Heat Sink Design for Power Electronics

Key Takeaways

• Effective heat sink design is crucial for ensuring reliable performance of power electronics while considering performance, cost, and various requirements.

• Material selection impacts heat sink performance; high thermal conductivity materials like copper and aluminum lead to better heat dissipation.

• Parameters such as fin geometry, airflow, and thermal resistances collectively influence the overall efficiency of heat sink design for power electronics.

General structure of a heat sink design system for power electronics

Designing the appropriate heat sink holds significant importance in guaranteeing the dependable functionality of power electronics. While working on or selecting a heat sink, thermal engineers strive to enhance contrasting performance and financial prerequisites. Apart from sustaining the ideal temperatures of electronic components, the ultimate heat sink design must also fulfill objectives related to dimensions, weight, robustness, and expenses. Read on as we discuss heat sink design for power electronics.

Heat Sink Architecture for Power Electronics

Heat is transferred through three primary mechanisms: conduction, convection, and radiation. The thermal effectiveness of a heat sink is established by summing up the thermal resistances within the network of interest (electronics, heat sink, and surroundings) and then multiplying this value (in oC/W) by the heat source's maximum power output (TDP in watts) to derive the total temperature increase of the heat sink under operation.

Within a heat sink, layers of thermal resistance collectively influence the overall thermal performance. These encompass the thermal interface materials (TIMs), the heat sink base, the fin surface area, and the airflow traversing the fins. Each component carries a thermal resistance, with lower values indicating superior performance.

• Interface to base (R_TIM): This thermal resistance denotes the heat transfer between the component's casing and the heat sink's base. This heat transfer's efficiency hinges on factors such as the material's conductivity, the thickness of the thermal interface material (TIM) connecting the case and base, as well as the area of contact, and applied pressure.

• Base to fin (R_base/fin): This thermal resistance manifests when fins are linked to a base and signifies the heat flow from the heat sink base to the fins. The efficacy of this heat transfer depends on the material used to attach the fins to the base. Solder is the favored choice, leading to low resistance. Epoxies can be employed for more substantial assemblies where soldering isn't practical or necessary. While typically modest, this value must be integrated into calculations.

• Fin to air (R_fin/air): This thermal resistance characterizes the heat exchange between the fin's surface and the adjacent surrounds —usually air. Convection primarily drives this heat transfer, with its effectiveness contingent upon the air's flow velocity, which governs the rate of heat flow (heat transfer coefficient).

• Air Temperature Rise (R_air/rise): In cooling systems, the heat generated by electronic components gets transmitted to the heat sink, dispersing it into the surrounding air. This induces a temperature surge in the air as it navigates through the heat sink, effectively augmenting the "ambient temperature" across the heat sink's length. This parameter is directly influenced by the overall bulk airflow coursing through the heat sink.

The cumulative thermal resistance of the heat sink (R_total) can be estimated by aggregating the individual thermal resistance layers.

Basic Heat Sink Design Tips

Operating on the principle of conduction, a heat sink functions by assimilating thermal energy from the surrounding environment, stemming from the inefficiency of electrical components. Below are basic tips to consider when designing a power electronics heat sink. 

• The optimal configuration for a heat sink design, considering its geometry, usually involves the incorporation of fins or pins to amplify the surface area available for facilitating heat exchange. Alternate materials and forced cooling through convection may also be useful.
• Several critical parameters factor into selecting a heat sink, including

1. Thermal resistance 
2. Airflow
3. Volumetric resistance
4. Fin density
5. Fin spacing
6. Width
7. Length

• Copper is a prime selection for heat sink materials due to its exceptional thermal conductivity. Nevertheless, aluminum takes precedence in usage due to its lower cost and relatively good thermal conductivity.
• A powered fan operates to augment the airflow rate over a surface, thus intensifying the pace of heat transfer across the surface and facilitating the extraction of greater amounts of heat energy.

Material Considerations

The process of choosing materials is crucial in the design of heat sinks, given its direct influence on the heat sink's thermal efficiency and the suitability of interconnecting surfaces (coatings/plating). A heat sink crafted from a material boasting high thermal conductivity exhibits reduced thermal resistance, thereby facilitating enhanced heat dissipation in contrast to a heat sink fashioned from a material characterized by lower thermal conductivity. See the table for potential heat sink materials and associated thermal conductivity.

Heat Sink Design Fin Geometry

In a general context, heat sink fin designs can be categorized broadly as either plate fins or pin fins. Plate fins are flat and parallel structures extending from the heat sink base, presenting a substantial surface area to facilitate heat transfer. On the other hand, pin fins are elongated cylindrical protrusions emerging from the heat sink base, enhancing the flow of air. When dealing with forced convection scenarios involving strong and directed airflow, plate fins often exhibit improved performance due to their extensive surface area and streamlined configuration. However, pin fins excel in conditions with multidirectional airflow by promoting enhanced air circulation around the fins.

Plate fin heat sinks typically possess a more condensed form, which can be advantageous in applications where space is restricted. Meanwhile, pin fin heat sinks offer superior performance per unit volume when subjected to lower airflow conditions and are more adaptable to irregular shapes. Pin fins might be preferred in environments prone to dust or debris accumulation, as they are less susceptible to blockages and easier to clean compared to plate fins.

Heat Sink Design for Power Electronics and Chips

The upper limit for the temperature at the junction (T_JMAX) that can be sustained is a crucial factor governing the power dissipation capacity of a device or chip. Electronics manufacturers establish T_JMAX, which typically hinges on the reliability of the die incorporated in the manufacturing process. 

The equation below portrays the customary formula used for computing dissipation:

With θ being the thermal resistance, T_K the junction temperature, T_A the ambient temperature and P_D the power dissipation. The maximum power dissipation of the device can then be found by rearranging the equation above to:

For specific power electronics, the datasheet might offer a maximum θ_JA and T_JMAX. In most scenarios, the device manufacturer's data sheet may also provide T_J, P_D, and θ_JA values. The ambient air temperature T_A for cooling the components relies on the intended operating environment. This temperature typically ranges from 35°C to 45°C when external airflow through a fan is employed and from 50°C to 60°C when the component is enclosed. The most trustworthy information can be directly obtained from material manufacturers.

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