In complex electronic systems, the issue of power resistor derating is often overlooked. The system designer can have many design issues to consider, and passive components, such as power resistors, are often the last consideration.
If long-term system performance and reliability are a concern, failing to consider the power rating of resistors under operational conditions is a mistake. If power resistor performance is compromised, it can damage equipment or compromise safety.
In this post, we discuss derating thick film power resistors in high-temperature applications.
The Impact of Temperature on Power Resistor Rating
We have discussed the impact of temperature on thick film power resistor performance elsewhere in this blog. Current flow through a resistor generates heat. Failure to control temperature rise in the resistor film can cause irreversible changes to the resistance value or, in extreme cases, complete resistor failure.
Resistor manufacturers assume an ambient temperature of 25°C when specifying power ratings. As ambient temperature increases, the resistor’s ability to dissipate heat to the environment decreases.
Power resistor derating reduces a resistor’s maximum power capacity as ambient temperature increases. The first step is to select the correct resistor for the application, then quantify the actual ambient temperature and derate accordingly. Failure to derate a power resistor correctly can lead to drift in resistance values, physical damage, and reduced reliability.
Various methods, including heatsinks, are used to dissipate heat away from a resistor, but their effectiveness depends on the temperature of the environment. Hence, system designers seek to limit ambient temperature using airflow, enclosure design, and forced cooling, but all have cost implications.
With so many variables in play, it can be difficult to measure the ambient temperature accurately. Hence, the power resistor derating factor can only be an approximation. In safety-critical applications, it is often safer to assume the worst case and select a derating factor accordingly.
How to Derate a Power Resistor
As the name suggests, derating a device means operating it at less-than-rated maximums. The rated load on resistor datasheets is usually quoted at 25°C (ambient). At higher temperatures, the power resistor is derated.
As mentioned above, the first step is to establish the ambient operating temperature. As this is often an estimate, it is wise to apply a safety margin, especially in fluctuating temperature environments. Then review the resistor’s datasheet for its maximum rated power, temperature limits, and derating curve.
The derating curve plots show the percentage power rating versus ambient temperature. They represent how a component’s maximum allowable power or current capacity decreases with rising ambient temperature. The derating curve graph X-axis displays ambient temperature, starting from a baseline (e.g., 25°C). The Y-axis (power or current) shows the maximum allowable power dissipation or current at a given temperature as a percentage of the component’s rated capacity.
Above a specific temperature (often 25°C), the allowable power typically decreases linearly with increasing temperature. At very high temperatures, the curve reaches zero, which means the resistor can no longer safely dissipate power.
The derating curve allows the designer to calculate the allowable power dissipation at the operating temperature. For temperatures above the rated threshold, it is important to reduce the power rating proportionally.
As a final step, It is worth validating performance under actual operating conditions to confirm the derating factor calculation. Many power resistor manufacturer datasheets show a derating curve for general and special cases. The special case environments include high-altitude applications, resistors within enclosures, and resistors grouped in restricted areas. These environments have their own derating curves.
If power resistor derating is not possible and/or there is no way to control ambient temperature, then the system designer can select a higher power resistor device. This usually increases resistor size and cost.
Power resistor performance and reliability in high-temperature applications depend on many factors. The resistor and substrate materials used to construct the power resistor, the design, and the manufacturing process all have an impact. If in doubt about power resistor derating factors, it is wise to consult a specialist thick film power resistor manufacturer.