By operating the transistor in the most efficient way possible, that is by choosing a class of biasing that gives high efficiency and is least wasteful of power. Minimising the problem of heat is approached in two main ways: The performance of a power transistor is closely dependant on its ability to dissipate the heat generated at the collector base junction. Power De-ratingįig 5.1.2 Power de-rating Graph for the TIP31įor example, a transistor such as the TIP31 having a quoted maximum power output P TOT of 40W can only handle 40W of power IF the case temperature (slightly less than the junction temperature) is kept below 25☌.
The maximum power output available from a power transistor is closely linked to temperature, and above 25☌ falls in a linear manner to zero power output as the maximum permissible temperature is reached. If too much power is dissipated, this junction gets too hot and the transistor will be destroyed, a typical maximum temperature is between 100☌ and 150☌, although some devices can withstand higher maximum junction temperatures. The maximum power rating of a transistor is largely governed by the temperature of the collector/base junction as can be seen from the power de-rating graph in Fig. As most heat is generated at the collector/base junction, the area of this junction is made as large as possible. They must also be able to dissipate heat very quickly so they do not overheat.
They must have low output resistances so that they can deliver large currents to the load, and good junction insulation to withstand high voltages. 5.1.1 handle larger currents and higher voltages, they have a different construction to small signal devices. There is not a clear cut difference between ‘ordinary’ transistors used in voltage amplifiers and power transistors, but generally Power transistors can be categorised as those than can handle more than 1 Ampere of collector (or Drain in the case of FETs) current.īecause power transistors, such as those shown in Fig.
Understand the need to connect the collector and metal case.Recognise power transistor construction.After studying this section, you should be able to:.
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