by Chris Warner, ECN Magazine
Ecnmag.com - February 01, 2004
The need to manage and dissipate heat in electronics is an inevitably important aspect of any design. As sure as Moore's Law is a fact of life, so too is thermal management. The incessant demands of high functionality, small size and higher speeds in electronic components and systems increases the potential for failure and places greater emphasis on thermal management.
Power supplies as well as electronics housed in enclosures generate physical heat that must be regulated. Densely packed components in confined spaces add to the challenge of cooling. Designers use a number of devices and monitoring techniques to achieve reliable thermal conditions. These include fans/blowers, heat sinks, heat pipes and temperature sensors, controller ICs and software. In addition, component selection, fan control and air circulation require considerable care. This article will provide an overview of these fundamentals of thermal management.
Software
Designers must address thermal management, also known as temperature control, as early in the design process as possible. Because space is already at a premium within an enclosure, re-arranging components due to thermal needs late in the process may be impractical or prohibitive. In addition, the designer faces many formulae and graphs plus physical models for each variable in thermal control under tight time-to-market constraints. When an engineer determines desirable thermal conditions, the design may have already changed for a variety of reasons not related to thermal management, and he would have to restart hand-calculating thermal conditions.
Thermal software can provide a model to calculate the amount of heat to be dissipated along with the desired airflow to do the job. The engineer can quickly determine air temperature, air pressure and airflow velocity anywhere within an enclosure. Many of the software tools provide 3-D graphics and take into consideration placement of components and their geometries and their affect on airflow. This cuts down on the steps for creating a physical model, and it allows engineers time to create alternative designs.1 Popular vendors of thermal software include Innovative Research, (www.inres.com), Flomerics, www.flomerics.com) and Fluent, Inc., (www.fluent.com). Simulation may show that an active method of moving air through a package is necessary.
Fans
For air circulation, the tools most people think of first are fans and blowers. These electromechanical devices are driven by motors and use rotating blades to circulate air. Fans vary in type, and the most prevalent are axial fans and blowers. The axial fan moves air in a direction parallel to the direction of the fan blade axis. They work well under low-pressure or low system impedance conditions. These fans are common in PCs and most electronic cooling systems. Blowers are centrifugal, ie., the air moves perpendicular to (or a 90-degree direction) from the axis. They are suitable in high-pressure applications such as high-end networking/telecom applications.
Fans can draw warm air out of, or blow cool air into, an enclosure. Because a system requires the same volume of air to remove a given amount of heat, designers can choose either intake or exhaust fans. Intake fans offer laminar -- continuous and uniform -- airflow. According to NMB Technologies' Fan Engineering 101 (www.nmbtech.com), this is important to removing stagnant air and eliminating hot spots. It adds that the turbulent airflow of exhaust fans is able to dissipate two-times that of a laminar flow.2 A disadvantage to an intake fan is that it can slightly warm incoming air, owing to the air dissipated by its motor.
Designers must consider dust in any system that moves air, because accumulated dust can restrict airflow and thus raise temperatures within an enclosure. Dust also can stick to components and reduce their ability to efficiently radiate heat. Intake fans use a filter to keep dust out of an enclosure, and because they slightly pressurize air in the enclosure they keep dust from infiltrating through small openings. Keep in mind that adding a filter reduces airflow slightly.
Fan speed is an important factor in fan selection because air must be sufficiently circulated to prevent components from overheating. A designer must determine the amount of airflow necessary under worst-case conditions, or the necessary airflow can be calculated and graphed using the provided formula (Q=1.76W/Tc where Q is the airflow required in cubic feet per minute (CFM), W is the heat dissipated in Watts, and Tc is the temperature rise above inlet temp in degrees Celsius.).
For example, 32 cfm of airflow is required for a system that dissipates 200W and allows a 20-degree temperature rise. Check the manufacturer's specifications/airflow curves when choosing a suitable fan.3
The vertical axis is the heat to be dissipated, and the horizontal axis is the airflow. Both axes are logarithmic. The temperature rise in degrees Celsius is represented by the sloping lines. To determine the required airflow, find the sloping line that represents the permitted temperature rise, and then find the point on this line that corresponds to the heat to be removed. The horizontal position of this point is the required airflow.4, 5
Fan speed also contributes to audible noise. Fan noise may not be tolerable in an end-product's working environment, such as an office or where noise can affect an end-user's concentration where safety hazards are nearby. The fan's motor speed, rotation speed, its number of blades and its placement within a design will affect noise level.
Other variables in fan selection include power (AC or DC), voltage, EMI/RFI noise and life expectancy. DC fans are most commonly used because they tend to create less EMI/RFI interference (most models are brushless) than AC fans, fit into smaller spaces, and they are more easily controlled. If the device is intended for use in an EMI/RFI-sensitive environment, DC fans are preferable. The lack of a DC power source and price considerations are among the criteria to use AC power. Voltage is an important consideration because speed is proportional to the DC voltage applied. AC relies on the supplied voltage in which it is used. This information is important when considering current levels and power dissipation. Life expectancy is affected by the type of bearings (ball or sleeve) used, and both cost and the manufacturer's quoted life are considerations.
Fan Control
Thermal management may rely heavily on the fan and how it is used; however, it is an ongoing process. With time, the fan and the device it is ultimately designed to protect may suffer from inefficiency, increased power consumption and increased noise (both EMI and audible). The fan itself may slow over time, or it may speed up when operating under worst-case conditions. These conditions will eventually give way to more catastrophic events such as component damage and mechanical failure. Therefore, proper monitoring must take place throughout the life of the unit being cooled.
The most prevalent method of monitoring fans is through sensors. Many fans have built-in speed sensors that give a digital output in which the frequency is proportional to the speed of the fan (for example, two pulses per revolution).6 Alarm sensors alert the user to an over-temperature condition or a fan failure, or they may even shut the system down entirely to prevent major damage.
Many of the leading semiconductor manufacturers offer online tutorials that provide an overview of silicon-based temperature sensors and fan controller ICs. As part of its Analog University, National Semiconductor’s (www.national.com ) Temperature Sensor Handbook provides an overview of RTDs (resistive temperature detectors), thermistors, thermocouples and temperature sensing ICs. Maxim’s Web site (www.maxim-ic.com) explains silicon-based controllers and the methods they employ to regulate fan speed. They include pulse-width modulation (PWM), in which the fan directly turns its power supply on and off at a fixed frequency; linear regulation whereby a DC regulator reduces a fan's torque by adjusting the DC voltage across the fan and DC/DC regulation, which also adjusts DC voltage across the fan. The DC/DC regulator, however, uses a switch-mode power supply.7
Heat Sinks and Heat Pipes
In some cases, cooling electronic components may require a simple solution such as placing a susceptible component in a cooler part of the enclosure or near a vent. However, many parts of the design may require additional tools to do the job. The heat sink is the most widely used form of thermal management. Its purpose is to dissipate heat from the component being cooled to the air moving across the device via a fan or other ventilation, which transports the heat safely away. A heat sink attached to a component increases its surface area and increases the amount of heat that can be dissipated. This transfer of kinetic energy away from a solid object is known as convection. A hot heat sink also releases heat through radiation.
Heat sinks are widely variable in the way they are manufactured, shaped and in the materials from which they are made. The most important factor to consider when selecting a heat sink is its surface area. The goal is to increase the surface area of the device being cooled so that a heat sink with an already large surface area will expose the device to more air and dissipate more heat. In fact, heat sinks are made with extrusions to greatly expand their surface area far beyond the area of a smooth surface. Most heat sinks are made from Aluminum, which is a good conductor of heat. Copper is a better conductor of heat, but it costs more than Aluminum. To remove heat faster or to provide an air stream specifically to a critical device, a small fan can be added to the heat sink, making it active.
While heat sinks rely on convection and radiation to dissipate heat, heat pipes use evaporation and condensation to transfer heat. They have a highly effective thermal conductivity and can be used to spread heat uniformly throughout the heat sink's condenser section, effectively increasing the heat sink's size. They are made from a Copper pipe or channel and are back-filled with a working fluid such as Helium, Nitrogen or Potassium, depending upon the intended application. They can be used in almost any heat transfer application. Thermacore's Web site (www.thermacore.com ) offers an informative Frequently Asked Questions section along with technical articles covering heat pipe applications. Liquid-cooled heat plates are another option in applications where a heat sink is not practical.
Finally, many applications require more than one fan. Multiple fans, in parallel, can be used to disperse more air than is possible using a single fan. They add redundancy, and since there is a back-up in place in case a fan fails, they make the system more reliable. An obvious drawback is that more fans also require more money. In addition, more motors can create more heat than is provided by the extra cooling. The fans cannot be controlled individually, so controller ICs and software are important tools for regulating and synchronizing multiple fan speeds.
Conclusion
Keeping components cool and systems running reliably has taken on greater importance as end-products continue to get smaller while components add more functionality despite their shrinking footprints. The topics discussed here and the resources provided are a starting point for thermal management, and they are just one part of the challenge of protecting an electronic circuit against failure. Circuit protection involves many of the devices and techniques mentioned here, and it encompasses many more components and design techniques. In the February Starter’s Kit, we will review circuit protection and the problems the engineer must overcome to create a reliable and safe design.
Endnotes
1. Prahbu Sathyamurthy, Rajesh Nair, Fluent, Inc., "Simple Thermal Models Lead To the Coolest Designs," Machine Design (March 11, 1999).
2,3,4 Fan Engineering 101, NMB Technologies, www.nmbtech.com.
5 Abe Arredondo, JMC Products, "Comparative Analysis of Power Supply Single Speed Air Mover and an Entire Electronics Enclosure System Fan," www.jmcproducts.com.
6,7 Bruce Denmark, Maxim Integrated Products, "Cooling Down with Fan Speed Control, EDN (September 28, 2000).
Other Sources
www.epanorama.net
www.siliconacoustics.com
www.thermacore.com
www.aavidprecision.com
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