Rev-Up Your Heat Sink Design Using CFD Simulations

Key Takeaways

• Passive heat sink performance can be enhanced by modifying its geometry according to the particular application.

• Heat sink CFD simulations open deep insights into air dynamics, which serve as an engineering resource in thermal management system design. This allows the over-engineering of heat sinks to be limited to a great extent.

• Using CFD/FEA coupled simulations, you can parameterize the heat sink material and geometry for the specific thermal requirements.

In passive cooling, the extruded-pin type heat sinks are most popular.

For a test experiment on cascaded inverters, I constructed two inverters that were identical in all respects except the heat sinks for the MOSFET switches. While conducting the experiment, one of the switches failed because of the wrong heat sink selection. Heat sink selection critically affects the life of electrical systems. There is a great deal of math behind a suitable heat sink design. As modern electronic technology makes our life convenient, its thermal management is viewed seriously for improving reliability.

Mounting heat sinks can prevent thermal runaways and premature failures of sensitive circuit elements. Heat sinks remove the heat flux from device surfaces by means of conduction, convection, or radiation. In most of the heat sinks seen in today’s electronics, air acts as the coolant.  By understanding the concepts of aerodynamics and thermal characteristics of the heat sink, efficient and fast cooling can be achieved.

Computational Fluid Dynamics (CFD) plays a vital role in keeping your electronics running irrespective of the heat generated. Heat sink CFD simulation is the virtual third eye of a thermal engineer.  The designer perceives the fluid approach velocity and its flow interactions using CFD simulations and applies the acquired knowledge into heat sink engineering for the best design. In this article, we’ll briefly see the basic passive heat sink operation and its design. We’ll also dive into the fundamentals of CFD simulation, which is the best tool available to study heat sinks from a thermal point of view.

Heat Sinks Are Indispensable

Why do the electrical component manufacturers specify the maximum operating temperature range in datasheets?  To be more precise, the junction to ambient and junction to case thermal resistance will be separately included under the thermal characteristics heading. This is to alert the user to the need for thermal management when powering these components. Well-designed heat sinks are effective in improving the heat transfer from the critical components to the surroundings. Heat sinks increase the mass and surface area of the hot device(s) and thus the heat dissipation process gathers speed. 

I always correlate the word heat sink to a silver-colored metal piece with fins. The extruded fin-type heat sinks are the most popular ones now available on the market. How does this passive heat sink work?  The heat generated in the component substrate is conducted to its casing and further to the surroundings via the heat sink fins. The buoyant force pulls the lighter hot air upwards and gravity pulls the denser cool air downwards, creating free flow of air within the fins. This air movement results in natural convection and thus, cooling is achieved. Without a heat sink, the device junction temperature exceeds the allowable safe operating limit and gets destroyed from overheating.

It is critical that the airflow profile controls the passive heat sink based cooling system. If you are knowledgeable about the behavior of air in motion, you can surely come up with the best heat sink geometry. This is the moment where we think about CFD simulations.

Passive Heat Sink CFD Simulation

In CFD, single-phase fluid flow is defined by the Navier-Stokes equation. As we are dealing with natural air convective heat removal, the same applies to passive heat sinks. In passive heat sinks, the airflow profile is dependent on the location, shape, size, and surface finish of the heat sink. The free area around the heat sink and the flow area between the fins also influences the free airflow. The orientation of the fins can obstruct the air movement if not placed properly. It is always advisable to keep the fins vertically aligned to enhance convective fluid flow and because this orientation also lowers the thermal resistance.

With this many dependent factors, there is no chance of obtaining a complete solution for CFD equations by following the traditional analytical methods. The availability of CFD-enabled simulation packages is trending in the fields of study involving fluid dynamics. A heat sink CFD simulation allows designers to view the variations in airflow in response to geometric modifications and initial conditions in the system under test. 

If you can model the heat sink with considerations for the electrical, thermal, and structural aspects, your design outcome will be outstanding. Total system analysis using electrical-thermal co-simulation employed in thermal management engineering is a move towards impressive designs. 

In these simulations, system modeling is approached using CFD and Finite Element Analysis(FEA) fundamentals. The application of CFD for convective fluid flow modeling and Finite Element Analysis (FEA) for heat transfer modeling establishes an accurate heat sink model to study thermal performance. These models can accurately correlate thermal constraints, output parameters, as well as initial and boundary conditions in the system to the simulation results. The CFD/FEA simulation and design adaptations derived from simulation results can direct your heat sink geometry to the best it can be. 

Plug and Play with Heat Sink Materials

The design and aesthetics of heat sinks are given utmost care as the demand for compact size gadgets is rising. The primary objective is to select a material with low thermal resistance and high thermal conductivity. The thermal properties of the heat sink material should enhance the airflow characteristics and heat transfer momentum. The CFD/FEA simulations offer you the provision to change the material properties and visualize the effect on heat sink efficiency. 

In Table 1 it is clear that diamond is the first preference for heat sink material, but this is not practically feasible. Aluminum and copper are the most frequently used heat sink materials. Nowadays, a combination of aluminum fins with a copper base is common. 

Heat sink CFD/FEA simulations allow you to run many iterations on a single design by editing the material properties. In this way, you can conduct a behavioral study on different heat sink materials without spending a single penny on production costs. Thermal stress analysis in a CFD/FEA simulation package can also predetermine the possibility of heat sink deformations.

Table.1 The thermal conductivity of various materials. 

Heat sink CFD/FEA simulation is performed to validate the heat sink design.

Optimize Your Heat Sink Geometry

If you are assigned optimizing the heat sink of the power electronic module (PEM) in an electric vehicle, how will you proceed? Here, the material is fixed and you are allowed to modify the geometry. Your optimization procedure can be simplified by developing an accurate 3D thermal model of the PEM module. Highly accurate 3D models leave no loopholes in the thermal solutions attained. A simulation conducted on CFD/FEA coupled models helps you to analyze the air dynamics, heat extraction, overall temperature control, and also the hotspot detections. Any physical design modifications, electrical input power variations, or changes in the boundary or initial conditions reflect immediately in the simulation results. The CFD/FEA simulation is the best practice to assure that you submit the thermal management assignment with full confidence.  

If you are looking for a real-world heat sink, you’ll want to incorporate the CFD/FEA simulation feature of  Celsius Thermal solver in  Allegro PCB Designer from Cadence into your design master plan. You can utilize the CFD and FEA analysis tools in the Celsius Thermal Solver to redesign the geometry until favorable aerodynamic conditions and heat dissipation rates are accomplished. Once your heat sink geometry is CFD/FEA simulation-approved, then it will be recognized as the best possible design.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.