New External Power Supply Efficiency Requirements On the Way?

July 02, 2012

Things could get very interesting for external power supply (EPS) designers if the U.S. Department of Energy (DOE) has its way.

For the last few years, the tightest EPS efficiency standards that designers worldwide have had to meet were based on the active and no-load mode requirements of  2008’s ENERGY STAR EPS v2.0 program for single output supplies under 250 W output. But that could change as the DOE has just proposed new standards that could significantly alter the EPS efficiency regulation landscape.

A few weeks ago, I attended the DOE’s stakeholder meeting in Washington, DC, covering the EPS efficiency requirements proposed in their Notice of Proposed Rulemaking (NOPR). If you don’t have time to read the 170+ page NOPR, I recommend reviewing the efficiency limit tables posted in the last few pages plus the definition section, 430.2 (page 18644). I’ve included a link to this and other relevant information at the end of my blog. The DoE is proposing tighter requirements than any other government EPS efficiency program. It also covers more types of EPSs, including multiple output and higher output power supplies. (The NOPR also proposes the first mandatory efficiency standards for battery charger systems; I’ll discuss that in a future blog.)

More Classes, Multiple Outputs, Higher  Power

Currently, minimum U.S. EPS efficiency limits are included in EISA2007*, and cover only single output voltage, AC-DC and AC-AC power supplies with an output power nameplate rating of ≤ 250 W. These are known as Class A EPSs and are identical to the types covered in other EPS country standards. This new proposal expands that to seven product classifications, as shown in Table 1 below. Class A has been split into Classes B, C, D, and E(low voltage EPSs are defined as output voltage < 6V and output current > 550mA). The proposal also differentiates between “direct” and “indirect” operation EPSs. Classes B, C, D, and E cover only EPSs that “directly” power a product, regardless of the charge condition of the application’s battery (if the application includes a battery). In applications where an EPS doesn’t directly power the consumer product without the assistance of the product’s battery, the EPS falls into Product Class N. Class N EPSs are regulated only by the battery charger system efficiency requirements. (See NOPR section 430.2to determine direct vs. indirect operation).

Finally, the DoE breaks new ground with Product Classes X and H, regulating efficiency in multiple output power supplies and those delivering > 250 W.


Table 1. Proposed EPS Product Classifications (Source: U.S. DOE March 2012 BCS EPS NOPR)

How High is the DOE Raising the Bar?

I wanted to see how the NOPR’s proposed limits for Class B and C EPSs compared with the minimum efficiency requirements of the European Commission’s (EC) Ecodesign Directive Lot 7, so I picked a few typical output power levels and did the calculations using the formulas listed in each standard. (The good news for engineers is that the DOE’s test and efficiency calculation methodology continues to harmonize with the rest of the world.) Table 2 shows the resulting efficiency required. In all cases, the DOE’s maximum allowable no-load power consumption reduces by more than half that currently required by the Ecodesign Directive, while the minimum average active mode efficiency increases by up to approximately 5% in some cases.

Table 2. Comparison of Minimum Efficiency Requirements – U.S. DOE Proposed vs. EC Ecodesign Directive Lot 7

Will these DOE limits change before becoming final? It’s possible, based on comments submitted by stakeholders. Timing wise, it’s typical for the DOE to allow at least 2 years between when a standard becomes final and when it becomes effective. Most likely, the EPS standard will be finalized later this year and become effective early 2015.

 A copy of the NOPR, the Technical Support Document, the May meeting presentation slides, and other support documentation can be downloaded at:

 * Energy Independence and Security Act 2007

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