Environmental considerations and the opportunity to reduce company expenses are fuelling the need for manufacturers of computer equipment to achieve higher efficiencies with their products over their expected lifetime. However, these efficiencies must be achieved with minimum disruption or changes to the habits of the end user (or impact on the employer's company policies). One approach that can contribute to addressing this challenge is to use Power State Indication (PSI) mode to lower mother board power usage at light loads and when the computer idles.

New studies show impact

From studies carried out on standard computer operation practices within the work environment we know that 60 percent of all computers are not switched off at night for various reasons, such as company policy, IT upgrades and time considerations. Of that 60 percent, we also know that half of these computers do not have any sleep modes enabled.

A survey carried out by 1E concluded that among employed adults who regularly use a PC at work:

Recorded user power down habits

Programs such as Energy Star set down regulations on energy consumption in off and sleep modes, with strict guidelines on the total energy consumption (ETEC) that must be adhered to in order to meet the Energy Star certification.

Where all Px are power values in watts, all Tx are time values in % of year, and the ETEC is in units of kWh. This represents annual energy consumption based on mode weightings in the table seen in this article.

However, it has been the practice for some companies not to activate hibernation modes or for individual users to disengage this function on their PC. This results in higher energy costs than would otherwise be achievable. Indeed, according to the Climate Savers Computing Initiative, some 30 percent of all computers sold to the work place potentially never enter a power save or off state.

Energy Star Guidelines for Desktop PC

If we take these figures and apply them to a company with 20,000 PCs, we can see that there could be 6,000 PCs which may never be switched off over their lifetime. And the resulting energy consumption has a very real cost in terms of money off the bottom line and environmental impact.

While the obvious and best solution is that companies employ a policy of switching off computers when not in use, the problem of habit and changing user's apathy comes into play. At the same time such a policy does not take into account the energy usage during active periods and how this can be reduced.

Power usage during the desktop's "ON" states can be a significant portion of the total energy consumption during the lifetime of a product. The graph below shows the total energy use of different appliances in active mode. As can be seen desktop computers are the highest contributor to energy consumption, equating to over 34TWh per year. This makes desktop computers one of the largest users of energy consumption in the workplace outside of lighting and climate control.

Total Energy Usage in Active Mode (TWh/Year)

Power State Indicator

Employing mechanisms for Power State Indication (PSI) and current monitoring provides a route to making significant reductions to the power consumption of desktop computers in both active and inactive states - without necessitating a change in user profile or behavior. Such techniques are now being integrated into power control ICs in which a PSI pin input is used to determine the operating state of the load. If this input is pulled low by a signal from the microprocessor, the load is in a low power state and the controller asserts the ODN (output disable logic) pin low, which can be used to disable phases and maintain better efficiency at lighter loads. The sequencing into and out of low power operation is maintained to minimize output deviations as well as providing full power load transients immediately after exiting a low power state [4].

The user can program how many phases are enabled when PSI# is asserted. By default only phase 1 is enabled. The number of phases enabled can be changed over the management bus. The actual phases enabled depend upon how many phases are enabled for normal operation. For example if four phases are enabled normally and two during PSI, then Phase 1 and Phase 3 will be enabled during PSI.

Efficiency curve improvement with PSI

The existing generation of ON Semiconductor's converter parts provides a dedicated current-sense amplifier (CSA) to monitor the total output current for accurate voltage positioning vs. load current, for the IMON (current monitoring) output and for current-limit detection. Sensing the load current at the output gives the total real time current being delivered to the load, which is an inherently more accurate method than peak current detection or sampling the current across a sense element such as the low-side MOSFET. To provide the best accuracy for sensing current, the CSA is designed to have a low offset input voltage. Also, the sensing gain is determined by external resistors to make it extremely accurate.

A significant difference both in system efficiency and overall energy savings can be made over the lifetime of a machine by deploying such techniques. With purchase of computer products that have PSI technology on board, for example, the company with 20,000 computers in house can save approximately $80,000* in energy costs over the course of computer lifetime without any compromises made to present company practice.

Reducing C02 emissions

Annual energy usage with and without PSI

While the benefit is great for the company involved, this simple change helps the environment as a whole reducing CO2 emissions by 112.95 tons pre annum. As a result, this single change in architecture equates to removing 21+ family cars from our roads per year.

While this figure may appear low, if we multiply this up for the total number of computers sold in the US in 2007 alone, we would see this would be the equivalent to some 150 thousand cars removed from circulation per year**. Taking this one step further, if all PCs sold in 2007 had PSI-capable parts on board it would equate to over 630,000 cars worth of CO2 removed from circulation per year.

Worldwide computer recorded sales and estimated future sales

Although the energy savings involved with running computers more economically help bottom line expenditures, there will also be associated affects, such as the possibility of air conditioning being turned down (or even off), which further reduces a company's running cost and ability to reduce green house gas emissions.

+: Figure is based on average CO2 car emissions of 164g/Km and average distance travelled of 32,000 Km per year.

**: Figure based on car sales figures of 7M in 2007

Auto PSI modes

Finally, it is worth noting that there are also devices that offer designers the option of an 'auto' PSI mode. ON Semiconductor, for example, offers an auto PSI mode with the NCP5392P, 4-phase desktop controller. The part automatically detects the voltage identifier (VID) transitions and directs the core voltage regulator in or out of low power states. As the part does not have to wait for a signal from the microprocessor, it can dynamically switch from 4 to 2 to 1 as or when the part decides energy-saving can be realized, allowing for even greater savings over the computer's life span.

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