Page 234 - Rashid, Power Electronics Handbook

13 DC-DC Converters 223

It can be seen that this transfer function has two poles and one voltage or current at a different level than the input source.
zero. The zero is due to the ®lter capacitor ESR. Buck-derived This dc transformation is performed by electronic switching
converters can easily be compensated for stability with second- means, not by electromagnetic means such as in conventional
order controllers. transformers. The output voltages of dc-dc converters range
The control-to-output transfer function of the boost from one volt for special VLSI circuits to tens of kilovolts in X-
converter is given by ray lamps. The most common output voltages are: 3.3 V for
modern microprocessors; 5 and 12 V for logic circuits; 48 V for
V r telecommunication equipment; and 270 V for main dc bus on
O C
T ðsÞ¼ÿ
p airplanes. Typical input voltages include 48 V, 170 V (the peak
ð1 ÿ DÞðR þ r Þ
C
2 value of a 120-V rms line), and 270 V.
ðs þ 1=Cr Þðs ÿð1 ÿ DÞ R=LÞ
C
2 2 : Selection of a topology of dc-dc converters is determined
s þ sðð1 ÿ DÞ CRr þ LÞ=LCðR þ r Þ not only by input=output voltages, which can be additionally
C
C
2
þð1 ÿ DÞ R=LCðR þ r Þ adjusted with the turns ratio in isolated converters, but also by
C
ð13:41Þ power levels, voltage and current stresses of semiconductor
switches, and utilization of magnetic components. The low
2
The zero ÿð1 ÿ DÞ R=L is located in the right-half of the s- part-count ¯yback converter is popular in low power applica-
plane Therefore, the boost converter (as well as buck-boost tions (up to 200 W). Its main de®ciencies are a large size of the
and ¯yback converters) is a nonminimum phase system. ¯yback transformer core and high voltage stress on the
Nonminimum phase dc-dc converters are typically compen- semiconductor switch. The forward converter is also a single
sated with third-order controllers. Step-by-step procedures for switch converter. Because its core size requirements are smal-
the design of compensating networks are usually given by ler, it is popular in low-medium- (up to several hundreds of
manufacturers of ASIC controllers in application notes. watts) power applications. Disadvantages of the forward
This section ends with a word on the behavior of dc-dc converter are the need for demagnetizing winding, and a
converters in distributed power supply systems. An important high voltage stress on the semiconductor switch. The push-
feature of closed-loop regulated dc-dc converters is that they pull converter is also used at medium-power levels. Due to
exhibit a negative input resistance. As the load voltage is kept bidirectional excitation, the transformer size is small. An
constant by the controller, the output power changes with the advantage of the push-pull converter is also a possibility to
load. With slow load changes, an increase (decrease) in the refer driving terminals of both switches to the ground, which
input voltage results in a decrease (increase) in the input greatly simpli®es the control circuitry. A disadvantage of the
power. This negative resistance property must be carefully push-pull converter is a potential core saturation in the case of
examined during the system design to avoid resonances. asymmetry. The half-bridge converter has a similar range of
applications as the push-pull converter. There is no danger of
transformer saturation in the half-bridge converter. It requires,
13.10 Applications of DC-DC Converters however, two additional input capacitors to split in half the
input dc source. The full-bridge converter is used at high
Step-down choppers ®nd most of their applications in high- (several kilowatts) power and voltage levels. The voltage stress
performance dc drive systems, for example, electric traction, on power switches is limited to the input voltage source value.
electric vehicles, and machine tools. The dc motors with their A disadvantage of the full-bridge converter is a high number of
winding inductances and mechanical inertia act as ®lters semiconductor devices.
resulting in high-quality armature currents. The average The dc-dc converters are building blocks of distributed
output voltage of step-down choppers is a linear function of power supply systems in which a common dc bus voltage is
the switch duty ratio. Step-up choppers are used primarily in converted to various other voltages according to requirements
radar and ignition systems. The dc choppers can be modi®ed of particular loads. Such distributed dc systems are common
for two-quadrant and four-quadrant operation. Two-quadrant in space stations, ships and airplanes, as well as in computer
choppers may be a part of autonomous power supply systems and telecommunication equipment. It is expected that modern
that contain battery packs and such renewable dc sources as portable wireless communication and signal processing
photovoltaic arrays, fuel cells, or wind turbines. Four-quad- systems will use variable supply voltages to minimize power
rant choppers are applied in drives in which regenerative consumption and to extend battery life. Low-output voltage
breaking of dc motors is desired, for example, transportation converters in these applications utilize the synchronous recti-
systems with frequent stops. The dc choppers with inductive ®cation arrangement.
outputs serve as inputs to current-driven inverters. Another major area of dc-dc converter applications is
The addition of ®ltering reactive components to dc chop- related to the utility ac grid. For critical loads, if the utility
pers results in PWM dc-dc converters. The dc-dc converters grid fails, there must be a backup source of energy, for
can be viewed as dc transformers that deliver to the load a dc example, a battery pack. This need for continuous power