A Perpetual Hot-Button Issue in Power

Nov. 1, 2007
Signs of cooler winter weather are starting to appear, but in power electronics, concerns over high temperatures never seem to fade. The continual shrinking

Signs of cooler winter weather are starting to appear, but in power electronics, concerns over high temperatures never seem to fade. The continual shrinking of circuitry at the component level, coupled with demands for higher performance, leads to higher levels of dissipation for many power devices. As a result, components may be running hotter and contributing to higher ambient temperatures. Then, too, there are the growing demands for electronics that can operate in harsh environments such as under-the-hood automotive systems and various industrial applications.

Improving the efficiency of power components is one way to alleviate thermal challenges. But then after you've made all the design tradeoffs to get the best efficiency possible, the challenge of cooling your devices is still there.

Although it's never been considered a glamorous issue (like anything digital), thermal management has always been a hot topic for power-supply designers. Several of the articles in this issue attest to the notion that thermal issues are becoming more pressing over time. These articles were developed with the understanding that designers need more-accurate methods for predicting thermal performance, design techniques that spread heat more evenly and better heatsinking options.

For example, in this month's cover story, “Beyond the Data Sheet: Demystifying Thermal Runaway,” Roger Stout addresses the sometimes-misunderstood issue of thermal runaway. Stout applies mathematical modeling to explain why semiconductors may fall victim to thermal runaway, even when they are operating at what appear to be safe junction temperatures. Understanding the mathematical model of a semiconductor device presented by the author will help designers understand why some systems are thermally stable and others are not. Knowing how to apply the model also should allow designers to more accurately predict their operating margins with respect to thermal runaway.

The threat of thermal runaway seems to be out of reach in the power-stage design described in Mark Hazen's feature, “Symmetrical Layout Enhances Power Controller.” Hazen discusses the design of a motor controller he undertook when he converted his Chevy S10 pickup to an all-electric vehicle. The focus of this article is the MOSFET power stage Hazen developed for driving the motor. Here, he takes a unique approach to the physical design of the power stage, mounting a series of MOSFETs around the perimeter of two large aluminum discs.

This novel layout creates a design that delivers equal and symmetrical gate drive to the MOSFETs, and helps distributes heat dissipation evenly across many transistors. As Hazen's results show, this physical design results in great thermal and electrical margins with the semiconductors running cool despite the high currents they carry.

Of course, there will be applications were semiconductors do run hot and where the methods used for device cooling become critical. In “Reticulated Metal Foams Build Better Heatsinks,” Burhan Ozmat discusses the properties of a new class of heatsink materials with an exceptional ability to remove heat from electronic components. As Ozmat describes, the characteristics of RMFs translate into lighter, smaller heatsinks.

With the subjects described above, the authors may be making assumptions about what levels of power dissipation certain components can handle, or about the maximum temperature levels those components can tolerate. However, in some cases, component suppliers are working to change those assumptions by increasing the operating temperatures their devices can tolerate. In this month's Analog Feedback, I look at some of the recent developments among discrete semiconductors, passives and magnetic components that allow these parts to run hotter and operate in higher-temperature environments.

Perhaps this sampling of high-temperature components reveals what may be an evolutionary trend toward higher operating-temperature capabilities in newer devices. If this trend is real, then there may be great opportunities ahead for those researching new materials, component designs and the manufacturing methods needed to build these more-durable, temperature-tolerant parts. Such developments will only fuel the trends that make thermal management a never-ending hot-button issue in power electronics.


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