Leverage latest power devices to improve data-center efficiency

Worldwide in 2005 alone, more than $7 billion was spent on powering data-center servers and the infrastructure supporting them, requiring the equivalent capacity of fourteen 1000 MW power generating facilities [1]. While data centers are migrating to low-cost clean energy from hydroelectric sources in Oregon and Washington [2], that does not reduce the waste associated with power conversion and conditioning, distribution, and environmental control including air conditioning.

In fact, in a typical data center, less than half of the power consumption goes into the computing function. Consequently, data-center operators are looking at every opportunity to improve power conversion and distribution efficiency, including eliminating the number of conversion stages through the distribution of high voltage direct-current sources [3].

This feature provides an update on the great strides have been made during the last decade in the development of power semiconductor devices and products to help reduce wasted energy through improved efficiency at every stage of the power delivery system. New high and low voltage power devices are pushing efficiency up through lower conduction loss and fast switching for reduced switching loss.

In the conventional conversion and distribution process, multiple conversion stages occur. In the U.S. the electric grid distributes ac power to local communities at 13,800 volts that is eventually stepped-down to 480 Vac using transformers that do not contribute significantly to energy loss. An uninterruptible power supply (UPS) is required to ensure that the data center does not fail when a power outage occurs. Commonly, a double-conversion on-line UPS that continuously regenerates a new clean sine wave output from the rectified AC main or the battery ensures no switch over interruption and provides conditioned 208Vac to the data center through a power distribution unit. This stage of the power conditioning may be at best only 70 percent efficient [3].

At the server rack the 208-Vac power is converted to 12- or 48-Vdc and subsequently stepped down to the bus voltages required for the processor and associated components like disk drives, and memory. A switched-mode topology converts the incoming AC power to regulated DC for distribution to the server main boards and peripheral components. Both of these conversion stages, the double-conversion UPS and switched-mode ac/dc converter benefit from high-performance semiconductor components like IGBTs and super-junction power MOSFETs for rectification, battery charging, and dc/ac inverting. Present embodiments of these advanced high-voltage semiconductor components have made great strides in recent years, enabling improved performance and higher efficiency in the UPS and air-conditioning functions in the data center.

Fig.1: Power conversion stages in a typical data center. The proper application of advanced high- and low-power semiconductors can greatly improve efficiency at every stage.

Once the dc voltage is applied to the server board additional dc/dc conversions are required to provide specific regulated power to the processor, memory, and other components. At full power, the processor may use in excess of 60W. At 90 percent efficiency the voltage regulator (VR) wastes 6 W providing power to the processor. A fully populated server rack with two processors per boards could waste as much as 500 W just in the VR with every percent gain in efficiency resulting in 50 W of energy savings. High-performance, low-voltage MOSFETs have enabled improved efficiency for these conversion stages with lower on-state resistance for reduced conduction loss with new device structures enabling lower switching loss. In the last decade, VR efficiency has increased by more than 5 percent and increased the output current rating by 5 times [5].

Buck converters of past generations may have used a Shottky diode and 60-V-rated power MOSFET for switching and power-loop return, achieving 80-85 percent efficiency. Even though processor input voltage decreased, present power MOSFET products can achieve greater than 90 percent efficiency.