That is an interesting consideration regards the beat frequency, thanks for the post. Something we have noticed several times with third-party 'bought in' mains-DC SMPSU (most people do that rather than design a bespoke one because the third-party has gained the necessary re safety compliance) is that people may specify the SMPSU for the maximum load out of their power budget for the apparatus to be driven by the SMPSU.
This usually is a sum/concatenation of all the highest current draw circuits/parts e.g. all LEDs on, activating sounder transducer, maybe driving a printer (usually thermal), radio module (Wi-Fi say) active, etc, etc. In fact the equipment does not do this 'maximum draw' too often and so the majority of the time the SMPSU is usually operating at lower capacity (a few LEDs on, idle printer, not actively TX/RX data packets). And it is then that EMC emissions created by the SMPSU (radiated and conducted) are actually at their worse. In some (probably more extreme) cases this can upset performance of the apparatus, or even co-located equipment, but also it can annoy when at the EMC test chamber with breaches of the emissions limit line. The downside of third-party power supplies is although they will have a nice declaration of conformity (for us in the EU (OK I know UK isn't any more but we've decided to carry on with it behind the scenes!) and claims for EMC compliance, never is any precise detail of how and the environment in which these tests were performed provided. A resistive load on the shortest possible DC output leads, with short mains input leads too I'm sure is the setup - happy to be proved wrong! Nearest I've found to such is this from Meanwell but it has some assumptions - large metal plates for one (but which one did they use for a particular model?!): https://www.meanwell.co.uk/knowledge-base/how-do-mean-well-test-for-electro-magnetic-interference-emi Anyway, as is probably known in this audience, but is a constant reminder to clients and so forth, you can't expect not to test a third party SMPSU even when it is 'compliant'. Thanks for giving me an excuse to ramble on :-) Kind regards, Matthew Wilson, GB Electronics (UK) Ltd. Matthew WilsonMIET Technical Director GB Electronics (UK) Ltd [email protected] www.gbelectronics.com +44 (0) 1903 244 500 Ascot House|Mulberry Close|Woods Way Goring-by-Sea|West Sussex|BN12 4QY|UK Certificate Number 10455 ISO 9001, ISO 14001 Disclaimer: This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please delete it from your system, do not use or disclose the information in any way and notify the sender immediately. The contents of this message may contain personal views which are not the views of the company, unless specifically stated. GB Electronics (UK) Ltd is a company registered in England and Wales under number 06210991. Registered office: Ascot House Mulberry Close, Woods Way, Goring By Sea, West Sussex, BN12 4QY. From: doug emcesd.com <[email protected]> Sent: Wednesday, July 3, 2024 10:55 PM To: [email protected] Subject: [PSES] Switching power supplies continued Hi All, When thinking about power supplies, one would think that they either work or not. But this is not the case. Switching power supply frequencies are usually much lower that today's system frequencies. This leads to a situation where it can take an hour or much more for an edge to hit a circuit in the system at a critical time and cause a problem. The problem looks like an intermittent one, but it is not intermittent! It is sort of like a beat frequency between a system signal and the power supply switching. This class of problems is very interesting. I have been tracking them down for about 40 years now and have developed some techniques for doing this. Tracking down what seem like intermittent problems can take a lot of time, but power supply interaction with a system is not an intermittent problem but sometimes takes a bit to track down a fix. I have seen these kinds of problems where a switching supply, not even connected to that part of the system, cause this kind of problem in a circuit a meter or more away from the supply. This is the kind of problem I love! Like a cat and mouse game, often requiring unconventional troubleshooting methods. Doug [https://lh4.googleusercontent.com/_HuR3Ky2TF_XhFHyxnYRmiq7nHQldnMsPNYFaLG6kb5T4y8MeCe-BDC_BscJtSFgszSSjssihHS-pjM3-jwNP8S0CwE-gN8fsRsPkojiAlmpBwb20vIVizS-siCUywW_jqrefbVr] ________________________________ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to [email protected]<mailto:[email protected]> All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe)<https://ewh.ieee.org/soc/pses/list.html> List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected]<mailto:[email protected]> Rick Linford at: [email protected]<mailto:[email protected]> For policy questions, send mail to: Jim Bacher at: [email protected]<mailto:[email protected]> ________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to [email protected] All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] Rick Linford at: [email protected] For policy questions, send mail to: Jim Bacher: <[email protected]> _________________________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1
