[Technology Report]
Optimized Power Supplies Beget Superior Data-Center Efficiency
Data centers are conspicuous consumers of power. Power-supply design that maximizes efficiency can deliver big payoffs.
LLarge data centers devour huge amounts of electrical power (see “Energy-Hungry IT Centers See Hope In Digital Power”). At the heart of efforts to reduce wasted energy, power-supply makers offer a host of ways to minimize their contributions to the problem.
In broad terms, the challenge is to deliver power to hundreds of servers’ processors at very low voltage levels from multiphase ac mains. The processors have three power rings—logic, memory, and I/O—all requiring very precisely regulated voltages on the order of 1 V, regulated to a precision of one one-hundredth of a volt.
The processor loads are highly variable, depending on how hard the processors are working, that is, how many gates are switching. Those loads vary from milliamps to tens of amps, with transient demands of hundreds of volts per microsecond.
A complicating factor is that the voltages to the different power rings on the processors must be managed in terms of regulation as well as sequencing. They must be applied and removed in a fixed order, with precise timing. The challenge is to create an efficient power-distribution network that can convert power from the grid to those dynamic low-voltage loads.
Five power distribution alternatives for data centers are either in use or under discussion (Fig. 1). But which is most optimal in terms of overall efficiency? Debate continues, but an interesting analysis, “AC vs. DC Power Distribution for Data Centers” by American Power Conversion’s Neil Rasmussen, provides a methodology for analysis.
Approaches a and b in Figure 1 represent, respectively, the configurations generally used in North America and the rest of the world for data centers. Approach c represents the configuration used in telecom central offices.
In the North American data-center configuration, the mains power goes through an uninterruptible power supply (UPS) and a transformer-based power-distribution unit (PDU), and then to the server rack. In the rest of the world, where ac mains voltages are higher, there’s no need for the PDU.
The UPS provides pre-regulated ac power, either from the ac line, or, when the utility power drops out, from a battery bank. Its first stage is simply an ac-dc rectifier whose unfiltered output is applied to the battery bank. Its second stage is an inverter. The ac inverter feeds the PDU, which provides power factor correction and steps up the ac voltage for distribution around the data center. Inside each equipment cabinet, a front-end converter rectifies the ac and steps it down for distribution across the backplane.
In the telco central office, the UPS has historically comprised a large, 48-V lead-acid array to provide the mandated “five nines” (99.999%) availability. Therefore, 48 V is distributed directly to backplane busses in equipment cabinets.
Challenging the traditional approaches, the configurations in d and e are based on producing high dc voltages at the front end/ UPS and busing that are around the data center, with proportionally lower I2R losses. The version in e would require the last stepdown to 48 V to occur in the equipment cabinets. Meanwhile, the version in d would use a common step-down converter and allow for conventional 48-V distribution on backplanes.
By discounting certain assumptions in other studies that favor the high-voltage dc concepts, Rasmussen’s analysis essentially concludes that the conventional “rest of the world” approach is optimum. Read the analysis yourself and decide what you think. Inside the equipment cabinets, the common model is the intermediate
bus architecture (IBA). The IBA takes that nominal 48-V dc backplane voltage and applies it to a step-down converter on each board, often in the form factor of a fractional “brick” (Fig. 2). This brick then isolates and transforms the 48 V to a somewhat regulated intermediate bus voltage. This voltage supplies a number of point-of-load (POL) dc-dc converters that step-down and tightly regulate the voltages for critical ICs. For the sake of efficiency and control, these are always switch-mode converters.
To simplify this explanation, we’ve been treating the intermediate bus voltage provided by the dc-dc brick regulators as if it were standardized at 12 V. In actuality, it might be 12, 8, or 5 V dc. It depends on the system design. A higher bus voltage suffers less from I2R losses, but the efficiency of each simple dc-dc buck regulator at each POL decreases with the ratio of voltages it has to drop. Thus, the choice of bus voltage involves some tradeoffs.
FLATTENING EFFICIENCY CURVES Even though improving POL efficiency results in small energy savings, the effects add up. In fact, they actually compound as they’re reflected back through the supply chain (Fig. 3). In the past, power-supply efficiency numbers on data sheets referred to peak efficiency, which usually happened between 50% and 80% of rated load. Actual efficiency would fall off slightly at full load and sometimes dramatically at loads around 20% or so. Recent standards worldwide recognize that loads are variable and call for high efficiencies (80% and more) across a range of loads from 20% to 100%.
Achieving and improving efficiency across the full load range has become a point of differentiation among power-supply manufacturers at all power levels, with the application of proprietary techniques and considerable squabbling over patents. Broadly, efficiency-boosting techniques include synchronous rectification, cycle skipping, and multiple phases and phase shedding.
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