48 Vdc Distribution; a Safe, Practical and Efficient Solution for Equipment Racks

April 5, 2013
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Power efficiency in supercomputersVicor’s new AC Front End Module has an isolated, regulated output of 330 W at 48 Vdc. Its applications are many and varied, including powering rack-mounted equipment in telecommunications or data centres. However designers for such applications sometimes ask why the DC output has been set at 48 V rather than 12 V, or even 5 V? Designers are looking for these voltages because they are building systems that include one or more legacy boards – typically CPU boards – that operate on 12 Vdc input. So, if they are basing their system architecture on such products, wouldn’t it be logical to design any additionally required boards to run on 12 Vdc too, to keep the power train simple?

Although this once could have been true, it almost certainly is not today. As data and telecoms resource providers come under continual pressure to deliver more processing power from the same space, they respond by installing newer, more power-dense electronics hardware. This causes a significant increase in the power drawn by a rack populated with, say, a set of blade servers. Under these circumstances, limiting the supply to 12 V inevitably increases the current and associated I2R losses in the rack backplane or bus bars. Today’s conditions dictate power transmission at a higher voltage for acceptable transmission efficiency.

Some equipment centres have responded by running a global voltage of 380 or 400 Vdc. However this is not necessarily desirable or safe for running within an equipment rack; and in any case, boards or plug-in servers will not accept such high voltages directly. A more practical alternative is for each cabinet to convert the incoming high DC voltage to 48 Volts. This is a safe level, yet high enough to address and reduce I2R losses across the backplane, connectors and possibly board tracks. AC-48V power supplies are typically more efficient that AC-12V power supplies, yielding savings not only in lower distribution losses, but in lower front end power conversion losses as well.

Another alternative is to bring AC mains voltage to an AC Front End in each cabinet or rack. This can then deliver isolated SELV 48 Vdc via a backplane or bus bars to boards in the rack. These boards will fit one of three categories: they are off-the-shelf or legacy 12 V, off-the-shelf 48 V, or custom/yet to be designed.

The 12 V boards can be connected into the 48 V system via a DC-DC converter mounted on the rack backplane. Regulation does not have to be particularly tight, as it will be handled within the recipient board. Existing 48 V boards can be directly connected, while new boards can be designed for improved power efficiency using power topologies such as Vicor’s Factorized Power Architecture (FPA). Board I2R losses can be minimised by routing the 48 V supply through to FPA devices located close to the POL. Vicor also offers a solution that will provide an Intel VR12.0 compliant solution directly from a 48V input, without the need for an intermediate conversion stage to 12V. This allows a user to eliminate power conversion stages from a system and realize improvements in end to end efficiency, as well as cost and power density.

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One Response to:
48 Vdc Distribution; a Safe, Practical and Efficient Solution for Equipment Racks

  1. Eddy Kilowatt on April 12, 2013 at 1:53 PM

    Glad to see Vicor supporting industry-standard voltage levels!

    Looking forward to application notes on switching and fusing at the 48 VDC level. Much easier than 380 VDC, but still not quite as easy as at 12 VDC!

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