By implementing harmonized standards that remove the barriers to free movement of electronic products, the European Union (EU) is rapidly realizing its goal of creating a single market. This unified market eliminates the confusion and cost associated with multiple local laws and standards. This move could potentially simplify the technical and commercial task of creating products for Europe, and make it easier to use products from any country in Europe.
The result is a major opportunity, but it does demand that designers be at least familiar with a completely new set of rules. The good news is that these rules have been written as a coherent set, building on national experiences gained over a number of years. However, the sheer volume of material, and the labeling system employed, can be confusing. In the context of power supplies, approximately 116 standards and amendments govern EMC alone. Of all the standards and recommendations, nowhere is the hierarchical structure of European standards clearer, but the application more open to interpretation, than in the CE mark.
The Basic Nomenclature
Before we get too deeply into the topic, it may help to clarify a few terms:
- Directive: The name given to an official EU document that defines the requirements.
- Apparatus: A finished product delivering an intrinsic function, and directly usable by the end user.
- CE Marking (93/68/EEC): Products subject to Directives must comply with those Directives, and be marked in a prescribed way with the CE logo to be legally offered for sale in any EU state.
- Competent body: A body recognized as fulfilling the criteria of Annex II of the EMC Directive, and responsible for issuing reports or certificates under Article 10.2 of that Directive.
- Notified body: A body specially defined to the European Commission by a member state as having responsibility for issuing EC-type examination certificates for radio communications equipment under Article 10.5 of the EMC Directive. The body is also required to match the criteria of Annex II of that Directive.
- Compliance: Unlike approval systems like UL, which always require testing, there are several ways to justify a claim of compliance with a Directive.
- EMC Directive (89/336/EEC amended 92/31/EEC): The EU specification for EMC emission and susceptibility (see Table 1).
- Low-Voltage Directive (LVD) (73/23/EEC): Safety requirements for equipment normally operating at from 50 to1000 V ac or 75 to 1500 V dc (see Table 1, again).
- New Approach: A set of Directives intended to harmonize product safety throughout the EU.
Equipment meant for distribution or use in any European Union country must comply with the applicable directives. The directives lay down equipment requirements, and leave it to the standards, primarily European harmonized standards, to define the technical requirements. The manufacturer, or its European representative, must ensure that the product complies with applicable directives and CE mark the product.
Note that the CE mark does not claim conformance with a particular standard. A plastic toy duck and a TV set will both need the CE mark, although they meet totally different standards (toy safety was, in fact, the subject of one of the early directives).
Power-supply designers and users must consider the EMC Directive, LVD, and CE-marking amendment together, remembering that since Jan. 1, 1997, it is mandatory to CE mark under the LVD if the directive is applicable (Table 2).
Even within Europe itself, there has been significant discussion about when a subassembly such as a power supply needs CE marking. Some guidelines have been established by the European Power Supply Manufacturers Association (EPSMA). A web site provides access to these recommendations, but they are summarized below (see Table 3).
All line-powered power-supply units fall within the LVD, and the EMC Directive applies to all standalone units. Thus both types of devices required the CE mark. Provided that their input and output are not within the voltage range of the LVD, dc-to-dc converters do not fall under the LVD regulations, so all that's left is the EMC Directive.
The basic requirement of the EMC Directive is that electrical equipment may not cause a disturbance, or be affected by reasonable disturbance levels in its environment. The directive covers radiated and conducted emissions, RF electromagnetic susceptibility, electrostatic discharge, and fast transients.
The guidelines to the Directive state that components performing no direct function, such as components of electrical or electronic circuits, shall not be CE marked, as the EMC Directive does not apply to them. This matches practical experience because the EMC performance will depend on the details of the components' installation in the final equipment. Although a recent revision of the guidelines modified the list of specified components having no direct function, and omitted the explicit reference to "power supplies intended to be incorporated in apparatus," the EPSMA recommends that the EMC Directive, and therefore the CE marking, is still not relevant to component power supplies such as dc-to-dc modules.
Unlike the process for approvals like UL, there are three routes to EMC compliance:
- Self-certification (89/ 336/EEC Article 10.1): Administratively, this is the simplest route, and may be achieved either with in-house testing or by a specialty test facility. The manufacturer declares that the products conform to the applicable harmonized European standards.
- Technical Construction File (TCF) (89/336/EEC Article 10.2): This requires a detailed record of drawings, specifications, design details, and full information to enable vetting by the authorities. The TCF should enable a competent body (usually an accredited laboratory) to vet the design and confirm that it would comply if retested independently. This route is used when the manufacturer has not applied the harmonized standards, or has applied them only in part, or applied them in the absence of relevant standards.
- EC-type examination by a notified body (89/336/EEC Article 10.5): This specialized route is applicable only to radio communications equipment.
The costs involved in using external services have led many companies to invest in in-house facilities to undertake so-called precompliance testing (see the figure). Work is done in-house until the company believes that a product will pass the first time in compliance testing.
The facilities required for precompliance testing are less stringent, and are therefore, less expensive than full testing. Nonetheless, they allow engineers to more easily make the iterative design changes often required to reduce EMC problems. As usual, however, it is better to design out as many problems as possible from the beginning.
Even though a dc-to-dc converter is outside the LVD voltage range, and does not require CE marking, any equipment into which it is built will need to comply with the EMC Directive. Therefore, it is essential that the module be designed with the regulations in mind. In fact, designing converters to have low emissions and low susceptibility is critical to meeting the short time-to-market window many designers are facing. Allowing designers to meet this window is one of the major benefits of using on-board dc-to-dc converters.
EMC is becoming almost an industry in its own right, and considering the power supply's place between the line and the equipment, the supply has a key role to play. Although the performance is determined more by the layout of the equipment than by the supply itself, a few tips can enhance the advantage of using on-board dc-to-dc converters.
Using converters with built-in radio-frequency-interference filtering will minimize the disturbance on the supply lines. Practical experience shows that in order for the final equipment to meet the typical requirement of EN55022 class B, the converter must be designed with EMC control in mind, ideally meeting EN55022 class A. This sort of performance is only possible with LC-type filtering in the power module. Check the specification first.
When planning decoupling, the goal is to minimize path length and loop area. Dc-to-dc converters offer a significant intrinsic ability to decouple fast transients close to the load, minimizing the antenna effect of long supply lines. Keep any additional output filtering below 100 µF/A to avoid longer settling times and reduced stability.
A number of smaller-value capacitors, evenly distributed around the board, will be more effective than a single component, especially for the high-frequency decoupling. Don't forget that while parallel capacitors are additive, parallel resistance and inductance reduce, providing a double gain in the time constant.
An earth or ground plane in the printed-wiring board will not only provide some screening, but improve the EMC performance. It does this by introducing distributed decoupling capacitance for the highest frequencies, and reducing the inductance in both the ground path and signal distribution.
Surface mounting offers significant benefits in terms of reduced lead inductance and high-frequency radiation from the leads and traces. With surface-mount dc-to-dc converters now available that can handle up to 15 W, this is an increasingly practical and effective option.
The SLIM Initiative And EMC
The European Commission has recognized the complexity of the legislation that has established the Single Market, and in May 1996, it embarked on a program of simplification known as the Simpler Legislation for the Single Market (SLIM). The activity is planned to be completed in phases, with the EMC legislation coming under attention in phase III.
The SLIM team's report is expected to be presented by the end of the year, complete with conclusions and recommendations, which it is hoped will streamline and reinforce the Single Market concept.
In closing, please remember that the EU legislation has teeth. Equipment found to contravene the EMC Directive can be seized and banned from sale throughout the EU. In addition, the penalties for knowingly supplying or CE marking non-compliant equipment include fines and imprisonment.