Note that when the ambient temperature is 25☌ the case temperature is likely much higher, so the maximum power rating must be lower. The power dissipated in the transistor is the voltage drop across the collector emitter junction times the collector current (neglecting the base current times. In your example, the maximum power dissipation when T C = 25☌ is 3.0 W and the maximum power dissipation when T A = 25☌ is 0.8 W. (b) For which value of RL, the power diss transistor is maximum What is the dissipation then 2. However, in actual use, it is very difficult to make the package surface temperature exactly 25☌, and if you also take derating into account, the allowable power will in fact be somewhere in between T A = 25☌ and T C = 25☌. Note that the ratings may include the note "with infinite heat sink". A bipolar junction transistor ( BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. ![]() So the power dissipation is the power dissipation across the collector and emitter. I'm taking a class in electronics and have the following equation in my notes (not sure if it helps): P PCE +PBE + Pbaseresistor P P C E + P B E + P b a s e r e s i s t o r. The output characteristics of a BJT with Fo 25 and VA 8 are shown in figure 2. I'm not sure how to calculate the power dissipation across the transistor. ![]() The C, B, and E terminals can be identified in figure 1. The specification at T C = 25☌ in the power ratings refers to the total power dissipation when the semiconductor element (case) itself has been forcibly cooled, i.e., when temperature of the package surface is kept at 25☌. We will use 2N3904, a general purpose npn BJT with a maximum working current of 200 mA and a maximum power dissipation of 625 mW. At any given time, the power dissipated by a transistor is equal to the product (multiplication) of collector current and collector-emitter voltage. ![]() In this case, the thermal resistance from the heat source to the ambient air is expressed as R th(j-a). The specification at T A = 25☌ in the power ratings refers to the total power dissipation of a discrete semiconductor element in an environment with an ambient temperature of 25☌. Normally transistor power dissipation is quoted for T A and T C.
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