The changing maze of power supply standards

Brexit altered the landscape of standards for power supply products. Here’s an update on the latest revisions.

Ron Stull CUI Inc.
The usual way of choosing a power supply is to search on the electrical performance parameters the application demands. But today, it is actually more efficient to start with the national or international standards the end product must meet.

Standards requirements can vary markedly among domestic, IT, audio, industrial and medical applications. And it can’t be assumed that compliance with what might be considered a ‘high-end’ application automatically is acceptable for more commercial end-uses. For example, a medically certified supply requires input fusing in both line and neutral for Class I applications (with a protective ground). But in domestic equipment, this kind of fusing is expressly forbidden.

Equipment manufacturers are also increasingly required to assess the risks of their products according to ‘hazard-based’ standards, which include an evaluation of where the product might be used. For example, a commercial laptop that is sold as ruggedized with an add-on rubber casing might reasonably be expected to be used in industrial or even outdoor environments, requiring its external charger be certified to a more specific standard.

Aspects controlled by standards fall into three main areas: safety, electromagnetic compatibility (EMC) and energy efficiency. Common safety standards that apply are set by the International Electrotechnical Commission (IEC).

Other standards apply for particular applications such as hazardous locations, railroad, test equipment, and so forth. Some areas haven’t yet standardized. For example, automotive on-board electrical safety currently relies on OEM internal rules and recommendations from automotive industry associations such as the Society of Automotive Engineers (SAE).

In addition, IEC standards are only a basis. Different countries or economic areas may choose to accept them, or not, and sometimes with local variations. One example: IEC 62368-1 becoming UL 62368-1 or EN 62368-1. Some countries require compliance with older versions of standards which may even have been withdrawn by the IEC. For example, IEC 62368-1 supersedes IEC 60950-1 which was withdrawn on Dec. 20, 2020. But countries such as China, Korea and Taiwan have yet to publish their versions of IEC 62368-1. They may require that imported products continue to comply and be marked to 60950-1 or comply with their standards, such as GB 4943.1-2011 and GB 8898-2011 in China.

IEC 62368-1 is already at its third edition with the fourth a work in progress. Countries outside the European Union may require compliance with the third edition. But within the EU, this edition is not formally published in the ‘Official Journal’ and therefore cannot be used as a basis for CE marking. The fourth edition is unlikely to be published until late 2021 or 2022. But as its contents become known, power supply manufacturers are likely to pre-empt the standard and ‘design to meet’ before it becomes mandatory.

To complicate matters further, different countries have different ‘grandfathering’ schemes allowing the continued sale of products after the withdrawal date of older standards. This is specifically allowed in the second edition of IEC 62368 (clause 4.1.1) for power supplies as components within end applications that meet IEC 60950-1. The third edition, however, removes this concession.

Special power supply construction

Geography can affect standards requirements as well. Later standards use altitude as an environmental limit that affects requirements for clearances and isolation test voltages. In IEC 62368-1 for example, a maximum of 2,000 m is assumed with safety clearance multiplied by a factor of 1.48 for operation up to 5,000 m.

This requirement is not exceptional; eight world capitals including Mexico City sit at an elevation over 2,000 m. Smaller towns and cities, including La Rinconada in Peru (population 30,000), sit at over 5,000 m. Power supplies manufactured or sold in China according to GB 4943.1-2011 for domestic use must all be rated for 5,000-m operation unless specifically marked otherwise. Products may also require thermal derated at high altitude to maintain their safety certification, due to the reduced heat transfer capacity of thin air.

Standards such as IEC 62368-1 also contain other conditions of use; ac supplies are defined to have ‘over-voltage’ (OV) categories and environments have ‘pollution degrees’ (PD). Thus a claim of ‘IEC 62368-1 certification’ should state the actual OV category and PD achieved in the design.

Compared with the plethora of safety standards, there are only a few standards for electromagnetic compatibility (EMC), and they have been consolidated in recent years. In Europe, for example, EMC standards for emissions from IT equipment and both professional and domestic audio-visual equipment were combined in 2017 into one standard, EN 55032. This standard calls up the limits and test methodologies defined in CISPR 32 (CISPR stands for the French words for the Special Committee on Radio Interference. It publishes a number of EMC standards used for a variety of product families}.

Top-level IEC standards for EMC are in the IEC 61000 series covering conducted and radiated emissions, immunity to RF and magnetic fields, surges, transients, dips, induced line harmonic currents, and electrostatic discharge (ESD). As with safety, IEC publications don’t cover automotive EMC standards but documents from the International Standards Organization (ISO) in the ISO 114542 series cover test methods for immunity testing. Again, the SAE publishes a range of documents covering the same areas.

EMC standards incorporate different limit levels depending on the environment or application. Many engineers are familiar with the ‘Class A’ and ‘Class B’ emissions limits for industry and domestic use, but there is more to consider.

For example, in the EN 61000 series, ‘severity’ levels are defined for immunity to ESD and conducted and radiated interference. Effects ‘criteria’ are also specified, ranging from ‘A’ for ‘no effect’ to ‘D’ for permanent loss of function or damage. The acceptable level depends on the application; critical medical equipment might demand uninterrupted operation with the highest severity level of interference. But a disposable cell phone adapter might be fine with a non-resettable fuse opening inside after a mains surge. In both scenarios, the products could be said to be EMC compliant but with very different outcomes.

Medical power supplies generally, whether in ‘operator’ or ‘patient connect’ applications, have enhanced EMC compliance requirements in the fourth edition of IEC 60601-1- 2, based on the IEC 61000 series but with higher severity levels or expanded scope.

Power converter efficiency

The efficiency of power converters at full and light load, along with no-load losses, has long been an economic concern. Environmental impact worries are now mandating tighter efficiency limits.

Standards defined by the U.S. Dept. of Energy are at Level VI. In the E.U., the Ecodesign Directive 2019/1782 sets mandatory limits substantially the same as those of the DoE with some extra labelling and documentation requirements. Outside the U.S. and E.U., countries mostly use the DoE specifications, sometimes mandatory at lower levels with voluntary compliance at higher levels.

The CE mark is an indication that a product meets all requirements for unrestricted sale throughout Europe. These requirements are set by European Directives and relate to safety, health and environmental impact. Marking is not required for all product categories, and in many cases, manufacturers can self-declare they comply. For common power supply products, the Low Voltage and EMC directives are most relevant referring to Euronorms such as EN 62368-1 for safety. But other standards may apply depending on the end-use, such as EN 60601-1 for medical applications.

Of course, certification and marking of power supplies changed in Jan. 2021 for manufacturers that sell into the UK and Northern Ireland. With the UK withdrawal from the European Union, a new UKCA mark is required to certify the product meets the relevant standards, similar to the CE mark. Initially, the same international standards will be used, with a BS prefix. But there is no guarantee the standards will remain in lock-step with IEC versions in future revisions. Northern Ireland now requires dual marking, CE and UK(NI), although longer-term requirements are unclear, subject to UK-EU trade negotiations.

Fortunately, many power supply manufacturers have products holding worldwide certifications for just about any application. Working with such suppliers can take the stress out of choosing a power supply.