When trying to select the correct thick film resistor it is important to understand why resistors fail under pulse conditions. High energy pulses may occur in a wide variety of applications. They may be predictable or random events. It is important to understand the nature of the pulse event, the application and environmental conditions and select a resistor accordingly.
Thick film resistors fail under pulse conditions because they are unable to dissipate the heat generated in the resistor device by the electrical energy of the pulse.
With a short pulse of sufficient amplitude, the temperature of the resistor material may reach hundreds of degrees Celsius but the short pulse width means there is insufficient time for the heat to be transferred through the mass of the substrate material. Failure to select the correct thick film resistor device can mean the high temperature either destroys the resistor material or causes a long-term degradation in performance.
When a single pulse occurs with a lower peak amplitude but a longer duration the average power is more of a concern. The substrate material will dissipate a proportion of the heat generated by the pulse event but it is important to ensure the maximum power rating of the resistor device is not exceeded.
For repetitive pulses, the first step is to calculate if the peak pulse amplitude may generate sufficient energy to damage the resistor track. With this concern eliminated the average power dissipation over the period of the pulses should be established. This value must not exceed the continuous power rating of the resistor.All calculations must take account of the ambient operating temperature of the application. Resistor manufacturers usually quote power ratings of resistors at 25͒℃. It is, therefore, important to derate the power rating based dependant on the actual expected ambient temperature.
Many manufacturers provide nomo charts to allow the system designer to understand the impact of single and repetitive pulse events on the choice of a pulse resistor. For simple pulse conditions these can be sufficient but for more complex situations it is important to consult the resistor manufacturer.
The energy curve generated by the surge condition should be properly understood and a resistor selected accordingly. For high pulse power and pulse current handling the width of the thick film resistor should be maximised to prevent overheating and burn-out. Where average power is more of a concern sufficient substrate mass is required to accommodate the pulse energy. A specialist resistor manufacturer may design a thick film resistor device to match the demands of a particular application.