Page 222 - Rashid, Power Electronics Handbook

13

DC-DC Converters

Dariusz Czarkowski, Ph.D. 13.1 Introduction ..................................................................................... 211
Department of Electrical and 13.2 DC Choppers.................................................................................... 212
Computer Engineering.
Polytechnic University, Six 13.3 Step-Down (Buck) Converter .............................................................. 213
Metrotech Center, Brooklyn, 13.3.1 Basic Converter 13.3.2 Transformer Versions of Buck Converter
NY 11201 USA 13.4 Step-Up (Boost) Converter ................................................................. 215
13.5 Buck-Boost Converter ........................................................................ 216
13.5.1 Basic Converter 13.5.2 Flyback Converter
Á
13.6 Cuk Converter .................................................................................. 218
13.7 Effects of Parasitics ............................................................................ 218
13.8 Synchronous and Bidirectional Converters............................................. 220
13.9 Control Principles.............................................................................. 221
13.10 Applications of DC-DC Converters ...................................................... 223
References ........................................................................................ 224

13.1 Introduction Therefore, high operating frequencies allow for achieving a
faster dynamic response to rapid changes in the load current
Modern electronic systems require high-quality, small, light- and=or the input voltage.
weight, reliable, and ef®cient power supplies. Linear power High-frequency electronic power processors are used in
regulators, whose principle of operation is based on a voltage dc-dc power conversion. The functions of dc-dc converters
or current divider, are inef®cient. This is because they are are:
limited to output voltages smaller than the input voltage, and
S
also their power density is low because they require low- to convert a dc input voltage V into a dc output voltage
V ;
O
frequency (50 or 60 Hz) line transformers and ®lters. Linear
to regulate the dc output voltage against load and line
regulators can, however, provide a very high-quality output
variations;
voltage. Their main area of application is at low power levels.
to reduce the ac voltage ripple on the dc output voltage
Electronic devices in linear regulators operate in their active
below the required level;
(linear) modes, but at higher power levels switching regula-
to provide isolation between the input source and the
tors are used. Switching regulators use power electronic
load (isolation is not always required);
semiconductor switches in on and off states. Because there
to protect the supplied system and the input source from
is a small power loss in those states (low voltage across a electromagnetic interference (EMI); and
switch in the on state, zero current through a switch in the off to satisfy various international and national safety stan-
state), switching regulators can achieve high energy conver-
dards.
sion ef®ciencies. Modern power electronic switches can
operate at high frequencies. The higher the operating The dc-dc converters can be divided into two main types:
frequency, the smaller and lighter the transformers, ®lter hard-switching pulsewidth modulated (PWM) converters, and
inductors, and capacitors. In addition, the dynamic charac- resonant and soft-switching converters. This chapter deals
teristics of converters improve with increasing operating with PWM dc-dc converters, which have been very popular
frequencies. The bandwidth of a control loop is usually for the last three decades, and that are widely used at all power
determined by the corner frequency of the output ®lter. levels. Topologies and properties of PWM converters are well
211
Copyright # 2001 by Academic Press.
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