Beating the budget

Sept. 21, 2007
New and existing standards impact the power budget approach to design, causing engineers to make a number of tough decisions.

Where product design is concerned, legislation and industry standards on energy saving tend to focus on power-conversion efficiency and ways to improve the power supply. However, if power is misused by the functional system, any gains will be squandered. This hasn't escaped the attention of regulators—new efforts are underway to establish energy budgets for common systems that apply to the whole product rather than just the power supply.

This brings a new dimension to the product design process: System designers and planners must trade off the market value of product features against not only their cost in material to implement, but also their marginal cost in power-supply efficiency required to meet the power budget. This article discusses the existing and upcoming standards in general terms and outlines how an engineer may begin to analyse and make educated choices.

The total energy budget method of regulating for efficient energy use works best in applications with a fixed feature set—such as printing one page of colour text, a 17-in. LCD computer display, a device for playing DVDs, and so on. Many companies make similar products that work substantially the same way. Therefore, regulators feel confident that they can bound the energy usage and provide incentives to the most efficient designs, or issue penalties for the least efficient. This "power budget" philosophy was already applied to standby power usage, yielding industry-wide improvements in the standby power consumption of certain product classes.

Power Integrations expects that regulators such as CEC, CECP, Energy Star, EU, and the Australian Green House Office will elect to regulate an increasing number of products. In the USA, Energy Star regulators are working on such budgeted standards for printers, personal computers, and televisions. In Europe, there's substantial focus on set-top boxes. Power Integrations tracks all of these activities closely to ensure ICs provided to customers meet or exceed all current and upcoming standards.

Take, for example, the ubiquitous inkjet printer. Products from various manufacturers deliver approximately the same amount of ink to, generally speaking, standard paper stock at a rate and resolution that increases continuously. Nonetheless, it's similar across makes and models of the same generation.

Our analysis shows that inkjet printers consume around 1W or less in standby mode, around 25W or 30W in operating mode, and between 70W and 80W when engaging motors to advance the paper. Such a large dynamic range creates significant power-supply design challenges that must be considered when employing the power-budget approach to product design.

When in standby mode, printers must perform various well-defined functions, which include checking the ON switch periodically for any activity and illuminating the "Power connected" LED. Also, printers often have a sleep mode that allows the device to power up quickly when a print job is issued, and monitor the PC interface to determine if it's time to wake up and print a document.

To optimise the power consumption of the printer across its entire power range, the power supply has to deliver efficient power in all operational modes, from standby to peak-power modes. Figure 1 shows such a power supply, designed using an integrated switch-mode IC with peakpower capability.

This power supply converts power from the primary to the secondary governed by the equation:

P ≈ 0.5LI2f Watts

where L = transformer inductance, I = current limit, and f = frequency.

The integrated switch-mode IC varies the average switching frequency to provide variable amounts of power across the range required by the printer, and selects from four currentlimit levels at various power thresholds to optimise switching efficiency. As a result, the power supply can maintain a high level of efficiency across a broad range of output requirements from standby to 300% peak load (Fig. 2).

The more or less straight line in the graph indicates practical "constant efficiency" capability over 3.5 decades of power output dynamic range. This ensures that the designer has a consistent and high proportion of power available at any mode. Here, regulators determine that a fixed input power budget should be enforced.

New regulations will require that equipment be designed to adhere to pre-defined power budgets. With these more exacting constraints on system-level power usage, the designer must work to derive energy savings from both the power supply and the functional system, and from the way these two elements interact. The power-budget approach to design doesn't just deliver power savings in standby mode, though. It can also lead to important efficiency gains in other operational modes, including no-load and peak-power modes.

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