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Benefits of designing in the negative space

29 Dec 2015  | Karim Wassef

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For example, with a 12-amp DOSA Pico-sized point-of-load (POL) module powering an 8amp, 1.2V load using a tunable loop, just 400 microfareds (uF) of external capacitance is needed to deliver the same transient response you'd find on a comparable 1,500-uF system. In addition to saving up to 75 per cent of component board space, this reduced capacitance provides additional bill-of-materials cost savings (figure 5).

Figure 5: Transient Performance Constrained.

Finding space in power distribution
Designing in the Negative Space also can be applied to how AC-to-DC power is delivered at a system level—from a server to a full rack cabinet. Power distribution constraints involving balancing three-phase power, for example, typically require three separate rectifiers and a power distribution unit (PDU). Newer, true three-phase AC-to-DC rectifiers can replace the three single-phase conversion units with one unit that balances all three phases. This single unit delivers the same amount of power in half the rack space of previous generation three-phase power solutions.

A typical server cabinet has 42 vertical rack units or about 74 inches of usable vertical space for server blades. Typical power conversion, however, consumes about four or five of these rack units. That's about nine inches of vertical rack space that cannot be used for data centre computing or networking because it is being used for power conversion.

By cutting the rectifier footprint in half and allowing data centre engineers to connect three-phase power directly to the power supply, twice the power is available in the same space (figure 6). That frees up critical server bay capacity and gives back space inside the rack for extra computing capacity. Multiply that times the hundreds of server cabinets in a typical data centre, and data centre operators can save significant capital and operating expenses in infrastructure and facility space.

Figure 6: "Single Integrated Rectifier"—Power Distribution Constrained.

Again, applying a "zero-based" Designing in the Negative Space philosophy, what if we could eliminate three-phase power conversion equipment completely from a server or telecommunications equipment rack? Looking at the few extra inches between the 19-inch interior width for equipment and the total 24-inch interior cabinet width, power designers are using power systems that place the PDU and single, integrated three-phase rectifiers in that unused inside corner space (figure 7). Putting power distribution and rectifiers into that unused space in the back corner of the server cabinet frees up the full 74 inches of vertical equipment space for data centre equipment, improving power density and computing capacity.

Figure 7: Using "Useable" Server Cabinet—Power Distribution Constrained.

As seen in the examples detailed in the article, we have more options in tackling power density than just forcing more power into smaller packages. Instead, by Designing in the Negative Space, we can reclaim PCB and device space, giving precious real estate back to designers to harness improved computing, processing and communications capacity.

About the author
Dr. Karim Wassef, general manager of Embedded Products for GE's Critical Power solutions, works with telecommunications and data centre customers to provide advanced power solutions that support massive communications, network and computing capacity.

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