MOSFET Thermal Resistance Calculator
Calculate the complete MOSFET thermal path from junction through package, interface material, and heatsink to ambient.
Reverse modes determine the maximum allowable total thermal resistance or the required heatsink θSA for practical component selection and thermal verification.
Engineering tool
MOSFET Thermal Resistance Calculator
Evaluate a junction-to-ambient thermal path or reverse-solve the required total and heatsink thermal resistance.
Steady-state device power converted to heat.
Worst-case air temperature around the heatsink or PCB.
Package thermal resistance from the datasheet.
Interface pad, grease, insulator, and mounting contribution.
Heatsink performance under the expected airflow and orientation.
Result console
- Total thermal resistance (θJA)
- 12°C/W
- Temperature rise (ΔT)
- 60°C
- Junction temperature (Tj)
- 85°C
Thermal path status
Acceptable
The nominal path estimate is below 125 °C. Verify it against the device Tjmax and worst-case power, ambient temperature, interface, and airflow.
Formula reference
MOSFET Thermal Resistance Formulas
The first-order steady-state model treats each thermal resistance in the heat-flow path as a series element. Use power in watts and thermal resistance in °C/W.
Total thermal resistance: θJA,total = θJC + θCS + θSATemperature rise: ΔT = P × θJA,totalJunction temperature: Tj = Ta + ΔTRequired total resistance: θJA,required = (Tjmax − Ta) / PRequired heatsink resistance: θSA,required = θJA,required − θJC − θCSVariable definitions
- P
- MOSFET power dissipation
- Ta
- ambient temperature
- Tj and Tjmax
- estimated and maximum junction temperatures
- θJC
- junction-to-case thermal resistance
- θCS
- case-to-sink interface resistance
- θSA
- sink-to-ambient thermal resistance
Worked Example
P = 5 W, Ta = 25 °C, θJC = 1.5 °C/W, θCS = 0.5 °C/W, and θSA = 10 °C/W.
θJA,total = 1.5 + 0.5 + 10 = 12 °C/W
ΔT = 5 W × 12 °C/W = 60 °C
Tj = 25 °C + 60 °C = 85 °C
The nominal path estimate is acceptable, subject to the selected MOSFET Tjmax and worst-case operating conditions.
Engineering Notes
Series thermal path
Total thermal resistance is the sum of the junction-to-case, case-to-sink, and sink-to-ambient path resistances.
Package contribution
θJC depends on the MOSFET package, die attach, and datasheet measurement conditions.
Interface contribution
θCS depends on the thermal pad, grease, electrical insulator, surface finish, and mounting pressure.
Heatsink contribution
θSA depends on heatsink size, airflow, fin orientation, enclosure conditions, and nearby heat sources.
Worst-case design
Use worst-case power and ambient temperature, then apply thermal margin below the absolute maximum junction rating.
Support reference
FAQ
What is MOSFET thermal resistance?
MOSFET thermal resistance describes how strongly the package and cooling path oppose heat flow. A value in °C/W indicates how many degrees the junction rises for each watt dissipated under specified conditions.
How do you calculate total thermal resistance?
For a MOSFET mounted to a heatsink, add the series path resistances: θJA,total = θJC + θCS + θSA. Multiply that result by power to estimate temperature rise.
What is θJC?
θJC is junction-to-case thermal resistance. It is primarily determined by the MOSFET die, internal construction, and package, and should be taken from the relevant datasheet conditions.
What is θCS?
θCS is case-to-sink thermal resistance. It includes the thermal interface pad, grease, electrical insulator, surface flatness, and mounting pressure between the package and heatsink.
What is θSA and how do I choose a heatsink?
θSA is sink-to-ambient thermal resistance. Choose a heatsink rated at or below the calculated requirement, then derate it for airflow, orientation, enclosure temperature, nearby heat sources, and installation conditions.
Documentation
Related Engineering Guides
Design notes, guides, and engineering articles linked to this tool.
Engineering Guide
Power Dissipation and Component Power Ratings
Calculate actual component dissipation and choose safe power ratings using thermal resistance, derating, junction temperature, SOA, pulse conditions, PCB heat spreading, and real operating conditions.
20 min · Intermediate
Engineering Guide
How to Choose the Right MOSFET
Choose MOSFETs by application, VDS, drain current, real gate voltage, RDS(on), gate charge, switching frequency, driver capability, power loss, thermal design, package, SOA, and layout.
20 min · Intermediate
Engineering Blog
10 Common Power Design Mistakes (and How to Avoid Them)
Avoid power design mistakes involving watts versus watt-hours, unit prefixes, RMS current, AC power factor, efficiency, derating, thermal design, startup current, voltage drop, batteries, PCB copper, and undersized components.
17 min · Intermediate
Related Calculators
MOSFET Junction Temperature Calculator
Estimate junction temperature, maximum power, and thermal margin.
MOSFET Total Power Dissipation Calculator
Combine conduction, switching, and gate-drive loss with thermal estimates.
MOSFET Conduction Loss Calculator
Calculate current-dependent loss using hot RDS(on) and duty cycle.
Transistor Power Dissipation Calculator
Estimate BJT or MOSFET power, temperature rise, and junction temperature.
Diode Power Dissipation Calculator
Calculate diode loss, thermal rise, and power margin.