• A heat sink is a passive heat ex-changer that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature at optimal levels.

• Heat sinks are used with high-power semiconductor devices such as power transistors and opto-electronics such as lasers and light emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature.

• In case of transistor or power transistors if the temperature of collector base junction increases due to increase in temperature, the collector leakage current increases. Due to this the collector current increases.

• The increase in collector current produces an increase in the power dissipated at the collector junction. This, in turn, further increases the temperature of the junction and so gives further increase in collector current. The process is cumulative. This action of increase in current and further increase in temperature eventually destroys the transistor. This is known as thermal runaway.

• To prevent thermal runaway,the power handling capacity can be increased if suitable provision for rapid conduction of heat away from junction. This is achieved by using Heat Sink.

• Power transistors are mounted on the heat sink, which are large metal pieces of different shapes. When heat sink is used, it quickly transfers heat produced by transistors into air due the large area of the heat sink.

• There is no electric contact between the transistor and heat sink but heat can easily follow between the transistor case to heat sink.

• For maximum efficiency, a heat sink should be in good thermal contact with the transistor with the transistor case, have the largest possible surface area, painted black, and mounted in a position such that free air can flow past it.

• Even if large heat sink is used, the case temperature cannot be held at the ambient temperature, and derating to certain extent has to be done. Therefore the derating curves provided by manufacturer.

The heat sink is as shown in fiugre

:

The relationship between thermal and electrical analogy:

Thermal Resistance: It is observed that steady state temperature rise at the junction of the transistor is proportional to the power dissipation at the junction. This means,

∆T is directly proportional to PD. Where ∆T is the temperature difference.

Therefore, ∆T = Ө PD where Ө is the constant of proportionality and it is known as thermal resistance. Higher value of thermal resistance indicates low conductivity of heat

Lowering the value of the thermal resistance will improve the conduction of heat. Therefore we can have the following analogy: