Voltage Sag Immunity Tests: Some Common Mistakes and How To Avoid Them Alex McEachern, Senior Member, IEEE [email protected] Power Standards Lab, Emeryville, California, USA TEL ++1-510-658-9600 ABSTRACT Two standards, SEMI F47 and IEC 61000-4-11, require voltage sag immunity tests for electronic equipment. Voltage sags (or dips) of specified depths and durations are intentionally applied to the equipment, and the equipment’s performance is verified. Several common testing mistakes have been identified. These mistakes can yield false positive results, incorrectly indicating that the equipment has passed, or they can give false negative results, incorrectly failing the equipment. The mistakes include having insufficient available current; trying to simulate phase-to-phase sags with phase-to-neutral equipment sag generators; and misunderstanding the requirements of the standards. FAX ++1-510-658-9688 Properly designed transformer-based sag generators can almost always provide sufficient pulse current. However, hand-operated or motor-driven variable transformers should be avoided, as the contact point limits available pulse current. Oversized transformers with multiple fixed taps are optimal. The switches in a transformer-based sag generator must be carefully understood. IGBT-type switches generally include current limiting in their drive circuit, which is good because it protects the IGBT, but bad because it limits the pulse current available to the load. For example, in a sag generator a 300-amp IGBT would be appropriate for loads up to 50 amps continuous. A smaller IGBT will give false positive results. Keywords: sag, dip, immunity, testing, SEMI F47, IEC 61000-4-11, mains, power line I. MISTAKE #1: INSUFFICIENT SAG CURRENT – FIRST TYPE The sag generator must be capable of providing at least 6 times the nominal current or volt-amp rating of the load for at least one cycle. For example, to test a 16 amp load, at least 96 amps must be available for one cycle from the sag generator; if the load is a 10kVA load, at least 60kVA must be available from the sag generator. Insufficient available current gives false positive results: equipment that fails with real-world sags will incorrectly pass with sag-generator sags. For this reason it is generally not possible to correctly do sag immunity testing with amplifier-based sag generators. (An exception: if the load requires less than 5 amps, and a large amplifier-based sag generator capable of delivering 50 amps or more is available, testing can be done correctly.) Fig. 1 – Voltage sag (upper graph) vs load current (lower graph). The load current typically increases by 500% or more, for 10-20 milliseconds, at the conclusion of the sag. This can blow fuses or cause other EUT problems. If the sag generator lacks sufficient current capacity, this current pulse will be limited, and there will be a false pass. For this reason, electronic sources should generally not be used for voltage sag testing. TRIAC-type switches have appropriately large pulse current ratings, but limit the switching angles to 0 degrees and 180 degrees. A combination of TRIAC-selected or electro-mechanically selected taps and IGBT-gated sags is optimal. ____________________________________________________ Page 1 of 4 Another source of false positive results comes from component variations in fuses and/or circuit breakers. If the tested sample has a barely adequate fuse to deal with current pulses at the end of a voltage sag, the fuse may not blow on the tested sample. This problem can be addressed by rapidly repeating test sags, and by testing with sags that are somewhat deeper and longer than the standard test requirements. Fig. 3 – Component variations in EUT’s can easily be overlooked. This typical electrolytic capacitor has a value tolerance of –10%+50%,. One must be careful about drawing conclusions from a single tested sample – its capacitors may have been at the high end of the tolerance range. IV. MISTAKE #4: MISUNDERSTANDING THE REQUIRED Fig. 2 – Typical fuse current-time curves for electronic equipment. A typical sag-induced 500% increase in current for 10 milliseconds can create significant problems – but only if the sag generator is capable of supplying sufficient current. WAVEFORMS Not all standards are intuitively clear regarding the test II. MISTAKE #2: INSUFFICIENT SAG CURRENT – SECOND TYPE For three-phase loads, it is typical for current to increase by 150% or more on the non-sagged phases during a voltage sag. As the test sag may last 10 seconds or more, is is essential that the sag generator be capable of providing at least double the nominal current or volt-amp rating of the load for an extended period of time. III. MISTAKE #3: FAILURE TO CONSIDER COMPONENT VARIATIONS Typically, the device under test stores energy during normal operation, then releases that energy as compensation during voltage sags. Usually this energy is stored in electrolytic capacitors. Large value electrolytic capacitors have a wide tolerances. If the sample being tested happens to have a capacitor at the high end of the tolerance range, the sample may well pass the voltage sag test, while production units with more typical capacitor values may fail, yielding a false positive result. Fig. 4 – The upper graph shows the required voltages and durations for sag testing in SEMI F47. The lower graph shows a well-meaning, but wrong, test waveform. The difference hinges on the subtle meaning of “duration” in the x-axis label of the standard graph – which makes it difficult for non-English readers to interpret correctly. ____________________________________________________ Page 2 of 4 waveforms, so it is possible for a well-meaning engineer to choose the wrong waveforms. Consult an expert in the standard of there are any doubts. V. MISTAKE #5: SIMULATING PHASE-TO-PHASE SAGS WITH A PAIR OF PHASE-TO-NEUTRAL SAGS Some sag generators are incapable of generating true phase-to-phase sags, so an attempt is made to simulate a phase-to-phase sag by generating two simultanous phaseto-neutral sags. This works, but only partially. In a true phase-to-phase sag, the phase-to-neutral voltages are determined by the load and source characteristics, and may be substantially unbalanced (for example, if there are large phase-to-neutral loads on one phase but not the other). Typically, simulated phase-to-phase sags are perfectly balanced. Simulating phase-to-phase sags with a pair of phase-toneutral sags can give false positive results. Use a sag generator that can create true phase-to-phase sags. Most of the problems in this area tend to be softwarerelated. For example, a compliant component my momentarily give an incorrect output during a voltage sag, but recover immediately at the end of the sag. The system software may interpret this momentary signal as an important error, and decide the shut the system down. System software should be designed to not respond instantly to error signals; instead, some intelligence should be applied to the required duration of the error. For example, a single incorrect temperature reading can often be ignored, if the preceding and following readings are within tolerance. (Safety-related signals, of course, should always get an immediate response.) CONCLUSION Voltage sag immunity testing is a useful, practical engineering tool. It produces products that are stronger and more reliable. Like any engineering discipline, experience leads to the discovery of pitfalls and incorrect shortcuts; this paper has identified a few of the most common ones. VI. MISTAKE #6: INCORRECT PASS-FAIL CRITERIA Sag immunity standards often have pass/fail criteria that are open to widely different interpretations. This is especially true when the same standard is applied to components, subsystems, and complete systems. A typical example: an appropriate pass/fail criteria for a complete system might be that the system is permitted to mis-operate during a voltage sag, but must recover without operator intervention. However, if the same criteria is applied to a system component, the system must be prepared to tolerate mis-operations by its components – something that is rarely possible. In general, components need to meet tougher criteria than subsystems, and subsystems need to meet tougher criteria than complete systems, which have the easiest criteria. If this hiearchy of criteria is not possible to achieve, information about how, and under what conditions, the subsystems and components misoperate is critical. VII. MISTAKE #7: ASSUMING THAT SYSTEMS BUILT FROM SAG-TOLERANT COMPONENTS WILL AUTOMATICALLY BE SAG TOLERANT TOO ACKNOWLEDGEMENTS The author gratefully acknowledges the useful advice, shared testing experiences, and interesting suggestions – not all taken, which undoubtedly will explain the remaining errors in this paper – from his friends and colleagues: Josef FOLDI and Uwe HALLER and Hassan IRAVANI (all of Applied Materials), Wilmer AWAYAN (Novellus), Mark FRANKFURTH and Byron YAKIMOW (both of Cymer), Bob DETTORRE (Comdel), Danny POLIDI (formerly of Nanometrics), Cliff GREENBURG (Nikon), Ron WAGNER (CPI), Norm NICOLAI (IGC Polycold), Robert MAXWELL and Jeff BRUNER (both of KLA-Tencor), Dave GRAHAM (NUMMI), Greg WILTERDINK (Schlumberger), Dan WOODALL (formerly of SCP Global), P.K. FENG (Brookhaven National Lab), Jor AMSTER (ADTEC), all the engineers at ENI Power, all the engineers at Advanced Energy (who are very good at catching my errors!), Kamran HAQUE (Verteq), the engineers at Siemens, and everyone else who has put up with all my endless questions and poking around. Thank you all. Remaining errors, of course, are solely my own responsibility. It is perfectly possible to build a system that does not meet a sag immunity standard (for example, SEMI F47) from components that do meet that standard. ____________________________________________________ Page 3 of 4 REFERENCES [1] SEMI F47-0200, Specification for Semiconductor Processing Equipment, Voltage Sag Immunity. SEMI, Santa Clara, California, USA. 02-2000 [2] SEMI F42-0600, Test Method for Semiconductor Processing Equipment, Voltage Sag Immunity. SEMI, Santa Clara, California, USA. 06-2000 [3] G.T. Heydt, Electric Power Quality, Stars in a Circle Publications, Lafayette, Indiana, 1991 Alex McEachern (M 1984, SM 1996) is the President of Power Standards Lab in Emeryville,California. Over the last 20 years he has taught graduate-level power quality courses and/or has supervised the installation of electric equipment in the United States, Canada, Croatia, Japan, Hong Kong, China, South Africa, Germany, France, Singapore, Switzerland, England, Scotland, Mexico, New Zealand, and Australia. [4] IEC 61000-4-11 Ed. 2.0, Testing and Measuring Techniques – voltage dips and short interruptions immunity tests. IEC Document 77A/336/CD [5] IEEE Standard 1100-1992, IEEE Recommended Practices for Powering and Grounding Sensitive Electronic Equipment [6] A. McEachern, Handbook of Power Signatures, Dranetz-BMI, Edison NJ 1997 ____________________________________________________ Page 4 of 4
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