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Thermal model and heat transfer optimization for enclosure

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Thermal management is critical for heat control as it limits the performance and reliability of components that impacts the cost and environment. Let's discuss some of the best practices to calculate exact thermal behavior. 

 

Electronics development has come a long way from limited capability devices to modern devices with high computational speed and power. The technological growth of the industry led to an exponential improvement in power densities, which in turn drove the innovation of smarter and smaller products. Advancement in technology and an increase in the need for miniaturization demanded innovation in the thermal management of electronic devices. Thermal management plays a vital role in improving reliability and enhancing performance by removing or suppressing the heat generated by the devices.

Unfortunately many component manufacturers still provide limited information on the thermal behavior of their products. Ample information on thermal analysis of component will not only improve the usability of components but also enhance the ecosystem. However, this gap can be filled with software solutions that help to solve the design problem related to thermal behavior.

In this blog, we are sharing the best practices we have learned during the development of complex edge devices for our various clients.

Background of Thermal Management of Embedded Equipment

‘Heat transfer (or heat) is thermal energy in transit due to a spatial temperature difference. Whenever a temperature difference exists in a medium or between media, heat transfer must occur.’ There are different heat transfer modes which are called conduction, convection, and radiation. Conduction is used as the term form temperature gradient for existing in a stationary medium. Temperature is transferred from the high-temperature region to the low-temperature region and the basic calculation for one-dimensional heat flow is given following:

Where k is known as thermal conductivity, L indicates the length of the conducting path.

Figure 1: One-dimensional conduction heat transfer 

The formula for three-dimensional conduction heat flows explained with energy balance for cartesian coordinates:

The right side of the equation gives the change in thermal energy storage in the element and the left side of the equation represents energy generated within the infinitesimal element and thermal energy for a control volume.

Formula to show the relationship between thermal conductivity and temperature difference according to the first law of the thermodynamics is given the following:

where the thermal resistance for conduction and contact interface is expressed by

Thermal models of the electronic enclosure are represented with this case study. The enclosure can consist of power boards, CPU boards, and whatnot. The study consists of,

1. Design of a chassis with the given geometrical and thermal constraints starts generation of the three-dimensional (3D) model.

2. Then, thermal analysis of the 3D model is conducted to determine if the given heat loads can be dissipated.

3. According to results, geometry and material of the 3D model are updated to reach an optimum cooling solution.

4. Correct cooling methods can reduce thermal stresses and provide the capability to system reliability. Also, it improves the performance of electronic components which is directly related to temperature as seen on components.

The main of the thermal analysis is that the geometry and flow domain consists of a flat circuit board with a heat-generating electronic chip mounted on it. Heat is conducted through the source (chip) and the board on which it is mounted. A laminar stream of air flows over the board and the chip, causing simultaneous cooling of the solid components and heating of the airstream due to convection. Thermal energy is also transported due to the complex flow field.

 

Figure 2: Enclosure 3D Model

To simulate the heat dissipation of chassis and see the temperature on electronic components, an analysis model is created. These arrangements and analyses were performed using ANSYS® (workbench) FLUENT program. Analyses are conducted for 70 C operational temperature tests. Computational fluid dynamics using the finite volume method has been used for modeling the conjugate heat transfer through the chip and the circuit board.

Figure 3: Draft Model

The draft model of the ATR chassis with side plate fins is shown in Figure 3.

To obtain the accurate results and convergence of simulation, 32 million mesh elements are used. 0.1 mm inflation with 10 layers is applied for not only outer flow but also the inner flow region.

Figure 4: Inflation Layers of Inner and Outer Flow Regions

Figure 5: Inner Flow Meshing

The heat transfer mechanism in electronics is a complex interdisciplinary subject that can be addressed at all design stages to achieve the right decisions and the engineering knowledge for further development of electronic products.

Heat transfer solvers are used to predict the temperature of the components and parts within an assembly. Both hot spots within the PCB layout or temperature distributions that exceed operational limits can be visualized. The choice of the heat transfer software ranges from simple analytical code numerical solvers. Results from the heat transfer are used to decide if the design is optimal or an optimization scenario must be considered. The concern of electronics heat transfer computation is that of the active components and their cooling. Active components (resistors, transistors, integrated circuits transformers) are the main heat sources. Heatsinks remove the highest possible heat flux, while the surface temperature of the heatsink remains at a low temperature. An optimal heat sink produces a uniform surface temperature with the least number of hotspots.

To obtain the hot spots causes problems or not, the junction temperature is used in analysis steps. Junction temperature is the highest operating temperature of the actual semiconductor in an electronic device. In operation, the junction temperature is higher than the case temperature and the temperature of the part’s exterior. The difference is equal to the amount of heat transferred from the junction to the case multiplied by the junction-to-case thermal resistance. When designing integrated circuits, predicting, and calculating the chip junction temperature is a particularly important task.

Steady-state thermal analysis is used to determine the temperature distribution in a structure at thermal equilibrium. Steady-state solvers assume that the loaded body instantaneously develops an internal field variable distribution to equilibrate the applied loads. In this study, thermal analysis is achieved by using steady-state conditions. In this term, components are working at full performance for 3 hours and the temperature of the enclosure reaches steady-state conditions. Therefore, thermal analysis is conducted on worst-case conditions, and given results for components at Table aa are the highest temperature that can be seen on the components.

Figure 6: Temperature Gradient of Processor

               

Figure 7: Other Components on CPU Board

Thermal management is critical for heat control as it limits the performance and reliability of chips and other microcontroller components which impacts the cost and environment. Due to the increasing demand for miniaturization, the need for reliability in harsh environments, and complex conditions, thermal management has become an integral part of the design process.

There are many innovative cooling solutions that emerged to meet the challenges of various scenarios. The latest thermal management solutions deal with basic heat transfer modes – i.e., conduction, convection, and radiation – and the development of technologies is moving from single-phase heat transfer to multi-phase heat transfer. In our next blog, we will cover more on this topic and discuss innovative solutions for heat dissipation.

In Trunexa, we pay great attention to thermal management and analysis to ensure good heat dissipation which enhances the performance of the electronic device. To find how Trunexa can help your design team with the same, please feel free to contact us.