The Principles of Switching Power Conversion
Introduction
Imagine we are at some busy “metro” terminus one evening at peak hour. Almost instantly,
thousands of commuters swarm the station trying to make their way home. Of course there is
no train big enough to carry all of them simultaneously. So, what do we do? Simple! We
split this sea of humanity into several trainloads — and move them out in rapid succession.
Many of these outbound passengers will later transfer to alternative forms of transport. So
for example, trainloads may turn into bus-loads or taxi-loads, and so on. But eventually, all
these “packets” will merge once again, and a throng will be seen, exiting at the destination.
Switching power conversion is remarkably similar to a mass transit system. The difference is
that instead of people, it is energy that gets transferred from one level to another. So we
draw energy continuously from an “input source,” chop this incoming stream into packets by
means of a ‘switch’ (a transistor), and then transfer it with the help of components (inductors
and capacitors), that are able to accommodate these energy packets and exchange them
among themselves as required. Finally, we make all these packets merge again, and thereby
get a smooth and steady flow of energy into the output.
So, in either of the cases above (energy or people), from the viewpoint of an observer, a
stream will be seen entering, and a similar one exiting. But at an intermediate stage, the
transference is accomplished by breaking up this stream into more manageable packets.
Looking more closely at the train station analogy, we also realize that to be able to transfer a
given number of passengers in a given time (note that in electrical engineering, energy
transferred in unit time is ‘power’) — either we need bigger trains with departure times
spaced relatively far apart OR several smaller trains leaving in rapid succession. Therefore,
it should come as no surprise, that in switching power conversion, we always try to switch at
high frequencies. The primary purpose for that is to reduce the size of the energy packets,
and thereby also the size of the components required to store and transport them.
Power supplies that use this principle are called ‘switching power supplies’ or ‘switching
power converters.’
‘Dc-dc converters’ are the basic building blocks of modern high-frequency switching power
supplies. As their name suggests, they ‘convert’ an available dc (direct current) input voltage
rail ‘VIN,’ to another more desirable or usable dc output voltage level ‘VO.’ ‘Ac-dc
Comments
Post a Comment