We used to call those SMPS "off-line switchers", because they didn't have a 60Hz (or 50Hz) isolation transformer between the AC line and the rectifier/capacitors. Isolation was done with a much smaller ferrite transformer driven at about 20kHz using bipolar transistors for the switching.
Ralph McDiarmid, AScT Compliance Engineering Group Xantrex Technology Inc From: [email protected] [mailto:[email protected]] On Behalf Of John Woodgate Sent: Tuesday, May 26, 2009 2:09 PM To: [email protected] Subject: Re: 120V mains voltage distortion In message <8A756AA14D6C4BC2BB8F3A84A59B7C55@Pete97219Compaq>, dated Tue, 26 May 2009, Pete Perkins <[email protected]> writes: > The introduction of SMPS into electronic equipment in large numbers >led to unexpected problems in electrical installations. > > The key technical problem initially identified was the development >of harmonics which were not properly accounted for in the >installations. Rectifiers driven from transformers also produce harmonic currents in the supply, but the direct-on-line rectifiers in SMPS produce more, because the impedance in series with the rectifier is lower. This raises efficiency, but TANSTAAFL - more harmonics. The triplen harmonic currents of all the SMPS, on all phases, add arithmetically in the neutral conductor, and if the cable is fully loaded on its line conductors with the most efficient practicable SMPS, without PFC, the neutral current is 2.8 times the line current. Goodbye, cable, and probably building as well. Of course this worst-case full load is unlikely, but 2.8 is a BIG factor, especially since neutrals in 3-phase installations used to be 70% size of line conductors. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk Things can always get better. But that's not the only option. 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]>
