As power transistors handle large currents, they always heat up during operation. Since transistor is a temperature dependent device, the heat generated
must be dissipated to the surroundings in order to
keep the temperature within permissible limits.
Generally, the transistor is fixed on a metal sheet (usually aluminium) so that additional heat is transfer to the Al sheet.
The metal sheet that serves to dissipate the additional heat from the power transistor is know as heat sink.
Most of the heat within the transistor is produce at the **collector junction. The heat sink increases the surface area and allows heat to escape from the
collector junction easily. The result is that temperature of the transistor is sufficiently lower.
Thus heat sink is a direct practical means of combating the undesirable thermal effects e.g. thermal runaway.
*Almost the entire heat in a transistor is produce at the collector-base junction. If the temperature exceeds
the permissible limit, this junction is destroy and the transistor is render useless.
** Most of power is dissipate at the collector-base junction. This is because collector-base voltage is much
greater than the base-emitter voltage, although currents through the two junctions are almost the same.
It may be note that the ability of any heat sink to transfer heat to the surroundings depends upon its
material, volume, area, shape, contact between case and sink and movement of air around the sink.
Finned aluminium heat sinks yield the best heat transfer per unit cost.
It should be realise that the use of heat sink alone may not be sufficient to prevent thermal runaway under all conditions. In designing a transistor circuit, consideration should also be give to the choice of (i) operating point (ii) ambient temperatures which are likely to be encounter and (iii)
the type of transistor e.g. metal case transistors are more readily cool by conduction than plastic
ones. Circuits may also be design to compensate automatically for temperature changes and thus
stabilise the operation of the transistor components.
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