🚀 go-pugleaf

henry@zoo.toronto.edu (Henry Spencer) writes:

>In article <OTTO.90Oct15004800@tukki.jyu.fi> otto@tukki.jyu.fi (Otto J. Makela) writes:
>>... talking about power supplies and stuff.  Suddenly I find out
>>that I don't know how a switching power supply works.  Could someone with the
>>necessary knowledge give me the baby-talk -type explanation...

>Here's one I posted some time ago:

>A switching supply replaces not just the transformer and diode bridge, but
>also the regulators that supply precise output voltages.

[Lot's of stuff deleted]

I thought that Henry's description was pretty good (but what do I know),
but I thought that he left out a description of an important component
which is key to understanding *how* these things work:  a big
inductor in series with the load.  My understanding of a switching
regulator (which would take as input unregulated DC such as from a
xformer/bridge), is that it consists of a series pass transistor
(as described) followed by a big inductor which dumps onto a big
filter capacitor.  The inductor wants to have constant current, so
a reverse bias diode to ground is provided to supply the current when
the transistor is switched off.  And that's basically it, except for
the feedback circuit to do the switching, typically pulse-width-modulation
at some high rate.

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
to have a higher voltage rating), and the inductor resistance
would provide current limiting (like the transformer) for
limited safety.


-- 

	Jeff Mulligan (jbm@eos.arc.nasa.gov)
	NASA/Ames Research Ctr., Mail Stop 262-2, Moffett Field CA, 94035
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