Page 11 - Power Electronics Handbook

4 Power semiconductor devices

(iii) Transportation applications, the largest being motor drives for areas
such as electric vehicles, locomotives, and fork-lift trucks. Equally
important are non-motor drive applications, such as traffic signal
control, vehicle electronic ignition and vehicle voltage regulation.
(iv) Aerospace and defence applications include VLF transmitters;
power supplies for space and aircraft; and switching using solid state
relays and contactors.

1.1.3 Power semiconductor operation
This chapter describes the construction and characteristics of several types
of power semiconductor devices. These can generally be operated in
different modes, due either to their construction or the application in
which they are used. Four operating modes are considered in this chapter:
One way of differentiating devices is whether they are capable of
being controlled, regarding their turn-on point. The power rectifier
cannot be so controlled, since it will conduct as soon as the voltage at
its anode is more positive than that at its cathode. All other power
devices described in this chapter, such as the transistor, thyristor,
gate turn-off switch and triac, can be turned on (and sometimes off)
by a control signal on an auxiliary input.
Some power devices can also be operated in a linear or a switching
mode. The transistor is the only component described here which is
capable of linear operation, so it is the obvious choice for this
application. Losses, caused by the product of current through the
device and voltage drop across it, are much higher when in the linear
mode. Switching devices can handle greater power, since their
dissipation is lower. Their power gain is also generally higher, so that
they need less drive current to control their operation.
The third operating mode is the type of signal required to control the
power semiconductor devices. Generally, this consists of an
electrical signal, although in a large class of devices optical energy is
used.
Finally, the voltage and current capability of power devices can be
considerably increased by operating several of them in series or in
parallel mode, so that the total voltage and current are shared across
several devices.
1.1.4 Device characteristics
Many of the power semiconductor devices described here have special
characteristics. There are, however, also many similarities, such as:
(i) The voltage drop across the device when it is carrying current.
(ii) The capability of the device to handle current. Both the steady state
current and the peak or overload current-carrying ability are
important, since overload capabilities often determine the need for
protection.
(iii) The capability of the device to block voltage, both in the direction
reverse to that in which it normally conducts, and in the conduction
direction, when it has not yet been turned on.