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designed to increase the surface area in contact with the cooling medium surrounding it. Transistor Power Rating The temperature at the collector junction places
a limit on the allowable power
dissipation P . Depending on the transistor type. Power Transistor Heat Sinking what Push_on heat radiates more quickly because of the increased surface area of the fins. provides a path out of the transistor
This metal tab can be fastened to the chassis of electronics equipment.
Because the chassis is a massive heat sink, heat can easily escape from the
transistor to the chassis. why _ integrated circuits are used for small-signal and low-power applications, most
high-power applications still require individual power transistors.
A power transistor is just this transistor built to carry more power.
it can handle more than 1 Ampere of collector current.
_ Improvements in production
have provided higher power ratings in small sized packaging cases. note average power dissipated may be approximated by
P =V . I D D CE C Minimizing the problem of heat is approached in two main ways:
1. 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.
2. By ensuring that the heat produced by the
transistor can be removed and effectively
transferred to the surrounding air as quickly
as possible. Even with an infinite heat sink (which, of course, is not
available), for which the case temperature is held at the ambient (air) temperature, the junction will be heated above the case temperature and a maximum power rating must be considered. types of heat sinks :- the collector connected directly
to the case to let heat escape
as easily as possible. continue _ Heat-sinks take many different forms, such as finned aluminum or copper sheets or blocks.
_ It is necessary to maintain electrical insulation between transistor and heat-sink a mica layer is used between the heat-sink and transistor. Mica has excellent insulation and very good heat conducting properties.
_ Good physical contact between the transistor and
heat-sink is essential.
derating factor It is not necessary to provide a derating
curve since the same information could
be given simply as a listed derating factor. thermal analogy
= transistor thermal resistance (junction to case)
= insulator thermal resistance (case to heat sink)
= heat-sink thermal resistance (heat sink to ambient)
= total thermal resistance (junction to ambient) JA JC CS SA electric voltage difference VS thermal difference. Thermal resistance. applying Kirchhoff’s law to obtain The value of thermal resistance from junction to free air (using no heat sink) is
= 40°C/W (into free air) JA with using heat sink Example team members :- _ Amr Shrif.
_ Eman Ahmed.
_ Mahmoud El-Ghobashy .
_ Mahmoud Hosam Elsheikh.
_ Mohammed Khaled.
_ Samar Mohammed. Where , , Thermal-to-electrical analogy. = decrease in power rating.
= derating factor.