Page 20 - Rashid, Power Electronics Handbook

1 Introduction 3

Switching devices are selected based on their power handling can lead to quick failure. The ®rst issue for reliability in power
rating Ð the product of their voltage and currents ratings Ð electronic circuits is that of managing device voltage, current,
rather than on power dissipation ratings. This is in contrast to and power dissipation levels to keep them well within rating
other applications of electronics, in which power dissipation limits. This can be challenging when power handling levels are
ratings dominate. For instance, a typical stereo receiver high.
performs a conversion from ac line input to audio output. The second issue for reliability is simplicity. It is well-
Most audio ampli®ers do not use the techniques of power established in military electronics that the more parts there
electronics, and the semiconductor devices do not act as are in a system, the more likely it is to fail. Power electronic
switches. A commercial 100 W ampli®er usually is designed circuits tend to have few parts, especially in the main energy
with transistors big enough to dissipate the full 100 W. The ¯ow paths. Necessary operations must be carried out through
semiconductor devices are used primarily to reconstruct audio shrewd use of these parts. Often, this means that sophisticated
information rather than to manipulate energy ¯ows. When the control strategies are applied to seemingly simple conversion
devices are used as switches instead, the power levels increase circuits.
considerably, as suggested by the following examples. The third issue for reliability is integration. One way to
avoid the reliability–complexity tradeoff is to integrate multi-
EXAMPLE 1.1. The 2N2222A is a popular bipolar tran-
ple components and functions on a single substrate. A micro-
sistor with a rated collector-emitter breakdown voltage
processor, for example, might contain more than a million
of 30 V, a maximum collector current of 0.8 A, and rated
gates. As all interconnections and signals ¯ow within a single
power dissipation of 0.5 W. In a conventional analog
chip, the reliability is nearly that of a single part. An important
circuit, it usually handles energy within its 0.5 W power
parallel trend in power electronic devices involves the inte-
dissipation rating. In principle, this device can manip-
grated module [6]. Manufacturers seek ways to package several
ulate the ¯ow of 0.8 A in a 30 V circuit and so a power
switching devices, with their interconnections and protection
electronics engineer would list its power handling rating
components together as a unit. Control circuits for converters
as 24 W. The ability to control up to 24 W, combined
are also integrated as much as possible to keep reliability high.
with good switching characteristics, makes this device
The package itself becomes a fourth issue for reliability, and
common as an auxiliary element in power supplies.
one that is as yet only partly understood. Semiconductor
EXAMPLE 1.2. The MTW20N50 is a metal-oxide-semi- packages include small bonding wires that can be susceptible
conductor ®eld-effect transistor (MOSFET) with a drain to thermal or vibration damage. The small geometries tend to
current rating of 20 A, a maximum drain-source break- enhance electromagnetic interference among the internal
down voltage of 500 V, and rated power dissipation of circuit components.
250 W. The power handling rating is 10 kW. Several
manufacturers have developed power electronic control-
lers for domestic refrigerators, air conditioners, and even 1.3 Trends in Power Supplies
electric vehicles based on this device and its relatives.
As costs of electronics decline, the power supply becomes a
The second part of the de®nition of power electronics in
larger fraction of system cost and design effort. One major
Section 1.1 points out that the circuits handle power at levels
manufacturer estimates that power supply cost will soon reach
much higher than that of the ratings of individual devices.
50% of the total cost of a typical electronic product such as a
In the ®rst Example, a 2N2222A might be used to handle
cordless telephone or personal computer. Thus, new technol-
24 W Ð as compared with its individual rating of 0.5 W. The
ogy developments in power supplies are critically important.
MTW20N50 is used to handle up to 10 kW, compared to its
In the past, bulky linear power supplies were designed with
rating of 250 W. These ratios, 48 and 40, respectively, are high,
transformers and recti®ers from the ac line frequency to
but not unusual in power electronics contexts. In contrast, the
provide low-level dc voltages for electronic circuits. Late in
same ratio in a conventional audio ampli®er is close to unity.
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
1.2.2 The Reliability Objective: Simplicity and
Integration today, an ac source from a wall outlet is recti®ed without any
transformation; the resulting high dc voltage is converted
High-power applications lead to interesting issues. For exam- through a dc-dc circuit to the 5 V, 12 V, or other level required.
ple, in an inverter the semiconductors often manipulate 40 These switched-mode power supplies are rapidly supplanting
times their rated power or more. A small design error, linear supplies across the full spectrum of circuit applications.
unexpected thermal problem, or minor change in layout A personal computer commonly requires three different 5 V
could alter this somewhat, perhaps to a factor of 45. This supplies, two þ12 V supplies, a ÿ12 V supply, a 24 V supply,
small change puts large additional stresses on the devices, and and perhaps a few more. This does not include supplies for