Page 21 - Rashid, Power Electronics Handbook

P. Krein
44 P . Krein
video display or peripheral devices. Only a switched-mode EXAMPLE 1.3. Consider the circuit shown in Fig. 1.4. It
supply can support such complex requirements without high contains an ac source, a switch, and a resistive load. It is a
costs. The bulk and weight of linear supplies make them simple but complete power electronic system. Let us
infeasible for hand-held communication devices, calculators, assign a (somewhat arbitrary) control scheme to the
notebook computers, and similar equipment. Switched-mode switch. What if the switch is turned on whenever
supplies often take advantage of MOSFET semiconductor V > 0, and turned off otherwise? The input and out-
ac
technology. Trends toward high reliability, low cost, and put voltage waveforms are shown in Fig. 1.5. The input
miniaturization have reached the point at which a 5 V power has a time average of 0, and root mean square (RMS)
p
supply sold today might last 1,000,000 hr (more than 100 yr), value equal to V peak = 2, where V peak is the maximum
3
provide 100 W of output in a package with volume <15 cm , value of V . The output has a nonzero average value
ac
and sell for a price of <$0:30 watt. This type of supply brings given by:
an interesting dilemma: the ac line cord to plug it in actually
takes up more space than the power supply itself. Innovative 1 ð p=2 ð 3p=2
concepts such as integrating a power supply within a connec- hv out ðtÞi ¼ 2p V peak cos y dy þ 0 dy
tion cable will be used in the future. ÿp=2 p=2
Device technology for power supplies is being driven by V peak
¼ ¼ 0:3183V peak ð1:1Þ
expanding needs in the automotive and telecommunications p
industries as well as in markets for portable equipment. The
automotive industry is making a transition to 42 V systems to and an RMS value equal to V =2. Since the output has
peak
handle increasing electric power needs. Power conversion for nonzero dc voltage content, the circuit can be used as an
this industry must be cost effective, yet rugged enough to ac-dc converter. To make it more useful, a lowpass ®lter
survive the high vibration and wide temperature range to would be added between the output and the load to
which a passenger car is exposed. Global communication is smooth out the ac portion. This ®lter needs to be
possible only when sophisticated equipment can be used lossless, and will be constructed from only inductors
almost anywhere. This brings a special challenge, because and capacitors.
electrical supplies are neither reliable nor consistent through-
The circuit in the preceding Example acts as a half-wave
out much of the world. While in North America voltage
recti®er with a resistive load. With the hypothesized switch
swings in the domestic ac supply are often < 5% around
action, a diode can be substituted for the ideal switch. The
a nominal value, in many developing nations the swing can be
example con®rms that a simple switching circuit can perform
25% Ð when power is available. Power converters for
power conversion functions. However, notice that a diode is
communications equipment must tolerate these swings, and
must also be able to make use of a wide range of possible not, in general, the same as an ideal switch. A diode places
backup sources. Given the enormous size of worldwide restrictions on the current direction, while a true switch would
not. An ideal switch allows control over whether it is on or off,
markets for telephones and consumer electronics, there is a
while a diode's operation is constrained by circuit variables.
clear need for ¯exible-source equipment. Designers are
Consider a second half-wave circuit, now with a series L-R
challenged to obtain maximum performance from small
load, as shown in Fig. 1.6.
batteries, and to create equipment with minimal energy
requirements. EXAMPLE 1.4. A series diode-L-R circuit has ac voltage-
source input. This circuit operates much differently than
the half-wave recti®er with resistive load. A diode will be
1.4 Conversion Examples on if forward biased, and off if reverse biased. In this
circuit, an off diode will give a current of zero. When the
1.4.1 Single-Switch Circuits diode is on, the circuit is the ac source with L-R load. Let

Electrical energy sources can come in the form of dc voltage
sources at various values, sinusoidal ac sources, polyphase
sources, and many others. A power electronic circuit might be
asked to transfer energy between two different dc voltage
levels, between an ac source and a dc load, or between sources
at different frequencies. It might be used to adjust an output
voltage or power level, drive a nonlinear load, or control a load
current. In this section, we consider a few basic converter
arrangements and discuss energy conservation as a tool for FIGURE 1.4 A simple power electronic system. (From Reference [2],
analysis. copyright # 1998, Oxford University Press, Inc.; used by permission.)