Alex, Are you speaking of radiation directly from a SMPS on a table? If this is the case, why, yes, you may, ASK for margin, but you may find few vendors willing to bid to that requirement. Few ITE makers would warrant their own boards to meet FCC limits outside a cabinet! But a SMPS should never be close to marginally compliant in radiated emission from its power cord and asking for margin is in such cases (I would say) a smart thing to do. How much depends on vendor process variability.
My own experience has been that radiated emissions above 30 MHz attributable to SMPS were readily shielded -- once the PS was installed in equipment. It took very little common-mode filtering to attenuate radiation due to conducted noise once in the box, and the box was well enough designed to avoid slots and openings. (Granted, that was earlier days, with switching frequencies of 100 KHz or less.) I did institute a practice of testing SMPS inside chassis they were designed to fit, and the reason is rather amusing. A PS we were testing with resistive load consistently failed conducted emissions. As was our practice at the time, we were testing with PS and load on a table, into LISN's installed in a shielded (not anechoic) chamber, thus ensuring we did not measure ambients. But the vendor was puzzled, as HIS tests showed everything within limits. Engineers flew in from Hong Kong and witnessed the tests. Yes, conducted emissions WERE too high. But efforts to reduce them were not too successful. About then I discovered that if we tested OUTSIDE the chamber, these PS's _passed_. Inside, fail. Outside, pass. The SMPS output wiring was was exciting resonance in the chamber and coupling onto the IEC power cord, whence they showed up as conducted emissions to the LISN's. >From that time forward, we built resistor load boards that fit in place of a motherboard, and were able to see a MUCH reduced level of conducted noise. (We also got a good idea how well the fan cooled things.) I do recommend testing subassemblies in a manner as similar to their intended use as possible. For one thing, we saved money on each PS that didn't need design changes. And THAT adds up. You make note of the effect of the SMPS on emissions it does not generate, coming from the EUT it powers. This is a bit of a different bird. I believe in not putting such emissions where a power supply will pass them, but (depending on your design process) you may have little choice. In this case, rather than specifying some margin for emissions to the PS maker (who does not KNOW what you apply to its outputs) why not an attenuation requirement? How much, may be derived by looking at RF current on the PS output, with a current clamp, and comparing this to what's seen on the power cord. This kind of need is not unique to SMPS's, as many peripheral cards, having been tested in a "quiet" computer, are offered for sale to a world of "unquiet" ones. Ad Astra per Aspirin! Cortland ------------------------------------------- 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: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
