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]>
