When choosing the correct resistor for a high-power application, power resistor stability over time should be a key concern. If a resistor is to retain its resistance value over the long term, it is important to go beyond a simple review of datasheet parameters.
Resistor Stability – A Definition
Resistor stability is expressed as a percentage variation from the stated resistor value at a defined temperature. It is a measure of the repeatability of a resistance measurement (the stated value) over time.
Power Resistor Stability – Key Factors
The three main issues that can affect the stability of a resistance value over time are thermal, electrical and mechanical stresses. The stability of thick-film power resistors is related to the choice of resistance film and the manufacturing process.
Comparing the Stability Of Resistor Technologies
Standard thin-film resistors are generally chosen for precision rather than high power or high pulse-energy operation.
Wirewound resistors can provide high power and surge robustness, but they are often larger and their inductance can be an issue. Carbon-film power resistors often perform well under certain pulse conditions, but their stability and suitability depend on pulse shape, energy, voltage stress, resistance value, package size and environmental conditions.
Thick-film technology is often preferred where a compact, low-inductance, custom geometry is required, especially when the resistor can be custom-designed around the application’s electrical, thermal and mechanical stresses. Hence, the remainder of the article focuses on thick film resistor design.
Temperature Effects
A resistor rated for a given wattage can only safely dissipate that power under specified thermal conditions (often an ambient temperature of 70°C). Above this temperature, the allowable power dissipation must be reduced according to the manufacturer’s derating curve.
However, it is important to understand the actual operating temperature of the resistor, not just the nominal ambient temperature. The resistor body temperature is influenced by power dissipation, PCB layout, mounting method, airflow, and the proximity of other heat-generating components.
Furthermore, the temperature of a thick-film resistor is not always uniform across the resistive element. It is therefore important to consider not only the average resistor temperature, but also the possibility of local hot spots within the resistive film.
Operating a resistor above its rated ambient temperature, or failing to derate the applied power appropriately, can cause the resistor film to overheat. Depending on severity, this may permanently shift the resistance value or lead to complete resistor failure.
This distinction between actual element temperature and nominal ambient temperature is important for thick-film resistor stability, because resistance drift is driven primarily by the resistive element’s temperature rather than the surrounding air temperature. It is also important to note, however, that this concerns long-term resistance drift, which should be distinguished from the temporary and reversible shift in resistance with temperature defined by the Temperature Coefficient of Resistance (TCR).
Electrical Stresses
A resistor can be within its continuous average power rating but still be damaged by high peak pulse energy. Pulse load conditions can degrade the resistor material and cause a permanent change in resistance without causing complete failure.
A resistor may survive one pulse but drift over time when exposed to repeated pulses. However, If Pulse energy, peak voltage, peak current, pulse duration, pulse shape, repetition rate, and cooling time between pulses are fed into the resistor design process at an early stage, their impact can be minimised.
Specialist thick film resistor manufacturers can improve drift performance under pulse conditions by selecting an appropriate substrate and optimising the thick film firing process.
Mechanical Stresses
It is important to consider the damage to the power resistor while in service and its impact on resistor stability. Vibration or shock can damage the resistor substrate, resistive elements, terminations and system board solder joints.
Mechanical stresses can cause permanent changes in resistance value. The change is directly related to the stress level, with extreme stress causing complete resistor failure. Minor stresses may cause negligible changes in resistance value, but their impact can be cumulative over time.
After firing, the resistance of a thick-film resistor is determined by the microstructure of the fired resistive layer: conductive oxide particles, glass phase, particle spacing, contact points and tunnelling paths. Mechanical strain, vibration or shock can disturb this network, create microcracks, alter local current paths or damage the bond between film, termination and substrate, producing a permanent change in resistance.
Power Resistor Performance Trade-offs
A perfect resistor will maintain its resistance value over its service life regardless of any external stresses applied. Unfortunately, the perfect resistor does not exist, which means there must be some level of compromise.
Performance characteristics are often closely linked. Changing one parameter can have a detrimental effect on another. For example, if the system board area is a concern, reducing the size of the power resistor could be a solution. But this can compromise heat dissipation, resistor functionality, pulse performance and long-term reliable performance. Choosing a specialist substrate material may maximise heat dissipation, but only at an increased cost.
It is important to consider resistor stability when choosing a power resistor. For critical applications, designers should define the real thermal, electrical and mechanical conditions early in the design process. If in any doubt, it is best to consult a specialist thick film power resistor manufacturer.