🚀 go-pugleaf

In article <OTTO.90Oct15004800@tukki.jyu.fi> otto@tukki.jyu.fi (Otto J. Makela) writes:
>and we got to 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 so that I won't
>get stuck in such an embarassing way the next time ;-) ?

   Switching power supplies are designed to use 'saturated switches', i.e.,
the active regulator element is either fully on or fully off. If one is
using a MOSFET, then the term saturated isn't directly applicable, but means
the same nonetheless.

   A simple form of switching power supply consists of a switching
element and a low-pass filter. The low pass filter is usually an inductor
and fairly small capacitor. Also in this power supply is the controller,
which switches the switching element on and off (ideally never in a linear
mode) at the correct duty cycle to produce the desired voltage at the
output of the low pass filter.

  More complicated switchers, such as the one in the AT, rectify and
filter the incoming mains power to generate a high voltage rail,
something like 170V in the USA, which is switched through a small,
efficient transformer at a much higher frequency than the mains
frequency.  The switches used in the power supply must switch a much
higher voltage than the previous example, but at a much lower current.
There may be multiple secondaries on the power transformer; the voltages
will be related to each other and usually the duty cycle of the switcher
is adjusted based on one of the output voltages (often the +5V output).
All the secondaries may have only a rectifier(s) and small filter cap(s),
with no other active elements.

  In each of these cases, the switches are operated in such a mode to
minimize the I^2*R power loss in the active device. In a linear
regulator, the output voltage is regulated by altering the voltage
drop across the actice element. If you have a +5V supply with +9V in and
a 2A load, the approximate power dissipated in the pass element will be
something like (Vin-Vout)*Iout or 8 Watts. With bipolar devices it can
be worse; the base current contributes to power dissipation in the
active element. With a MOSFET device, the gate current is essentially
nil, and the active device dissipates exactly (Vin-Vout)*Iout.
In terms of efficiency, this linear supply is running 18W in and 10W
out; this is about 56% power efficiency.

  The efficiency of switchers is a more complicated subject, depending
on the configuration. Suffice it to say a well-designed switcher can
achieve 90% power efficiency, although I don't think the switchers
in PCs are designed for maximum efficiency so much as lightest weight
and lowest cost.

   Of course, *I think* this is all correct :-)

/*
 * Dana H. Myers KK6JQ 		| Views expressed here are	*
 * (213) 337-5136 		| mine and do not necessarily	*
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