----- Original Message ----- From: "Pat Lawler" <[email protected]> To: "EMC-PSTC" <[email protected]> Sent: Wednesday, July 30, 2003 7:31 AM 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?). Some observations and opinions: 1. Some relatively new techniques advance the idea of spectrum spreading which involves either modulating the switching frequency, random frequency switching, or a different modulation technique that inherently changes the frequency such as delta modulation to broaden the frequency spectrum, reduce the energy at any one frequency, and reduce the total noise. These techniques all work to varying degrees. This would seem to weigh in favor of non-synchronous. My guess would be that you will be a few dB better off with different frequencies. If your customer is a telecommunications customer and has an IF or clock frequency at which he operates it may be good to synchronize all your frequencies to one in which none of the harmonics are close to the customer's IF or clock frequency. 2. The dependence of the emissions on the load depends largely on the converter interface to the outside, i.e., a boost converter has non-pulsating line current and the emissions are independent of load at the line terminal, similarly for the output emissions from a buck or forward converter, i.e., the emissions will depend on the current delta, but not on the load current which is just the dc component of the output current. Changing the dc current level does not effect the noise, it is the ac component that determines noise levels. For a converter whose interface to the outside is pulsating the emissions are very much a function of the load current. At different load levels the pulsating/non-pulsating nature of the current may change, but, in general, when the a non-pulsating current changes to pulsating at light loads the emissions drop as the load drops. 3. I have heard the argument before about beat frequency generation and I am not convinced, but on the other hand I have also not done any measurements. Beat frequencies are the result of multiple switching frequencies and a non-linear circuit element or non-linear circuit, so it appears as a distortion component which would typically be much reduced from the fundamentals, so I would expect to see some beat frequency energy and I would expect it to be much smaller in magnitude than that generated at the switching frequency and its harmonics. Ernie Wittenbreder > > 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? It will not hurt. I would expect this to help at the lower harmonics (some cancellation due to the phase differences), but have little effect at higher harmonics. This is typically what you see with a multi-phase interleaved converter system. The results are highly dependent on the degree of current sharing in a multi-phase system, so that if the magnitudes are not well matched do not expect to pick up much improvement. > > 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
