I agree with you. Unsynchronized, the emissions will add randomly as the square root of the sum of the squares. Synchronized, they add in phase, coherently. The difference between these two techniques is
20 log (n) - 10 log (n) = 10 log (n) in dB, where n is the number of PWMs, so the difference in dB between coherent and random phase addition depends on the number of PWMs. This means the difference increases exponentially with increasing number of PWMs. > From: Pat Lawler <[email protected]> > Reply-To: Pat Lawler <[email protected]> > Date: Wed, 30 Jul 2003 07:31:14 -0700 > To: EMC-PSTC <[email protected]> > Subject: Synchronizing DC-DC converters to reduce emissions? > > > We are designing a switching power supply for a customer that has > multiple outputs. Due to the tight regulation requirements, all > outputs have their own PWM modulators and control loops. > > The customer feels the RF emissions (as measured by CISPR 11) will be > reduced by synchronizing the frequencies of all the converters. I > think synchronizing the PWM sections will make the total power supply > emissions as high as possible (emissions are coherent?). > > 1) What has been your experience with controlling noise from multiple > switching power supplies? Is synchronizing a good idea? > > 2) If the supplies are synchronized, would a phase shift between > converters (avoiding simultaneous switching of all converters) help? > > Thanks, > --- > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
