Since all mammals do indeed have certain traits in common, I will attempt to answer the question at admittedly a conceptual level.
The SMPS switching device operates between two extreme potentials: the rectified mains potential, and ground. Further, there is capacitance between the switch and the equipment case, which is typically grounded through the green or safety ground. Placing an alternating potential on that capacitance drives current into the ground. That current is sourced from the mains conductors. To make the picture concrete in my mind, I like to picture a TO-3 can transistor heat-sinked to equipment chassis. That concretizes the capacitance as being that of a parallel plate capacitor consisting of the hot transistor case on one side, chassis ground on the other side, and the thermally conductive but electrically isolating dielectric in between. Clearly the common mode current will be given by I = C dV/dt In real situations the capacitance will not be as clearly defined as in my example, but that is the mechanism. There are other places you can get parasitic capacitance, such as across a transformer winding, but the basic principle is that current is driven into ground by an alternating potential driving current through a parasitic capacitance. Might be the Forrest Gump explanation - but Forrest would have recognized that mammals have a lot in common even though they occupy all sorts of different evolutionary niches. Ken Javor Phone: (256) 650-5261 > From: John Woodgate <[email protected]> > Date: Wed, 10 Dec 2008 16:53:01 +0000 > To: <[email protected]> > Subject: Common-mode emissions from SMPS > > 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. > -- > OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk > Either we are causing global warming, in which case we may be able to stop it, > or natural variation is causing it, and we probably can't stop it. You choose! > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK > > - > ---------------------------------------------------------------- > 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]>
