Power adapter designs must comply with energy efficiency standards and EMC compliance requirements.
A power adapter must be safe to use and isolate the user from lethal AC mains voltage. Adapters or external power supplies must also not damage the environment with unnecessary power draw in use and unloaded modes. Moreover, they must not damage or disturb other equipment with conducted or radiated electromagnetic emissions.
Standards also apply to these considerations, some mandatory, others voluntary. The application and country of use also need review. Similarly, power adapters must include immunity to conducted and radiated emissions as specified in international standards with various levels specified, representing different end-use environments.
This article summarizes the requirements for adapter efficiency and no-load/standby losses. It also explains which standards currently apply in the United States, Europe, and other regions around the world. Standards for electromagnetic compatibility (EMC) also need consideration; again, there are regional and application variances. Examples include products that comply with international standards and are suitable for markets around the world.
Understanding power adapter efficiency
In an ideal world, power adapters would be 100% energy efficient. However, the process of power conversion—be it from AC to DC or DC to DC—involves the use of many discrete components, of which some create energy losses. Typically, in any switched-mode power supply design, the overall losses are caused by lots of small amounts of losses across several different parts. Inductors and the semiconductors used for the switching contribute to the losses but are by no means the only components.
Power electronics designers can calculate the energy efficiency of an AC/DC power adapter by dividing the output power by the input power and show the result as a percentage. For example, if an adapter delivers a full load output of 12 VDC at 4 A, this equates to a 48 watts output. On the AC input, assuming a power factor of 1, a 220 VAC and a 0.25 A current give a 55 watts input power. The power adapter, therefore, is 87% efficient.
In this example, the difference between the input and output power results in 7 watts of waste heat that needs dissipating. Waste heat is a significant factor in power adapter design. Firstly, the ambient temperature of an adapter can have a negative influence on component reliability. The more efficient the power adapter, the less waste heat there is to dissipate, so the more reliable the adapter is during operation. Increased component reliability prolongs the adapter’s lifetime.
Also, keeping power adapter efficiency high means that no fan-assisted cooling is required, and the waste heat created is removed by conduction cooling alone. When selecting an AC/DC power adapter, design engineers will find the energy efficiency rating quoted in the adapter’s datasheet.
Another consideration in terms of power adapter efficiency is that it’s not static. The power efficiency of any conversion circuit changes as the load imposed on it changes. Typically, the lower the power load on the adapter’s output, the less efficient the power conversion process is. The efficiency is also dependent on the input AC line voltage, so checking the datasheet for the AC input range it can accommodate is critical.
Also, the operating temperature will influence efficiency, and some adapters will derate their maximum power output as the temperature increases. For the product design engineer, it is essential to understand the worst-case efficiency rating in case any additional cooling methods are needed.
Energy efficiency standards
With energy use a global concern, there is far more attention to how efficient a power adapter is. Energy efficiency legislation first appeared in 2004 when the California Energy Commission (CEC) set out the first formal energy efficiency standard. Now, most regions around the world have mandatory energy efficiency standards that stipulate the minimum levels of efficiency required for any power supply or adapter. In some cases, certain countries have adopted the U.S. or European standards instead of establishing separate legislation.
Since the CEC’s initial specification, there have been many iterations to the permissible efficiency limits. The current standard within the United States is the Department of Energy Level VI (DoE Level VI), and within Europe, since 1 April 2020, it’s covered by the Ecodesign 2019/1782 directive. These standards apply to not only the power adapter but the whole system comprising the power adapter and the end product.
Over the years, there has been an increasing concern regarding the amount of energy consumed when the powered product is in standby mode. It’s often assumed that the amount of power consumed in standby mode—also termed a no-load condition—is relatively small, but unfortunately, that is often not the case. The DoE Level VI and Ecodesign 2019/1782 standards are, generally speaking, similar but with one exception. Ecodesign 2019/1782 require testing efficiency at a 10% average load condition (Figures 1 and 2).
Figure 1 The U.S. DoE Level VI energy efficiency parameters are outlined for a single voltage external AC/DC power adapter. Source: CUI
The European Code of Conduct (CoC) Tier 1 standard introduced the 10% load power consumption limit in 2014, and the requirements were less demanding than the DoE Level VI specification. The Ecodesign 2019/1782 standard became law in April 2020 and matches the DoE Level VI requirements for average active efficiency, although it is less stringent than the CoC Tier 2 standard which is still in place. Ecodesign includes the 10% load efficiency test but does not impose a requirement.
Figure 2 The European CoC Tier 2 specification outlines stringent energy efficiency requirements. Source: CUI
Improving power adapter efficiency
Recent semiconductor process technology advances are aiding power supply design engineers to deliver even more efficient power adapters. As mentioned earlier in the article, one of the primary sources of efficiency losses within a power adapter is the switching transistors. Switching transistors are now available based on wide band-gap (WBG) semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN).
The WBG-based transistors have much lower switching and conduction losses than silicon counterparts, in addition to exhibiting significantly higher breakdown voltages. The use of GaN-based semiconductors also yields further benefits in terms of allowing the conversion process to operate at higher switching frequencies. Increasing the switching frequency means that some of the bulkier components, typically inductors, can be significantly reduced in size.
An example of a power adapter that uses GaN switching transistors is the SDI200G-U desktop external power adapter. It can continuously deliver a 200-watt output power and, with its 95% energy efficiency rating, it conforms to the DoE Level VI and CoC Tier 2 energy efficiency specifications. With an increased switching frequency compared to silicon-based power adapters, the SDI200G-U desktop adapter size has been reduced in half, and it weighs 32% less.
EMC and EMI considerations
Another set of international standards stipulate the maximum levels of electromagnetic emissions a power adapter may generate. In any switched-mode power supply or adapter, the majority of electromagnetic noise emissions come from the switching circuitry. Emissions fall into two distinct categories: conducted and radiated.
Conducted emissions occur when the electromagnetic noise exits the adapter by being propagated along the connecting wires to the DC output. As a result, unintended emissions may interfere with the correct operation of the product the adapter is powering. Such emissions are usually low frequency in the range of 10 kHz to 30 MHz. Above 30 MHz, the adapter’s internal conductors behave as antennas, resulting in the radiation of the unwanted noise signals.
Power adapters need to conform to the relevant EMC standards. As the amount of electronics-based equipment and devices increase in homes, offices, and cars, there is an increasing need for electromagnetic compliance testing so that one piece of equipment does not interfere with or disrupt the operation of another.
Within the United States, the Federal Communications Commission (FCC) Part 15 standard sets the limits for conducted and radiated electromagnetic interference (EMI). Within Europe, the CISPR 32 standard, harmonized with FCC Part 15, applies. Both define limits for Class A and Class B equipment. Class A covers a broad range of equipment for use in commercial and industrial locations and Class B for use in residential environments.
Figure 3 indicates the European CISPR 32 field strength limits for both conducted and radiated spurious emissions.
Figure 3 The CISPR 32 standard defines the field strength limits for conducted and radiated emissions. Source: CUI
Power adapter designers use a variety of filtering techniques and components to reduce unwanted emissions. As shown in Figure 4, capacitors across the AC line form common-mode and differential-mode filter arrangements to attenuate any noise conducted out of the adapter.
On the DC output side, capacitors across the positive and negative lines reduce any unwanted noise emissions. Series inductors in the output, together with ferrite beads, are frequently used to limit any radiated emissions.
Safety is not the only consideration when selecting a suitable AC/DC power adapter. Ensuring the power adapter complies with the energy efficiency requirements of the countries in which it is likely to be used is an essential requirement. Also, conformance to the appropriate EMI standard will ensure that power source does not interfere or disrupt the operation of another item of equipment.
This article was originally published on EDN.
Ron Stull is power systems engineer at CUI Inc.