Electrical issues traditionally take precedence in the design of notebook computers. But, as system functionality per square inch shoots skyward, thermal issues are now moving closer to the forefront. With heat-generating microprocessors and other electronic components being packed into a limited space with restricted airflow, heat inevitably becomes a problem. It can cause anything from erratic behavior to complete system failure.
To date, methods to alleviate thermal effects range from an extensively modeled, optimum component layout that minimizes system cost to a sophisticated high-power fan with speed control to maximize battery life. Since every notebook is different, however, a unique thermal solution exists for each system. Heatsink manufacturers, thermal-modeling software providers, and others active in thermal issues can assist the designer in this complex area. In addition, the global requirements for notebooks may suggest the use of a thermal consultant with global expertise and support. Whatever method is chosen, space constraints, battery life, weight, and system cost all demand that the thermal issues be addressed during the early phases of system design.
Design Constraints
Thermal design in notebooks is particularly challenging because system heat tends to increase the temperature of all components—even those that generate little heat themselves. The heat-removal rate is driven by the temperature differential between the inside and the outside of the notebook. The chassis itself can dissipate only a limited amount of heat, as it must remain comfortable to the touch. Furthermore, long battery life is a selling feature for notebooks, so optimal solutions must minimize the draw on the battery. Portability also is necessary, so the thermal design must not add significant weight to the package. To add to this issue's complexity, notebooks restrict the maximum height of the thermal solution to under one inch.
Heat from low-end notebooks can be eliminated with proper component placement and inexpensive heatsinks. Yet the high-end notebooks, with their greater power draw, require high-performance fan systems as well as strategic vent placement to optimize airflow. To preserve battery life, the smallest fan possible must be used. This minimal airflow requires that the heatsinks and pipes conform to tight operational tolerances. Ease of installation also is important. Attachment of the heatsink and heat pipe usually occurs after the CPU has been installed. These thermal components must align with both the CPU and the chassis, so they must also conform to tight physical tolerances.
Thermal Solutions
Heat pipes and heat plates: Solutions for low-end notebooks can be very cost-effective. Here, the primary heat generator is the CPU. Simply keeping the CPU away from the hard drive maintains hard-drive reliability at essentially no cost. The heat from the CPU can then be piped out using a 3-mm-diameter heat pipe that's connected to a stamped aluminum plate on the back of the keyboard. The aluminum plate can typically absorb 2 to 4 W without significant temperature rise. The heat is then dissipated through the keypad.
A straight, round heat pipe can dissipate up to 15 W. Maintaining the low profile of the more expensive notebooks requires more intricate designs. These designs increase the need to bend and flatten the heat pipe, decreasing the amount of heat dissipated. Adding 90° bends or flattening the pipe can reduce effectiveness to as little as 6 W. These low-profile notebooks require additional thermal solutions.
Heatsinks: For low-end to mid-range notebooks, small, inexpensive extruded heatsinks provide sufficient additional cooling when the heat pipe cannot carry the whole load. Finned heatsinks dissipate the most power in the smallest amount of space, incrementally increasing the ability of the designer to dissipate heat passively. This is a low-cost solution. But for some applications, more fin density may be needed. This, in turn, requires more space. Folding the fin increases the surface area available without increasing the space occupied. The folded-fin heatsinks are more expensive than the simple extruded variety. Still, the amount this adds to the cost of manufacturing is more than compensated for by the increase in the retail price of the higher-end notebook.
Airflow control: Properly placed vents provide airflow to remove the remainder of the heat from the system. As the notebook's complexity increases, simple fans can raise the airflow, thereby improving heat transfer out of the system. A variable-speed control fan is an innovative solution to battery life limitations, offering additional margin on high-end systems. Whenever the user accesses high levels of computing capacity, and the CPU is generating the maximum amount of heat, this type of high-performance fan operates at maximum speed. When computing levels decrease, the fan slows down to conserve battery power. This solution also reduces acoustic noise, since the noise associated with the fan is proportional to fan speed. Though these fans are approximately 10% more expensive than the conventional ones, the extension of the battery life increases the marketability of the notebook, justifying the added cost.
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