When considering the power estimation of a design, understanding the
efficiency of the thermal solution is crucial. The lower the junction temperature, the
lower the static power of a design.
AMD recommends using lidless
packaging if it is available for your device. Lidless packaging offers a more efficient
thermal solution and allows direct contact with the heat source, removing a thermal
interface material (TIM) layer. AMD lidded and
lidless parts have the same handling and manufacturing requirements. The following
figure compares the heat sink application for a lidded and lidless device.
Thermal Tip:AMD recommends between 20 and 50 pound-force per
square inch (PSI) for the heat sink, which ensures the smallest bond line thickness
(BLT), and recommends using 4-hole mounting to ensure even pressure for both lidded and
lidless devices. For more information on lidless techniques, see
Mechanical and Thermal Design Guidelines for Lidless
Flip-Chip Packages (XAPP1301).
Figure 1. Heat Sink Example
AMD also recommends thermal simulation to ensure that there is
adequate margin and accurate power estimation. In the Xilinx Power Estimator (XPE) spreadsheet tool (download at www.xilinx.com/power), you have control
over the following thermal settings:
Junction Temperature Tj
You can override this setting to a desired junction
temperature to match your thermal simulation. If you are not running a
thermal simulation, set the junction temperature to the worst case.
Effective ΘJA
Describes the thermal efficiency of a thermal solution, the
units are measured in degrees Celsius per watt (°C/W). For example, an ΘJA
of 2.1°C/W means that for every watt dissipated in the device, the junction
temperature increases by 2.1°C. For a 10W design, the increase is 21°C above
the ambient temperature.
Note: You can obtain the ΘJA through thermal
simulation using the following formula:
ΘJa = (Tj
– Ta)/
PowerDissipated
The following figure shows the recommended flow for thermal
validation.
Figure 2. Recommended Thermal Validation Flow
After the junction temperature is within specification and sufficient
margin is considered, the thermal solution is considered effective.
Thermal Tip: Add the results of the
power estimation and thermal simulation to the Vivado design constraints. You can use the
following XDC constraints, which you can export from the XPE, as described in the
Xilinx
Power Estimator User Guide (UG440):
# Standard Constraints:
set_operating_conditions -process Maximum
set_operating_conditions -design_power_budget <value>
#If thermal simulation completed
set_operating_conditions -ambient_temp <value>
set_operating_conditions -thetaja <value>
#Else if no thermal simulation completed
set_operating_conditions -junction_temp <value>