On Sep 21, 2018, at 6:03 PM, Noel Chiappa via cctalk <cctalk@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
