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Heat flows from a semiconductor's junction out to the surrounding air through a series of thermal "resistances," exactly like voltage divides across resistors in series. θJC is the resistance from junction to case (fixed by the device package), θCS is case-to-heatsink (through any thermal interface material), and θSA is heatsink-to-ambient. Each stage adds a temperature rise equal to P×θ for that stage, and the junction temperature is the sum of all these rises added to the ambient temperature.
| Quantity | Formula |
|---|---|
| Total thermal resistance | θtotal = θJC + θCS + θSA |
| Junction temperature | Tj = Ta + P × θtotal |
| Temperature rise per stage | ΔTstage = P × θstage |
| Maximum safe power | Pmax = (Tj(max) − Ta) / θtotal |
If there is no heatsink, use the device's single lumped θJA (junction-to-ambient) value directly in place of the JC+CS+SA sum — the datasheet quotes both figures separately for exactly this reason. Keeping Tj comfortably below the maximum rated junction temperature (often 125–175 °C) is essential for reliability; running near the limit dramatically shortens component life.
It is the temperature at the actual semiconductor die inside a package — the hottest point in the device. It cannot be measured directly in normal operation and must be calculated from the power dissipated and the thermal resistance path to ambient.
Tj = Ta + P×θtotal, where Ta is the ambient temperature, P is the power dissipated, and θtotal is the sum of all thermal resistances between the junction and ambient air.
Junction-to-case thermal resistance, a fixed property of the device package (found on the datasheet) that describes how well heat moves from the silicon die to the outer case surface.
θCS (case-to-sink) is the resistance through the thermal interface material between the case and heatsink; θSA (sink-to-ambient) is the heatsink's own resistance to the surrounding air, which depends on its size, fins and airflow.
θJA (junction-to-ambient) is a single lumped value for when there is no separate heatsink — it combines all the thermal path resistance in one number, quoted for the device mounted on a standard test board.
θSA is usually much larger than θJC or θCS unless a large, well-finned heatsink with good airflow is used, so most of the total temperature rise typically happens in that final stage.
Stay comfortably below the datasheet's maximum rated Tj (commonly 125–175 °C for silicon, higher for SiC/GaN). Many designers target 20–30 °C of margin below the maximum for long-term reliability.
Rearrange the formula: Pmax = (Tj(max)−Ta)/θtotal. This tells you how much power the device can dissipate at a given ambient temperature before exceeding its rating.
Since Tj=Ta+P×θ, a higher starting ambient leaves less temperature "budget" before reaching the maximum Tj, so the allowed power (and hence output) must be derated at higher ambient temperatures.
Yes, significantly. Semiconductor reliability generally follows an exponential relationship with junction temperature (roughly doubling failure rate per 10–15 °C rise), so keeping margin below the maximum rating meaningfully extends product life.
It gives the correct steady-state (long-term average) temperature for continuous power. For pulsed or transient loads, a more detailed transient thermal-impedance model (using Zth curves) is needed to capture short-term temperature spikes.
This tool computes Tj from a given thermal chain; the Heat Sink calculator works the reverse direction, solving for the θSA a heatsink needs to keep Tj at a target value — use them together in a design.
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