Electrical & Electronics
Transistor Thermal & Heatsink Calculator
Estimate junction temperature, thermal margin, maximum dissipation and required heatsink resistance.
Your inputs
Your results
- Junction temperature estimate
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- Temperature margin
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- Temperature rise
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- Maximum dissipation estimate
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- Maximum heatsink thermal resistance
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—Estimate junction temperature and compare heatsink resistance
Estimate steady-state junction temperature for one transistor or another device with a known dissipation. Choose a bare-package model or a series heat path through case, mounting interface and heatsink. Compare temperature rise, margin and maximum modeled dissipation. The calculation does not simulate transient heating, thermal coupling between devices or parallel heat paths through the PCB.
Step by step
- Choose with or without heatsink and enter device dissipation in W. Use heat generated in the device, rather than the load's delivered power. A link from the MOSFET calculator can prefill its transistor-loss estimate; check that value against your intended operating condition.
- Enter ambient temperature and your chosen junction temperature limit in °C. Without a heatsink, enter junction-to-ambient resistance θJA. With a heatsink, enter junction-to-case θJC, case-to-heatsink θCS and heatsink-to-ambient θSA in °C/W.
- Read junction temperature, temperature rise and margin together. Compare the entered heatsink resistance with the maximum calculated requirement. Change dissipation, ambient or thermal resistance to examine the result, then allow appropriate operating margin and verify the physical assembly.
Settings and limits
- Thermal path and ambient
- The bare model uses θJA; heatsink mode sums θJC + θCS + θSA. Junction temperature is ambient plus dissipation times total thermal resistance. Use local operating ambient near the assembly. Thermal resistance depends on the package, PCB, mounting interface, airflow and installation; a datasheet value needs compatible conditions.
- Junction limit and power margin
- The junction limit is a value you select, not a rating inferred from a part number. Margin is that limit minus estimated junction temperature. Maximum modeled dissipation is the available temperature rise divided by total thermal resistance, bounded at zero when ambient is already above the limit.
- Required heatsink resistance
- The maximum allowed θSA is (junction limit − ambient) / dissipation − θJC − θCS. A lower positive θSA gives a smaller temperature rise in this model. The required value changes with ambient and power; it does not select a heatsink or account for another device sharing one.
Worked example
Choose heatsink mode with 5 W dissipation, ambient 40 °C and junction limit 125 °C. Enter θJC = 2 °C/W, θCS = 0.5 °C/W and θSA = 10 °C/W. Total resistance is 12.5 °C/W, temperature rise 62.5 °C and junction temperature 102.5 °C.
| θ | TJ | ΔT | Tlimit − TJ | Pmax |
|---|---|---|---|---|
| θSA = 10 °C/W | 102.5 °C | 62.5 °C | 22.5 °C | 6.8 W |
| θSA = 20 °C/W | 152.5 °C | 112.5 °C | −27.5 °C | 3.77778 W |
| θJA = 20 °C/W | 140 °C | 100 °C | −15 °C | 4.25 W |
Margin is 22.5 °C, maximum modeled dissipation 6.8 W and maximum heatsink resistance 14.5 °C/W. Changing only θSA to 20 °C/W gives junction temperature 152.5 °C and margin −27.5 °C. Without a heatsink, an entered θJA of 20 °C/W would instead give 140 °C junction temperature for the same power and ambient.
Questions and troubleshooting
Can I reuse a datasheet θJA value for any PCB?
θJA includes heat transfer through the specified board and surroundings, so it is not a universal package-only constant. Use a value justified for your PCB, airflow and enclosure, or characterize the assembly. Changing board copper or nearby heat sources can change the effective path.
What does no finite passive heatsink mean?
The remaining allowed heatsink resistance is zero or negative after the junction-to-case and interface terms consume the temperature budget. Improving the heatsink alone cannot satisfy those modeled inputs with a finite positive resistance. Reduce power or ambient, improve the other path terms or revise the justified limit.
Why does zero dissipation show no heatsink requirement?
With zero heat in this steady-state model, junction temperature equals ambient and there is no power-based heatsink resistance to calculate. Ambient must still be compared with the junction limit. The tool accepts zero dissipation but requires a positive total thermal resistance.