IEEE Solid-States Circuits Magazine - Fall 2021 - 12

CIRCUIT INTUITIONS
Ali Sheikholeslami
Buck and Boost Converters
W
Welcome to the 30th article in the " Circuit
Intuitions " column series. As the
title suggests, each installment provides
insights and intuitions into circuit
design and analysis. These articles
are aimed at undergraduate students
but may serve the interests of other
readers, as well. If you read this article,
I would appreciate your comments and
feedback as well as your requests and
suggestions for future installments in
this series. Please email me your comments:
ali@ece.utoronto.ca.
Most electronic devices, including
cell phones, laptops, and cameras,
use a single battery to power
up their internal functional blocks,
such as microprocessors, digital signal
processors, displays, and analog
circuitry, where each requires a different
voltage. Some blocks require
voltages higher than the battery voltage,
and some require voltages that
are lower than the battery's. The battery
voltage may be 1.5 V, with the
digital blocks requiring a 1-V supply
and the analog blocks requiring a 3-V
supply. This is a real challenge for
battery-powered devices and motivates
the question of how to create a
dc voltage higher or lower than that
of the battery.
A circuit that can create a dc voltage
higher than that of a battery is
referred to as a step-up dc-dc converter,
and one that can create a dc
voltage lower than that of a battery
is known as a step-down dc-dc converter.
A switched-mode step-up converter
is called a boost converter,
Digital Object Identifier 10.1109/MSSC.2021.3110306
Date of current version: 17 November 2021
12
and a switched-mode step-down converter
is a buck converter. There are
other types of switched-mode dc-dc
converters, such as the buck-boost
converter, which is able to create a
step-up or step-down dc voltage of
opposite polarity (e.g., creating -2 V
from a 1.5-V battery), and the noninverting
flyback converter, where
a single circuit creates both a stepdown
and a step-up dc voltage of the
same polarity (e.g., creating either
2 or 1 V from a 1.5-V battery) [1]. In
this article, we focus on the basics of
buck and boost converters.
Buck Converter
Let us begin with the simplest stepdown
converter that we are all familiar
with. It is a simple resistive divider,
shown in Figure 1(a), formed by a
resistor R in series with the load
resistor
R .L
If we choose a proper
value for R, we can effectively create
any voltage at the output between
0 V and the battery voltage
V .B
For
example, to deliver 0.5 V from a 1.5-V
battery to a functional block whose
input resistance is
choose R 2kΩ=
1k ,Ω we need to
to form a step-down
factor of /. /.
13().
=05 15 The resistive
divider is a simple solution and
a common method today to produce
a step-down voltage. However, it has
two major issues. First, if the load
current changes as a function of time
(e.g., in periods of higher or lower
activities), a fixed R will result in a
time-varying voltage across the load,
deviating from the nominal dc level.
Therefore, we must develop a mechanism
to adjust R as the load current
changes with time.
FALL 2021
IEEE SOLID-STATE CIRCUITS MAGAZINE
(b)
Vout
VB
L1
L2
(c)
FIGURE 1: Three attempts at designing a
step-down converter: (a) using R to form a
resistive divider with the load, (b) using a
capacitive divider, and (c) using an inductive
divider.
RL
(a)
Vout
VB
C1
C RL
2
R
VB
RL
stant, say R 1kΩL
tor ()R 2kΩ=
=
Second, even when the load is con,
the added resisconsumes
twice the
power than the load does. In other
words, only one-third of the battery
power is delivered to the load, and
the remaining two-thirds are wasted
in R. The ratio of the power delivered
to the load to the power generated
by the battery is known as the power
efficiency of the converter. In our
example, the power efficiency of this
simple dc-dc converter is equal to the
step-down ratio. If we were to produce
0.1 V from a 1.5-V battery, we would
have a power efficiency of 6.6%.
We can correctly attribute this
wasted power (low efficiency) to the
use of resistors in a voltage divider.
Perhaps if we use capacitors, as in
Vout
IEEE Solid-States Circuits Magazine - Fall 2021

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