Hi Brian:
When evaluating field failures due to surges, I find it very helpful to carefully inspect each unit returned from the field to try and identify the path that the surge took through the equipment. Then, the components in that path are carefully examined for clues about what happened. Usually, much can be learned by carefully studying units that suffered unexpected damage in the field. I agree with Courtland Richmond that in your particular case, it is not 100% clear that the damage is surge related. Even so, careful inspection of failed units should provide some useful clues. I know of one case where the manufacturer, after suffering the type of frustration you are having, instituted a policy that every-single-unit that failed in the field was sent to one engineer for analysis. Failed units were analyzed and photographed before being discarded. Certain patterns became apparent, and it turned out that there was more than one failure mechanism taking place. Over time, each mechanism was systematically identified and steps were taken to upgrade the design accordingly. One thing I don’t think you have mentioned is whether the failures appear to be limited to the line side of the isolation barrier, or involve an actual failure of the isolation barrier. These would point to different mechanisms. One last comment. You mention testing up to 5 kV, but it’s not clear how the surge is applied (such as common mode or differential), and the waveforms and short-circuit currents are not identified. Without more information I can’t say for sure how much your tests are stressing the power supply. In general, though, I consider 5 kV to be a rather low number in terms of the open-circuit voltage of a surge test for an AC mains interface. This topic can be debated endlessly, but a brief review of IEEE C.62.41 will provide some perspective on the problem of identifying appropriate surge-tolerance requirements for equipment that connects to the AC mains. In general, I do not think that simply complying with the applicable regulatory standards will guarantee reliable performance in the field. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) <mailto:[email protected]> [email protected] <http://www.randolph-telecom.com> http://www.randolph-telecom.com From: Kunde, Brian [mailto:[email protected]] Sent: Wednesday, September 16, 2015 2:54 PM To: [email protected] Subject: Re: [PSES] Unexplained High Fallout of Power Supplies Thanks to all for your input and suggestions. You all have given us several areas to look at. The power supply failures have been on six different models or power supplies and two different manufacturers. We have also seen a couple line filters fails which are unusual. The failures have occurred in both new products and in products that have been operating in the field for several years. Because of the architecture of our products, many have large high current AC Line Filters upstream from these small AC/DC power supplies (generally 150-300 watt 24Vdc output). We are aware of the phenomena where the Surge pulse gets amplified due to the large cores in the line filters. This is why we pre-qualify all power supplies with the line filter intended to be used. We will also test surges up to 5kV (test to failure) so we can see how the Power Supply fails under those conditions. Some models pass, but when power supplies do fail during this test they generally just such down and won’t restart. No BANG, no smoke, no flash. However, the power supplies failing in the field are blowing their guts out (described earlier). We will be looking again at the Inrush currents observed during a momentary voltage dropout. We perform the voltage dropout test with a programmable power supply and large transformer to be able to generate a limited amount of Inrush Current. However, our 300 watt power supplies are used in a product that is plugged into a 30A – 50A single phase branch circuit. I assume that such a circuit in a commercial or industrial environment can produce much larger inrush currents than is possible with our test equipment in the EMC Lab. Maybe high inrush currents are over-taxing the bridge rectifier causing its self-destruction. We have something to play around with. If you have any other ideas, please let me know. Thanks for your help as always. The Other Brian From: Ken Wyatt [ <mailto:[email protected]> mailto:[email protected]] Sent: Wednesday, September 16, 2015 9:33 AM To: Pat Lawler; Kunde, Brian Cc: <mailto:[email protected]> [email protected]; Brian Dahl Subject: Re: [PSES] Unexplained High Fallout of Power Supplies Hi Brian, Pat offers some great advice with respect to filter stability and resulting ringing. This is not something normally tested using a transient generator, so should be examined. I’d also like to add an observation. During the time I worked for HP/Agilent in their scopes division, we also started noticing in the late-1990s that power supplies had become the #1 field failure. We pretty much determined (maybe more of a guess at that point) that due to the variety of failures across different products and vendors, that it was most likely a reliability issue, rather than due strictly to over stress. In fact, in the mid-1970s, the NAVY had come to the same conclusion and had instituted similar reliability research on power supplies. Our environmental test engineer, Brian Dahl (the “other other Brian”), who was also an expert (a black belt) in reliability, instituted two major changes in our power supply vendor selection process: 1. HALT testing of all power supplies (as well as whole products) 2. Life Testing of all power supplies For those unfamiliar, HALT (highly accelerated life testing) is a combination of very fast changing temperature extremes (high to low and visa versa in 1-2 minutes) in combination with three-axis vibration. He would run HALT on samples of power supplies prior to our selecting a final vendor. Those supplies that passed the HALT were then subjected to long term life testing in a large walk-in temperature chamber. He’d take a sample of 10 to 20 power supplies from each potential vendor, place them under full resistive loads and cycle the temperature every 5 to 10 minutes for 1 to 2 months. After starting this program, field failures of power supplies dropped almost off the list. Here are some references that might be helpful: http://www.cui.com/catalog/resource/reliability-considerations http://www.astrodynetdi.com/pdf/MTBF_and_Power_Supply_Reliability.pdf http://www.sre.org/pubs/Mil-Hdbk-338B.pdf http://snebulos.mit.edu/projects/reference/MIL-STD/MIL-HDBK-217F-Notice2.pdf I might add that Brian was a nominee for EDN's “Test Engineer of the Year” back then. If anyone would like to pick his brain, he may be reached at [email protected] <mailto:[email protected]> . I’m copying him on this email. Cheers, Ken _______________________ I'm here to help you succeed! Feel free to call or email with any questions related to EMC or EMI troubleshooting - at no obligation. I'm always happy to help! Kenneth Wyatt Wyatt Technical Services LLC 56 Aspen Dr. Woodland Park, CO 80863 Phone: (719) 310-5418 <mailto:[email protected]> Email Me! | <http://www.emc-seminars.com> Web Site | <http://design-4-emc.com/> Blog <http://www.edn.com/blog/The-EMC-Blog> The EMC Blog (EDN) <http://www.emc-seminars.com/Newsletter/Newsletter.html> Subscribe to Newsletter <http://www.linkedin.com/in/kennethwyatt> Connect with me on LinkedIn On Sep 16, 2015, at 6:50 AM, Pat Lawler <[email protected] <mailto:[email protected]> > wrote: Hi Brian, - Have there been changes to the design of your system, specifically the AC EMI filter? Increased inductor values, either by design or vendor change (filter manufacturer, or type of core used in the filter) could cause excessive ringing at the input of the power supply at turn-on. - Try putting high-voltage scope probes at the input of the power supply to see the common and differential mode voltage at turn-on, or during line surge testing at 200V levels. I had one case where the filter caused ringing to 2x the applied voltage! - Is your equipment installed differently now, either by installation or removal of isolation transformers or grounding changes? I once had a customer claim that our power supplies no longer met the IEC 61000-4-5 line surge requirement, and the equipment was resetting. After many heated conference calls, they admitted the equipment was actually a redesign, with the addition of a Wi-Fi module. Not knowing what else to try, I asked if they disconnected the Wi-Fi module, returning the system to it's previous design configuration. It turns out the noise from the line surge test was being picked up by the Wi-Fi module and getting conducted into the rest of the system. Pat On Tue, Sep 15, 2015 at 1:53 PM, Kunde, Brian <[email protected] <mailto:[email protected]> > wrote: Our company’s Service Department provides monthly field repair reports to our R&D department who looks for patterns and high fallout of components. Over the last 6 to 12 months, we have noticed a high fallout of Power Supplies in the United States. However, we have not been able to find the reason for the fallout. The power supplies have all shown arc damage to the AC front end, signs of arcing and traces burned or vaporized, blown fuses, and shorted FETs and/or Rectifiers. These failures have occurred on several different locations, on different power supply models, different manufactures and on different instruments. Some instruments have been in service for years; some for only a few weeks before they fail. Some instruments even have surge suppression modules installed and though the power supplies fail the surge modules tested out fine. The failures did not occur during any known lightning storm or any other known transient. Very strange. Of those of you who read these emails, have you experienced an unusual increase in such field failures in the last year? Anyone have an idea of what might be causing this increase? We test our products and power supplies to the IEC 61000-4-4 and 4-5 fast transient and surge immunity tests. We actually test beyond what is required for CE in Europe (often to the limit of our test equipment which is 5kV) and we audit every family of products about once a year. We have performed additional testing of production power supplies of known models that have failed in the field yet no unusual problems have been found. We also perform radiated and conducted RF immunity tests, ESD, voltage dips and dropouts, frequency variation testing, and harmonic and inter-harmonic immunity tests with no discernable problems found. We have AC Line Analyzers running for months at several customer locations and have not detected any unusual transients or reason for the high fallout of power supplies. Two weeks ago we had a power supply blow in one of our own labs on an instrument that had been running for several years. Two R&D engineers were sent over to investigate. They changed out the power supply and verified the instrument was running properly. As they turned to walk out of the room, POW!!, the power supply in the instrument next to the one they just fixed blew up. A power line analyzer has been running ever since but not unusual transients have been detected, yet. Are we missing something? Is there additional transient tests that we are not performing that we should be? Is there something we should be looking for and are not? Has this last year been unusually bad for power grid problems, sun spots, alien transmissions, ?? We are stumped. Thanks for any advice and comments. The Other Brian ________________________________ LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. 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