MOSFET Junction Temperature Calculator
Estimate MOSFET junction temperature and thermal margin from device power, ambient temperature, and junction-to-ambient thermal resistance.
Reverse modes calculate the theoretical maximum allowable power or the required thermal resistance for package, PCB copper, airflow, and cooling decisions.
Engineering tool
MOSFET Junction Temperature Calculator
Estimate junction temperature and thermal margin, or reverse-solve maximum power and required θJA.
Steady-state MOSFET loss converted to heat.
Local air or PCB environment around the device.
Junction-to-ambient thermal resistance for the actual package and PCB.
Absolute maximum junction temperature from the datasheet.
Result console
- Temperature rise (ΔT)
- 80°C
- Junction temperature (Tj)
- 105°C
- Thermal margin
- 45°C
Thermal status
Safe
The steady-state estimate has useful thermal margin. Confirm datasheet test conditions and worst-case board-level thermal performance before release.
Formula reference
MOSFET Junction Temperature Formulas
These equations provide a first-order steady-state thermal estimate. Use power in watts, temperature in degrees Celsius, and thermal resistance in °C/W.
Temperature rise: ΔT = P × θJAJunction temperature: Tj = Ta + ΔTThermal margin: Tmargin = Tjmax − TjMaximum power: Pmax = (Tjmax − Ta) / θJARequired thermal resistance: θJA,required = (Tjmax − Ta) / PVariable definitions
- P
- MOSFET power dissipation
- Ta
- ambient temperature
- Tj
- estimated junction temperature
- Tjmax
- maximum rated junction temperature
- θJA
- junction-to-ambient thermal resistance
Worked Example
Power dissipation P = 2 W, ambient temperature Ta = 25 °C, θJA = 40 °C/W, and Tjmax = 150 °C.
ΔT = 2 W × 40 °C/W = 80 °C
Tj = 25 °C + 80 °C = 105 °C
Thermal margin = 150 °C − 105 °C = 45 °C
The nominal steady-state result is Safe, subject to datasheet conditions and design derating.
Engineering Notes
Power creates temperature rise
Junction temperature is higher than ambient because MOSFET power dissipation flows through a finite thermal resistance.
Layout-dependent θJA
θJA depends heavily on package, PCB copper, thermal vias, airflow, orientation, and mounting.
Datasheet conditions
MOSFET thermal ratings assume specific test boards and cooling conditions that may differ from the final hardware.
Derating
Always design below maximum junction temperature to cover operating variation, measurement uncertainty, and aging.
Transient loads
Pulsed or transient loads require transient thermal impedance analysis rather than only steady-state θJA.
Support reference
FAQ
How do you calculate MOSFET junction temperature?
Multiply steady-state MOSFET power by junction-to-ambient thermal resistance to find temperature rise, then add ambient temperature: Tj = Ta + P × θJA.
What is θJA?
θJA is junction-to-ambient thermal resistance in °C/W. It describes the temperature rise per watt from the semiconductor junction through the package, PCB, and surrounding air under specified test conditions.
What is a safe MOSFET junction temperature?
A safe design remains below the datasheet maximum junction temperature with margin for component variation, ambient extremes, layout differences, and measurement uncertainty. Many designs target substantial derating rather than operating close to Tjmax.
How do you reduce MOSFET temperature?
Reduce conduction or switching loss, increase PCB copper area, improve airflow, use thermal vias or a heatsink, select a lower-loss device, or choose a package with a better thermal path.
Why does PCB copper affect MOSFET temperature?
PCB copper spreads heat away from the package and increases the effective area available for convection. Copper area, layer count, thermal vias, and nearby heat sources can substantially change real θJA.
Documentation
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Design notes, guides, and engineering articles linked to this tool.
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