Healthcheck prepares the way for the first-time EMC pass
By Ken Webb, TUV Product Service and Tim Williams, Elmac Services
The consequences of leaving EMC testing to the very end of the product development cycle have been well documented and long debated. Missed marketing opportunities, tarnished images and costly re-designs have all blighted manufacturers’ plans just as they thought they were ready to take products to market. A study has shown that as many as 60 percent of new designs fall at the final hurdle.
Based on the premise that preventing this disaster is more cost-effective than subsequently curing design faults, suppliers of EMC testing services are evolving new provisions for assisting manufacturers – by bringing EMC-related design issues into the spotlight earlier in the design cycle, and by providing timely advice on design best practice to ensure a higher first-time pass rate at compliance testing.
The unhelpful “sorry, your product has failed, we recommend you insert a filter at point A” only raises questions – what filter? how will I make it fit? what if it’s too expensive? – and fails to assist continuous improvement in design for EMC. The challenges are to provide advice at the optimum point in the development cycle, to ensure that advice is detailed and practical, and to encourage a trend in electrotechnical manufacturing industries towards the adoption of design best practice.
Successful design for EMC demands dedicated skills which are beyond the resources of many manufacturers. Unfortunately, the resources of test houses also generally fall short of the design skills and experience to make a significant difference. What is needed is an integrated approach to design and test, applied at the right time and conveying the right information. This might take the form of an EMC healthcheck that confirms that all is well prior to submitting a product for compliance testing, and accompanied by precise recommendations on correcting errors.
The rationale behind EMC healthchecks
Most private healthcare companies offer medical healthchecks in which they perform a range of tests, assess results in the light of genetic predispositions and lifestyle, and recommend ways in which an individual can work towards a healthy future – prevention better than cure.
An EMC healthcheck includes a range of design checks focusing on EMC control measures, that is those particular areas of design that have most impact on the EMC performance of the product. Recommendations will be provided to improve the EMC performance. When all design checks have been completed satisfactorily, the manufacturer can submit his product for compliance testing confident of a high degree of probability that it will pass EMC testing first time around.
The most significant factor is that the design consultancy will not only provide advice on the product under test, but can also assess predispositions within a manufacturing company to poor EMC design. Its recommendations on ways in which the designer, and his design colleagues, can work towards a healthier future for their company in EMC terms, can help to eliminate design bad practice from an entire organisation, paving the way for fast, troublefree compliance for all future products.
Two factors are required to ensure that an EMC healthcheck more than pays for itself: the level of detail in the healthcheck report, and a method of delivery which ensures that the optimum number of people will benefit.
Detailed reporting
The healthcheck report should focus on a number of design points (Fig. 1) if it is to thoroughly characterise EMC performance.
Partitioning
The design consultant will look at the general layout of the board with respect to circuits and the components they include. For design best practice, circuits that are sensitive to rf radiation, for example low-level analogue circuits, should be separated from relatively noisy generators such as power supplies, motor drives, etcetera. Consideration must be given to proper segregation of analogue, digital and power circuitry and the cables connecting them to ensure that, where the different sectors communicate, common mode interference currents are controlled. The positioning of the interfaces through which these interference currents migrate must be such as to optimise this control.
Components and circuit design
Components must be selected with EMC in mind. For example, at the high frequencies associated with EMC phenomena, components can depart from their intended behaviour, and parasitic elements may behave differently. While a designer will automatically select a particular component on the basis of, say, its voltage rating, the need to consider other characteristics such as parasitic effects can be overlooked.
PCB layout
The layout of the board must also be thought out in terms of its capability to provide a low-impedance ground system. On complex, multi-level boards there will be a number of ground planes, and attention must be paid to the layout of high-speed signal tracks to minimise emission and immunity problems.
Cables
Cables must be selected for the intentional signals they must convey, but also for EMC. The choice must clearly be on the basis of the type of signal or data they are carrying, but it must also be ensured that, for instance, any shield is terminated correctly to maintain shielding integrity.
Interface protection
Where cables enter or leave the product, good design practice is necessary to ensure that the interface is protected with filters or transient suppression components. These interfaces must be designed carefully in order to deal with emission and immunity issues while allowing the intended currents to flow unhindered.
Grounding
The grounding scheme must be carefully thought out to present a low-impedance path for the return currents associated with intended signal and, at the same time, a controlled path for unwanted rf and transient currents.
Shielding
If the product is contained in a metallic enclosure, the designer must have given consideration to the size of the apertures which will allow rf energy to penetrate or escape from the closure. There are many ways of achieving isolation, but the choice depends on cost, as well as screening performance. The cost of engineering products for EMC is an interesting issue which will be addressed later.
Structural design
Both shielding and grounding require consideration of the mechanical aspects of the product’s design. An understanding of the electrical performance of a physical structure is needed in order to deal with features such as resonances and uncontrolled ground impedances. This calls for a synthesis of both electrical and mechanical design skills.
General
The value of an EMC healthcheck embracing all of these design issues depends on the provider’s sound understanding of the practicalities of implementation in terms of manufacturing viability and costs.
It is not uncommon for manufacturers to over-engineer new products for EMC testing to enable them to launch on time, and subsequently to apply a cost reduction process to improve their profitability. An EMC healthcheck has the potential to ensure first-time pass without over-engineering, saving development costs. On the other hand, if a product has already moved into a cost reduction process, an EMC healthcheck can be used to economically assess its effects on EMC performance.
When is the best time for an EMC healthcheck?
An EMC healthcheck is best invoked prior to submitting a design for compliance testing (Fig. 2), and with sufficient time in hand to implement modifications. The design must be clear enough to enable all the relevant factors to be assessed, but not so fixed as to foreclose the most cost-effective control options. At this time, an EMC healthcheck will reduce costs by ensuring a shorter stay in the test house, and will help to secure first-pass compliance and the maximum probability of meeting launch schedules.
Where is the best place for an EMC healthcheck?
The best place for an EMC healthcheck is at the manufacturer’s site. This enables the design consultant to sit down with the designer and discuss any reasons for failure and the best methods of rectifying errors. A day’s consultation will be enough for most products, making it extremely cost-effective for the manufacturer to validate a design and determine any changes that are needed. Importantly, the healthcheck will provide an opportunity for the designer of the product to benefit from a few hours’ training from an expert in design for EMC, leading to a more thorough approach in the future.
In addition, on-site consultation allows other design engineers to gather round and pick up valuable understanding and experience, elevating the general level of EMC design expertise within the company.
Market demand
The idea of an EMC healthcheck came from a survey of manufacturers submitting their equipment for a one-day safety check service operated by TUV Product Service. Over 60 percent of users of this service said they would welcome a similar service for EMC. In order to take on board the necessary high level of design expertise, TUV Product Service sought a partnership with Elmac’s Tim Williams, a universally acknowledged expert on EMC design, with several books published on the subject and years of providing EMC training under his belt. The formation of this test/design partnership provides the basis for the EMC Healthcheck service now available from TUV Product Service and delivering
- a shorter time in the test house
- the best chance of meeting product launch dates
- reduced costs
- a client-focused learning opportunity, ensuring improved EMC design for the future.
For further information contact TUV Product Service Limited, Segensworth Road, Titchfield, Fareham, Hants PO15 5RH.
Tel: (01329) 443322. Fax: (01329) 443421.
Email: info@tuvps.co.uk. URL: http://www.tuvps.co.uk/
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