Formula Used
Hot RDS(on): Rhot = R25 × [1 + coefficient × temperature rise]
Conduction loss: Pcond = Irms² × Rhot × duty
Switching loss: Psw = 0.5 × VDS × Isw × (tr + tf) × frequency
Gate drive power: Pgate = Qg × Vgate × frequency
Output capacitance loss: Pcoss = 0.5 × Coss × VDS² × frequency
Body diode loss: Pdiode = Vf × I × dead time × frequency × events
Reverse recovery loss: Prr = Qrr × VDS × frequency × events
Linear mode loss: Plinear = VDS(linear) × Ilinear × duty
Junction temperature: Tj = Tambient + total power × thermal resistance
How to Use This Calculator
Enter the switching voltage and total circuit current first.
Add the MOSFET RDS(on) value from the datasheet.
Use a hot resistance estimate for safer results.
Enter rise time, fall time, and switching frequency.
Add gate charge, capacitance, diode, and recovery values.
Set thermal resistance, ambient temperature, and maximum junction limit.
Press Calculate to view losses above the form.
Use CSV or PDF buttons to save the result.
MOSFET Power Dissipation Guide
Why Heat Matters
A MOSFET may look efficient on paper. Yet heat can rise fast in real circuits. Current, switching speed, gate charge, and body diode time all create loss. That loss becomes heat inside the package. If heat cannot escape, the junction temperature rises. High junction temperature reduces reliability. It can also shift resistance, timing, and safe operating limits.
Main Loss Sources
Conduction loss is usually the first check. It depends on drain current and on resistance. The calculator uses I squared times RDS(on). It also adjusts resistance for temperature when you enter a coefficient. Switching loss appears during turn on and turn off. Voltage and current overlap for a short time. Faster edges reduce this loss, but may add noise. Gate drive loss comes from charging the gate each cycle. Output capacitance loss is stored energy lost during switching. Reverse recovery loss matters when the body diode or another diode recovers. Diode conduction loss matters during dead time.
Thermal Design
Power loss is not the final answer. You must connect it with thermal resistance. The junction rise equals power times thermal resistance. Add ambient temperature to find junction temperature. Then compare it with the maximum junction rating. A good design keeps margin. It should also account for airflow, copper area, heatsinks, and enclosure heat. Parallel devices can share loss, but sharing is never perfect. Layout, gate resistance, and device matching matter.
Practical Use
Use worst case values when possible. Choose the highest current. Use hot RDS(on), not only the room value. Add realistic rise and fall times. Include duty cycle and frequency. Review each loss line, not only the total. A high switching number suggests slower frequency, better driver strength, or another device. A high conduction number suggests lower resistance or more devices. A weak thermal margin suggests a heatsink, larger copper, lower ambient, or a package change. This calculator is an engineering aid. It does not replace datasheet limits, bench tests, or safety review.
Data Quality
Small input changes can move the result. Keep units consistent. Record assumptions beside every run. Save CSV files for reviews. Use the PDF report for field notes. Recheck values after selecting a final part. Document test results.
FAQs
What is MOSFET power dissipation?
It is the power converted into heat during operation. It includes conduction, switching, diode, recovery, and sometimes gate drive losses. The final value helps estimate junction temperature.
Why does RDS(on) change with temperature?
MOSFET channel resistance usually rises as temperature increases. Hot resistance gives a safer conduction loss estimate. Use datasheet curves when available.
Should gate drive power be counted?
Gate drive power is often dissipated in the driver and gate resistance. This calculator lists it separately. You can include it in the total budget when needed.
What causes switching loss?
Switching loss occurs when voltage and current overlap during turn on and turn off. Higher voltage, current, frequency, and transition time increase this loss.
What is body diode dead-time loss?
In bridge circuits, current may flow through the body diode during dead time. The diode voltage drop creates heat. Shorter safe dead time can reduce this loss.
How do parallel MOSFETs affect loss?
Parallel devices divide current. This can reduce conduction loss per device. Real sharing is imperfect, so the sharing factor estimates the hottest device.
What is thermal margin?
Thermal margin is the maximum junction limit minus estimated junction temperature. A positive value is required. Extra margin improves reliability.
Can this replace datasheet testing?
No. This calculator is an estimate. Confirm limits with datasheets, thermal measurements, layout review, and real operating tests.