Mumbai University > Electronics Engineering > Sem 4 > Discrete Electronic Circuits

Marks: 5M

Year: May 2015, Dec 2015

• If temperature of the collector base junction increases, the collector leakage current $Icbo$ increases. Because of this, collector current increases.
• The increase in collector current produces an increase in the power dissipated at the collector junction. This, in turn, further increases the temperature of the junction and so gives further increase in collector current. The process is cumulative.
• It may lead to the eventual destruction of the transistor. This is described as the thermal runaway of the transistor.
• For transistors handling small signals, the power dissipated at the collector is small. Such transistors have less chance of thermal runaway. In power transistors, the power dissipated at the collector junction is larger. This may cause the junction temperature to rise to a dangerous level.
• The power handling capacity can be increased if suitable provision for rapid conduction of heat away from junction. This is achieved by using a sheet of metal called Heat Sink.
• As the power dissipated within a transistor is predominantly the power dissipated at its collector-base junction, sometimes the collector of the power transistor is connected to its metallic case.
• Connecting a heat sink to a transistor increases the area from which heat is to be transferred to the atmosphere. Heat moves from the transistor to the heat sink by conduction, and then it is removed from the sink to the ambient by convection and radiation.
• For maximum efficiency, a heat sink should be in good thermal contact with the transistor with the transistor case, have the largest possible surface area, painted black, and mounted in a position such that free air can flow past it.

Figure 1: Different kind of heat sinks with power transistors.

• The case of the transistor is then bolted on to a sheet of metal as shown in Figure 1(a). This sheet serves as the heat sink.
• Figure 1(b) consists of a push fit clip. This clip is pushed on to the transistor. To increase the surface area of the heat sink, it is usually given a ribbed structure. Because of this structure, the heat sink does not occupy much space within the equipment.