In message <AFA9C97E2D25491B8AEEBBFE93556FC9@PC323541548743>, dated Thu, 18 Dec 2008, rn...@san.rr.com writes:
>But, no one provided me with a Thevenin equivalent >circuit for the SMPS when looking up the ground >wire. I hoped to get some info from an SMPS designer, but it was not successful. There was confusion between the actual source and the path the disturbance takes to escape from the printed board and become an emission. > >What is the circuit, and where is the generator? > >When looking up the ground wire, is the source a current source or a >voltage source? Please ignore the fact that the LISN is a 50-ohm load. >Instead, >please consider that the ground wire is open- >circuited at the wall end. I think the source must be the switching device, normally a FET. We can regard it as a voltage generator in series with a resistor, but that can also be regarded as a current generator in parallel with the same resistor. The two ends of that circuit have *different* stray capacitances to the PEC. If the capacitances were exactly equal, no disturbance current would flow into the PEC. Fixed capacitors elsewhere in the SMPS circuit, that may appear to be in parallel with the stray capacitances, are actually largely isolated from them at disturbance frequencies by fixed and stray inductances. I suppose it's not difficult to measure the capacitances and the disturbance voltages at their source (balanced input or differential oscilloscope - must have high impedance input), but it may be more difficult to determine the resistor value. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk All these closing-downs have been caused by dozing clowns 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 <emc-p...@ieee.org> 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 <emcp...@ptcnh.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
