Page 90 - Rashid, Power Electronics Handbook

76 I. Batarseh

Today, switching devices are manufactured with amazing made between reduced size, weight and cost of compo-
power handling capabilities and switching speeds as will be nents versus reduced switching power dissipation,
shown later. The availability of different devices with different which means inexpensive low switching frequency
switching speeds, power handling capabilities, size, cost etc., devices.
makes it possible to cover many power electronics applica- 5. Reduced component and device reliability.
tions. As a result, trade-offs are made when it comes to
For more than 30 years, it has been shown that switching
selecting power devices.
(mechanical or electrical) is the best possible way to achieve
high ef®ciency. However, unlike mechanical switches, electro-
nic switches are far more superior because of their speed and
6.2 The Need for Switching in Power power handling capabilities as well as reliability.
Electronic Circuits We should note that the advantages of using switches do not
come without a cost. Because of the nature of switch currents
As already stated, the heart of any power electronic circuit is its and voltages (square waveforms), high-order harmonics are
semiconductor-switching network. The question arises here as normally generated in the system. To reduce these harmonics,
to whether we have to use switches to perform electrical power additional input and output ®lters are normally added to the
conversion from the source to the load. The answer, of course, system. Moreover, depending on the device type and power
is no, as there are many circuits that can perform energy electronic circuit topology used, driver circuit control and
conversion without switches, including linear regulators and circuit protection can signi®cantly increase both the complex-
power ampli®ers. However, the need to use semiconductor ity of the system and its cost.
devices to perform conversion functions is very much related
EXAMPLE 6.1. The purpose of this example is to inves-
to converter ef®ciency. In power electronic circuits, the semi-
tigate the ef®ciency of four different power circuits
conductor devices are generally operated as switches, that is,
whose functions are to take in power from 24-V dc
either in the on-state or the off-state. This is unlike the case for
source and deliver a 12-V dc output to a 6-O resistive
power ampli®ers and linear regulators where semiconductor
load. In other words, these circuits serve as a dc
devices operate in the linear mode. As a result, a very large
transformer with a ratio of 2 : 1. The four circuits
amount of energy is lost within the power circuit before the
shown in Fig. 6.1a,b,c,d represent the voltage divider
processed energy reaches the output. Semiconductor switching
circuit, Zener regulator, transistor linear regulator,
devices are used in power electronic circuits because of their
and switching circuit, respectively. The objective is to
ability to control and manipulate very large amounts of power
from the input to the output with a relatively very low power calculate the ef®ciency of these four power electronic
circuits.
dissipation in the switching device. Their use helps to create a
very highly ef®cient power electronic system. SOLUTION 6.1. Voltage divider dc regulator: The ®rst
Ef®ciency is considered an important ®gure of merit and circuit, the simplest, forms a voltage divider with
has signi®cant implications for overall system performance. R ¼ R ¼ 6 O and V ¼ 12 V. The ef®ciency de®ned
L o
Low ef®ciency power systems, large amounts of power are as the ratio of the average load power P to the average
L
dissipated in the form of heat, which results in one or more of input power, P
in
the following implications:
1. Cost of energy increases due to increased consump- P L
Z ¼ %
tion. P in
2. Additional design complications might be imposed, R L
¼ % ¼ 50%
especially regarding the design of device heat sinks. R þ R
L
3. Additional components such as heat sinks increase
cost, size and weight of the system, resulting in low-
In fact, ef®ciency is simply V =V in %. As the output
o
power density.
voltage becomes smaller, the ef®ciency decreases propor-
4. High-power dissipation forces the switch to operate at
tionally.
low switching frequency, resulting in limited band-
width, slow response, and most important, the size and Zener dc Regulator: Since the desired output is 12 V, we
weight of magnetic components (inductors and trans- select a Zener diode with Zener breakdown V ¼ 12 V.
Z
formers) and capacitors remain large. Therefore, it is Assume the Zener diode has the i-v characteristic shown
always desired to operate switches at very high in Fig. 6.1(e); as R ¼ 6 O, the load current I ,is2 A.
L
L
frequencies. However, we will show later that as the Then we calculate R for I ¼ 0:2 A (10% of the load
Z
switching frequency increases, the average switching current). This results in R ¼ 5:27 O. The input power is
power dissipation increases. Hence, a trade-off must be P ¼ 2:2A 24 V ¼ 52:8 W and the output power is
in