Power resistors play an important role in medium-power (up to 200kVA) auxiliary power supplies. They provide overcurrent protection, limit inrush current, damp switching transients, and deliver harmonic damping, all of which can impact the long-term reliability of the overall power conversion system.
Auxiliary Power Supply Applications
Auxiliary power supplies are “secondary” in function, but often critical in practice. They do not power the main asset (example: large motor-driven industrial machine) but functions such as HVAC, battery charging, lighting, control electronics and other auxiliary loads.
Power Resistor Selection Considerations
When considering resistors for auxiliary power supply applications, the key design issues are not limited to resistance value and nominal power rating alone. The resistor must be selected according to thermal behaviour, overload requirements, transient conditions, physical size and long-term stability, all of which interact in ways that can undermine reliability if not addressed as a whole.
Central among these is the requirement to withstand not just steady-state current and voltage, but also short-duration overloads, inrush currents, startup surges and repetitive pulses. Resistor selection must therefore account for both continuous power dissipation and short-term energy absorption.
Underlying all of the above is thermal performance. A resistor’s power rating is only valid under defined operating conditions. In practice, its temperature rise depends on enclosure design, airflow, nearby heat sources and mounting surfaces. Physical size and mounting arrangement matter here too, since they directly influence thermal behaviour.
All of the above can impact long-term power supply stability. Elevated temperature, repeated pulsing, thermal cycling, humidity and contamination can all cause resistance drift and reliability issues
Custom Power Resistors vs Standard Resistor Product
Standard resistors might not survive in applications subjected to high temperatures, chemicals and dust, high moisture levels, vibration and/or shock. Custom thick film resistors bridge this gap by tailoring the resistor component to the demands of a given application.
Where vibration or shock is a concern, substrate design, material selection, and specialised leadframes can help reduce the potential impact. In particular, planar (flat geometry) resistor formats perform better where mechanical stress is an issue.
Similarly, material choice and encapsulation can mitigate chemical attack, while overglaze coatings and anti-corrosion terminations help protect against moisture ingress.
Where high temperature is a reliability issue, custom resistors can be printed directly onto high-thermal-conductivity materials such as Aluminium Nitride. This approach maximises heat dissipation within confined enclosures, lowering the component’s operating temperature.
Beyond environmental stresses, inappropriate final resistor trim techniques can create localised current crowding and thermal hot spots during surge events. Custom thick-film components can utilise optimised trimming patterns to evenly distribute pulse energy across the entire resistive layer, drastically reducing the risk of catastrophic open-circuit failures caused by inrush currents.
Finally, by optimising the resistor paste formulation for specific voltage and temperature coefficients, custom thick-film resistors can minimise resistance drift over time, hence improving the long-term reliability of the auxiliary power supply.
Standard resistors are often the best choice unless environmental or electrical factors could impact reliability, in which case, a custom power resistor designed to meet the specific demands of the application may provide a better solution.
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