Connect Tech PCI-104 Power and Thermal Considerations, Reference Design Fpga power analysis

Connect Tech FreeForm/PCI-104 User Manual

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 poi nt of failure.

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