🚀 go-pugleaf

In article <17660122@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes:
>One other *disadvantage of switchers that I believe Henry forgot to mention
>is a typically atrocious power factor, as the switching action plays merry
>hell with the input current waveform.  This has become quite a concern at
>sites which have a large number of independent switchers (such as a bunch
>of PCs or workstations) on their AC lines.

Actually, I believe the problem here is common to both linears and switchers.
It's not the switching action per se, given that there is a diode bridge and
a bit of a filter capacitor in a typical switcher.  (I didn't overlook this,
by the way, just considered it an irrelevant detail.)  The problem occurs
in any supply which gets its initial DC by rectification followed by a
filter capacitor:  the rectifier diodes conduct *only* when the input
voltage exceeds the voltage on the capacitor (ignoring details like diode
drop).  This typically means that the current waveform resembles a sort
of interrupted sawtooth, with no drain until the AC wave is close to peak,
then a big surge as the diodes fire, then a gradual taper as the capacitor
charges, and then cutoff and nothing again as the AC voltage heads for zero.

This is actually much worse than what one would get from the switching
action, because large numbers of independent switchers would not generally
maintain a consistent phase relationship in their switching circuits, and
so things would average out.  But *everybody* is sucking current madly at
the AC peaks, linears and switchers alike.

There is a move afoot towards supplies with better power factors, if only
because desktop computers are getting bigger and want more power, and
people want to go on plugging them into ordinary 15-amp circuits.  The
lousy power factor of conventional supplies makes it impossible to use
the full capacity of a 15A circuit.
-- 
The type syntax for C is essentially   | Henry Spencer at U of Toronto Zoology
unparsable.             --Rob Pike     |  henry@zoo.toronto.edu   utzoo!henry