As costs of electronics decline, the
power supply becomes a larger fraction of system cost and design
effort. One major manufacturer estimates that power supply cost will
soon reach 50% of the total cost of a typical electronic product such
as a cordless telephone or personal computer. Thus, new technology
developments in power supplies are critically important.
In the past, bulky linear power
supplies were designed with transformers and rectifiers from the ac
line frequency to provide low-level dc voltages for electronic
circuits. Late in the 1960s, use of dc sources in aerospace
applications led to the development of power electronic dc-dc
conversion circuits for power supplies.
In a typical power electronics
arrangement today, an ac source from a wall outlet is rectified
without any transformation; the resulting high dc voltage is
converted through a dc-dc circuit to the 5V, 12 V, or other level
required.These switched-mode power supplies are rapidly supplanting
linear supplies across the full spectrum of circuit applications.
A personal computer commonly requires
three different 5V supplies, two ‡12 V supplies, a ΓΏ12 V supply, a
24 V supply, and perhaps a few more. This does not include supplies
for video display or peripheral devices. Only a switched-mode supply
can support such complex requirements without high costs.
The bulk and weight of linear supplies
make them infeasible for hand-held communication devices,
calculators, notebook computers, and similar equipment. Switched-mode
supplies often take advantage of MOSFET semiconductor technology.
Trends toward high reliability, low
cost, and miniaturization have reached the point at which a 5 V power
supply sold today might last 1,000,000 hr (more than 100 yr), provide
100Wof output in a package with volume <15 cm3, and sell for a
price of <$0:30 watt.
This type of supply brings an
interesting dilemma: the ac line cord to plug it in actually takes up
more space than the power supply itself. Innovative concepts such as
integrating a power supply within a connection cable will be used in
the future.
Device technology for power supplies is
being driven by expanding needs in the automotive and
telecommunications industries as well as in markets for portable
equipment. The automotive industry is making a transition to 42 V
systems to handle increasing electric power needs.
Power conversion for this industry must
be cost effective, yet rugged enough to survive the high vibration
and wide temperature range to which a passenger car is exposed.
Global communication is possible only when sophisticated equipment
can be used almost anywhere.
This brings a special challenge,
because electrical supplies are neither reliable nor consistent
throughout much of the world. While in North America voltage swings
in the domestic ac supply are often <5 % around a nominal value,
in many developing nations the swing can be #25%—when power is
available.
Power converters for communications
equipment must tolerate these swings, and must also be able to make
use of a wide range of possible backup sources. Given the enormous
size of worldwide markets for telephones and consumer electronics,
there is a clear need for flexible-source equipment.
Designers are challenged to obtain
maximum performance from small batteries, and to create equipment
with minimal energy requirements.
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