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Choose the decision you need from the same junction-to-ambient model.
A preset fills two fields; it is not a substitute for the selected part's datasheet.
Changing the unit converts the value without changing the physical heat load.
The junction temperature rise begins from this local air temperature.
Manual edits switch the package starting point to Custom.
Use more margin for tolerance, aging, sealed products, or uncertain airflow.
Package resistance before heat reaches the mounting interface.
°C/W
Use a preset for orientation, then enter material or assembly evidence when available.
Manual edits switch the interface starting point to Custom.
°C/W
The candidate enables temperature, margin, and maximum-power comparisons.
Manual edits switch the heat-sink candidate to Custom.
°C/W
Neutral datasheet rating is the identity default and leaves the entered θSA unchanged.
Choose one, two, or three decimal places without changing the thermal model.
Thermal measureValueDesign meaningCopy
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Design guidance

Candidate thermal fit

Compare the candidate path with the working junction target.
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What to check next

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This is a steady-state series-path estimate. Verify device, interface, heat-sink, orientation, airflow, and enclosure assumptions on the real assembly before release.

Power vs. junction

See how the candidate thermal path raises junction temperature as device heat increases.

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Heat leaves a semiconductor junction through several physical boundaries before it reaches the surrounding air. Each boundary resists that flow, so a heat sink cannot be chosen from wattage alone. The same device can run safely on an open bench and overheat inside an enclosure because local ambient temperature, mounting pressure, interface material, fin orientation, and airflow all change the path.

Thermal resistance is measured in degrees Celsius per watt (°C/W). A lower value carries more heat for the same temperature rise. Three series terms describe the simplified top-side path:

  • θJC covers junction to package case and normally comes from the device datasheet.
  • θCS covers case to heat sink, including the thermal interface material and mounting quality.
  • θSA covers heat sink to ambient air under a stated airflow and orientation.

The available temperature rise begins at the local air seen by the heat sink, not necessarily room temperature. A nearby power supply, recirculating fan exhaust, or closed case can make that local ambient much hotter. The working junction target should also sit below the absolute maximum rating by a deliberate design margin for tolerance, dust, aging, fan degradation, and changing load.

A paper result is a first-order design check. Datasheet θJC values, catalog θSA ratings, and interface estimates are measured under particular conditions, while real assemblies have board conduction, spreading resistance, contact variation, radiation, bypass airflow, and transient power. Prototype measurement or a fuller thermal model remains necessary when reliability or safety depends on junction temperature.

How to Use This Tool:

Select the decision first, then replace every preset with evidence from the intended device, interface, and airflow condition.

  1. Choose whether to solve for required heat-sink θSA, check a candidate, or find the candidate's maximum modeled power.
  2. Enter device heat dissipation, local ambient temperature, maximum junction temperature, and a design margin. Power may be entered in watts or milliwatts; temperatures may use Celsius or Fahrenheit.
  3. Enter datasheet Junction-to-case θJC and the mounted interface's Case-to-sink θCS. Presets are orientation aids, not substitutes for part or assembly data.
  4. Enter the candidate heat sink's rated θSA and choose the rating context that best represents its use.
  5. Review the required θSA ceiling, estimated junction temperature, working margin, and maximum power. A lower candidate θSA is better, provided the rating applies under the same airflow and orientation.

Interpreting Results:

A candidate should have θSA at or below the Required heat-sink θSA. A negative required value means the junction-to-case and interface path already uses more than the entire temperature budget; no positive heat-sink rating can fix that combination. Reduce power or ambient temperature, improve θJC or θCS, or relax only a margin that engineering evidence supports.

Candidate has margin means estimated junction temperature is at least 10°C below the working target. Candidate is thermally tight covers 0°C through less than 10°C of margin, including exact equality at 0°C. Candidate exceeds target means margin is negative. Even a green result needs measurement under worst-case supply, load, enclosure, orientation, and fan tolerances.

Technical Details:

The calculation uses a steady-state series path in which most modeled heat travels from junction to case, through the interface, through the heat sink, and into ambient air. Temperature differences in Celsius and kelvin have the same numeric size, so °C/W is used throughout after absolute Fahrenheit temperatures and Fahrenheit differences are converted correctly.

Formula Core

The working junction target is the absolute maximum less the chosen design margin. Dividing the available temperature rise by power gives the allowed junction-to-ambient resistance:

Ttarget=TJmax−M,θJA,required=Ttarget−TambientP

The heat-sink ceiling is what remains after package and interface resistance:

θSA,required=θJA,required−θJC−θCS

The selected rating context multiplies the entered heat-sink rating. Installed junction temperature and maximum modeled power then follow the same series path:

TJ,estimated=Tambient+P×θJC+θCS+θSA,effective,Pmax=Ttarget−TambientθJC+θCS+θSA,effective

No internal rounding is applied to the model. Display precision is one, two, or three decimal places.

Rule and Lookup Core

Context or boundaryFactorMeaning
Entered datasheet rating1.00×No planning adjustment.
Horizontal fins or weak chimney path1.20×Raises effective θSA to explore weaker natural convection.
Enclosed or obstructed airflow1.45×Raises effective θSA to explore restricted airflow.
Reliable forced airflow0.65×Lowers effective θSA to explore active airflow.
Required θSA < 0—Package path exceeds budget.
Installed margin ≥ 10°C—Candidate has margin.
0°C ≤ margin < 10°C—Candidate is thermally tight.
Installed margin < 0°C—Candidate exceeds target.

The four airflow factors are repository-authored scenario assumptions, not vendor derating curves. Use them for sensitivity checks only and replace them with a candidate rating measured under the actual airflow when available.

Accuracy Notes:

This is a steady-state, single-path approximation. It does not solve transient thermal impedance, heat spreading, parallel board conduction, thermal coupling, fan curves, altitude, radiation, contact-pressure variation, or temperature-dependent power. Manufacturer θ values may describe standardized test boards rather than the final enclosure. Use worst-case datasheet values where appropriate and verify the assembled design with calibrated measurements or detailed simulation.

Worked Examples:

Seven-watt TO-220 design

With 50°C local ambient, a 125°C junction limit, and 20°C design margin, the working target is 105°C. At 7 W the allowed θJA is about 7.86°C/W. Subtracting 4.0°C/W for θJC and 0.2°C/W for θCS leaves a required heat-sink ceiling of about 3.66°C/W. A 2.5°C/W candidate estimates 96.9°C junction temperature and 8.1°C margin, so it is classified as thermally tight rather than comfortably margined.

References: