In message <c569ab7a.33aed%[email protected]>, dated Sat, 13 Dec 2008, Ken Javor <[email protected]> writes:
>I didn't understand that part of Mr. Gremmen's post. Say there is no >line-to-ground capacitance at all between phase and neutral wires and >the grounded equipment chassis. Or more accurately, that the reactance >of the line-to-ground capacitance is a much higher impedance than 50 >Ohms. I suppose you are looking at the testing situation, whereas I am looking at the field situation, with no LISN and no fixed 50 ohm measurement termination. >Given the FET switch capacitance to ground, you are still going to >drive current into the green wire and that current will be drawn from >both phase and neutral conductors. That's where we have got to think very clearly, and all along the same lines. Does the FET act as a disturbance voltage generator in series with two capacitors, drain to ground and source to ground? If so, making them equal minimises the CM injection. You may have another model in mind. > >The purpose of Y caps is of course to keep the cm current circulating >within the equipment and not in the LISNs, but it isn't obvious to me >that you get zero cm current if the two Y caps are perfectly balanced, >and some net cm current if there is a disparity. The Y-caps may well be too 'far', electrically, from the disturbance source to be very effective at higher frequencies. My idea is to try to minimise the disturbance current very close to its source. > >In fact, you typically need a cm choke to give you cm filter insertion >loss at the low end, because leakage current requirements place an >upper bound on Y cap values, which means they are ineffective at lower >frequencies. Agreed, but the CM choke has practical limits on its inductance, and isn't necessarily as effective in the field as it is when connected to a LISN. The real power network typically (in Europe, anyway) has resonant peaks in its impedance in the audio-frequency range. -- 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 <[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]>
