Obtaining Thermal and Power Targets to use with Thermal Simulation

Designing Heatsinks and Thermal Solutions for Xilinx Devices (XAPP1377)

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Prior to running a thermal sim, the power dissipation and temperature targets for the device need to be understood. As for thermal targets, it is best to first understand the minimum and maximum device operating temperatures for the device. This can be obtained from the appropriate Xilinx product selection guide or data sheet. In general, most devices fall into one of four classifications.

Table 1. Operating Temperatures for Xilinx Devices
Ordering Code Temp Grade Operating Range
C Commercial 0 to 85°C
E Extended 0 to 100°C 1
I Industrial –40 to 100°C 1
M Military –55 to 125°C
Q Q-grade –40 to 125°C
  1. These devices can offer an excursion temperature to 110°C depending on device.

For devices offering excursion temperature operation, the junction temperature might be able to operate at higher temperatures for short periods of time. To use this thermally beneficial feature, a mission profile needs to be generated and evaluated for the design. For more information on generating a mission profile and using an excursion temperature, refer to Extending the Thermal Solution by Utilizing Excursion Temperatures (WP517).

For devices using HBM memories, the additional step of setting temperature goals for HBM operation must also be taken. The maximum operating temperature for HBM memories is 95°C for constant operation. For the Virtex® UltraScale+™ and Versal® devices with –2 LE speed grade, excursion temperature operation can be up to 105°C for a limited time. Higher operating temperatures can impact refresh rates that affect the operational bandwidth, so that should be taken into consideration when setting HBM memory target temperatures.

Determining the target temperature for simulation depends on design goals. Often the target is set to the maximum operating temperature of the device or the calculated excursion temperature if allowed by the device and design. However, sometimes a lower target temperature is desired. For instance, if minimum operating power is desired, further reduction of the junction temperature results in less power dissipation in the devices. Another example is in the case of a hand held unit. Sometimes lower operating temperatures are necessary so as to not burn the user or keep battery or other component temperatures within reasonable limits. In any case, one of the first steps prior to thermal simulation is determining the appropriate junction temperature target for the application.

Xilinx devices often can have a wide range of power depending on use, and thus Xilinx provides the Xilinx® Power Estimator (XPE) tool to understand operational power that can be used during thermal analysis and regulator selection. XPE allows a user to provide high-level parameters for how they intend to operate the device and can provide a power calculation for thermal simulation. The tool can be obtained from the Power page of the Xilinx website. For Kria SOM users, the Power Design Manager (PDM) tool is used to determine power dissipation and apply predicted power to the SOM thermal model. The tool can be obtained from the Kria K26 System-on-Module page of the Xilinx website. After the tool is properly filled out, you can find the power to be applied to the simulation in the Total On-Chip Power section of the tool.

Figure 1. Example Total On-Chip Power Results from XPE

For an accurate estimation, use the determined thermal target for the device by forcing the junction temperature to that value. This can be done on the Summary page in the Environment section.

Figure 2. Example of Overriding Junction Temperature to Excursion 110°C for Power Estimation in XPE

For devices using HBM memories, the HBM target temperature and power summaries can be found in the following summary table on the Summary page. Two power entries should to be provided to the simulation model: one for the FPGA and one for the HBM stacks. Both can be obtained from the summary table in XPE. Even for devices that contain multiple die or multiple HBM stacks, only a single power value for each should be provided to the model comprising the total power of all die or stacks.

Figure 3. Example HBM Power and Thermal Information from XPE