A (bit simplistic) summary of thermal resistance, thermal characterisation parameter and how to use them to estimate junction temperature.

Summary

  • Thermal resistance of certain path (such as junction-case top) = (temperature difference ) / ( actual power conducted through this path )
  • Characterisation parameter between junction and case top = ( temperature difference ) / ( total power dissipated )
  • Characterisation parameter by definition is not thermal resistance, therefore a different letter Ψ is used.
  • Characterisation parameter is more suitable for estimating junction temperature in applications (chips mounted on a circuit board), while thermal resistance could be used to compare the thermal performance of different packages.

Formulae

For example, estimating junction temperature using junction-case top parameters.

$P_T$ is the total dissipated power, and $P_{j-ctop}$ is the portion of power conducted through the junction-case top path.

Using characterisation parameter:

$$T_{j}=T_{ctop}+P_{T}\psi_{j-ctop}$$

Using thermal resistance

$$T_{j}=T_{ctop}+P_{j-ctop}R_{\theta jc}$$

  • Pay attention to the test condition (such as PCB size, number of vias)

Resource

Semiconductor and IC Package Thermal Metrics (Rev. C) (ti.com)

How to Evaluate Junction Temperature Properly with Thermal Metrics (Rev. B)

The complexity of the thermal conduction path in an actual system determines that junction temperature cannot be simply estimated by the total dissipation power and thermal resistance parameters.