A wide range of high voltage resistor devices is available to a variety of quality standards. It is important for the designer to understand the trade-off between cost and reliability and to fully understand the application if resistor failure is to be avoided.
A cheaper high voltage resistor device may well be appropriate for many applications but the low cost generally means some compromises have been made in the resistor design and manufacturing process. In this article, we cover the potential impact on resistor performance of external environmental and mechanical issues. We discuss how the impact of these stress factors may be reduced via appropriate high voltage resistor design and an optimised resistor manufacturing process.
Key High Voltage Resistor Performance Parameters
The key parameters to be calculated and managed to ensure long term and stable operation of a resistor are:
-Load life stability
The Designer must always be aware of thermal and mechanical stresses and their impact on the resistor. The temperature coefficient and voltage coefficient of materials must be taken into account to avoid high voltage resistor performance and stability issues in the end application.
Current flow through the resistor device will generate heat that must be dissipated in some way to prevent device overheating. If overheating occurs it can change the resistance value and/or damage the resistor track or substrate.
Errors in the high voltage resistor manufacturing process can result in pin holes in the resistor track. Abrading of the resistor track to set the resistor value can cause a non uniform resistor track pattern. Both of these issues can cause hot spots that may result in unexpected failure of the resistor film or changes in performance.
To prevent in service failure (or degradation) of the high voltage resistor a detailed understanding of the resistor application is required. Significant single transient (pulse) events or multiple voltage pulses can degrade the resistor performance or cause resistor failure. Appropriate design and manufacture of the high voltage resistor may reduce the impact of over voltage events but cannot ensure the device will survive in all circumstances.
Load Life Stability
When using high voltage resistors one of the most important, but often the most neglected, parameters is load life stability. This defines the ability of the resistor to retain its initial resistance value, within specified limits, over an extended period of time.
The particle to particle contact in the resistor film may be disrupted by thermal or voltage stresses causing a permanent change in resistance value. The granular structure also makes the device susceptible to damage by mechanical stress. Resistor stability is directly related to the manufacturing process, resistance film selection and its susceptibility to applied voltage and thermal stresses.
Although the makeup of the resistor film material does make thick film technology more susceptible than some other resistor technologies to damage, selection of a specialist manufacturer can reduce the impact of the problem. Careful resistor design and selection of materials can minimise issues caused by transient thermal and mechanical events.
A specialist high voltage resistor manufacturer will often build safety margins into the design and manufacturing process to minimise the risk of drifts in performance or product failure. If required, fail safe measures may be incorporated in the resistor product. For example, thermal laser cuts in the ceramic side opposite to the resistor element will ensure that the products breaks and safely fail due to excess thermal conditions.
Hot spot issues may be minimised by designing products with as long resistor path as possible within a given available area. This process can also minimise voltage stresses on the resistor materials.
If resistor reliability is of paramount importance in any given application then a close partnership between the designer and the resistor manufacturer is required to ensure all application issues are covered and the high voltage resistor device is manufactured appropriately to address those issues.