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Lec 43: Choosing Heat Sinks
NPTEL IIT Guwahati
Overview
This video explains how to select an appropriate heat sink for thermal management in power electronic converters. It covers essential terminology like LFM and CFM for fan specifications, and details the process of calculating required thermal resistance using device parameters and ambient conditions. The lecture demonstrates how to interpret heat sink manufacturer datasheets, which include graphs relating power dissipation to temperature rise and air velocity to thermal resistance, enabling engineers to choose a heat sink that maintains device temperatures below critical limits, both for natural and forced air cooling scenarios.
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Chapters
- Thermal design aims to cool electronic devices and maintain their operating temperatures below maximum limits.
- Heat sink selection involves choosing the right cooling method (natural, forced air, or liquid) and a suitable heat sink.
- Familiarity with terms like thermal resistance, impedance, time constants, and fan specifications (LFM, CFM) is crucial.
Understanding the fundamental goals and necessary terminology is essential before diving into the calculations and selection process for effective thermal management.
- Fans are often specified by Cubic Feet per Minute (CFM), a measure of volumetric airflow.
- Linear Feet per Minute (LFM) represents the air velocity through a specific cross-sectional area.
- LFM can be calculated from CFM by dividing CFM by the fan's cross-sectional area in square feet (LFM = CFM / Area [ft²]).
Correctly interpreting fan specifications ensures that the airflow provided by the cooling system is accurately accounted for in thermal calculations.
An 80 CFM fan with a 6-inch by 6-inch (0.25 ft²) cross-sectional area provides an airflow of 320 LFM (80 CFM / 0.25 ft²).
- The simplest approach uses steady-state thermal models, often represented by electrical equivalent circuits.
- Key parameters include total power dissipation (Pd), maximum junction temperature (Tj_max), ambient temperature (Ta), and thermal resistances (Rθjc, Rθcs).
- The required sink-to-ambient thermal resistance (Rθsa) is calculated using the formula: Rθsa = (Tj_max - Ta) / Pd - (Rθjc + Rθcs).
This calculation determines the maximum allowable thermal resistance of the heat sink needed to keep the device within its safe operating temperature limits under natural convection.
Given Tj_max=150°C, Ta=50°C, Pd=5W, Rθjc=3°C/W, and Rθcs=0.5°C/W, the required Rθsa is (150-50)/5 - (3+0.5) = 16.5°C/W.
- Heat sink datasheets provide performance graphs and physical dimensions.
- Graphs typically show temperature rise vs. power dissipation (for natural cooling) and air velocity (LFM) vs. thermal resistance (for forced air cooling).
- Datasheets also detail dimensions, materials (e.g., copper), mounting instructions, and part numbers for different series of heat sinks.
Datasheets are the primary source of information for selecting a heat sink; understanding their graphs and specifications allows for accurate performance prediction.
A graph for a specific heat sink shows that at 5W power dissipation, the temperature rise is 75°C, yielding an Rθsa of 15°C/W (75°C / 5W).
- Forced air cooling significantly reduces thermal resistance compared to natural convection.
- Heat sink datasheets provide graphs showing how thermal resistance decreases as air velocity (LFM) increases.
- By using a fan to achieve a specific LFM, a heat sink with a much lower Rθsa can be utilized, allowing for higher power dissipation or lower operating temperatures.
Forced air cooling offers a substantial improvement in heat dissipation, enabling the use of smaller heat sinks or higher power devices.
With forced air at 200 LFM, the same heat sink from the previous example might have an Rθsa of 2.6°C/W, reducing the temperature rise for 5W dissipation to 13°C (5W * 2.6°C/W).
- Calculate the required Rθsa based on device limits and ambient conditions.
- Consult heat sink datasheets to find a part with an Rθsa at or below the calculated value for natural cooling.
- For forced air, use the air velocity vs. thermal resistance graph to determine the required fan LFM and select a heat sink that meets the criteria.
- Verify the chosen heat sink's performance using the datasheet graphs for the expected power dissipation and cooling method.
This practical example demonstrates the step-by-step process of applying the learned concepts to select a suitable heat sink for a real-world application.
A calculated required Rθsa of 16.5°C/W is compared against a heat sink's datasheet. For natural cooling, a heat sink with Rθsa of 15°C/W is suitable. With forced air at 200 LFM, the same heat sink offers Rθsa of 2.6°C/W, significantly reducing temperature rise.
Key takeaways
- Effective thermal design is critical for the reliability and performance of power electronic converters.
- Understanding thermal resistance is key to selecting appropriate cooling solutions.
- Fan specifications (CFM and LFM) must be correctly interpreted to assess forced air cooling effectiveness.
- Heat sink datasheets provide essential performance data through graphs and specifications.
- Forced air cooling drastically reduces thermal resistance, offering significant performance benefits over natural convection.
- The selection process involves calculating required thermal resistance and comparing it against available heat sink options using datasheet information.
- Always consider the maximum allowable junction temperature and the expected ambient temperature for your application.
Key terms
Thermal DesignHeat SinkPower Dissipation (Pd)Junction Temperature (Tj)Ambient Temperature (Ta)Thermal Resistance (Rθ)Junction-to-Case Resistance (Rθjc)Case-to-Sink Resistance (Rθcs)Sink-to-Ambient Resistance (Rθsa)Cubic Feet per Minute (CFM)Linear Feet per Minute (LFM)Natural CoolingForced Air Cooling
Test your understanding
- How does the calculation for required sink-to-ambient thermal resistance differ between natural and forced air cooling?
- What is the relationship between fan specifications (CFM, LFM) and the airflow available for cooling a heat sink?
- How can you use the graphs provided in a heat sink datasheet to determine if a particular heat sink is suitable for an application with a specific power dissipation?
- Why is it important to consider both the maximum junction temperature and the ambient temperature when selecting a heat sink?
- Explain how increasing air velocity in forced air cooling affects the thermal resistance of a heat sink.