Conformity Magazine - September 2007 - (Page 26)

setup also requires electrical isolation between the EUT and coupling plane. The test is performed by applying discharges to the EUT in two ways, first, directly into the EU’s exposed surfaces, and then indirectly via coupling planes located a small distance from the EUT. Exposed conductive surfaces are tested in the contact mode. Nonconductive surfaces are tested with the air discharge mode. Discharges are applied to horizontal and vertical coupling planes in the contact discharge mode for the indirect application of discharges. Discharges are applied as single events with a minimum of one second between subsequent pulses. Both positive and negative polarity discharges are used. Because the relationship between pre-discharge voltage and discharge current of realworld discharges is not linear, testing is required at the test limit voltage and at predefined lower test levels. Simple, Effective, Not Perfect This test standard has found wide-spread use largely because of the advantages it offers. The test is relatively simple to perform. The ESD generator is small enough to be held in the test operator’s hand, allowing discharges to be applied to a large number of locations on the EUT fairly quickly. Most commercially-available ESD generators can also be mounted on a test stand or tripod for automated testing or situations where test operator fatigue may be a factor (for example, in cases where a large numbers of discharges are applied to each test point). The types of equipment to which the test can be applied are nearly limitless. The cost of test equipment is reasonable, compared with test equipment for other EMC tests. Perhaps most important is that equipment that meets the generally accepted test limits when following IEC 61000-4-2 usually exhibit acceptable immunity to ESD events in actual use. For all its simplicity and wide use, or perhaps because of them, the test method does have a few areas of concern. From the perspective of its practical application, these areas of concern may best be summarized as reproducibility of test results. Some equipment tested to this test standard exhibit a variation in the charge voltage at which failures first occur that is dependent on the particular brand and model of ESD generator used for the test or when tested with essentially identical test setup at different physical locations. [2] Pre-Discharge Voltage (kV) 2 4 6 8 Peak Discharge Current (A) 7.5 15 22.5 30 Rise Time (nS) 0.7 – 1 0.7 – 1 0.7 – 1 0.7 - 1 In some cases, the variations can be a simple nuisance, such as making it difficult to determine the true pass/fail level in a test. In other cases, the impact can be more serious, such as passing when tested with an ESD generator from one manufacturer and failing when tested with a generator from a different manufacturer. This second case can cause real problems when one needs to determine if a product complies with a mandatory regulatory requirement (such at the European Union’s EMC Directive) or in contractual disputes between an original equipment manufacturer and their prospective OEM customer. The IEC Takes Action The IEC is aware of the concerns with its basic ESD standard and has an active maintenance project going on within its technical subcommittee SC77B. Maintenance team MT12 of SC77B was formed in 2004 specifically to revise 61000-4-2. One of MT12’s goals is to revise the standard in a way that will improve the reproducibility of test results. At the same time, any changes introduced need to be proven to have the desired impact. Unproven changes, especially those that would increase costs for test labs, should not be introduced. MT12 began its work by describing the concerns with the standard, and by creating a list of the elements of the standard to consider for change during the maintenance process. A list of those elements that should remain unchanged was also created. The rationale for each element in these lists was also discussed. Comments on the proposed plan of action were then solicited from the National Committees participating in SC77B. The subsequent activity of MT12 is based on this proposed plan of action, and on comments from the National Committees. At the time this article was written, the IEC’s maintenance schedule is to publish edition 2.0 of 61000-4-2 in 2009. Two committee drafts (CDs) have been circulated to the National Committees for comment. [3 and 4] Several reasons for the variation in test result reproducibility have been considered. The main causes are postulated to be the specification of the test generator’s discharge current, lack of specification on electromagnetic fields emanating from the generator, calibration of the test generator, and elements of the test setup. Table 1: Discharge current parameters 26 Conformity September 2007 While much research has been performed on radiated fields from ESD generators, required quantitative data and calibration methodologies are not Current at 30 nS Current at 60 nS sufficiently mature to support a mandatory specification for fields in an international standard. Research 4 2 on fields continues, and adding 8 4 a specification on fields will be 12 6 considered as part of a future revision project. For now, proposed changes to 16 8 the ESD generator are limited to the discharge current.

Table of Contents Feed for the Digital Edition of Conformity Magazine - September 2007

Contents Editor’s Note FCC Extends Disability Access Requirements to VOIP Commission Fines Importer $3 Million Over Non-Compliant TVs FCC Gets Tough on Retailers Over Analog TV Tuner Labeling Analog Cellular Service Sunset Date Reaffirmed Commission Adopts Recommendations of Katrina Panel Free-Space Antenna Factors Through the Use of Time-Domain Signal Processing Editorial Correction ESD Open Forum An Update on Revisions to IEC 61000-4-2, the Basic Standard on ESD Practical Advice for Meeting the DFS Testing Requirements of the FCC The EMC Impact of Embedded Software ESD Design Buyer's Guide Tate Renominated to the FCC EPA Releases Report on U.S. Electronic Waste Additional Standards For The EU’s Toy Directive Medical Device Manufacturer Enters Consent Order Updated Guidance Document on Laser Products Issued FDA Offers Advice on Defective Heating Pads CPSC Issues China Product Safety Program Report Other CPSC Actions In The News IEC Standards Update Product Reviews Product News UL Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - September 2007

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