Electronic Design

Energy Star Mandates Power-Efficient PCs

In reaction to the U.S. Environmental Protection Agency's (EPA) recently announced Energy Star 4.0 guidelines, power efficiency has become the dominant issue within powermanagement design. Hewlett-Packard will offer 80% efficient power supplies as an option for the recently introduced HP Compaq dc7700, dc5700, and dc5750 series desktop business PCs this month. Not only that, the optional supplies let HP meet the Energy Star guidelines six months before they take effect.

The most important factors in achieving the Energy Star goals depend on the choice of power MOSFET and the ICs that drive it. One approach is to use a switching regulator IC with integrated MOSFETs, which is usually feasible for up to about a 100-W load. Another approach is to employ external MOSFETs, which have lower on-resistance and thus higher efficiency than integrated MOSFETs.

Another efficiency-related design decision is whether to use a switching-regulator IC with integrated MOSFET gate drivers or external gate drivers for the external MOSFETs. Newer, high-power processors usually require external MOSFETs and sometimes external gate drivers to handle loads reaching hundreds of watts. The specific application then determines the optimum energy-efficient configuration.

To realize the impact of high-efficiency power-supply design, consider a typical commercial building, where the power supplies in electronic equipment drive as much as 10% to 15% of the overall energy bill. The new 80% efficient power supplies are 33% more efficient than current supplies because they reduce the power drawn in waste heat that's dispersed into the environment.

This reduction is based on data from the 80 PLUS program, which focuses on integrating energy-efficient power supplies into desktop computers and servers. HP is the first major PC manufacturer to support this program.

HP laboratory tests reveal its existing business desktop customers could improve power efficiency as much as 45% by switching to a next-generation system with an 80% efficient power supply. These new power supplies could save tens of thousands of dollars each year for enterprises with thousands of PCs.

The Snohomish County Public Utility District (PUD) near Seattle, Wash., tested an 80% efficient power supply in its newest HP business PCs and found significant savings. Initial tests indicated an average annual savings of about 125 kWh per PC using the more efficient power supplies. In addition, power factor correction reduced power-line harmonics, allowing Snohomish PUD to connect almost twice as many PCs to each ac power circuit.

80 PLUS Program
Sponsored by Ecos Consulting with electric utility funding, 80 PLUS is an incentive program to employ more energy-efficient power supplies. An independent testing laboratory must verify a power supply's efficiency before a manufacturer can obtain certification. The 80 PLUS performance specification requires power supplies in computers to be 80% or more energy efficient at 20%, 50%, and 100% operating loads. An official Energy Star specification will replace the present 80 PLUS program.

Idle power is one of the major question marks in the Energy Star specification, because it depends on a computer's efficiency. Present data doesn't capture this accurately, since almost all supplies use conventional power-supply technology operating at roughly 65% to 70% efficiency during idle state (due to low operating efficiencies). Therefore, most idle power specifications must still be determined.

Also, the power factor of power supplies in today's desktops fall in the 0.5 to 0.6 range. In response, 80 PLUS certified power supplies require at least 0.9 power factor, reducing input current requirements by as much as half. Higher power factor lowers power-line harmonics, which puts less stress on neutrals and increases the life of power distribution transformers.

Improved energy efficiency offers other value-added benefits. Reduced heat output increases computer system reliability up to 40%. Greater reliability reduces maintenance costs and IT support while improving worker productivity. Enhanced reliability and lengthened equipment life combine with energy savings to cut the total cost of ownership (TCO) of PC networks. And, reduced heat output puts less of a burden on the air-conditioning systems within a building.

Reference Designs
A wide range of ac adapters is used in electronic systems as well. ON Semiconductor's GreenPoint reference design for a 60-W notebook adapter addresses applications in which high efficiency and adapter size are crucial. The enabling device for the reference design is the NCP1337 enhanced pulsewidth modulation (PWM) current-mode controller (see the figure).

The NCP1337 combines a true currentmode modulator and a demagnetization detector that ensures full borderline/critical conduction mode in any load. It eliminates the need for an external signal by providing internal transformer core reset detection. Frequency is internally limited to 130 kHz to facilitate the compliance with electromagnetic-interference (EMI) conducted emission standards, the lowest limit being 150 kHz.

By monitoring the feedback pin activity, the NCP1337 enters ripple mode as soon as the power demand falls below a pre-determined level. Because restart is softened by an internal soft-start and the frequency can't fall below 25 kHz, it emits no audible noise.

Power-Supply Trends

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