Manufacturers of high power semiconductor devices offer power modules, which are easy to use to build a power electronics converter. Power modules offer two or more devices typically of the same type interconnected in a certain way to facilitate building a given converter topology. The power modules also naturally have increased voltage and/or current ratings due to internal series and/or parallel connection of several semiconductors. Converter legs for a single-phase and three-phase converter are also available and in many cases a modular converter including not only the three-phase inverter legs, but also the front-end three-phase diode rectifier and other integrated components.
Many other components must be used to make a converter topology function properly to shape the voltage and current supplied from the source to the ones required by the load in a regulated manner and in many cases allow the power flow 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, ceramic, 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.
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, information and display boards to name a few.
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 Tj determines its reliability performance. 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 convection. 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.
λ is the thermal conductivity of the material in [W/m . °C]
T1,T2 are the temperatures in [°C]
A is the surface area in [m2]
d is the length in [m]
The energy flow per unit time by convection is
α is the convection coefficient
A is the surface area in [m2]
T2, T2 are the temperatures in [°C]
Finally, the energy flow per unit time by radiation is
S is the Stefan-Boltzmann constant
E is the emissivity of the material
A is the area in [m2]
T1, T2 are the temperatures in [°K]
In all previously mentioned cases, the heat transfer is dependent upon the surface area of the object. To increase the surface area, a heat sink is used to mount the device. The heat generated in the device is transferred first from the semiconductor to the heat sink and then to the ambient air if no other means are provided. In heat sinks all modes of heat transfer exist, namely, conduction between the semiconductor, and the heat sink, convection between the heat sink and the air, and radiation from the heat sink and semiconductor to the air. The efficiency of the transfer mode also depends upon the medium used for cooling when forced mechanisms are used. To improve the heat transfer due to conduction, the contact pressure between the semiconductor and the heat sink surface may be increased and conductive grease or soft thermal padding may also be used.
However, in most cases of low power electronic equipment a fan is placed at the bottom of the enclosure and slotted openings are provided to allow circulation of the air. In converters of significant power level and when the power-to-weight-ratio is very high, other means of forced cooling are used. For instance, oil similar to the one used in transformers is used to remove the heat from the converter.
In many cases water forced through hollow pipes is used as a cooling means. The heat-pipe coolers are composed of
- An aluminium base with elements clamped to conduct heat.
- Cooling plates composed of copper or aluminium with surfaces.
- Heat pipes that provide the thermal link between the aluminium base and the plates.
Another kind of heat pipe may include insulation between the evaporator and the condenser allowing water as a coolant or water and glycol and not necessarily chlorofluorocarbons (CFCs).
Figure 5.13 shows some heat sinks with power semiconductors mounted on them to illustrate the previously discussed points.
Fig. 5.12 Heat sinks: (a) air cooled extruded heat sink (velocity = 5 m/s, Rsa = 0.2 C/W, weight = 2.6 lb/ft); (b) air cooled fabricated heat sink (velocity = 5 m/s, Rsa = 0.02 C/W, weight = 21 lb/ft); ant (c) liquid cooled heat sink (flow rate = 5 lt/s, Rsa = 0.002 cm, weight = 4.5 lb/ft). (Courtesy of R-Theta, Inc., Mississauga, Ontario, Canada.)
The layout of the converter is very important as high voltage and current are switched at high frequencies. This generates a great deal of EMI and voltage spikes, and care should be taken to minimize the inductance so that the electric noise effect is also kept to the minimum. Of course the worst-case scenario would be wrong triggering of a device that may result in short circuit, which will probably cause destruction of the system.
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