Altera Cyclone II PowerPlay Early Power Estimator - Manual (Page 42)

3–26 Altera Corporation
PowerPlay Early Power Estimator User Guide For Cyclone II FPGAs May 2006
Power Analysis
Thermal Analysis
In the thermal analysis section, the device’s ambient temperature, the
airflow, the heat sink solution and the board thermal model are
considered to determine the junction temperature (T
J
) in degrees Celsius.
T
J
is the maximum recommended operating junction temperature based
on your device and thermal conditions.
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 varies
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 (
θ
JA
). In this case there are two significant junction-to-
ambient thermal resistance paths. The first is from the device through the
case to the air and the second is from the device through the board to the
air.
Figure 3–22 shows the thermal representation without a heat sink.
Figure 3–22. Thermal Representation without Heat Sink
In the model used in the PowerPlay Early Power Estimator spreadsheet,
power is dissipated through the case and board. Values of
θ
JA
have been
calculated for differing air flow options accounting for the paths through
the case and through the board.
Figure 3–26 shows the thermal model for
the PowerPlay Early Power Estimator spreadsheet without a heat sink.
C ase
¸
JA
Therm al R epresentation w ithout H eat S ink
B oard
D evice