Passive and Active Thermal Management

Keeping electronics cool is an important goal for designers, and they have a wide variety of options with active and passive thermal management. With power densities continuing to increase, and microprocessors drawing more power for meeting the demands of computational and functional requirements made by high-powered applications, thermal management is, even more, a requirement.

Today, densely populated circuit boards use high-power components to create systems that generate more heat than ever before. Therefore, designers face an upward challenge in implementing effective thermal design and management, as heat is the major cause of electronic failure.

The operational life and functionality of components can be compromised severely when the application exposes them to high localized temperatures, or when they operate at excessive temperatures. Prolonged operation in these extreme conditions can also lead to degradation of the components and ultimately, failure.

Designers ensure reliability and proper performance by arranging to transport the excess heat away from system hot spots and critical components. By dissipating this heat to the ambient environment, they manage to keep the system temperature within acceptable limits. Thermodynamics offers three basic methods of heat transfer that designers use to their advantage. They derive the thermal solutions from the basic principles of conduction, convection, and radiation.

Conduction allows heat to flow from an area of higher temperature to one with a lower temperature, provided both are within a single medium, which can be a solid, liquid, or gas. The process of conduction attempts to equalize thermal differences. The process also transfers heat between media in direct physical contact, such as between two touching solids.

Convection is the process of transfer of heat between a solid body and a fluid, which can be liquid or gas. The bulk motion of the fluid helps in the heat transfer. During the natural convection process, cold ambient air assimilates heat from any heated surface nearby. The air becomes warmer and, therefore, less dense, causing it to rise and creating low pressure. Cooler air from the surrounding areas rushes in to balance the low pressure created by the rising warm air, which, in turn, rises when it warms up. This creates a natural cycle of airflow, removing the heat.

In the process of radiation, a hot body radiates thermal energy as electromagnetic waves. The surface condition of the radiating surface and its temperature determines the amount of emission. Likewise, a cold body can absorb electromagnetic waves, and the amount it can absorb depends on its surface condition, its temperature, and the temperature of the surrounding environment.

All the above are examples of passive thermal management. These are commonly in use as they are easy to implement and are the least expensive. In some situations, however, passive thermal management is not adequate for removing excess heat. These situations call for active thermal management, such as using a fan to force an air-flow movement over the heated surface. Controlling the airflow may require a change in the fan size and its configuration. Optimization of the flow path may require proper placement of the fan and its orientation.