🚀 go-pugleaf

>I don't understand how a switcher "replaces" the bridge,
>since the AC mains still have to be converted to DC.
>I suppose that the transformer can be eliminated since no
>voltage step-down is required (but then the pass transistor has

In fact, there IS a rectifier after the AC mains in switchers - I think
Henry got a bit carried away with that one.  Also, one will very often
find a transformer included in a switching supply - but located just
"after" the switch, and operating at the switching frequency (so it's
considerably different than the typical 60Hz power transformer of linear
designs).  For that matter, there's really no *need* for a power transformer
in a linear design - but it'd take a helluva pass transistor to get from the
300 VDC rail down to, say, a +5 VDC output! :-)  A true "transformerless"
design in either case usually only comes about when the desired output
voltage is within a reasonable fraction (up or down) of the DC rails existing
immediately after the rectifier.

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.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.