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Ixiasoft

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Ixiasoft

## 6.2. Example 2: Solution with Temperature-Dependent Power

_{J}=100° C; however, the cooling solution keeps the T

_{J}at 96° C, meaning that the static power is lower and the total power dissipation is less than what was used in the analysis.

To get a more accurate result you must run an iterative analysis using a variable power for the FPGA as a function of temperature. To do this, you must enter the power of the FPGA as a function of the case temperature, T_{CASE}, and enter it into your CFD analysis. You can run the PTC for several T_{J-MAX} values at 5° C increments and record the T_{CASE} value for each increment. In this way you can build your power graph for CFD analysis. This method is particularly useful if the FPGA has high static power consumption, or if there are several FPGAs in your system with varied Ψ_{CA}.

The following figure shows the power curve for this example. The power and T_{CASE} values for this instance are obtained from the PTC by setting the T_{J-MAX} values to 100⁰ C, 95⁰ C, and 90⁰ C. The graph also shows the conversion point for the analysis, indicating a case temperature, T_{CASE}, of 74.5⁰ C and thermal design power (TDP) of 12.7 W.

The following equation illustrates the calculation of the new junction temperature, based on the new results:

T_{J}= T_{CASE}+ TDP * Ѱ_{JC}

T_{J}= 74.4 + 12.7 * 1.617 = 95° C

These results show that the junction temperature drops by a degree; also notice that in the new calculation the value of Ψ_{JC} changed. As stated before, this value is not constant and for this case it corresponds to when the T_{J-MAX} entered in the PTC is 95° C.

Temperature-dependent calculations may not be necessary in all cases, however they are recommended for increased accuracy and when changes in static power can affect the result.