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Re: [RFI] New switching wallwarts - RFI potential?

To: <rfi@contesting.com>
Subject: Re: [RFI] New switching wallwarts - RFI potential?
From: "Paul Christensen" <w9ac@arrl.net>
Date: Tue, 12 Jan 2010 12:40:06 -0500
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
>> 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.


I can 100% understand why this would be the case where a chopper is used 
ahead of the transformer primary (e.g., typical SMPS) and the secondary is 
rectified and filtered.  The faster rise time results from the chopper 
located between the common-mode AC line filter and the transformer primary 
with the intent to up-concert 60 Hz to some higher frequency to make the 
transformer smaller and lighter with smaller smoothing components after 

But what I don't understand is why the diode type makes a difference on a 
perfectly sinusoidal 60 Hz secondary feeding the rectifier diodes in a 
linear power supply.  That appears to be the type of supply being modified 
in that link from Dallas Lankford.  I'm really lost on that one.

Paul, W9AC 

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