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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: Switching Power Supplies
Message-ID: <8835@utzoo.UUCP>
Date: Tue, 27-Oct-87 16:26:59 EST
Article-I.D.: utzoo.8835
Posted: Tue Oct 27 16:26:59 1987
Date-Received: Tue, 27-Oct-87 16:26:59 EST
References: <1340@bgsuvax.UUCP>
Organization: U of Toronto Zoology
Lines: 42
Keywords: Theory?

> ... switching power supplies.  I know these supplies
> dissipate less heat and work by somehow increasing the input frequency,
> but are these supplies considered new technology to replace humming          
> transformers?  More importantly can anyone take the time to explain the theory
> behind how these supplies actually work, and why they are used rather than    
> the traditional transformer/diode bridge type?

A switching supply replaces not just the transformer and diode bridge, but
also the regulators that supply precise output voltages.  That's important.
The standard sort of regulator for the old linear (transformer/diode) supply
holds its output at a given voltage by imposing enough resistance internally
to drop the input voltage to the desired output voltage.  The trouble is that
passing high currents through a resistance generates a lot of heat and wastes
a lot of power.

A switching supply, on the other hand, turns the flow on and off at a high
frequency such that the *average* output voltage is the desired one, and
then filters the result to get a (more or less) smooth DC voltage again.
The key point here is that the control transistors are either fully off (no
current flowing, so no power loss) or fully on (minimum voltage drop through
the transistor, so minimal power loss), not halfway between as they usually
are in a linear supply.  This is much more efficient.  As a side issue, if
one does want to use a transformer -- usually desirable, partly for
isolation from the AC line and partly to make design easier -- the switching
can be done at high frequencies where physically-small transformers are
efficient.  This eliminates the great lump of metal in the corner. :-)

Both types of supplies have their advantages.  Switchers are smaller,
lighter, and more efficient.  On the other hand, their output tends to be
noisier, they have to be carefully designed to minimize radiated interference,
they are more complex, and they often regulate poorly when asked to run at
much less than full output.  Generally, switchers come out better for
powering big pieces of digital equipment (which have semi-predictable power
demands and don't mind a bit of noise) and linears come out better for lab
work (highly variable demands), analog work (noise sensitivity), and small
equipment (lower complexity).

I think the above is relatively accurate, although I'm not an expert on
this and may be wrong on some details.
-- 
PS/2: Yesterday's hardware today.    |  Henry Spencer @ U of Toronto Zoology
OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry