23 Sep
2018
23 Sep
'18
5:59 p.m.
On Sep 21, 2018, at 6:03 PM, Noel Chiappa via cctalk <cctalk at classiccmp.org>
wrote:
(Although I guess the coil
for the buck might be cheaper than the transformer - even though the use of a
high frequency would reduce the size of the latter - making the buck approach
superior.)
To put it another way, there's no _necessary_ connection between the switching
concept, and the buck converter is there? Does that mean it is in theory
possible to stick a buck converter on the output of a linear supply to do the
V1I1-> V2I2 conversion? (Although I know it's probably a stupid design, because
you'd still need some sort of switcher for the buck converter, so the linear
supply would be basically pointless.)
Noel,
here?s one way to think about buck converters. The coil (inductor) serves as a way of
giving the flow of electrons ?inertia?. It?s not really inertia, it?s energy built up in
the magnetic field of the inductor, but it acts like inertia.
You want the electrons to have (say) 3.3 V of energy per electron - that translates into
an equivalent ?speed? of the electrons. So if the power supply can put out a train of
electrons moving at that speed (plus or minus a little), the load will be happy.
But, the available (battery, or whatever) energy source drops electrons out at (say) 5V.
The buck converter lets the 5V shove the ?train? of electrons forward for a while, until
it?s a little above the 3.3V ?speed? at the load, then stops the 5V shove and lets the
train run on ?inertia? (stored energy in the inductor) for a while - the voltage will be
dropping at the load, but only slowly. When it drops low enough, the converter opens the
switch to the 5V and starts speeding up the train again. The bigger the inductor, the more
?inertia? the current flow has, and the longer the train takes to speed up and slow down,
and the slower the switching rate can be for a give above and below voltage tolerance at
the load.
The inductor serves both to slow the rate at which the train speeds up (fighting against
the 5V source and storing energy into its field) and to slow the rate at which the train
slows down (powering the load out of the stored energy).
This is a slightly dangerous analogy in some ways - electrons do *not* have any useful
inertia in circuits, for example. I think Brent or any of the regulars can probably point
out other conceptual problems, as well. But, hopefully it?ll help in thinking about what a
buck supply is trying to do.
- Mark