SMPS without PFC front ends can cause high common-mode and differential mode conducted emissions, and significant radiated emissions due to common-mode cable currents. PFC is not a necessary element to this problem, although it does introduce additional challenges. The basic mechanism that was described - dV/dt coupling into parasitic (and/or intentional) capacitances - is valid, but so is it's corollary - dI/dt coupling into loops. This dI/dt is often left out of SMPS EMI discussions, but I've seen it be the more predominant source and a hard one to mitigate, especially where the source is not the grid but a low voltage DC source from which you demand and hard-switch high currents (ie high dI/dt).
Another piece of the SMPS EMI puzzle that I focus on is the rather nice radiated emissions generator set up when you take a device switching in the 10's or 100's of kHz and put a set of cables on each end of it = dipole antenna with an RF source in the middle. Jim Eichner, P.Eng. Compliance Engineering Manager Xantrex Technology Inc. e-mail: [email protected] web: www.xantrex.com Confidentiality Notice: This email message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message. From: [email protected] [mailto:[email protected]] On Behalf Of Fred Townsend Sent: Wednesday, December 10, 2008 3:20 PM To: John Woodgate Cc: [email protected] Subject: Re: Common-mode emissions from SMPS First you seem to condemn all SMPS to common mode emissions. Far from true. Most switching power supplies (at least the smaller ones) rectify the line and then use higher frequency DC to DC converters. It saves iron and with modern fast components you don't lose much in efficiency due to using higher frequencies (20 to 500KHz). The converter frequencies don't propagate much because they have to go through the big rectifier filer first. The problem occurs when we add power factor correction (PFC). Instead of straight rectification the AC current is taken out of the line in chunks. This produces lots of harmonics. Symmetry tends to cancel out the even harmonics but the odd ones propagate, particularly along the neutral in three phase systems. Since 3 phase neutrals are not expected to carry much current they are sometimes built smaller than the line circuits. It's all the perfect formula for emissions. Lack of symmetry and the UNCOMMON mode means they are very hard to filter. So it's not the SMPS and it's not common mode, it's the PFC. No good deed goes unpunished. Fred Townsend DC to Light John Woodgate wrote: > SMPS seem to be very good at producing common-mode emissions on all > the cables attached to them and the products they power. Would someone > please give me a simple explanation of how these common-mode emissions > are generated? > > I don't get involved with the internals of SMPS, and I've been given > some explanations that are both contradictory and, taken one at a > time, not terribly credible. - 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: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - 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: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
