INTRODUCTION

The function of a heatsink is to increase the surface

area available for the transfer of heat from a

component or device thereby increasing the amount of

heat that can be dissipated.

The following products/services are available:

1 A range of Standard heatsinks and accessories

which permit optimum flexibility in the design of

Electrical and Electronic equipment requiring

heatsink applications.

2 Custom designed heatsinks. We have facilities

available which enable us to provide design and

manufacturing of custom and specialised heatsinks

to your requirements.

The main factors to consider when selecting a

heatsink are:-

1 Geometry

2 Thermal Resistance

3 Cost

Defining the necessary heatsink performance.

In order to calculate the maximum acceptable thermal

resistance for the heatsink so that the device being

cooled does not overheat it is first necessary to define

the thermal parameters under which it is to operate.

The basic equation for thermal equilibrium is:-

Temperature difference

across the system

Power dissipated =

Sum of all the thermal

resistance in the heat

flow path.

Thus

PD =

Tj - Ta

Øjc + Øcs + Øsa

Where

PD = Power dissipation (W)

Tj

= Max allowable junction temp (ºC)

(specified by device manufacturer)

Ta

= Ambient temperature (ºC)

Øjc = Thermal resistance junction to

case (ºC/W)

(specified by device manufacturer)

Øcs = Thermal resistance, case to

heatsink (ºC/W)

Øsa = Thermal resistance, heatsinks to

ambient air (ºC/W)

The maximum value for thermal resistance heatsink to

air (sa) is usually determined by rearranging

equation 1 to the following:

Øsa =

Tj - Ta

- (Øjc + Øcs)

PD

The result of the above equation provides a thermal

resistance value which must be equalled or bettered

by the heatsink selected.

A semi-conductor device is to be operated with its

junction temperature not exceeding 80ºC whilst dissi-

pating 16 watts to ambient air at a temperature of40ºC.

The thermal resistance, junction to case, is specified

by the manufacturer as 1.25ºC/W and the thermal

resistance, case to sink (using an insulating washer and

thermally conductive compound) is taken as 0.50ºC/W.

Øsa = 80 - 40 - (1.25 + 0.50)

16

= 0.75ºC/W

The heatsink therefore must have a thermal resistance

which does not exceed 0.75ºC/W.

HEATSINK SELECTION

EQUATION 1

EQUAT ION 2

EXAMPLE

Thermal Performance of any heatsink is influenced by

many factors and for this reason all performance fig-

ures quoted should be treated as indicative only. It is

recommended that the effectiveness of any heatsink is

tested in the specific operating environment in which

it will be subjected.

2

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