Rich Miron, Technical Content Engineer, Digi-Key
Just when you thought you had it nailed, along comes another set of energy efficiency standards for external power supplies. Yesterday’s standards are no longer good enough and manufacturers cannot afford to be complacent.
Meeting energy efficiency standards is not a one-time fix. The pressure on governments to deliver on global agreements aimed at mitigating climate change has seen numerous regulations that ratchet-up the minimum efficiency requirements of products like external power supplies. The onus is on equipment manufacturers not only to meet these standards but also to keep ahead of the game, especially if they want to ensure the widest possible market for their products.
Currently the most demanding mandatory requirement is the Level VI standard specified by the US Department of Energy (DoE), which became effective in February 2016 and applies to products supplied into its domestic market. However new standards that currently exist as European Union ‘Code of Conduct’ requirements are set to become mandatory Ecodesign rules in the EU within the next two years. The initial tranche is broadly harmonized with Level VI but the next tier sets more stringent targets.
Manufacturers of equipment that includes external power supplies typically want the flexibility to supply their products to global markets without having to worry whether shipments to the US, Europe or elsewhere meet the respective regulations in those countries. For this reason leading external power supply manufacturers aim to ensure that their products meet up-coming standards well ahead of mandatory compliance deadlines.
The need for regulation
A study conducted in 1998 by the University of California’s Berkeley Laboratory estimated that as much as 5% of America’s residential electricity consumption could be accounted for by the standby operation of domestic appliances. This amounts to an estimated annual energy cost of more than $3 billion. Applying the same percentage to the 1.29 billion megawatt hours (MWh) of residential electricity consumption reported by the US Department of Energy in 2004 reveals a staggering 64 million MWh of wasted energy - equivalent to the output of 18 typical power stations.
Early initiatives to improve the energy efficiency of electrical appliances were voluntary and include the Energy Star program that was originally directed at the IT industry but subsequently embraced consumer products and the wider commercial market. However, it was not until 2004 that the first legislation was passed by the California Energy Commission to implement energy efficiency standards. This was soon followed by equivalent standards across the US, Europe and other parts of the world as can be seen in the timeline graphic (see Infographic), which also shows efforts to harmonize legislation with an international marking protocol that identifies the energy efficiency level of a power supply.
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Infographic: The evolution of energy efficiency standards from 2004 through to the latest 2016 US DoE Level VI regulations and the impending EU CoC and Ecodesign requirements (Courtesy of CUI Inc.)
External power supplies (EPS), of which there were estimated to be more than one billion in the early 1990’s, were notorious for wasting energy with efficiencies as low as 50%, largely due to the linear technology utilized at that time. Worse still, they continued to draw power when the end-equipment was turned off or disconnected. Unabated, the growth in external power supplies was projected to account for 30% of total energy consumption.
Consequently, much of the focus of later legislation has been on no-load power consumption and standards implemented by the US government in 2007 aimed to avoid some 200 million metric tons of carbon dioxide emissions, comparable to the greenhouse gas emissions of nearly 40 million automobiles.
Today’s regulatory state-of-play
The challenge for original equipment manufacturers (OEMs) is keeping track of the continually evolving efficiency regulations and understanding which are mandated by law in particular territories. Currently Canada mandates Level IV while in Australia this is still voluntary. The European Union requires the more demanding Level V standards while Level VI has been a legal requirement in the United States since February 2016.
More often, for OEMs wishing to serve a worldwide market, the best strategy to avoid the complexities of multiple product variants, inventory and logistical issues, is to ensure its products meet the latest and most rigorous standards. For the moment, in terms of mandatory compliance, this means Level VI although, as the next section outlines, more stringent rules are in the offing.
As one would expect, compared to Level IV and Level V, Level VI does impose tighter limits on the average efficiency and no-load power consumption that an EPS must achieve. So, for example, a supply with a rated power output between 49 W and 250 W now needs to be better than 88% efficient, up from 85% and 87% respectively, and consume less 210 mW under no-load conditions, down from 750 mW and 500 mW. These requirements are detailed in the table below (see Table 1).
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Table 1: US Department of Energy Level VI minimum active mode average efficiency and no-load maximum power consumption regulations for direct operation external power supplies
Furthermore, the Level VI specification now embraces AC-AC supplies, includes a higher power AC-DC converter category (>250 W) and separates out supplies of 1 W or less. It also introduces a distinction between what are now categorized as basic-voltage supplies and low-voltage supplies, which are those with a nameplate output voltage less than 6 volts and an output current ≥550 mA.
For the first time, multiple-output power supplies are regulated under the DoE’s standards. A distinction is also made between direct and indirect operation such that only direct operation supplies that function in the end product without the assistance of a battery are included in the regulations – an indirect operation EPS, which is defined as a supply that is not a battery charger but which cannot operate without the assistance of a battery, is still governed by earlier EISA2007 efficiency regulations.
Aside from not including indirect operation supplies, Level VI defines some specific exemptions for EPS products. Notably these include: medical applications where the supply requires Federal Food and Drug Administration listing and approval as a medical device; and direct operation AC-DC supplies, with a rated output voltage <3 V and output current ≥1 A, that charge the battery of a product that is fully or primarily motor operated.
For Level VI compliance, an EPS operating in active mode needs to achieve a minimum efficiency that is determined as the average of four efficiency measurements taken at defined load levels of 25%, 50%, 75% and 100%. Invariably lower loads result in worse efficiency and, in order to meet the tougher Level VI limits, EPS designers have needed to look at ways to improve low-load efficiency and re-examine the circuit topologies employed.
While supplies with higher power outputs operating at higher voltages can continue to use LLC resonant converters, and established flyback designs may be fine for more modest power output supplies, lower voltage power supplies that need to provide higher output currents require a different solution. For example, changing the design of the secondary-side circuitry from simple diode rectification to synchronous rectification helps improve both the average efficiency and no-load performance but does require a secondary-side controller to turn on and off the synchronization FETs.
Another approach, that one manufacturer, CUI, has used to achieve Level VI compliance, is to reduce the power supply’s switching frequency. As in Level V-generation products, the control IC normally operates at 65 kHz, but reducing this to 22 kHz under light and no-load conditions significantly reduces power loss and improves efficiency as can be seen in Figure 1.
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Figure 1. The use of a reduced switching frequency under light and no-load conditions helps meet Level VI average efficiency specifications
The global stage and harmonization
As discussed earlier, the implementation and adoption of energy efficiency regulations across the globe has been rather fragmented and piecemeal with a mix of voluntary and mandatory regulations applicable in different geographies. Unfortunately, despite the good intentions of governments and other authorities to harmonize the various standards, the onus still lies with power supply manufacturers and OEMs to ensure the compliance of their products.
Rather than harmonization, the process we’ve seen seems more a game of leapfrog as countries or regions seek to catch up with and then surpass their neighbors. The European Union is the latest challenger with its Code of Conduct (CoC) rules for EPS energy efficiency. Its Tier 1 standard, published in October 2013 and effective on a voluntary basis from January 2014, took the existing Ecodesign directive, which harmonized the EU with Level V in 2011, and raised its requirements to be broadly equivalent to DoE Level VI.
The CoC Tier 1 standard is currently under review but its adoption as an Ecodesign rule is expected to be mandated by January 2017. Next up is the EU’s CoC Tier 2 standard that goes beyond Level VI – this became effective on a voluntary basis in January 2016 and is also subject to review before potentially becoming mandatory as an Ecodesign rule in January 2018.
Once again, understanding the subtle differences between Level VI and CoC Tier 1, and then appreciating how much tougher Tier 2 gets, are vital from a manufacturer’s perspective, especially if their aim is to achieve compliance across all these standards by meeting the most rigorous specification.
Regarding no-load power consumption when compared to Level VI, it is perhaps surprising to find that the Tier 1 limit for basic voltage external supplies is less stringent. A maximum of 150 mW applies to supplies rated between 0.3 W and 49 W, and 250 mW for supplies from 49 W to 250 W, compared respectively to the 100 mW and 210 mW limits for equivalent Level VI units. Less surprisingly, the Tier 2 no-load power consumption limits are set lower than Level VI, at 75 mW (0.3 W < Pout ≤ 49 W) and 150 mW (49 W < Pout ≤ 250 W).
A more significant difference is that the CoC rules introduce a minimum efficiency in active mode at 10% of the full rated output current. This is in addition to the four-point average method that Level VI uses to measure efficiency, which, as we’ve noted earlier, presents a greater challenge. Like Level VI, in respect of efficiency measures, the CoC rules treat AC-DC and AC-AC power supplies the same, but there remains a distinction between basic voltage and low voltage supplies, which have different limits. As with the no-load criteria, we see that CoC Tier 1 average efficiency requirements are slightly less stringent than Level VI while Tier 2 is a little more demanding.
Solutions that ensure worldwide compliance
Original equipment manufacturers have a responsibility to ensure compliance with relevant energy efficiency legislation for any product they make that includes an external power supply in whatever territory it is sold. Clearly, including an EPS that meets the most rigorous prevailing regulations avoids the complexity of having different product configurations for different geographic markets. The benefit for OEMs that buy power supplies to bundle with their end products is that they are more easily able to choose a vendor whose products satisfy the latest and most demanding efficiency standards.
CUI is a technology company operating at the leading edge of power supply design. It introduced external power supplies that met the Level VI standard requirements more than a year before those requirements became mandatory. These products range from 5 W to 150 W power adaptors and include desktop, wall plug and USB units as illustrated in Figure 2. CUI’s stated intention is to “stay ahead of the rapidly evolving global efficiency regulations to ensure that our customers’ designs are properly future-proofed in the event that tighter efficiency rules become law”, so we can reasonably expect to see announcements concerning CoC compliant products very soon.
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Figure 2. A CUI SWI12-E 12 W wall plug series