Early Power Estimator User Guide

ID 683272
Date 7/16/2021
Document Table of Contents Not Using a Heat Sink

When you do not use a heat sink, the major paths of power dissipation are from the device to the air. You can refer this as a junction-to-ambient thermal resistance. In this case, there are two significant junction-to-ambient thermal resistance paths:
  • From the device through the case to the air
  • From the device through the board to the air
Figure 6. Thermal Representation without a Heat Sink

In the model used in the EPE spreadsheet, power is dissipated through the case and board. The θJA values are calculated for differing air flow options accounting for the paths through the case and through the board.

Figure 7. Thermal Model in the EPE Spreadsheet without a Heat Sink

The ambient temperature does not change, but the junction temperature changes depending on the thermal properties; therefore the junction temperature calculation is an iterative process.

The following equation shows the total power calculated based on the total θJA value, ambient, and junction temperatures.

Figure 8. Total Power