[Editor's note: we are pleased to continue our series on the vital and sometimes unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert Daryl Gerke of Kimmel Gerke Associates. Note that there are links to all previous entries here.]
This is the final installment in the mini-series on EMI/EMC troubleshooting. Often seen as an EMC stepchild, power disturbances are becoming increasingly important.
At the design level, we’ve seen a significant 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 specifications
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 EFT (Electrical Fast Transient) 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.
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.
There are unique PQ specifications for other industries, too. Sometimes these are separate documents, and sometimes they are separate chapters in detailed equipment specifications. Several of the European Norms address PQ concerns for European power mains as well.
Troubleshooting power disturbances
There are two methods when troubleshooting power disturbances—failure forcers and monitors. You may need a combination of both to isolate and fix a problem.
To force failures, both the EFT and Surge tests are good starting points. Start at low levels and work your way up. And remember, the surge can cause damage, so don’t do surge testing on a valuable one-of-a-kind prototype.
For monitoring power at the equipment, power disturbance analyzers (PDAs) are very helpful. If you don’t own one, these can be rented and left in place for a period of time. These devices will check numerous various parameters, such as over/under voltages, outages, transients, and more. They date/time stamp the events, and even capture the waveforms for later evaluation.
One caveat with a PDA: due to the bandwidth, they may miss EFT events, so you may want to augment one with a storage oscilloscope. The bandwidth should be 100 MHz or higher.
Here are five quick power disturbance suggestions:
- Add a modular power filter at the input. If in a metal enclosure, be sure to locate directly at the power entrance, and provide a low-impedance ground connection between filter and enclosure.
- Add transient protection at the input. Install both differential mode and common mode devices. For the surge, MOVs are adequate. For the EFT, you will usually need faster silicon devices, with short connections.
- If EFT upsets occur, try adding a multi-turn common-mode ferrite (3-4 turns through the core) to the input power line. Note that single-turn ferrites may not be adequate.
- If resets occur due to EFT, try adding 0.01 µF capacitors and multi-turn ferrites right at the reset circuit. Inputs are particularly vulnerable, but be sure any reset/voltage monitor devices are also well decoupled at the chip.
- If lightning damage occurs, try to assess the failure path. Consider isolation transformers and transient protectors designed to for the full lightning-surge levels.
At various times, we’ve done all of the above. Next time, we’ll start looking at some EMC shielding problems and solutions.
Also relevant to this topic:
Debugging: The 9 Indispensible Rules for Finding Even the Most Elusive Software and Hardware Problems (Chapter 5, Part 3 of 3) (and see its preceding sections, which are linked within)
About the author
Daryl Gerke, an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist, and their EDN Designer's Guide to EMC (1994) is still relevant and in demand. He can be reached via http://www.emiguru.com or his other blog at http://www.jumptoconsulting.com/.