Distributed Power Goes from Utilities Down to ICs
A discussion of distributed power starts at the power line and progresses to individual components.
by Jon Titus, Senior Technical Editor
Distributed power means different things to engineers. Some think of it only as the distribution of different DC voltages on a circuit board or in a rack of equipment. But good power-distribution engineering starts at the power company's connection to a building's wiring and goes all the way down to IC power inputs.
For a typical commercial enterprise, say a data center with racks of servers, a power company delivers 480-V three phase AC power that connects to a large uninterruptible power supply (UPS). "But servers need 208-V AC (line-to-line) or 120-V AC (line-to-neutral) single-phase power, explained Gary Anderson, marketing manager for AC power systems at Emerson Network Power. "So engineers must distribute the proper power to the server racks as efficiently as possible. They can do that in one or two stages."
A one-stage distribution system would route power from the UPS to a power distribution unit (PDU) that provides a step-down transformer with a 208-V output and many circuit breakers. Each breaker feeds a large power strip in a rack that servers plug into. That technique works well, but it doesn't give the data-center operators much flexibility because they have many large cables to route from the PDU to the individual power strips. "We used to see 27 single-cord servers in a rack," noted Anderson. "Newer servers use two power supplies to provide a redundant source of power, so with 42 dual-cord 1U servers in a rack, you now have 84 power plugs to deal with."
A two-stage technique simplifies power distribution. "You still have a 208-V PDU, but we place a distribution cabinet in a row of servers or around the walls of a data center," said Anderson. "Each cabinet connects to the 208-V PDU with one cable and then distributes power to individual server racks. So, you have fewer cables to buy and install. Fewer cables also improves air flow for cooling through the ceiling or raised floor." The cabinets don't have transformers, so you have only small power losses in the new cables and added circuit breakers.
Distribute a Higher Voltage
Engineers have other power-delivery options. "Recently people have started to distribute 600-V AC power in data centers," said Anderson. "That lets them pull more current through the same conductors they used for lower AC voltages. People also distribute 480-V, three-phase AC power that they reduce to 277 volts AC with a transformer. But that voltage falls outside the 100- to 250-V operating range of server power supplies. And distribution cabinets would need special circuit breakers and panels rated for 277-V operation. Today, most breakers have a maximum 250-V rating. Server power-supply manufacturers say they can make a supply that runs on 277 volts but they haven’t done it yet, so practical 277-V distribution is a ways off."
"On the DC side of the power equation, engineer talk about converting line voltage to 48V or 380V DC and distributing one of those voltages to equipment," said Anderson." Distribution of 48V DC makes sense in five or six racks of telecom equipment, but not across 200 server racks in a data center. That would require a lot of copper to handle the needed currents with low power losses. Some engineers look at a 380-V distribution system as a way around the losses and large conductors in a 48-V system. But, so far no commercial products work with 380-V DC power."
Take Advantage of Telecom Standards
Standardizing on a 48-V DC power-distribution bus makes sense within a rack or system because the telecom industry already had the necessary cables, sockets, connectors, and hot-swap technologies available for that voltage. "That’s a reasonably high voltage, so you lose only small amounts of power in the distribution buses and you can use standard 48-to-X-volt 'brick' converters," said Stephen Oliver, vice president of sales and marketing at Vicor. "You route the 48 volts close to where you need it and then convert from 48 down to 5V, 3.3V and any other voltage you require. That's a distributed-power architecture. These bricks furnish well-regulated outputs." (A full-brick module measures 4.6 x 2.4 x 0.5 inches, or 116.8 x 61.0 x 12.7 mm. Half-, quarter-, and eighth-brick sizes are available, too).
Another approach involves using an in-between 12-V bus in an intermediate-bus architecture (IBA). In an IBA system, you convert from 48 to 12V on a board or subsystem and then convert from 12 to 3.3V, for example, near the load. In this way, a well-regulated 12-V rail provides power to a board and individual converters produced well-regulated lower voltages near the loads. "Then someone said, 'Hang on, we don't have to regulate the power twice, first from 48-to-12 volts and then again from 12 to, say, 2.5V,'" said Oliver. "So we moved to a bus convertor that takes in 48 volts and produces 12 volts unregulated. Think of it as a 'transformer.' That technique saves power because the power circuit regulates the voltage only once, right at the load."
In 2003, Vicor developed its factorized power architecture (FPA) that separates--or ‘factorizes’--the transformer from the regulator stages in a DC-to-DC converter. "Those two functions are usually intertwined, so it's tricky to optimize the converter and you reach an efficiency ceiling," remarked Oliver. "In the factorized power architecture, embodied in our V.I chips, we separate the voltage-transformation module, or 'transformer,' and the regulator circuits and optimize each separately to create a set of building blocks. That means you can go from AC to, say, 48V DC, but instead of having a 48-to-12-V brick, you use a simple regulator and separate voltage-transformation module to go directly to 0.8 volts for a processor or memory array." This power-conversion technique improves efficiency, but a discussion of the technologies goes beyond the scope of this article.
Engineers use this type of factorized-power topology in automatic test equipment. The main equipment rack produces 48 volts and voltage-transformation modules near the device under test produce regulated voltages needed to run tests.
Move up to 380 Volts
Recently, some of the mainframe computer makers examined the AC-to-48V conversion step and took it a large step up to AC-to-380V DC. Why 380 volts? Because power-factor-correction (PFC) equipment can easily produce a 380-V output at high efficiency from many line voltages around the world. (In some cases, PFC equipment will produce a 350-V DC output.) This gives two options; a) convert from 380V to 48V and then directly to load voltages, or b) go from 380V DC to 12V and then use voltage-regulator modules to produce the lower voltages right at loads.
More Voltages Demand More Converters
According to Karim Wassef, product line manager for non-isolated power at Lineage Power, the most important reason for heightened interest in distributed power is the increasing number of voltages new circuits demand. More voltages require more converters.
"In a distributed-power architecture, designers place the last stage of power conversion, often from 12V or 5V down to a lower voltage, close to the actual load," noted Joseph Thottuvelil, director of application engineering and technical marketing at Lineage Power. "This proximity reduces voltage drop and provides good transient response. A distributed architecture also lets designers easily reposition point-of-load converters as they manipulate a high-density PCB layout."
"In distributed-power systems, we find 12 volts predominates because of the large 'infrastructure' built for the PC market," stressed Thottuvelil. "Many systems have primary power supplies that take line power and go directly to 12 volts. You don’t need to go up to 48 volts unless you have a high-end system in which the higher voltage bus helps reduce power-distribution losses."
Optimum Bus Voltage Appears Elusive
"Some engineers think they can determine an optimum bus voltage for a system," said Thottuvelil. "But their voltage will depend on the distribution circuits, connector losses, protective devices on the bus, and so on. They can realize the benefits of a distributed architecture even if they just pick 12 volts. We tell engineers if they need more than three or four load voltages and require more than 100 watts, a bus architecture will yield cost savings and perhaps other savings, too. If they have only three or four voltages and require under 100 watts a hybrid architecture would work well."
"Often a system has a 48-V input and it produces a 12-V bus voltage that goes only to POL converters," said Thottuvelil. "Nothing else connects to the 12-V bus. In a hybrid architecture, typically 5 or 3.3V, some loads connect directly to the bus voltage. We find some 5-volts and some 3.3-volts hybrid buses in wireless and telecom systems. Fiber-optic equipment, for example, tends to use 5 volts because that voltage can directly drive laser diodes."
Do you Isolate or Not?
Engineers have a choice between isolated and non-isolated converters. The former isolate the primary and secondary sides with a transformer, but the latter do not. The non-isolated converters require only 1/5th to 1/10th the number of components as an isolated converter. That reduction in circuit complexity cuts cost and increases reliability. "If a system needs many voltages, engineers should choose an isolated converter to go from 48 to 12V and then use non-isolated converters to go from 12V to lower voltages at the loads," said Wassef of Lineage Power. "When you have three or four voltages, that approach saves money. And you need an isolation barrier somewhere, but not at every converter."
"When engineers connect bus power to a POL converter, they must think about filtering, too" stresses Thottuvelil. "We have some products and application tools that help them optimize the amount of input and output filtering."
For further reading
"Factorized Power Architecture and V.I Chips," Vicor 5-2007. http://www.vicorpower.com/fpa101/fpa101.pdf.
"Power Management Strategies for High-Density IT Facilities and Systems," Emerson Network Power, 2007. http://www.liebert.com/common/ViewDocument.aspx?id=670.
Wagner, Mike, "The Efficiency Challenge: Balancing Total Cost of Ownership with Real Estate Demands," Cherokee International, 1-2008. http://1u-cherokee.com/content/Efficiency Challenge.pdf (Information about 380V conversion.)