Thermal analysis
Junction Temperature Calculator
Estimate component junction temperature, thermal margin and allowable power in still air, with forced airflow or with a heatsink.
Operating point
Use the losses converted into heat inside the component, which may differ from its total electrical power.
Package thermal data
Component dimensions
Optional · surface-density calculationLive result
Estimated junction temperature
Thermal summary
Assumptions to verifyRθJA only applies to the manufacturer test conditions: package, PCB copper, orientation and surroundings can materially change the result.
Thermal design guide
From thermal resistance to junction temperature
Junction temperature is the active internal temperature of a semiconductor. It follows from ambient temperature, actual dissipated power, and the thermal path to air or a heatsink.
Method and assumptions
At steady state, the basic calculation is TJ = TA + P × Rθ. In still air, RθJA represents the complete junction-to-ambient path under the datasheet test conditions. With a heatsink, the path is split into RθJC + RθCS + RθSA. Airflow mode only provides a screening model when no manufacturer curve is available.
Inputs to verify
Dissipated power
Enter losses converted into heat inside the component. For a linear regulator, for example, they are close to (Vin − Vout) × Iout, with possible additional internal losses.
Thermal resistances
Use values matching the intended package, PCB copper, orientation, and airflow. Do not mix RθJA and RθJC within the same path.
Local ambient temperature
Use the air temperature near the component or at the heatsink inlet, not merely the nominal room temperature.
Maximum TJ and derating
Maximum TJ is an absolute limit, not an operating target. Keep suitable margin for transients, tolerances, and the intended service life.
Package dimensions (optional)
Length and width provide projected footprint and average density in W/cm² or mW/mm². This is useful for comparison but does not describe die hot spots.
Recommended workflow
- 1Calculate worst-case dissipation and choose the maximum local ambient temperature.
- 2Select the thermal path matching the real assembly, then enter datasheet thermal resistances.
- 3Check TJ, margin, and maximum power, then validate the prototype by measurement in its final enclosure.
Example: package in still air
With TA = 25°C, P = 2 W and RθJA = 45°C/W, the rise is 90°C and TJ reaches 115°C. With a 125°C limit, only 10°C of margin remains, so improving the PCB, reducing loss, or adding cooling is prudent.
What the model does not capture
The calculation is steady-state with constant parameters. It does not model transient thermal impedance Zθ(t), hot spots, detailed radiation, air recirculation, or power variation with temperature.
Frequently asked questions
Why does RθJA change with the PCB?
A substantial part of the heat often flows through pins or the exposed pad into PCB copper. Copper area, thermal vias, layer count, and thickness therefore change the measured resistance.
Can I add RθJA to heatsink resistance?
No. RθJA already describes a complete path to ambient. For a heatsink, use the junction-case-interface-sink path: RθJC + RθCS + RθSA.
Is the airflow estimate enough to validate a design?
No. It supports an initial comparison. Geometry, orientation, obstructions, and inlet temperature then require a manufacturer curve, simulation, or measurement.
References to consult
- Component datasheet: RθJA, RθJC, maximum TJ, and any derating or airflow curves.
- Heatsink and thermal-interface datasheets: RθSA and RθCS under the intended mounting conditions.
- JEDEC JESD51 family for thermal characterisation methods of electronic packages.
Original educational content, reviewed for technical clarity on 16 September 2026. Always verify datasheets, applicable standards, and your design before power-up or manufacture.