In many high-power applications such as motors and power supplies power resistors are used in the main power line. Their purpose is to prevent damage or to deliver a level of control.
Resistors in these applications are subjected to a constant, relatively high, current flow. As current flows through the resistor, it generates heat. To prevent damage to the resistor element this thermal energy must be dissipated to the environment.
Power resistors dissipate heat primarily via convection. The amount of heat dissipated is, therefore, directly related to the surface area of the resistor. One method to increase the surface area is to use a heatsink.
A resistor heatsink is a device that dissipates heat from the power resistor to the surrounding environment, allowing for efficient cooling. It typically consists of a thermally conductive material, such as aluminium alloys, copper or steel.
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.
Heatsinks are generally chosen to match the precise requirements of a particular resistor and application. Materials are selected to have a high thermal conductivity.
The surface area of the heatsink should be much larger than the resistor. This means heat can be efficiently dissipated by convection (natural or forced), conduction or liquid cooling. For a given material and cooling method the larger the heatsink the more heat it may dissipate. Fins can be used, to increase its surface area.
In some applications, the resistor can be immersed in transformer oil or deionised water (non-conductive), with these materials kept at a suitable constant operating temperature. However, oil and deionised water needs to be filtered, or even exchanged, at intervals as they can become conductive over time.
Thermal Resistance
Resistors for heatsink applications are designed to dissipate their heat primarily in one plane – their rear face. The thermal resistance of a thick film power resistor component is determined by its internal design. It specifies how much hotter the internal resistor is relative to its case or backplate. A thermal resistance value is given on most datasheets.
Thermal resistance is a measure of heat transfer efficiency between two locations. It is modelled as a series of resistances to heat flow. 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 system board and/or the heatsink.
Thermal resistance issues can be managed by modifying the power resistor mounting method, mounting force and the interface between the resistor and substrate/heatsink.
Choosing A Heatsink
Power resistors are a small part of an overall system. They are used with other electronic components, some of which can be heat sensitive. In many applications, space will be limited. These factors have a significant impact on heatsink selection.
The design and choice of the heatsink should match the power rating and operating conditions of the resistor. A higher power rating will generally require a larger heatsink with better thermal conductivity.
Size can be reduced by using materials with higher thermal conductivity, but this increases cost. Cooling via natural convection, forced air, water, or oil (see above) can reduce the size of the heatsink, but (again) at a cost.
There are many resistor thermal issues to consider when trying to optimise power resistor performance. Take care when interpreting datasheet parameters. If there is any doubt, consult the device manufacturer or a power resistor design specialist.