Power Supply Efficiency Increase: Requirements and Trends

The industry drive towards smaller and lighter electronics as well as raising energy cost stimulate continuous efforts on more efficient power conversion. However, specific levels of PSU efficiency used to be virtually unregulated worldwide and were left up to the manufacturers and their customers.

Although U.S. Environmental Protection Agency (EPA) along with the U.S. Department of Energy (DOE) since 90's promoted the voluntary labeling program called Energy Star®, this program focused primarily on sleep mode requirements of appliances. To qualify for the label a personal computer needed a capability to enter a low-power sleep mode after a period of inactivity. As a stimulus for high computer power supply's efficiency this program was mainly useless since it did not address active mode and allowed to consume as much as 10% of the PSU's power rating in sleep mode. Furthermore, in practice the sleep mode is disabled on most office computers: the studies found that this mode is actually enabled on only about 5% commercially used PCs.
To stimulate efforts on reduction of PC power consumption in operational mode, Ecos Consulting in partnership with a group of electric utilities has recently created another voluntary program called 80 PLUS®. To qualify for 80 PLUS, a power supply must demonstrate efficiency ≥80% at 20, 50 and 100% of rated load, and power factor >0.9 at rated load. They test power supplies and offer manufacturers of desktop computers and desktop-derived servers $5 and $10 rebates respectively for every unit with a certified power supply sold in participating utilities territories. EPA incorporated 80 PLUS requirements in their new revised Energy Star® Computer Specification (version 4.0) effective July 20, 2007. Under this revision PCs, notebooks, workstations, and game consoles will all have to meet both active mode efficiency and "idle power" consumption requirements to qualify for the label.
EPA also revised ENERGY STAR® specification for single output external power supplies (casually called adapters) for laptops, mobile phones, printers, scanners, digital cameras and others appliances. Under rev.1.1 the minimum active mode efficiency of an adapter with nameplate output power Po is ranging from 0.49*Po for Po below 1W to 0.84 for Po above 49W. Slightly higher Tier 2 requirements may become effective January 1, 2008. Similar programs harmonized with Energy Star exist in other countries including EU, Japan, and China.

Although both 80 PLUS® and ENERGY STAR® programs remain voluntary, mandatory standards for power supply efficiency are currently under consideration in US and various counties. Particularly, The Energy Policy and Conservation Act (EPCA) directed the Department of Energy to determine by August 8, 2008 whether energy conservation standards shall be developed for battery chargers and external power supplies.

California Energy Commission has already adopted mandatory Appliance Efficiency Regulations, that among other things include new ENERGY STAR® requirements for external power supplies.

Further energy loss reduction needs joint efforts of manufacturers of PC PSU and computer components- obviously you can't address all energy issues with power supplies alone.
For example, in a white paper presented at the 2006 Intel Developer Forum, Google suggested to switch from multi-voltage PC power supplies to a single output 12-volt open standard and generate all other voltages required by motherboard components via VRMs. They argued that most of the voltages produced by multiple output PC power supplies are no longer used directly in today’s PCs. Although additional voltage conversion would still take place on the PC motherboard, the single-output 12V PSU would make it easier to achieve higher overall efficiencies. Google servers already use this approach.

What else can be done to further reduce energy losses in computers? One of the computer PSU requirements that affects its efficiency is 16 ms hold-up time. To meet this requirement, turns ratio of the power transformer is normally reduced to provide secondary voltages overhead. This in turn requires higher voltage rating of output rectifiers, higher peak currents of primary FETs and results in lower efficiency. If we set the turns ratio higher (closer to what is needed to regulate 12V at nominal PFC boost voltage) the power supply will be 1-3% more efficient, but during one-cycle AC line interruption the 12V bus may drop to about 8-9 V with typical values of storage capacitance. This voltage is still high enough to allow VRM's and POL's maintain regulation of any voltages 5V and below. The only problem would be with the computer components that use 12V directly. If they were designed to briefly operate at reduced voltage levels or at least to not latch up or loose data, the computer system could be made more efficient and could still ride through the one-cycle line interruptions. In my humble opinion, if computer industry eventually accepts single 12V bus standard, it should also require wider operating range of components that use 12V and respectively modify PSU holdup requirement by allowing 12V drop by some 25-30%.