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Benefits of designing in the negative space

29 Dec 2015  | Karim Wassef

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Another way to reclaim board space is to consolidate multiple features previously housed in separate components. Remember when we used to have all those fancy stereo components taking up shelf space in our house? Eventually the market consolidated them into one compact unit. Today we're even carrying the capability to listen to thousands of songs on the smart phone in our pocket.

That's the principle behind taking multiple features and combining them into a single power module to free up valuable space. For example, traditional circuit board design might require additional discrete components for accurate current, voltage, and temperature measurement with tight set point controls.

New power conversion designs can consolidate all of those features within a single power module product (figure 2). What used to require 987 mm2 of board space has now been reduced to a compact 755 mm2, allowing PCB designers to recapture 23 per cent of that space.

Figure 2: Feature Constrained.

"Under-think" the problem by changing sides
Another example of harnessing unused PCB space involves literally flipping the placement of the power component. Instead of placing a traditional high-current, 6-12 mm-tall power module on top of the board, a new slim-profile power module measuring just 2.8 mm to 2.9 mm in height lets designers place it under the board where the clearance is usually just a mere 3 mm (figure 3). New conductive cooling materials and techniques allow for this placement where no airflow is available.

Figure 3: Mechanically and Thermally Constrained.

The low height of such power modules also means they can be placed underneath the mezzanine, or middle level of the board. This ability makes the low height area of the main PCB usable for power. Traditional, taller high-current power modules do not easily fit in this space where height is constrained by the overhanging daughter board. The compact, low-height packaging approach also allows for top-board mounting near, or even under, traditional heat sinks, further conserving space and managing thermal challenges.

The fact that AC-to-DC power conversion generates heat creates another core thermal constraint. For many high-density power systems, that heat is dissipated with a small on-board fan. That's one more component taking up space, using energy and impacting reliability.

A tenant of Designing in the Negative Space philosophy is always asking, "What if?" So, rather than asking how to make the cooling fan more effective, ask if you can eliminate the fan altogether. We accomplished this in our designs by placing the heat-intensive integrated circuits on the bottom of the board, which was previously "unused" space. This placement also allowed us to employ conductive cooling through the system chassis, replacing the need for a separate fan (figure 4).

Figure 4: Thermally Constrained AC/DC.

The net result of these space-saving techniques is higher watts-per-cubic-inch. They achieved a reduction in size from a traditional 21 cubic inches to 11.2 cubic inches, while providing the same 200 watts of output. This doubles the density of the unit to 17.85 watts per cubic inch—which essentially equates to the same output in almost half of the space.

Addressing transient performance constraints
Another major power design constraint involves transient performance. One of the priorities when managing power conversion is to ensure consistent transient performance during load changes for sensitive loads like digital signal processors and field-programmable gate arrays. Transient response can be managed with newer tunable loop technologies. By adding a small tuning capacitor and a resistor, tunable loop technology achieves the same target transient performance of traditional designs with far fewer external capacitors.

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