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. 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.
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. 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. Ken Javor Phone: (256) 650-5261 > From: John Woodgate <[email protected]> > Date: Sat, 13 Dec 2008 16:49:07 +0000 > To: <[email protected]> > Subject: Re: Common-mode emissions from SMPS > > In message <[email protected]>, > dated Sat, 13 Dec 2008, Gert Gremmen <[email protected]> writes: > >> the two capacitances (+ wire and - wire) are inverse, you will need to >> know the difference between the tow caps and use that cor the CM >> current. If both capacitances are equal, the CM current will be >> essentially zero. > > So why not add a small capacitor so as to *make* the capacitances equal? > Adjust for minimum CM emission. > > Remember, you read it here first! (;-) > -- > 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]> - 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]>
