MOSFET Selection Guide: 7 Key Parameters Every Engineer Should Check

Why MOSFET Selection Isn’t Just “Pick the Right Voltage”

MOSFETs are the workhorses of modern power electronics — from motor drivers and LED lighting to DC-DC converters and battery management systems. But a MOSFET that works perfectly on paper can fail catastrophically in reality if you overlook one critical parameter.

The 7 Parameters

1. Vds — Drain-Source Voltage

Always choose a MOSFET rated for at least 1.5x your maximum operating voltage. A 24V system should use a 40V or higher MOSFET to handle voltage spikes and ringing.

2. Rds(on) — On-Resistance

This is the resistance when the MOSFET is fully on. Lower is better — but it’s a tradeoff: lower Rds(on) typically means higher gate charge. Calculate your conduction losses: P = I² × Rds(on).

3. Vgs(th) — Gate Threshold Voltage

This is the voltage at which the MOSFET just starts to turn on. For 5V logic-level drive, look for Vgs(th) of 1-2V max. For 3.3V microcontrollers, you need sub-1V threshold.

4. Qg — Total Gate Charge

Higher Qg means slower switching and more drive current needed. For PWM applications at >100kHz, minimize Qg to reduce switching losses.

5. Id — Continuous Drain Current

The headline number on datasheets is often optimistic (tested with infinite heatsink at 25°C). Derate by 30-50% for real-world conditions.

6. Thermal Resistance (RθJA)

This tells you how much the MOSFET heats up per watt of power. Multiply your power loss by RθJA and add ambient temperature to estimate junction temperature. Stay under 125°C for reliability.

7. Package Type

SOT-23 for signal-level, SO-8 or DFN for medium power, TO-220 or D2PAK for high current. Browse our discrete semiconductors →