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In many high-power applications, such as motors and power supplies, power resistors are found 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. This thermal energy must be dissipated to the environment to prevent damage to the resistor element without impacting nearby components.

Thick film power resistors can reach temperatures that exceed safe operation within seconds. It is important to keep the operating temperature range within maximum specified limits – typically 50 – 60⁰C.

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 ⁰C/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 selected to match the precise requirements of a particular resistor and application. Materials are chosen to have a high thermal conductivity. The TO style (leaded) package is the most common resistor package style in heatsink applications. It is available in many standard sizes with part numbers (e.g. 1206, 0805, 0402) representing the package dimensions in inches.

The surface area of the heatsink should be much larger than the resistor. This means heat is 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. Excellent thermal bonds between the resistor element and heatsink are required to avoid thermal fractures of the resistor.

In some applications, the resistor is immersed in transformer oil or deionised water (non-conductive), with these materials kept at a suitable constant operating temperature. However, oil and deionised water must be filtered, or 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 provided 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 substrate and 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 resistor generally requires a larger heatsink with better thermal conductivity.

Size is 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 optimising power resistor performance. Take care when interpreting datasheet parameters. If there is any doubt, consult the device manufacturer or a power resistor design specialist. Ultimately the responsibility for heatsink selection rests with the system designer and not the resistor manufacturer.