MOSFET Knowledge Base
Clear answers to common questions about channel type, structure, voltage class and first-pass product selection.
Common questions, concise answers.
What is an N-channel MOSFET?
An enhancement-mode N-channel MOSFET turns on when its gate is driven sufficiently positive relative to its source. The threshold voltage marks the start of conduction; it does not mean the device is fully on. Check the datasheet’s RDS(on) at the gate voltage your driver can actually supply, then account for the higher resistance at operating temperature. For high-side use, confirm that the driver can raise the gate above the source when needed.
What is a P-channel MOSFET?
A P-channel MOSFET turns on when its gate is driven sufficiently below its source. This can make some high-side switches simpler, especially in low-voltage circuits, but the choice is device- and design-specific. Compare its voltage rating, hot RDS(on), current, package and switching needs with an N-channel option and the required gate-drive circuit.
How should I choose a MOSFET voltage rating?
Start with the highest voltage the device may see, including startup, load dump, ringing and other transients. The MOSFET’s VDS rating must leave suitable margin for the real circuit; the required margin depends on the topology, clamping, layout and transient behavior. Check measured or simulated waveforms and the device’s safe operating area. See the MOSFET voltage rating guide before comparing voltage classes.
How do Planar, Trench, SGT and Super Junction MOSFETs differ?
Planar and Trench describe gate and channel structures; SGT (shielded-gate trench) is a trench approach that uses a shield structure to influence capacitance and switching behavior. Super Junction devices use charge-balanced regions to improve the voltage and resistance trade-off, commonly in higher-voltage silicon applications. These labels do not guarantee a particular performance level, and naming can vary by manufacturer. Compare the actual datasheet values for VDS, hot RDS(on), total and reverse-transfer gate charge, output capacitance, safe operating area and package. Read our guides to MOSFET operation and MOSFET types and applications.
When should I evaluate a SiC MOSFET?
Evaluate SiC when the system’s voltage, switching frequency, efficiency or thermal targets may justify a wide-bandgap device. Compare total system losses and cooling needs, not just the switch’s headline rating. Check gate-drive voltage limits, switching speed, layout parasitics, EMI, short-circuit requirements, availability and cost. A SiC MOSFET is not automatically a drop-in replacement for silicon; validate the driver and power stage together. Explore SiC MOSFET options and the DC-DC converter application.
What does RDS(on) mean, and how does it affect loss?
RDS(on) is the drain-to-source resistance in the on state under specified gate-drive and temperature conditions. A first-pass conduction-loss estimate is Pcond ≈ IRMS² × RDS(on, hot). Use the maximum datasheet value at the gate voltage available in your design, and account for temperature rise and current ripple. Lower RDS(on) can reduce conduction loss, while a larger die may increase gate charge or capacitance, so check switching loss too.
How do I estimate switching loss?
For a hard-switched device, a rough first estimate is Psw ≈ 0.5 × VDS × ID × (tr + tf) × fsw, where tr and tf are switching transition times and fsw is switching frequency. This is only a starting point: actual loss depends on the gate driver, load current, voltage, parasitic inductance, output capacitance, reverse recovery and topology. Use datasheet curves and measured waveforms to refine the estimate. In synchronous converters, also compare dead-time and body-diode losses; our synchronous rectification guide explains that trade-off.
What information should I send for a product recommendation?
Share the model number or target specifications, maximum bus voltage and transients, continuous and peak current, switching frequency, topology, available gate-drive voltage, package or thermal limits, application, and whether you need samples or production volume. A schematic or datasheet for the existing part helps identify constraints. You can also review the low-voltage and high-voltage MOSFET ranges.
A practical first-pass MOSFET selection checklist
Use these checks to build a shortlist before comparing individual part numbers. The final choice should be validated in the actual power stage and against the manufacturer’s datasheet.
- Set the voltage requirement. Record the normal bus range and the highest transient at the MOSFET terminals. Check VDS margin, clamping and ringing in the real layout.
- Estimate current and conduction loss. Use RMS current for thermal loss estimates, include ripple and duty cycle, and calculate Pcond ≈ IRMS² × RDS(on, hot). Compare the maximum hot RDS(on) at your available VGS.
- Check switching and gate drive. Match gate voltage, driver strength, total gate charge and reverse-transfer charge to the switching frequency. Estimate switching loss and account for dead time, output capacitance and layout parasitics.
- Verify thermal and reliability limits. Check junction temperature, package and PCB cooling, safe operating area, avalanche or short-circuit needs where applicable, and the expected operating environment.
- Validate the shortlist. Compare complete datasheets, review the application topology, then measure temperature and switching waveforms on a prototype. Browse the application guides or send your operating targets to dtPower Semi for a device discussion.
For a broader introduction, start with how MOSFETs work, then compare the relevant low-voltage, high-voltage or SiC product family.
Still deciding where to start?
Send the application and operating targets. We will reply within 24 hours on business days.