Differential versus common-mode SMPS EMC filter
Differential versus common-mode SMPS EMC filter
(OP)
I notice in the book "power supply cookbook" by Marty Brown that he concludes every smps design example with the design of a second order common mode EMC filter.
He states that if this isnt sufficient a differential filter stage should be added to make it 3rd order.
I would have thought that the differential filter stage was the most crucial one in an SMPS and should have been the starting point.
-Since the differential filter is the one that actually filters the power switching current -those strong current harmonics which will otherwise give bad conducted emissisons.
The common mode choke will not filter these harmonics. It only filters that bit of energy which gets capacitively or inductively coupled to earth from the SMPS PCB.
So i am wondering why the common-mode filter is given presedence in this book?
(It seems worth remembering that in mainland Europe, the mains is only two-wire, hot & neutral, with no earth....and a SMPS connected to this would have no earth connection , and a common-mode choke would be useless.)
He states that if this isnt sufficient a differential filter stage should be added to make it 3rd order.
I would have thought that the differential filter stage was the most crucial one in an SMPS and should have been the starting point.
-Since the differential filter is the one that actually filters the power switching current -those strong current harmonics which will otherwise give bad conducted emissisons.
The common mode choke will not filter these harmonics. It only filters that bit of energy which gets capacitively or inductively coupled to earth from the SMPS PCB.
So i am wondering why the common-mode filter is given presedence in this book?
(It seems worth remembering that in mainland Europe, the mains is only two-wire, hot & neutral, with no earth....and a SMPS connected to this would have no earth connection , and a common-mode choke would be useless.)





RE: Differential versus common-mode SMPS EMC filter
A differential mode filter provides a degree of common mode rejection. I have examined designs where a lot of differential mode was incorporated, and no common mode was present.
I'm presently designing a push-pull converter for a specialty application. Yet, when I've looked at some of the 'overview' power supply books, the entire coverage of push-pull might be one paragraph indicating it's a special case of a buck topology, and continued with flyback converters as if the entire world consisted of flyback. I wish such authors would add to the subtitle indicating that the primary topic is flyback, so I would know to not buy the book.
Some authors become focused on doing things their way. Good authors will provide alternate approaches to a design.
RE: Differential versus common-mode SMPS EMC filter
I agree that differential mode filters provides some common mode rejection.
This is interesting though, on page 5 of this application note (half way down on the left)....it says that differential mode filtering is implemented as a common mode filter........
http:
....that one really has me confused.
RE: Differential versus common-mode SMPS EMC filter
RE: Differential versus common-mode SMPS EMC filter
RE: Differential versus common-mode SMPS EMC filter
Flux walking can also be a problem with half and full bridges. I tested a push pull once and one fet had been connected SDG instead of GDS.....it still worked and passed all the tests. We reckon one fet was repeatedly "flashing over" but it worked allright. it was only a 10w output. The FETS were also big well de-rated fets for some reason that i never found out