Electronic Design

Power: Power Supplies

Stand By For More-Efficient Power Sources

It's a given that power-supply development always moves toward higher power density, or in the case of low-voltage supplies, higher current density. Naturally, this trend favors the development of more-efficient switching power-supply architectures built with better ICs, power semiconductors, passives, and interconnects, as well as improved cooling devices and thermal management techniques. For a long time, the focus was on improving the supply's efficiency so that it could power larger loads—a trend that will certainly continue.

But now there's a growing recognition that the efficiency of the supply is also important when the load is barely there or absent altogether. The power consumed by many types of electronic equipment under "no-load' or "standby" conditions is often significant. Even more crucial is how the energy wasted by nonoperating devices adds up when all of the loads on the ac mains are counted.

To combat this energy loss, various organizations have implemented energy-saving programs that will strongly influence the development of low- to medium-power supplies for consumer equipment. In particular, there will be pressure to replace the low-cost, 5-W or less linear-transformer-based ac-dc adapters with more-efficient switchers. Although switchers generally cost more, semiconductor developers are making strides to develop cost-effective switcher designs, and this trend should continue.

In computing applications, the demands for "always on" operations continue to drive the development of UPSs. One twist on this theme is the extension of UPS-backed power across the network through power-over-LAN power distribution. Supported by the emerging IEEE 802.3af standard, power-over-LAN will likely be driven to the market by voice-over-IP applications, which will demand telecom-level reliability.

Reliability concerns also play into the application of signal-monitoring and control functions via the I2C bus. Power supplies now under development are exploiting this serial bus to ensure the dependable operation of the supply. As systems designers begin to appreciate the advantages offered by I2C, they probably will insist on more sophisticated control and diagnostic functionality from their power supplies, forcing supply vendors to build in greater intelligence and flexibility.

TOP TEN
>SEVERAL INITIATIVES TO CUT STANDBY POWER consumption will influence power-supply development in 2003. Overseas, there's the European Commission's "Code of Conduct on Efficiency of External Power Supplies." As of the first of this year, this program limits no-load power consumption for external supplies up to 75 W to just 0.75 W. In Japan, the Top Runner program creates competition among equipment makers that promotes the lowest attainable standby power in consumer products. In the U.S., a Presidential Executive Order now requires that the federal government purchase electronics equipment that consumes 1 W or less in standby mode whenever practical. Also, Energy Star, a program traditionally focused on the efficiency of appliances, will require cordless phones and answering machines to consume under 1 W in standby by 2004.

>AS VOICE-OVER-IP IS DEPLOYED more widely, demand for power-over-LAN will grow as a means of ensuring system reliability. The IEEE 802.3af standard defines the electrical specifications for power-over-LAN, including a protocol for distribution of —48 V dc plus data over unshielded twisted-pair wiring. In the coming year, this standard will drive development of power-over-LAN power supplies by power-supply vendors and load-detection circuits by semiconductor vendors. Power-supply designers must meet some unique requirements, such as higher-than-usual safety isolation (2250 V dc) between the —48 V bus and other working voltages. Another challenge will be the 100-mV noise and ripple specification. Initial requirements for a 24-port networking switch call for about 300 W of power-supply output at a power density of 10 W/in.3 But as the number of peripheral devices increases in time, power-supply developers will be pushed to improve their efficiencies and thermal-management techniques to further raise power density.

>STANDARD POWER-OVER-LAN power supplies will emerge this year. At least one vendor plans to introduce an ac-input, power-over-LAN supply with 600 W of output. That power will be distributed between a 48-V output and a secondary voltage such as 12 or 5 V. Also look for development of 48-to-48-V power-over-LAN bricks that provide isolation as well as EMI shielding and filtering.

>POWER DENSITY FOR NETWORKING POWER SUPPLIES in the popular 1 U format will rise. In terms of an ac-dc supply with 48-V output, at least one vendor offers a 600-W, 10.4- by 4- by 1.58-in. unit. Those values translate to a power density of about 9 W/in.3 Performance for similar supplies could reach 12 W/in.3 in 2003.

>WIDESPREAD DEPLOYMENT OF BLADE SERVERS could challenge UPS manufacturers to meet the power distribution and cooling requirements of new server systems. With 1 U style servers, power consumption is about 6 kVA for racks equipped with 42 servers. The problems of supplying sufficient power and cooling to such a system have been overcome. But these problems could be roadblocks when blade servers dramatically raise the number of servers and overall power demands of the system. Look for more-sophisticated cooling schemes to address these problems.

>THE I2C BUS, a two-line serial port (data and clock), will replace traditional monitoring signals on many power supplies. It may be used to detect ac input failures, output status, and temperature, as well as fan status and speed. I2C may also be employed to enable or disable the supply.

>Li-POLYMER BECOMES A MORE ATTAINABLE option for designers at large, as vendors like Sony Electronics ramp up production of cells. In addition, other manufacturers, such as LG Chemical,Samsung SDI, and Sanyo plan to introduce their Li-polymer batteries to the market this year. These batteries are thin (3 to 4 mm), light, and resist leakage and swelling. However, their energy density ratings are not quite as high as similarly shaped Li-ion prismatics.

>IMPROVEMENTS IN SEALED LEAD ACID (SLA) BATTERIES (better amp-hour ratings) have made it possible for UPS developers like American Power Conversion to offer 3000 VA and 3000 W of output in a 2U high UPS. Barring an unexpected breakthrough in battery performance that level of power density shouldn't improve much in the coming year, although the 1:1 ratio of watts to VA will be extended across different product lines. However, there will be a continued focus on developing UPSs that satisfy requirements for modularity, scalability, and redundancy.

>ONGOING DEVELOPMENT OF LARGE, high-power Li-polymer batteries for hybrid electric vehicles (HEVs) should move these batteries closer to commercialization. Down the road, such batteries could also improve UPS performance dramatically since both the HEVs and UPSs require high-power batteries that can discharge quickly—within 5 to 10 minutes for a typical UPS. But batteries of this type are still in the development stage. Meanwhile, the development of large supercapacitors for HEVs will also spell better UPS performance. These components will experience dramatic price cuts in the next few years, making them more viable in the UPS

>AMONG TRADITIONAL Li-ION CYLINDRICAL and prismatic batteries developed for portable products, advances in cell chemistry may push volumetric energy density to 500 Wh/l in the near future. In terms of cell capacity, industry-best performance for the popular 18650 cylindrical could jump from 2200 mAh to 2400 mAH. Among Li-ion prismatics, 6-mm thick cells are currently the most popular. However, with portables getting thinner, it's expected that 4- to 5-mm thick cells will soon be dominant.

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