> -----Original Message-----
> From: piclist-bounces@mit.edu 
> Sent: Thursday, December 15, 2005 5:37 AM

> First off...I would think that switching power supplies would 
> create more of a lagging power, but we have a case where a 
> large number (100's) of switchers in a computer room are 
> causing, at least what we are told, a leading power situation.
>    
> Anyone familiar with standard PC type switchers confirm 
> that?  Not getting a lot of response from the vendor on this.'
>    
> And if it is leading....how can this be corrected?  Lagging 
> I know they add capacitors....so add inductance?
> 

As far as I know, the main problem of poor power factor
in PC switching power supply is the harmonics (high THD,
or total harmonic distortion) and not the displacement 
factor (the leading or lagging of the fundamental current). 
Higher power PC/server power supply will normally have some 
simple PFC (power factor correction) function built-in
in order to be sold in the European market where the
standard are more strict but I think other markets will
follow as well. 

For large number of PCs, you can try either passive
harmonic traps or active power filter (APF). 
Comparing to equip each PC with a PFC, it will
be cheaper to equip the whole system with a
single APF.

Regards,
Xiaofan
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