Spehro Pefhany 14:26 2005-11-20:
>Diodes 101 (or maybe 301). It is due to "snap-off" of the diode reverse-recovery
>current-- 

In other words: the diode continues to conduct during a short period even after the current have reversed.  Then, 

>the reverse recovery current can drop abruptly to almost zero as the
>minority carriers are removed from the junction). The rapid change in current
>(di/dt) can cause conducted and radiated EMI 

, and the ringing is pretty HF as it is made up basically only of the capacitance of the diodes and parasitic inductance in the transformer winding (plus leads)

>(and in some cases can cause
>spikes that exceed the breakdown voltage of the diode due to external
>inductance).

(But not in a bridge recitfier  - unless there is large wiring inductance on the DC side - as they protect each other.)

>You can also control the problem with soft-recovery diodes.

Or shottkys that shut off very quickly.  Shottkys also loose less power so might sometimes be economical choice as you get more electrical power out, and less heat to get rid of.

As first step I would guess it is cheaper to use good diodes, than simple diodes + capacitor.

For best performance maybe use R in series with the C.  If even that does not help also use good diodes as fast soft recovery or schottkys.

Alternatively use a RC snubber across rectifier AC side instead of diodes across every diode.

/Morgan
--
Morgan Olsson, Kivik, Sweden

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist