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Why digital power calls for more standards

Posted: 04 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:power supply? AC-DC? digital power? POL converters? PMBus?

There are numerous reasons why power system designers have increasingly been turning to digital power over the past decade. The principal drivers have been the demand to provide sophisticated power management functions, along with a desire to reduce board space and the number of external components. This is especially true in the distributed power architectures that now dominate the design of power systems for high performance datacom and telecom networks, and data center equipment.

The change from centralized power, with a simple AC-DC power supply feeding the cards in a system rack, came about by necessity to avoid the reduced power system efficiency that would otherwise result from unacceptable I2R losses in backplane wiring and circuit board tracks due to the higher current requirements of more powerful ICs, operating at ever lower voltages.

Figure: The proliferation of supply rails at the board level is another reason for using a distributed power architecture with multiple POL converters on the system board.

This trend is continuing as data center power levels rise from 300W to 1200W per board since the 1980s, and are forecast to reach 5kW by 2015 as network IP traffic growth is driven by video communications, cloud-based services and the Internet of Things (IoT). According to the Ericsson Mobility Report, annual IP traffic will reach 7.7 zettabytes by the end of 2017, up from 2.6 zettabytes in 2012. Furthermore, this demand for digital power and point-of-load (POL) solutions is proliferating into other application areas, such as in medical, industrial, and test/measurement equipment, as advanced processors and FPGAs become commonplace.

The adoption of digital power has accelerated in recent years by the ready availability of off-the-shelf digital power converter modules. Digital converters provide the same functionality as their analog counterparts but can dynamically adapt to changes in operating conditions in real time. Digital control is a particular benefit in distributed power systems during low load conditions when power supplies are normally relatively inefficient. By extending the digital control loop to encompass both the intermediate bus and POL converters the intermediate bus voltage can be reduced under low-loads, increasing the conversion efficiency of the POL converters.

While the technology of digital power and the benefits of a distributed architecture are widely accepted, and there is a good choice of AC-DC and DC-DC converter modules from many different vendors, there is still one issue that is holding back the market. This is the need for true multisource solution C something many equipment manufacturers with concerns about supply chain reliability regard as essential.

Such demands have led to various alliances between vendors and the formation of trade associations to provide a degree of interchangeability between products from different manufacturers. In reality, this has achieved little more than agreeing to standard footprints and pin-outs for certain categories of power converter, such as non-isolated and isolated DC-DC converter modules. Even the coalition formed by a group of semiconductor suppliers to develop an open standard communications protocol dedicated to power systems, known as PMBus, has proved to have its limitations. While PMBus provides a common set of commands to enable communication between the main components of a power system, the interpretation of these commands by digital ICs from different manufacturers doesn't guarantee interoperability when those commands are transferred over secondary serial busses.

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