John, Although that sounds fine on the surface and assuming that space is not a consideration, you are still doubling the parasitic resistance and inductance of the filtering path. In fact since the larger value capacitance device often requires even larger packages, the parasitics may be more than double.
These parasitics degrade the decoupling characteristics of the capacitor (or the coupling characteristics if you are looking at the issue differently). If you can visualize the impedance vs. frequency curve for a capacitor (a "V" shape), increased resistance will raise the entire curve up and the increased inductance will move the minimum impedance down in frequency. Neither effect is generally considered beneficial for EMI. ...Marko From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: Monday, January 14, 2008 2:05 PM To: [email protected] Subject: Re: Y1 cap info In message <[email protected]>, dated Mon, 14 Jan 2008, Marko Radojicic <[email protected]> writes: >Two Y-caps in series = lower capacitance, twice the parasitics, less >filtering, and thus more conducted EMI. > >There is no free lunch: "Bomb-proof" Y cap design = reduced EMI >performance, unfortunately. If I needed a 33 nF Y-cap, I might well use two 68 nF in series. No reduced EMC. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk For very important information, please turn over. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
