The purpose of a high power resistor heatsink is to keep the temperature of the resistor element below its maximum operating temperature but choosing the right heatsink can be a difficult process.
Heatsinks are generally chosen to match the precise requirements of a particular resistor and application. The choice of heatsink depends on three main factors
- Dimensions (Including fins)
- Thermal resistance
Taking each in turn.
Heatsink materials are chosen to have a high thermal conductivity. Aluminium alloys and copper are common choices with steel used in some applications.
When choosing a material there is a trade off to be made between cost and performance. Materials such as gold have excellent thermal conductivity but at a high cost
Copper is an option for some applications as it has a thermal conductivity of almost twice that of aluminium. However, it is expensive when compared to aluminium and it is relatively heavy. Aluminium is also easier to use than copper in the heatsink manufacturing process.
The surface area of the heatsink is much larger than the resistor. This means heat can be efficiently dissipated by convection (natural or forced), conduction or liquid cooling.
Heatsinkable resistors are designed to dissipate their heat primarily in one plane – their rear face. The ability of the heat sink to conduct heat away from the resistor is measured in C/W of power dissipation which in turn depends on the heat sink material and its properties, the size and finish of the heatsink and the cooling method. Heatsink design can be complex but commercially available heat sink design software can simplify the process.
For a given material and cooling method the larger the heatsink the more heat it may dissipate. Fins may be used to increase its surface area. If the heatsink plate is water cooled it will be significantly smaller than if it is cooled by natural (rather than forced) air flow.
The cooling ability of the heat sink depends to an extent on whether it is oriented vertically or horizontally because of natural convective air cooling.
In some applications the resistor may be immersed in transformer oil or deionised water which are non conductive, with these materials kept at a constant suitable operating temperature for the resistors. However, oil and deionised water does need to be filtered, or even exchanged, at intervals as can become conductive over time.
Thermal resistance is a measure of heat transfer efficiency between two locations. It is modeled as a series of resistances to heat flow. The objective should be to conduct heat away from the resistor. Thermal resistance limits heat conduction and should, therefore, be minimised.
The total thermal resistance is therefore the thermal resistance of the resistor (resistor element to base or case) which is fixed and the thermal resistance between the resistor base or case and the heatsink which can be managed depending on the power resistor mounting method (and force) and the interface.
Surface defects on the rear face of the resistor and the face of the heatsink increase thermal resistance at the resistor/heatsink interface. Thermal grease is often used as a filler to deliver improved thermal conduction.
Thermal management is essential to ensure a high power resistor continues to perform to specification over the long term. It is important to choose the right combination of resistor and heatsink for the application.