Altera PowerPlay Early Power Estimator - Manual (Page 32)

3–20 Chapter 3: Using Cyclone III PowerPlay Early Power Estimator
Power Analysis
PowerPlay Early Power Estimator User Guide for Cyclone III FPGAs © June 2009 Altera Corporation
The device can be considered a heat source and the junction temperature is the
temperature at the device. For simplicity, we can assume that the temperature of the
device is constant regardless of where it is being measured. In reality, the temperature
varies across the device.
Power can be dissipated from the device through many paths. Different paths become
significant depending on the thermal properties of the system. In particular, the
significance of power dissipation paths vary depending on whether or not a heat sink
is being used for the device.
Not Using a Heat Sink
When a heat sink is not used, the major paths of power dissipation are from the device
to the air. This can be referred to as a junction-to-ambient thermal resistance (
JA
). In
this case, there are two significant use the symbol for junction-to-ambient thermal
resistance's acronym paths. The first is from the device through the case to the air. The
second is from the device through the board to the air.
Figure 3–14 shows the thermal representation without a heat sink.
In the model used in the PowerPlay Early Power Estimator, power is dissipated
through the case and board. Values of 
JA
have been calculated for differing air flow
options accounting for the paths through the case and through the board.
Figure 3–15 shows the thermal model for the PowerPlay Early Power Estimator
without a heat sink.
The ambient temperature does not change, but the junction temperature changes
depending on the thermal properties. Since a change in junction temperature affects
the thermal device properties used to calculate junction temperature, calculating
junction temperature is an iterative process.
Figure 3–14. Thermal Representation without a Heat Sink
Case
Thermal Representation without Heat Sink
Board
Device
θ
JA
Figure 3–15. Thermal Model in the PowerPlay Early Power Estimator without a Heat Sink
T
J
JA
T
A
Power (P)
Heat
Source
θ