Junction-temperature estimator

Assumptions to check

• The thermal metric is valid for the same reference temperature and physical setup.
• Dissipation is steady-state.
• Power and ambient conditions are representative of the operating corner.

A thermal metric needs its reference

Junction temperature is calculated from a temperature reference plus power times a thermal metric. The equation is simple, but the chosen metric matters: case, board, and ambient references are not interchangeable, and RθJA is strongly tied to the board and airflow used to obtain it.

This tool shows the rise and optional limit margin. It is a first-pass thermal budget, not a package rating or a promise that the assembly stays within limit.

Check the hot corner

Use maximum dissipation, the relevant ambient or board temperature, and a metric valid for the actual mounting. Then compare the selected part’s data, board model, and temperature measurement under representative conditions.

Match the thermal path to the hardware

First identify the temperature reference in the entered thermal metric. Junction-to-case, junction-to-board, and junction-to-ambient values describe different paths and cannot be swapped into the same equation. Use a power estimate that represents the hot electrical corner, then select an ambient, board, or case reference that is actually available in the assembly. The resulting rise is meaningful only for the same package, board area, copper, orientation, airflow, and mounting condition as the metric.

A positive nominal margin is not a release criterion. Check maximum dissipation, supply and load variation, switching loss, neighboring heat sources, airflow, thermal-interface material, and temperature dependence of loss. Use the selected device data, a board-aware model, and representative measurement to validate a persistent hot condition. For pulsed or cyclic power, assess transient thermal impedance and lifetime separately from this steady-state screen.

Common mistakes

• Mixing RθJC, ΨJT, and RθJA references.
• Using nominal rather than maximum loss.
• Treating a data-sheet board condition as the final product environment.

Model limit and handoff

Return to the exact device thermal data, board environment, and representative measurement or thermal simulation.

FAQs