Page 19 - Rashid, Power Electronics Handbook

P. Krein
22 P . Krein
system. To put this in perspective, consider that a typical
American household loses electric power only a few minutes a
year. Therefore, energy is available 99.999% of the time. A
converter must be even better than this if system degradation
is to be prevented. An ideal converter implementation will not
suffer any failures over its application lifetime. In many cases,
extremely high reliability can be a more dif®cult objective than
that of high ef®ciency.

1.2.1 The Efficiency Objective: The Switch
A circuit element as simple as a light switch reminds us that
the extreme requirements in power electronics are not espe-
cially novel. Ideally, when a switch is on, it has zero voltage
FIGURE 1.1 Control, energy, and power electronics are interrelated. drop and will carry any current imposed on it. When a switch
is off, it blocks the ¯ow of current regardless of the voltage
across it. Device power, the product of the switch voltage and
‘‘applications-driven'' ®eld. This transition has been based on
current, is identically zero at all times. The switch controls
two factors: advanced semiconductors with suitable power
energy ¯ow with no loss. In addition, reliability is also high.
ratings exist for almost every application of wide interest; and Household light switches perform over decades of use and
the general push toward miniaturization is bringing advanced
perhaps 100,000 operations. Of course, a mechanical light
power electronics into a growing variety of products.
switch does not meet all practical needs. A switch in a power
supply often must function 100,000 times each second. Since
1.2 Key Characteristics even the best mechanical switch will not last beyond a few
million cycles, semiconductor switches (without this limita-
All power electronic circuits manage the ¯ow of electrical tion) are the devices of choice in power converters.
energy between some sort of source and a load. The parts in a A circuit built from ideal switches will be lossless. As a
circuit must direct electrical ¯ows, not impede them. A general result, switches are the main components of power converters,
power conversion system is shown in Fig. 1.2. The function of and many people equate power electronics with the study of
the power converter positioned at the middle is that of switching power converters. Magnetic transformers and loss-
controlling energy ¯ow between a given electrical source and less storage elements such as capacitors and inductors are also
a given load. For our purposes, the power converter will be valid candidates for use in power converters. The complete
implemented with a power electronic circuit. As a power concept, shown in Fig. 1.3, illustrates a power electronic system.
converter appears between a source and a load, any energy Such a system consists of an energy source, an electrical load, a
used within the converter is lost to the overall system. A power electronic circuit, and control functions. The power
crucial point emerges Ð to build a power converter, we should electronic circuit contains switches, lossless energy storage
consider only lossless components. A realistic converter design elements, and magnetic transformers. The controls take infor-
must approach 100% ef®ciency. mation from the source, load, and designer and then deter-
A power converter connected between a source and a load mine how the switches operate to achieve the desired
also affects system reliability. If the energy source is perfectly conversion. The controls are usually built up with conven-
reliable (it is on all the time), then a failure in the converter tional low-power analog and digital electronics.
affects the user (the load) just as if the energy source had
failed. An unreliable power converter creates an unreliable

FIGURE 1.2 General system for electric power conversion. (From
Reference [2], copyright # 1998, Oxford University Press, Inc.: used by FIGURE 1.3 A basic power electronic system. (From Reference [2],
permission.) copyright # 1998, Oxford University Press, Inc.; used by permission.)