[RFI] New switching wallwarts - RFI potential?

[Jim Brown]

On Tue, 12 Jan 2010 11:22:26 -0500, Paul Christensen wrote:

>> "...The mechanism that causes all of that noise is the rectifier diodes. 
>> If you use diodes that have a long minority carrier lifetime, no problem, 
>> but with the faster diodes that are being used nowadays..."

>That's a new one on me. 

To the extent that "faster diodes" shorten the rise time of the waveform, 
that makes some sense. Another primary mechanism is a current loop that is 
physically large, thus generating a strong magnetic field. A third mechanism 
is a "pin 1-like problem" with external wiring that puts noise on that 
wiring as a common mode signal. The capacitors across the diodes suggested 
in the reference are essentially reducing the HF current that excites these 
mechanisms. 

FWIW, I don't find the cited reference all that wonderful. On the other 
hand, there's an excellent treatment of RFI in switching power supplies in 
Henry Ott's latest book, the third edition of what has been "the EMC bible" 
since its initial publication in 1976.   

http://www.hottconsultants.com/  

73,

Jim Brown K9YC