Heatsinks for Electronics Equipment

- an overview or tutorial giving the basic facts about heatsinks or heat sinks, what they are and the thermal calculations required to choose the correct type.

Heatsinks are used in many pieces of electronics equipment to ensure that heat can be removed from pieces of equipment or particular components within them. For example with microprocessors running very hot these days, they normally have heatsinks clamped or attached to them, and in addition to this fans are also included in PCs to ensure that the components stay cool and operate within their operating temperature ranges. It is not only PCs that contain heat sinks. Many other pieces of equipment contain them. In fact heatsinks are used wherever there are sources of heat, and this heat needs to be removed.

Heatsinks can take a variety of forms, and they are widely available for electronics applications. If only a small amount of heat needs to be removed, small or simple heatsinks can be used. However if significant amounts of heat need to be removed, then more complicated heat sinks are needed. Using some relatively simple thermal calculations, it is possible to determine which heatsinks may be applicable for a given application. They are specified in a way that makes it possible to determine their performance.

Types of heatsinks and their specifications

Heatsinks come in a variety of sizes and shapes. The main purpose of a heat sink is to remove heat from the source where it is generated as efficiently as possible. To achieve this a heat sink must have as large an area as possible over which the heat can be transferred to the air. Often the heat transfer takes place assisted by convection, but fans may also be used, and this considerably improves their performance.

Normally a heat sink is specified in terms of its dissipation for a given temperature rise, i.e. its thermal resistance. The heat dissipation is given as a number of watts, and the temperature rise in terms of degrees Celsius. Thus a particular heat sink may have a rating of 10 watts per degree Celsius. This means that if it dissipates 10 watts of power then its temperature will rise 1 degree. Similarly if it dissipates 100 watts, its temperature will rise 10 degrees.

These figures are essential for any thermal calculations that are needed. By knowing the performance of the heatsink, the correct one can be chosen for the required heat dissipation, and allowable temperature rise.

Simple thermal calculations

It is possible to undertake some simple thermal calculations to determine the required performance for a heatsink. While some thermal calculations can become very involved, the thermal calculations needed for choosing heat sinks is very easy and quite straightforward.

The first step in any thermal calculation is to determine the amount of power being dissipated. This is simply done using one of the three equations below:

Power (Watts) = Voltage (Volts) x Current (Amps)

Power (Watts) = Voltage ² / Resistance (Ohms)

Power (Watts) = Current ² / Resistance (Ohms)

With the power dissipation of the component calculated, the next stage in the thermal calculations can be undertaken. This is to calculate the required thermal resistance of the heatsink.

The thermal resistance of the heatsink = (Max temperature rise / watts dissipated) - (junction to case resistance + case to heatsink resistance)

The above equation includes terms used for semiconductor devices, the items that most commonly need heatsinks. Often it is not possible influence of receive data for the junction to case resistance, or for the case to heatsink resistance. Thus it is safest to leave a small margin to cover these. This means that by leaving a margin the equation simplifies to:

Thermal resistance of the heatsink = Max temperature rise / watts dissipated

Summary

Heatsinks are an essential element of many electronics designs. For components that dissipate large amounts of heat, a heatsink is an essential requirement, and often fans may also be needed to assist in the cooling. The thermal calculations needed to select he require heatsink can be quite straightforward, although some thermal calculations to determine more complex thermal issues can be very involved. Fortunately these are rarely needed for home designs.

Written by Ian Poole .
Experienced electronics engineer and author.