30 Aug

Electrical and electronic components are sometimes housed in enclosures with ventilation openings to allow ambient air into the enclosure to more efficiently cool heat generating components. The most common ventilation configuration is to have openings at the top and bottom of the enclosure as shown in figure 1.

Figure 1. Ventilated enclosure cooled via natural convection

The air flow through the enclosure is driven by the difference in air density of the cooler air outside the enclosure and warmer air inside due to the heat transferred from the heat generating components. As the air flows vertically through the lower ventilation openings and across the heat generating components its temperature increases. As a result components mounted at higher locations in the enclosure will be subjected to air temperatures above the ambient air temperature. Determining the internal air temperature surrounding these components will allow you to identify and relocate components that have a maximum ambient operating temperature lower than the surrounding air temperature. The size or number of the ventilation openings can also be adjusted to lower the internal ambient temperature.
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01 Aug

Heat sinks are used to cool light emitting diodes (LEDs) in a similar manner to many other semi-conductor devices such as MOSFETs, CPUs and simple diodes. However LEDs because of their varied applications can be installed in various orientations unlike other semi-conductor devices which typically have a fixed orientation. The heat sinks to which LEDs are attached must be able to maintain the LED junction temperature below the specified limits in any of these orientations. If a sheet of aluminum or copper is used as the LED heat sink the thermal performance of this sheet will change depending on its orientation.

Figure 1. LED flat plate heat sink
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25 May

Without some basic understanding of the factors affecting heat sink performance the heat sink you select may be inadequate to meet the thermal requirements of the device being cooled. This leads to an increased number of designs and testing iterations ultimately delaying the completion of the product development. There are 3 common mistakes that are made when evaluating a heat sink to be used.

1. Using the manufacturer’s thermal resistance to predict performance

The thermal resistance of a heat sink is the most common way to assess the performance of a heat sink for a given application. By multiplying the heat sink thermal resistance, Rth by the power dissipation, Q of the device being cooled and adding the ambient temperature, Tamb to the results, the case temperature of the device, Tc can be determined, as shown in equation 1. The typical method by which heat sinks are selected is to first calculate the required heat sink thermal resistance using equation 1. A search of commercially available heat sinks with published thermal resistances less than or equal to the calculated value is then conducted.

$Latex formula$1

The thermal resistances provided by the heat sink manufacturers is typically determined through testing of a heat sink with a square heat source, usually 25.4 mm x 25.4 mm (1″ x 1″) attached to the center of the base of the heat sink with a predetermined heat dissipation value. The temperature difference is measured and using equation 1 the thermal resistance is calculated.

Using this measured heat sink thermal resistance value can often times results in the selection of a heat sink that may not meet your thermal needs because the thermal resistance of the heat sink is not a constant. The thermal resistance of the same heat sink  will change based on the size of the heat source relative to the base area of the heat sink.  If the heat source you are using is significantly smaller than the heat source used by the manufacturer during testing of the actual heat sink the thermal resistance may be much higher than the manufacturer’s tested value. This is due to the difference in thermal spreading resistance which is caused by the flow of heat from the smaller area of the heat source to the larger area at the top surface of the heat sink. The smaller the heat source area relative to the base area of the heat sink the higher the thermal spreading resistance which then increases the overall thermal resistance of the heat sink. The effect of spreading resistance is most pronounced on heat sinks undergoing forced convection.

Figure 1. Sample heat sink
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10 Jan

Often times electrical or electronic components are housed in sealed enclosures to prevent the ingress of water, dust or other contaminants. Because of the lack of ventilation in these enclosures all of the heat generated by the internal components must be dissipated through the walls of the enclosure via conduction then from the external surface of enclosure to the environment via radiation and natural convection as shown in figure 1.

Figure 1. Heat transfer from a sealed enclosure with heat generating components

Accurately calculating the temperature rise of each component housed inside the enclosure is a complicated task that is best accomplished using computational fluid dynamics and heat transfer software. However in many cases being able to estimate the average air temperature within the enclosure based on the dimensions and the material of the enclosure is sufficient information to allow you to develop a design that can be further refined through testing.
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30 Aug

A heat sink is a part that conducts heat from a heat generating component to a larger surface area to dissipate the heat to the surroundings thus reducing the temperature of the component. Based on this definition anything from a rectangular sheet of metal to a complex finned copper or aluminum extrusion can be used as a heat sink. In situations where there is ample space and/or the heat dissipated by the component is low an aluminum or copper plate can be used as an effective heat sink. The heat sink can be a simple plate or the metal wall of the enclosure housing the component as shown in figure 1.

Figure 1. Flat plate heat sink dimensions

To estimate the dimensions of the flat plate heat sink you need to determine the path of heat flowing to the surroundings and the magnitude to which that path resists the flow of heat. The thermal resistance circuit shown in figure 2 will be used to represent the path of heat flow. Let’s examine each of the thermal resistance elements:

Figure 2. Thermal resistance circuit of a flat plate heat sink
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27 Jul

Often times the role radiation plays in the design of a heat sink is overlooked. There are many references that state a percentage value for the heat dissipated from a heat sink. As with most phenomena in physics and engineering the effect of radiation cannot be generalized with one constant number.

There are several factors that determine the influence of radiation on the performance of a heat sink. Before investigating those factors a brief description of radiation is necessary.

Thermal radiation is electromagnetic waves emitted from all matter that has a temperature above 0 Kelvin (absolute zero). The maximum heat (Watts) that can be emitted from a surface due to radiation is given by:

$Latex formula$1

where:
$Latex formula$ is the surface area of the radiating surface
$Latex formula$ (Stefan-Boltzman constant)
$Latex formula$ is the surface temperature in Kelvin
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05 Jun

Sizing a heat sink can be daunting tasks for any one who does not have much experience in thermal analysis. There are commercially available software that would allow you to design and analyze a heat sink to meet the thermal requirements of  the device(s) to be cooled. If that type of software is not available to you some quick calculations can be done to get an estimate of the size of the heat sink required.

By making  a few simplifying assumptions you can conduct the heat sink analysis by hand or using a spreadsheet. The output of these calculations will be the dimensions of the heat sink required to maintain the required source temperature.

Figure 1. Plate fin heat sink dimensions

Figure 1 shows a typical plate fin heat sink used to cool common electrical and electronic components such as LEDs used in lighting applications, MOSFET used in digital circuits and microprocessors. There are six dimensions that would need to be determined to design an appropriate heat sink for your needs. In order to reduce the complexity of the calculations the following assumptions will be made:
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31 Dec

We have added some exciting features to the HeatSinkCalculator as well as updated the user interface. The feature additions have been made based on user feedback from surveys conducted.

Brand new dashboard
In order to simplify the dashboard and thus make it easier for the user to navigate the various input options the dashboard was separated into 5 sections: Dimension/Optimization, Air Flow, Power Source, Material, Calculate/Results. Only the input option of selected section is displayed allowing the user to focus on one section at a time.

04 Sep

The HeatSinkCalculator includes an option to input a fan curve of the fan used to cool the heat sink. This option can be exploited to allow the simulation of the heat sink and fan in a ventilated enclosure as was outlined in part 1 of this article.

It is assumed that the readers of this article already reviewed part 1. Recall from the previous article equation 1 that defines the total pressure difference around the flow network loop.

$Latex formula$                 (1)

The pressure differences across the fan and two ventilation openings can be summed to create a single parameter ΔPfan* that defines the total pressure difference across all three components.  The parameter ΔPfan* represents the modified fan curve.  This calculation is carried out at the specified volumetric flow rate of each point on the fan curve to generate the modified fan curve. Equation 1 is now reduced to equation 2. Please note that the pressure difference across the fan is considered positive since the pressure increases along the direction of the flow and the pressure difference across the ventilation opening is negative because there is a pressure decrease along the direction of flow.
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31 Aug

There are many situations in which the heat sink and the device(s) it is cooling are located in an enclosure with ventilation openings. The ventilation openings will allow the inlet of air at the outside ambient temperature and the exit of heated air. When the heat sink is being cooled by forced convection using a fan or blower the size and configuration of the ventilation openings can negatively impact the ability of fan/heat sink to cool the power generating devices when compared to the same fan/heat sink combination located in an open environment outside of the enclosure. This is because the ventilation openings which are typically perforated plates, louvered openings or mesh screens restrict the flow of air. Understanding what impact this restriction has on the volumetric flow rate of the air through the heat sink will allow you to select a heat sink and fan combination to meet the thermal requirements of the device being cooled.

Shown in figure 1 is an enclosure with ventilation openings at both ends. There is a heat sink inside the enclosure with a fan mounted on or very close to the heat sink such that it can be assumed that all of the air flow from the fan will pass through the heat sink.

Figure1.  Heat sink and fan in a ventilated enclosure
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