Connect Tech FreeForm/PCI-104 User Manual
Revision 0.02 20
Power and Thermal Considerations The FreeForm/PCI-104’s Virtex-5 FPGA is a versatile, flexible device, with many built-in features like
termination, PLLs, and high speed gigabit transceivers. The drawback of these on-chip features is that they
consume a lot of power and hence dissipate a lot of heat.
As a result Connect Tech, is recommending the installation of a heatsink, included with the product (see
section Heat Sink Installation). As well, the FPGA designer must perform power analysis on their design to
determine that they are not stressing the Virtex-5 component (i.e. exceeding the junction temperature).
Power analysis can be performed using the Xpower Analyzer (part of the ISE design suite) and the XPE
spreadsheets (Xilinx Power Estimator Spreadsheets).
http://www.xilinx.com/products/design_resources/power_central/
Reference Design FPGA power analysis Power analysis was performed on the FCG001 when configured with the reference design. The Virtex-
5 XPE spreadsheet was used to determine an effective junction to ambient thermal resistance
(θJA_effective). The following parameters are entered into the spreadsheet to determine θJA_effective.
Device
Part XC5VLX30T
Package FF665
Grade Industrial
Process Typical
Speed Grade -1
Stepping Stepping - 1
Thermal Information
Ambient Temp (°C) 50
Airflow (LFM) 250
Heat Sink Custom
Custom ΘSA (°C/W) 8 (*)
Board Selection Small (4"x4")
# of Board Layers 12 to 15
(θSA is the surface to ambient temperature for a heatsink with dimensions 27 mm x 27 mm x 6.4 mm and 250 LFM airflow. The
θSA improves (decreases) with a taller heatsink. )
Three scenarios were developed and the XPE parameters Airflow and Custom ΘSA were varied. The
θJA_effective was entered into the Xpower Analyzer yielding a Juction Temperature @ 50 °C and a
maximum ambient temperature. The following table summarizes the scenarios and the results. For
complete details of the scenarios, see Appendix B.
Scenario θJA_effective (°C/W) Tambient_max Tjunction at 50 °C
Heatsink attached, 250 LFM 4.9 82.7 67.3
No Heatsink, 250 LFM 6.4 72.7 72.7
No heatsink, 0 LFM 9.7 65.1 84.9
Calculation details:
Tjunction = Tambient + (PFPGA * θJA_effective) = 50°C + (3.53W * 4.9 °C /W) = 67.297°C
Tambient_max = Tjunction_max - (PFPGA * θJA_effective) = 100°C - (3.53W * 4.9 °C /W) = 82.7°C
Note Tjunction_absolute_max = 125°C is not used, since this is the absolute point of failure.