Check MOSFET On-Resistance at the Available Gate Voltage

A MOSFET with a threshold voltage below a microcontroller’s output voltage is not automatically a suitable directly driven switch. Threshold identifies a specified onset-of-conduction condition. The relevant on-resistance may be guaranteed at a substantially different gate-to-source voltage, and the available drive may fall below its nominal value.

Start from the guaranteed resistance conditions

Find the on-resistance entries and their gate voltage, drain current and temperature conditions. Nexperia AN90059 explains why gate drive must be assessed against on-resistance rather than threshold alone. Typical curves help explain behavior but do not replace the guaranteed limits needed for a worst-case design.

Record the minimum gate voltage the driver can provide under its actual supply and load. Measure gate voltage relative to the MOSFET source. In a high-side circuit, a voltage measured relative to board ground can give a misleading impression of gate drive.

Calculate conduction loss with relevant values

For an illustrative fully enhanced DC switch carrying 3 A with 50 milliohms resistance, conduction loss is 0.45 W. If the effective resistance were 100 milliohms, that loss would be 0.9 W. These values demonstrate the sensitivity to resistance; they do not establish a particular device’s resistance at an unspecified gate voltage.

Include temperature dependence and the thermal path through the package and PCB. A room-temperature resistance calculation can understate operating loss. Check the actual package and mounting conditions rather than interpreting a headline power rating as heat that any board can dissipate.

Check transition behavior separately

Gate charge and driver capability affect switching time. A device with attractive conduction resistance can require more charge than the chosen GPIO or driver can move within the desired interval. Estimate and verify switching losses for the intended frequency and load.

Review the off state, startup and loss of driver power. Provide the intended gate bias and check leakage, coupling and the maximum permitted gate voltage. Staying below the maximum voltage rating is necessary but does not by itself establish reliable switching behavior.

Measure the final circuit

Capture gate-to-source voltage, drain-to-source voltage and relevant current with suitable equipment. Compare the observed transitions and temperature with the assumptions. Test the lowest intended driver supply and representative load conditions rather than only a lightly loaded nominal bench case.

Keep the drive-voltage budget, selected resistance limits and thermal evidence with the part choice. For a substitution, compare these conditions again. Matching package, drain voltage and threshold can still change conduction loss, drive requirements and startup behavior enough to invalidate the original review.