A new and tougher power supply efficiency specification is coming into force to save energy and reduce greenhouse-gas emissions. OEMs must act now to ensure compliance by February 2016.
Jeff Schnabel, CUI Inc.
Market enthusiasm for new high-tech devices is undiminished. The Consumer Electronics Association, in its July 2015 U.S. Consumer Electronics Sales and Forecasts report, predicted total industry revenue will rise 2.4% to $285 billion. Major contributors include continued growth in smartphone and notebook/netbook sales, and successful new categories giving buyers more reasons to spend. These include 4K Ultra High-Definition televisions, connected-home technologies, and wearable devices.
On the other hand, governments are trying to limit demand for electrical energy, both to ensure grid stability and to combat climate change. As sales of high-volume consumer electronics continue upwards, the squeeze on resources demands greater energy efficiency. A key concern has been the energy wasted when electrical devices are in standby modes. In 1998, this was estimated at about 5% of all electricity generated.
Eco-design initiatives such as Energy Star in the U.S. and ErP in the E.U. have evolved to limit both the active and standby power of equipment such as televisions, set-top boxes, computers and domestic appliances. The external power adapters of equipment such as notebook computers and office machinery have also come under scrutiny. In 2004, the state of California became the first authority to introduce mandatory specifications to reduce the energy dissipated by external power supplies.
Today, the International Energy Efficiency Marking Protocol for External Power Supplies is the globally accepted framework setting out limits on standby consumption and average efficiency for such adapters. Within this framework, the toughest specification in use today is Level V, which is currently only enforced in the E.U. Things are about to change, however, as the U.S. Dept. of Energy (DOE) has announced it will raise the bar to Level VI in February 10, 2016. Any external power supply manufactured after this date and shipped into the U.S. must meet the new efficiency targets. The DOE says this will save 47 million tons of CO2 emissions annually.
New Specification, Tougher Targets
The new Level VI specifications demand higher average efficiency and lower no-load power consumption compared to current Level V power supplies. Figure 1 compares the Level VI average efficiency specification for power supplies up to 50 W against earlier Level IV and Level V specifications.
The new Level VI specification is also significantly more complex than previous generations. Several categories of power supplies are now defined, and new classifications will be regulated for the first time, including multi-voltage power supplies as well as single-voltage power supplies over 250 W. By including these high-power adapters, Level VI could have a significant impact in markets for industrial equipment not previously covered by the marking protocol.
Table 1 summarizes the specifications for all categories defined in the Level VI specification. Note that low-voltage power supplies are defined as having output voltage less than 6 V and output current greater than 550 mA. Basic voltage refers to a power supply that is not a low-voltage power supply.
The new specification also introduces a distinction between direct operation and indirect operation, and applies only to units intended for direct operation. Direct power supplies are defined as being able to function in the end product without a battery. An indirect power supply is not a battery charger, but cannot operate the end product without the assistance of a battery.
Some classes of direct power supplies are exempt from the need to comply with Level VI. These include devices that require FDA approval as a medical device in accordance with section 360c of title 21, and ac/dc power supplies with nameplate output voltage less than 3 V and nameplate output current greater than or equal to 1,000 mA that charges the battery of a product that is fully or primarily motor operated.
Changes Under the Skin
CUI began introducing Level VI compliant adapters in late 2014 to address the coming regulation. Some important design changes have been needed to satisfy the new, stricter targets, based on CUI’s established topologies. For units under 120 W, this is a flyback topology, while adapters over 120 W use an LLC resonant topology.
To meet the tougher standards, low-voltage/high-current models now feature synchronous rectification in the secondary side. Replacing conventional rectifier diodes with low-RDS(ON) MOSFETs has eliminated diode losses resulting in a net saving when the power to run the associated MOSFET controller IC is taken into account.
The PWM control strategy is significantly different compared to the previous generation. In CUI’s Level V adapters, the main control IC typically operates at a fixed frequency of 65 kHz, but the latest controllers for Level VI units improve efficiency as load decreases by reducing the switching frequency to 22 kHz. Improving light-load efficiency is important because the IEC-approved test method for assessing average efficiency (AS/NZS 4665) calls for power to be measured at 25%, 50%, 75% and 100% of rated load. The arithmetic average of the data from all four points is then calculated to determine overall average efficiency. Because the efficiency at 25% load is the lowest of the four points, improving performance at light load results in better average efficiency.
Figure 2 compares the efficiency of equivalent Level V and Level VI power supplies, measured at 25%, 50%, 75% and 100% of full load (7.5 V, 4 A). Efficiency is improved at all levels, and particularly at lower loads.
In addition the latest controllers draw lower quiescent current than their predecessors, which helps towards meeting the aggressive Level VI no-load targets.
Careful management of Power-Factor Correction (PFC) circuitry is another feature of the new Level VI units. Although PFC is mandatory for power supplies rated at 90W or over, the circuitry can be disabled at lower loads. This eliminates significant energy losses, thereby helping to improve both the average efficiency and no-load consumption. Ripple noise can be slightly higher when the PFC circuitry is de-activated, but this should not be a problem for most applications.
Despite changes throughout the design, most of the new Level VI units retain the same case sizes and external appearance as the earlier models. On the other hand, greater average efficiency has helped to reduce typical working temperatures, resulting in an improvement in reliability.
Less than Six Months to Comply
The new Level VI efficiency standards for external power supplies are to be mandatory initially only in the U.S., but in practice will require OEMs worldwide to adjust their purchasing and supply-chain arrangements. In any case, historical patterns suggest that other territories such as the E.U. are likely to adopt the Level VI specification themselves.
Since it is usually more economical and straightforward to ship products with the same power supply type to all markets globally, it makes sense to ensure that all units comply with the highest standard currently in force. This helps to minimize product-management challenges and avoid any potential for costly shipping errors.
Since Level VI power supplies are already available in the market, OEMs can and should begin preparing immediately for the transition.
References
Level VI standard info, www.cui.com/efficiencystandards
