Drivers are ideal voltage sources. Body diodes are drawn but stay reverse-biased in these orientations. Switching transients, temperature and gate charge are not modelled; the primer covers those.