Choosing a MOSFET for a switching application, such as a buck converter, motor driver or load switch, is a trade-off between conduction loss, switching loss and drive requirements. Three datasheet parameters do most of the work: Rds(on), gate threshold and drive voltage (Vgs), and gate charge (Qg).
Rds(on) and conduction loss
Rds(on) is the drain-source resistance when the device is fully enhanced. Conduction loss is simply I² x Rds(on). Remember that Rds(on) rises with junction temperature, often by 50-100% at 125 C, so use the hot value from the datasheet curves rather than the headline 25 C number. A lower Rds(on) usually means a larger die, which brings higher capacitance and gate charge.
Vgs: threshold versus drive voltage
The threshold voltage Vgs(th) is where the device just begins to conduct, typically at a tiny drain current such as 250 uA. It is not the voltage at which the part is fully on. Look at the Rds(on) rating conditions: a part specified at 10 V needs a 10 V driver, while logic-level MOSFETs are specified at 4.5 V or even 2.5 V for direct microcontroller drive. Also respect the maximum Vgs, commonly plus or minus 20 V, and watch for ringing that can exceed it.
Gate charge and switching loss
Qg tells you how much charge the driver must supply to switch the device. Drive power is roughly Qg x Vgs x fsw, and switching speed depends on the driver's peak current: transition time is about Qg divided by gate current. At high frequency, the figure of merit Rds(on) x Qg is a useful way to compare devices. For a high-side or high-frequency design, prioritize low Qg and Qgd (the Miller charge); for a low-frequency, high-current design, prioritize low Rds(on).
Other considerations
- Vds rating: leave 20-30% margin above the maximum bus voltage, plus any inductive spikes.
- Body diode: reverse recovery charge matters in half-bridge circuits.
- Package and thermal path: a tiny Rds(on) is useless if the package cannot shed the heat. Check RthJA and the copper area on your PCB.
- SOA: linear-mode operation, such as hot-swap, needs a safe operating area check.
Putting it together
Start with voltage and current, pick a drive level your controller can provide, estimate conduction and switching losses at your frequency, then verify with thermal calculations. When you are ready to shortlist candidates, you can browse a range of MOSFETs and compare Vds, Rds(on) and package side by side. Always confirm the final choice against the datasheet and measure switching waveforms on the bench.
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