Page 180 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH005.3D ± 168 ± [153±176/24] 17.11.2001 10:15AM

168 Power semiconductor devices and converter hardware issues

to be bidirectional. Components such as inductors, capacitors and resistors must be used
not only as part of protection devices in the case of snubbers, but also as filter elements.
Different technologies are available depending upon the power level and the
function of the component. For instance, electrolytic, paper, paper±film, film, cer-
amic, mica, aluminium electrolytic, and oil filled capacitors are widely used in power
electronics systems.
In the case of resistors, carbon composition, metal film, low voltage resistors, high
voltage resistors, wire-wound resistors, and resistance wire materials are used in
various ways and cases.

5.5 Ancillary equipment

A number of ancillary equipment is also used to build power electronics systems.
These include support equipment as many of the components of the system are quite
heavy. Also included are cabinets, copper bars, heat sinks, drive and control circuits
as appropriate, isolation equipment, protection systems, diagnostics, fuses, informa-
tion and display boards to name a few.

5.6 Cooling systems

Power losses associated with the operation of the semiconductors reduce their
thermal capacity. High temperatures of the wafers drastically reduce the electrical
characteristics of the devices, namely their maximum blocking voltage, switching
times, etc. In order to increase the life expectancy and the reliability of power
electronic equipment, adequate cooling means must be provided. Needless to say
that overheating may cause total destruction of the device and the converter at large.
The temperature of the semiconductor junction T j determines its reliability per-
formance. Its maximum allowable value is specified by the manufacturer in the data
sheets. It is therefore necessary to keep this temperature within a certain limit and for
that reason, depending upon the application, a number of cooling mechanisms are
available to the design engineer.
There exist three different mechanisms of heat transfer as follows:
. Conduction. The mode of heat transfer in solids or fluids, that are in conduct with
one another, and heat can be transferred from the warm object to the cooler one.
. Convection. The mode of heat transfer between a solid object and the surrounding
air. These mechanisms can be further divided into two subcategories, namely the
natural convection and the forced one. The first one occurs naturally when a
cooler non-moving air surrounds a warm object. The second one occurs when
the air flow around the warm object is forced by a fan or other mechanical means.
This method is more efficient and faster when compared with the natural convec-
tion. Of course other means such as liquid, i.e. oil or water can be used to remove
heat from a given object.
. Radiation. The mode of heat transfer due to electromagnetic emission when a
transparent medium surrounds a warm object.