• A heat sink is a passive heat exchanger 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.

• To design a heat sink for particular application we need to perform following procedure:

• Generally the manufacturer specification, thermal resistance from case to air i.e ӨCA is specified.

• From the thermal equivalent circuit we can write that,

ӨCA= ӨCS+ӨSA

Or ӨCA = ӨJA - ӨJC

Thermal equivalent circuit and analogy between electrical and thermal consideration is as shown in the figure:

ӨJA = total thermal resistance (junction to ambient)

ӨJC = transistor thermal resistance (junction to case)

ӨCS = insulator thermal resistance (case to heat sink)

ӨSA = heat sink thermal resistance (heat sink to ambient)

TA = Ambient Temp.

TJ = Junction Temp

• The junction to ambient thermal resistance is given by,

ӨJA = ӨJC +ӨCS+ӨSA

The power dissipation PD is given by,

PD = VCE . IC

But ӨJA = TJ –TA /PD

• If the maximum junction temperature i.e. TJ max is known and TA is known then we can calculate the value of ӨJA

• If the of value ӨJC for transistor is specified, then we can obtain the value of ӨCA required.

• Proper heat sink which has a ӨCA less than this value is then selected.