In many high-power applications, such as motors and power supplies, the power supply resistors are located in the main power line. Their purpose is to prevent damage or provide a degree of control.
In these applications, the resistor is subjected to a constant, relatively high current. When an electric current flows through a resistor, it generates heat. This thermal energy must be dissipated into the environment to prevent damage to the resistive elements without affecting nearby components.
Thick film power resistors can reach temperatures that exceed safe operating conditions in a matter of seconds. It is important to keep the operating temperature range within the maximum specified range (typically 50 60 C).
Power resistors dissipate heat primarily by convection. Therefore, the amount of heat dissipation is directly related to the surface area of the resistor. One way to increase the surface area is to use a radiator.
A resistive heat sink is a device that dissipates heat from a power resistor to the surrounding environment to achieve efficient cooling. It is usually composed of thermally conductive materials, such as aluminum alloy, copper, or steel.
The heat sink's ability to conduct heat away from the resistor is measured in the power dissipation of c w, which in turn depends on the heat sink material and its properties, the size and finish of the heat sink, and the cooling method.
Heat sinks are selected to meet the precise requirements of specific resistors and applications. Choose a material with high thermal conductivity. The TO (leaded) package is the most common type of resistor package in heat sink applications. It is available in a variety of standard sizes, and the part number (for example) indicates the package size in inches.
The surface area of the heat sink should be much larger than that of the resistor. This means efficient heat dissipation by convection (natural or forced), conduction, or liquid cooling.
For a given material and cooling method, the larger the heat sink, the more heat it is likely to emit. Fins can be used to increase their surface area. Excellent thermal bonding is required between the resistive element and the heat sink to avoid thermal breakage of the resistor.
In some applications, the resistor is immersed in transformer oil or deionized water (non-conductive), which is kept at a suitable constant operating temperature. However, oil and deionized water must be filtered or replaced at regular intervals as they become electrically conductive over time.
Resistors used in heat sink applications are primarily designed to dissipate heat on one flat surface, i.e., its backside. The thermal resistance of a thick film power resistive element is determined by its internal design. It specifies the temperature of the internal resistor relative to its housing or backplate. Thermal resistance values are provided in most datasheets.
Thermal resistance is a measure of the efficiency of heat transfer between two locations. It is modeled as a series of heat flow resistances. Thus, the total thermal resistance is the thermal resistance of the resistor (resistive element to the base or housing), which is fixed, and the thermal resistance between the resistor base or housing system board and or heat sink.
Thermal resistance issues can be managed by modifying the power resistor mounting method, mounting force, and the interface between the resistor and the substrate heat sink.
A power resistor is a small part of the overall system. They are used with other electronic components, some of which may be heat-sensitive. In many applications, space will be limited. These factors have a significant impact on the choice of heat sink.
The design and selection of the heat sink should match the resistor's power rating and operating conditions. Higher power ratings typically require larger heat sinks and better thermal conductivity.
The size is reduced by using a material with higher thermal conductivity, but this increases the cost. Cooling by natural convection, forced air, water, or oil (see above) can reduce the size of the radiator, but (again) at a cost.
There are many thermal issues that need to be considered when optimizing the performance of a power resistor. Be careful when interpreting datatable parameters. If you have any questions, consult your device manufacturer or EAK power resistor design specialist.