Originally Posted by AV Doogie
It is clear that you are not trying to help here. I am trying to explain the basic principles behind a switching supply and point out how it can be possibly affected. Your modern incarnation simply adds additional information to an already confusing situation. And 'virutually' all SMPS supplies does not mean 'All of them'. I find it interesting that you know what type of switching power supply each manufacturer is using in each piece of equipment. If you want to help someone understand the situation...great. Otherwise, bugger off.
When looking for line noise and analyzing power quality, what kind of equipment do you use?
You are simply wrong about how modern SMPS operate. I work on many different brands and types of displays and none operate the way that you described. Early switching designs might have, but I can't recall any consumer goods that would likely be used today that do. Check up on a few. You will find that all of them rectify before conversion.
When I have tried to identify noise passing from the a.c. line to the secondary of power supplies, I have used my 200mHz scope, observed the noise on the primary side, then searched the secondary side for that noise or harmonics of it. I have seen noise filtered by line conditioners, infact I use an old Adcom filter on my bench. It is not very sophisticated compared to regeneration technology, but it cleans up some noise generated by other equipment. Regardless, I have never seen the effect of that noise when not using it. It simply does not get past the power supplies in the displays. So regardless of how effective the filtering might be, if it never gets past the convertor, why will it make a difference?
You incorrect characterization of how SMPS operate is what causes confusion and is not helpful. There is one case that I can think of that might be affected by noise at the transition point, however, and this may be what you are thinking of or what has confused your understanding. Some units use a zero crossing detector to minimize the effects of inrush surges at startup, and IIRC some use it for power factor correction, but I have never heard of a case where an a.c. line caused problems with these. Poorly generated a.c. from a cheap UPS might, but we never use these, anyway.
I do wonder about the possibility of common mode noise and ground problems affecting display operation, but I have not experienced it directly. I suspect there may be some merit to balanced systems in this regard but they have their own potential ground implications.