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THERMAL CONSIDERATIONS IN ELECTRONICS DESIGN

by George Novacek

Start ý Why Worry? ý Reliability ý Design Steps ý Practical Example ý Heatsink Selection ý Sources and PDF

Until the first speedy microprocessors arrived, most electronics designersý experience with thermal management (thatýs the classy way of referring to cooling) was limited to power supplies and an occasional linear power output stage. Quite often, a rule of thumb was used to fit the few hot components with a heatsink and little further thought was given to the subject, unless problems arose. Things have changed drastically since those days and cooling of electronic components is (or should be) on everybodyýs mind.

The millions of transistors crowded within a minuscule space of a modern microprocessor chip, switching at continually higher clock frequencies, generate a great deal of heat. The power supplies, on the other hand, are expected to provide for these low-voltage, high-current hungry circuits clean, rock-steady power. A common present-day 500-W switcher working with 90% (excellent, by todayýs standards) efficiency will turn 50 W into pure heat. Low-voltage, high-current regulators in particular struggle to achieve high efficiency.

There are two basic methods at your disposal to manage this sorry situation. First, the most obvious method is to address the cause and reduce the heat generation, whether it is as a result of the voltage drop in regulators, high saturation voltage, switching losses, or so forth. Unfortunately, 100% efficiency is merely a pipe dream, and there is only so far we can go with the laws of physics and the limits of technology. In the end, the majority of the effort will go to treating the symptomýto conducting the excess heat away from the device.

How can the heat the equipment generates be reduced? By circuit design, efficiency can be optimized. Switching instead of series regulators can be used. But, nothing is free, you pay for the high efficiency of switchers with EMI headaches. Digital, as opposed to analog processing affords better energy efficiency in most applications. Logic circuits and microprocessors, operating at lower voltage, dissipate less heat per transistor.

Starting with 5-V TTL levels many years ago, 3-V devices came along, followed by lower voltages today. It is expected that in 10 years, the operating voltage will drop to 0.5 V, with gate thickness of a mere 10 atoms. However, the lower heat dissipation per transistor is always amply offset by increased functionality paid for by more, smaller transistors crowded inside the chip. Present-day microprocessors running with 1-GHz clocks easily put out 50 W in heat. The trend towards higher integration, smaller, more crowded cabinets, and higher clock speeds will only make the future heat dissipation problems worse.

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