Introduction to Conduction Cooling in Electronic Systems

Why Should I Care About Thermal Management?

Electronic systems are becoming more powerful each year. With additional processing power enclosed in such a small package, the power density has increased dramatically in recent years. Since heat generation is a result of current flow, now more then ever the thermal management of electronics is prevalent in order to achieve maximum performance and preserve the life of the electrical component.

What are the key parameters to control?

Electronic components are typically accompanied with a datasheet specifying the acceptable junction temperature, where the junction of an electrical component is the site of heat generation. This temperature value is determined as the acceptable limit such that the component performs at maximum potential and does not throttle performance. In order to maintain this thermal limit, various thermal cooling techniques are employed to ensure the junction temperature is kept below its limit with suitable margin. Techniques such as conduction, natural convection and radiation, forced air convection, and liquid cooling are all techniques used to cool an electrical component. For the purposes of this post we will be looking at the basics of conduction cooling.

Conduction Cooling

As stated earlier, wherever there is current flow there will be heat generated. By the first law of thermodynamics, we know the energy input into a system is equal to the energy output so long as the operation is steady. Since the only form of energy leaving the electrical component is heat, we can conclude that the magnitude of the heat dissipated from the electrical component is equal to the power consumption. The units are typically in Watts (W).

This generated heat, like all forms of energy travel be it fluid flow or vibration, likes to travel in the path of least resistance. Once the heat generated and the heat dissipated are equal, the electrical component will stabilize at temperature. Eventually the thermal load will dissipate into atmospheric air in some capacity, it is therefore a challenge to the engineer to determine how to create a heat transfer path that involves the least amount of thermal resistance. Depending on the application this can be quite challenging as there are various requirements that need to be considered. Many times electrical design requirements are contradictory to that of an efficient thermal design. Furthermore, the magnitude of the heat generated, the reliability requirements, the environmental conditions the electronics are exposed to, and cost are all factors in what the most optimal design path will be taken.

Formula for Steady, One-Dimensional Conduction Heat Transfer

The heat conduction through a plane wall is shown as a function of the thickness L, heat transfer surface area of A, and a thermal conductivity of k. The formula is shown below, with the units being in Watts.

An important form of this formula is the thermal resistance. The thermal resistance is the value that engineers must optimize to the lowest possible value in order to maintain a high performing design. The thermal resistance formula is shown below, with the units being in degrees Celsius per Watt.

Within a heat transfer path there can be multiple thermal resistances that vary in severity. This can be modeled and analyzed similar to an electrical or fluid circuit. Areas with the largest thermal resistances will show the largest temperature drops. It is the job of the engineer to ensure the temperature drop from the electrical component junction to atmospheric air is as small as possible. This will ensure optimal performance, a longer lasting product, and a happy customer in the end.

In my next blog post on thermal cooling I will go over some applications of conduction cooled electrical components as it relates to a heat sink. Visit our home page at www.designwithdreamspace.com as well to learn more about the valuable engineering design services we provide at DreamSpace Engineering Consulting!

Warmest Regards,

Thomas A. Smith