Heatsinks are devices which dissipate heat generated in some components in circuitary by exchanging
the heat from the component to atmosphere. The heat transfer is accomplished by three physical phenomena, viz; Conductions, Conviction and Radiation.
Heatsinks are essential for maintaining temperature control of semiconductors such as transistors, rectifiers etc.. These components contain a critical region called a junction, which cannot exceed a certain temperature without malfunction or destruction of the semiconductor. The maximum junction
temperature is specified by the manufacturer of semiconductor. A proper choice of heatsink ensures the safety of the semiconductor device by keeping the junction temperature below the maximum
The guidelines for the selection of proper heatsink are
1) Surface Area and (2) Thermal Resistance of the heatsink.
1] Surface Area : The surface area or area exposed to atmosphere is made to a maximum for particular particular design considerations. Theoratically 1800 Sq.mm. of totally exposed aluminium is
sufficient to dissipate one watt of heat generated without appreciable change in temperature. This can
be used as a rough guideline keeping in mind the actual design of the heatsink and actual area
exposed to atmosphere. Closely spaced fins from heat pockets, thus obstruction convection flow
They also adversely affect heat transfer by radiation
2] Thermal Resistance : This is more reliable base, if certain set of conditions are maintained Thermal
resistance of a heatsink can be obtained by, Qr = (Ts-Ta)/P
Where Qr is the thermal resistance, Ts is temperature of the hottest point on the heatsink (nearest to
the mounted semiconductors), Ta is the ambient temperature and P is power applied to the heat
generating semiconductor. Thus lower the value of thermal resistance, the more effective is the
There are other factors which affect the efficiency of the heat transfer, which could be listed below;
1] Mounting of Heatsink : Mounting should help expose as much area as possible with fins vertical and away from other heat generating components (Transformers etc.)
2] Surface Treatment : Black anodised heatsinks are known to increase the efficiency by atleast 25% in case of natural convection. Painted surface affects the efficiency adversely. Whereas, heatsink surface can be finished in Black, Natural Anodizing or could be chromotised
3] Contact Resistance (Thermal) : Between the semiconductor case and heatsink. Proper 25% in case of natural convection. Painted surface affects the efficiency adversely. Whereas, contact resistance considerably thus increasing the efficiency.
4] Ambient Temperature & Air Movement : Higher atmospheric temperture naturally decreases heat transfer efficiency. Similarly adequate air movement or forced convection by blowing air over heatsink or sucking hot air from heatsink improves heat transfer abilities to great extend