Power Resistor Derating For High Temperature
In complex electronic systems, the issue of power resistor derating for high temperature can often be overlooked. The system designer may have many design issues to consider. Passive components, such as power resistors, are often the last to be evaluated, if at all.
The Impact Of Temperature On Power Resistor Rating
If exposed to higher than its rated (specified maximum) temperature a power resistor will not operate as expected. The rated load on resistor datasheets is usually quoted at 25C (ambient). Higher temperature operation requires derating (a reduction) of the resistor’s rated load or the selection of a device with a higher power rating. This usually means a physically larger device.
A linear relationship between temperature rise and derating factor (percentage) exists that is quoted either on the resistor datasheet or in common industry standard specifications. The challenge is to quantify the actual temperature a power resistor is exposed to in a particular application. Without an accurate maximum temperature, the power resistor derating factor can only be an approximation. Common factors affecting the ambient temperature include:
•Enclosure volume and material
Altitudes below 5,000ft are generally not an issue but if equipment must operate above this level there is no choice but to derate accordingly. However, the other three key factors are all manageable to some extent.
Controlling Temperature Rise
The enclosure material, its thickness, finish and the number and position of openings should all be considered to optimise both airflow and the conduction of heat through the enclosure walls to the outside environment. Maximising the enclosure volume can also help.
The power resistor device should be placed as far away from other heat generating components (both active and passive) as possible. The orientation of devices should be considered to reduce the temperature rise and to maximise airflow. Hot spots across the resistor element due to poor airflow over some areas in respect to others should be avoided.
In general, airflow is the easiest element to control. The amount of flow required can be estimated and heat sinks and vanes positioned to maximise the transfer of heat away from the resistor component and enclosure.
Cooling The Power Resistor
One solution is to size the resistor appropriately (sufficient mass) to dissipate the heat generated. However, using a physically larger resistor device can be more costly and impacts on the system board area available for other components. The alternative is to cool the resistor device in some way.
The main options are air cooling or liquid cooling, with or without the use of a heat exchanger. Air cooling is the cheapest method to employ and is the process utilized in most high power systems. Typically this involves combining air flow management with the use of appropriate heat sinks.
There are applications where the heat capacity of air is simply inadequate to transfer the required amount of heat away from the power resistor device. Liquids (water and oil are the most common) are much denser than air and have a much higher thermal capacity. They are therefore far more efficient than air at transporting heat away from the resistor and/or at transporting heat to a secondary cooling surface such as a heat exchanger.
However, they add additional resources to the system and therefore come at a significantly higher cost than air cooling systems. Placing the high power resistor device in a metal tube then immersing that tube in the path of deionised water flow is an efficient method to cool the resistor.
Water is cheap and has a high heat capacity enabling it to transfer heat out of the system or to a heat exchanger or plate. It is also non-flammable and easy to source when required which are key advantages over oil cooling.
Deionised water reduces the risk of short circuit and reduces contamination and scaling that can decrease the insulating resistance of the water. The major disadvantage of water is its boiling point. Although pressurising the water can increase the boiling point.
In some applications water is not a viable solution. Oil is the best alternative as it has a much higher boiling point than water and it is an electrical insulator. However, unlike water, it can degrade (particularly at higher temperatures), it is flammable and it has only around half the specific heat capacity of water.
Like water, the oil must be checked and changed as required as its insulation resistance degrades over time. With appropriate management of the system design, layout and cooling the ambient air temperature around power resistors may be set at a level that reduces the impact of derating to an acceptable level.