How to Choose a MOSFET: Vds, Rds(on), Gate Charge and Heat
A practical MOSFET selection checklist covering breakdown voltage, on-resistance, gate charge, threshold voltage, package thermal design and logic-level drive.
Key parameters to check first
Start with the maximum drain-source voltage Vds and add a safety margin (commonly 1.5–2× the bus voltage). Then evaluate Rds(on) for conduction loss, continuous drain current Id, and gate charge Qg for switching loss.
For direct drive from a 3.3V or 5V microcontroller, select a logic-level MOSFET whose Rds(on) is specified at Vgs = 4.5V or 2.5V—not only at 10V.
Thermal and switching considerations
Estimate conduction loss as I²R and switching loss from Qg and switching frequency. Verify the package thermal resistance and add a heatsink or copper pour when junction temperature could approach limits.
In half-bridge and motor-drive designs, also check body-diode reverse recovery and use complementary N/P-channel or dedicated high-side drivers as needed.
Frequently Asked Questions
Why does my MOSFET run hot at low gate voltage?
A standard MOSFET may not fully enhance at 3.3V, leaving Rds(on) high. Use a logic-level part rated at the actual drive Vgs.
How much voltage margin should Vds have?
A 1.5–2× margin over the maximum bus voltage is common to absorb inductive spikes and improve reliability.