To calculate the thermal information of the device correctly, you are required to enter the ambient air temperature for the device in the EPE spreadsheet. Ambient temperature refers to the temperature of the air around the device. The temperature of the air around the device is usually higher than the ambient temperature outside of the system. To get an accurate representation of ambient temperature for the device, you must measure the temperature as close to the device as possible with a thermocouple device.
Entering the incorrect ambient air temperature can drastically alter the power estimates in the EPE spreadsheet. The following figure shows a simple system with the FPGA housed in a box. In this case, the temperature is very different at each of the numbered locations.
For example, location 3 is where the ambient temperature pertaining to the device should be obtained for input into the EPE spreadsheet. Locations 1 and 2 are cooler than location 3 and location 4 is likely close to 25 °C if the ambient temperature outside the box is 25 °C. Temperatures close to devices in a system are often in the neighborhood of 50–60 °C but the values can vary significantly. To obtain accurate power estimates from the EPE spreadsheet, you must get a realistic estimate of the ambient temperature near the FPGA device.
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