Inputs
Example Data Table
These sample runs show typical inputs and computed junction temperature.
| Method | Base Temp | Ptotal (W) | Rθ total (°C/W) | ΔT (°C) | Tj (°C) | Notes |
|---|---|---|---|---|---|---|
| Ambient | Ta=25 | 2.0 | 35 | 70 | 95 | Small IC on modest PCB copper. |
| Case | Tc=70 | 12.0 | RθJC 1.8 + RθCA 6.5 | 99.6 | 169.6 | Power device with heatsink and TIM. |
| Ambient | Ta=40 | 0.6 | 62 | 37.2 | 77.2 | Warm enclosure, low dissipation sensor. |
Calculation History
Up to 30 recent runs are stored in your session.
| Time | Method | P (W) | Rθ total | ΔT | Tj | Status |
|---|---|---|---|---|---|---|
| No saved runs yet. Submit the form to add one. | ||||||
Formula Used
Use when you know surrounding air temperature and an applicable RθJA value.
- P = Pstatic + Pswitch + Pother
- ΔT = P · RθJA
- Tj = Ta + ΔT
Use when you can measure case temperature and model the heat path to ambient.
- Rθtotal = RθJC + RθCA
- ΔT = P · Rθtotal
- Tj = Tc + ΔT
Notes: Thermal resistance values are condition-dependent. Match airflow, mounting torque, PCB copper, and measurement points to the datasheet test setup.
How to Use This Calculator
- Select Ambient-based or Case-based method.
- Choose temperature units and keep inputs consistent.
- Enter power losses (static, switching, and other) in watts.
- Enter the relevant thermal resistance values from your model or datasheet.
- Optionally enter Tj,max to get a safety margin.
- Press Submit to show results above the form.
Why Junction Temperature Matters
Junction temperature is the internal silicon temperature that most strongly drives reliability. For many devices, lifetime halves for each 10°C rise. This calculator converts your measured conditions into an estimate of Tj so you can compare designs, validate heatsinks, and set derating rules for production.
Thermal Resistance Inputs and Assumptions
Thermal resistance links heat flow to temperature rise: ΔT = P·Rθ. Use RθJA when the board, airflow, and orientation match the datasheet test. Use RθJC plus an estimated RθCA when the case point is well defined and the heat path includes a heatsink, interface material, and convection.
Power Loss Breakdown and Measurement
Total dissipation is the sum of static, switching, and other losses. Static includes conduction and bias losses, switching includes transition and capacitive effects, and other can cover gate drivers, magnetics, or a safety add-on. If you have a waveform-based estimate, enter the average power in watts for each component.
Selecting Ambient or Case Calculation
Ambient mode uses Ta as the base and is fast when only air temperature is available. Case mode uses Tc as the base and often reduces uncertainty for power packages. In both modes, the calculator adds the temperature rise to the chosen base temperature, keeping units consistent in Celsius or Kelvin.
Reading Status, Margin, and Sensitivity
If you enter a maximum rating, the tool reports margin (Tj,max − Tj). A negative margin shows an over-limit condition, while a small positive margin indicates limited headroom. The Plotly curve shows how Tj increases linearly with power, highlighting how even small power reductions or Rθ improvements can restore margin.
Using Exports and Trend Checks
CSV exports capture single results and your session history for design reviews. The PDF report summarizes inputs, computed rise, and final Tj for documentation. For trend checks, run several scenarios with different airflow, copper area, or heatsink choices, then compare the history table and the plotted slope (Rθ) to see which change delivers the best reduction. Typical limits are 125°C, 150°C, or 175°C depending on process and package. Always confirm whether the rating assumes continuous operation, short pulses, or a specific mounting condition. When uncertain, design for at least 10°C to 20°C additional margin at worst-case ambient. In practice.
FAQs
What is junction temperature?
Junction temperature is the estimated silicon die temperature inside a semiconductor. It is higher than case or ambient because heat must flow through package and interfaces, creating a temperature rise proportional to power and thermal resistance.
Which method should I use?
Use Ambient-based when you only know air temperature and have a valid RθJA for your board and airflow. Use Case-based when you can measure Tc and have RθJC plus a reasonable RθCA estimate.
Can I mix Celsius and Kelvin inputs?
Keep all temperature values in one unit for a run. The calculator supports either Celsius or Kelvin, but it does not convert between them automatically. Thermal resistance temperature units must match your selected temperature unit.
Why does my result change a lot with small Rθ changes?
Because Tj increases linearly with Rθ: ΔT = P·Rθ. At higher power, a small resistance error produces a larger temperature shift. Validate mounting, airflow, and interface material thickness to improve confidence.
How accurate is the estimate?
Accuracy depends on how well power loss and thermal resistance match real conditions. Datasheet Rθ values can differ from your layout, airflow, and heatsink. Treat the output as an engineering estimate and confirm with measurement when near limits.
What does Near Limit mean?
Near Limit indicates the computed margin is 5 units or less above your entered Tj,max. It is a caution state that suggests derating, better cooling, or measurement validation before finalizing the design.