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EMC Basics #10: How to troubleshoot power disturbances

Posted: 19 Sep 2011 ?? ?Print Version ?Bookmark and Share

Keywords:EMC? troubleshooting? Power Quality?

Often seen as an EMC stepchild, power disturbances are becoming increasingly significant. At the design level, we've seen a considerable increase in recent years with power disturbances. These problems are driven by improved power components, which allow faster switching rates, higher power levels, and unfortunately, increased EMI problems.

We've seen an increase in power disturbance problems at the systems level, too. The recent 2011 IEEE EMC symposium even held a full-day special session on EMC issues and the "Smart Grid". The session was well attended and promoted a lot of discussion. As one wag observed, "megawatts are finally meeting gigahertz."

But back to the design issues. Due to these problems, mandatory power-disturbance testing is now required for EMC qualification on a wide range of products. The specific tests vary with industry, platform, and even location. One size does not fit all when it comes to power disturbances.

Power disturbance specs
There are power-disturbance specifications for electronics used on AC mains (often varies with country), military platforms, vehicles, telecommunications facilities, commercial aircraft, and more. Most are unique to the environment, and are typically based on empirical data.

Some specifications are part of the general EMC requirements, while others may reside in separate documents. In the latter case, the popular term is "Power Quality," or PQ. While the EMC requirements focus on transients, the PQ requirements usually address longer term power perturbations like sags, over/under voltages, outages, and others.

EFT and surge
Two very popular commercial EMC requirements are the Electrical Fast Transient (EFT) and the lightning surge. These are applied to the AC inputs. In the real world, these are two of the more common causes of equipment malfunctions and damage. Other industries, such as military, vehicular, and telecommunications, have similar requirements for both AC and DC inputs.

The EFT tests simulate arcing at contacts, which results in short bursts of very fast transients. The EFT is described in ANSI/IEEE C62.41; the corresponding CE test requirement is EN61000-4-4. The individual transients uses a 5 nsec rise time, which is pretty close to the nominal 1 nsec for ESD. As such, upsets such as resets or other "bit-flipping" is common.

The surge tests simulate a lightning hit on the power mains. The surge is also described in ANSI/IEEE C62.41; the corresponding CE test requirement is EN61000-4-5. These transients (both voltage and current) are much slower but with much more energy than the EFT. As a result, both upsets and damage are common.

Power quality
An excellent resource for PQ on the AC mains in North America is IEEE Std-1000, also known as the "Emerald Book." This guide is put out by the IEEE Power Engineering Society, and focuses on wiring practices for computer equipment. As such, it is an excellent place to start. We have even used this as the basis for developing internal power specifications.

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