Pat, There is one problem which was not mentioned, yet - when you power multiple PWM converters from a single power source, they all draw power at their respective switching frequencies. In case of DC-DC on-board converters you will deal with discontinuous, pulsed input current, there is no reasonable way to isolate these pulses from the bus with just capacitive decoupling (there is a reason they say that you cannot decouple one capacitor with another). If these frequencies are close, you will see a so-called "beat frequency", a difference between them. If your power source impedance is not sufficiently low, this beat frequency will modulate the source voltage. Proper filtering of the noise at the input to an individual supply will help. You need a series component like a loosely wound common-mode choke at the input of each converter, where the leakage inductance will suppress some of the differential components of the noise. The goal is to minimize the amount of noise "this" supply injects into the common power bus, keep the noise localized near the converter. The beat frequency can be very low, thus it is difficult to build an LC filter to minimize its effect.
Boris Shusterman, EMC Corp. EMI Compliance Engineering From: Pat Lawler [mailto:pat.law...@verizon.net] Sent: Wednesday, July 30, 2003 10:31 AM To: EMC-PSTC 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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
