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.
Xilinx 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. Xilinx 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.
- 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.
- Ambient Temperature Ta
- Sets the maximum ambient temperature.
- 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.
After the junction temperature is within specification and sufficient margin is considered, the thermal solution is considered effective.
# 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>