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EDA/IP??

Manage thermal optimisation during PCB design

Posted: 09 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:computational fluid dynamics? CFD? simulation software? response surface optimisation? PCB?

U7 (upstream) and U8 (downstream) are modelled as 2R compact thermal models, and the other components are modelled as lumped blocks with distributed heat. The specific parameters relevant to the cooling design are:
???RJC: 0.5C/W
???RJB: 20C/W
???TIM: 0.5Cin2/W
???Heatsink material: 201 W/mK

For this cPCI card, we'll run through various scenarios to analyse the following cases:
???Case 1: Minimise the mass of U7 and U8 heatsinks, which are identical, to maintain U8 junction temperature at 100C.
???Case 2: Minimise U8 junction temperature. U7 and U8 heatsinks are identical.
???Case 3: Minimise the mass of U7 and U8 heatsinks, which are the same extrusion but with different fin lengths, which maintains U8 junction temperature at 100C. Additionally limit the difference in U7 and U8 junction temperatures to 1C.

Design of experiments
Before optimisation, we'll use the Command Centre design of experiments (DOE) option to create 50 simulation models to be analysed with CFD. For this study, the following input parameters are used:

Additional constraints used during the DOE include:
???Input constraint limiting the maximum height of the heatsink to 15.5 mm (Cases 1-3).
???Linear constraints to ensure U7 and U8 heatsinks remain centred on respective components as heatsink base dimensions change (Cases 1-3).
???Linear constraints to ensure all dimensional parameters of the U7 heatsink are identical to the U8 heatsink, that is, the same heat sink (Cases 1-2).
???Linear constraints to ensure all dimensional parameters of the U7 heatsink except fin height are identical to the U8 heatsink, that is, the same extrusion (Case 3).

Figure 3: The distribution of input parameters over the design space for Cases 1 and 2.

Figure 3 represents the distribution of input parameters over the design space for Cases 1 and 2. A separate DOE is performed for Case 3 where U8 and U7 fin heights are allowed to vary independently (figure 4).

Figure 4: A separate DOE is performed for Case 3 where U8 and U7 fin heights are allowed to vary independently.

Downstream components will naturally run hotter than upstream components, which requires the fins to be taller to operate at the same temperature, and the program can be given a range to run simulations with independently varying fin heights. An input constraint example is where the base has to be a certain width and fin certain height. Output constraints also can be used, such as the output temperature has to be less than certain value.

Figure 5: The results from the simulations.

Figure 6: The results from the simulations.


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