Universal collage of engineering and technology
Subject – Electronics device and circuits
Subject code - 2131006
Student names – 1. Prajapati Sachin 150463111014
2. Gajjar Umang 150463111005
Collector Power Dissipation
• Assume that a transistor is operating in its
active region.
• So a large voltage (Vce) appears across the
transistor in its on state and a large current
(IC) flows through it.
• Due to a large voltage appearing across and a
large current flow throw the power gets
dissipated in the power transistor.
Collector power dissipation
• The power dissipation in the transistor is given
by
Pd = Vcc * Ic
• This is also called as the collector power
dissipation.
• The power dissipated is measured in watts.
• The dissipated power increase the
temperature of the power transistor.
Symbol of the
collector power
dissipation
•More the value of power
dissipated higher is the
temperature of the device,
•If the device temperature
rises above a maximum
temperature Tj max specified
by the manufacture then the
device can be damaged.
•To avoid this the collector
power dissipation should not
be allowed to exceed beyond
its maximum permissible
value as specified by the
manufacturer.
•This can be achieved by
using a Heat sink.
Derating Factor
• The derating factor tells us about hoe much
reduction we need to do in the power rating
of the device (Pd (max)).
• With increase in temperature of the device
power derating essential. For example if the
derating factor of a transistor is 5 mW/C then
we have to reduce its power rating by 5 mW
for every degree C rise in temperature.
Heat Sink
• Heat sink are large metal pieces of different as
shown in fig.
• The power transistor are mounted on some
form of heat sink.
• This is due to large area of heat sink now
available for the radiation of heat.
• The transistor is mounted on the heat sink but
there is no electrical contact between the
transistor and the heat sink.
Different types of heat sink
Heat Sink
• But the heat can easily flow from the transistor
case to the heat sink.
• The heat sink are painted black because the black
coloured objects can radiate more heat as
compared to the objects of other colors.
• The heat sink are made from aluminum because
aluminum is a very good conductor of heat.
• So it can efficiently take away the heat from the
device mounted on it.