High power switching can cause surge or pulse conditions. A pulse resistor is often used to suppress the pulse and prevent damage to sensitive components. Applications for pulse resistors include power supplies and motors (inrush current), Defibrillators, welding equipment, medical equipment and medical cabling.
A key design choice is between pulse voltage resistors and those capable of withstanding a high current. The design of one is the exact opposite of the other. In this post, we only consider high current pulse resistor design.
Understanding The Threat
When pulse power is dissipated to a thick film resistor resistive element, it generates heat. Overheating can damage the resistive element, leading to a resistance change or (worse case) an open circuit failure.
There is a time lag between the application of a pulse, heating of the resistor element, and dissipation of that heat to the surrounding environment. The amount of heat dissipated depends on the structure of the resistor component. It is also related to resistor cooling (if any), and the ambient temperature.
Heat generation and transfer in the resistor takes time. Therefore, a resistor’s pulse load capability depends on the pulse duration. A high pulse load with a short duration may not have a significant heating effect. Whereas a lower pulse load with a long duration can cause more damage.
The shape of the pulse and the duration between pulses also requires careful consideration. Pulse shapes can vary from rectangular or triangular to the typical exponential decay curve.
Due to their transient nature, pulse events can be difficult to identify and quantify. It is important to understand the energy curve generated by the surge condition. Consider the application environment and select a resistor accordingly. Most pulse resistors are designed for repetitive known pulse conditions and are designed to handle these without failing.
Thick Film Resistor Pulse Withstand Calculations
Parameters, such as peak power, pulse duration or period must be identified. These should be compared to the resistor’s specified pulse load capability.
Pulse load diagrams are generally defined at room temperature. If a pulse resistor must operate at a higher ambient temperature, then select a resistor with a higher pulse load capability.
For short pulses there will be no time for heat dissipation and the heat will remain in the resistive element. The resistor will, therefore, withstand peak pulse loads higher than its rated dissipation.
For long (single) pulses, there will be time for some heat dissipation. This depends on the mass of the resistor, heatsinking and cooling. Depending on the nature of the pulse, there may be a significant temperature rise in the resistive element. Hence, for extended pulse durations, the permissible peak pulse load approaches rated dissipation.
The difference between a single pulse load and a continuous pulse load is a function of the number of pulses and the time interval between them. For short, sharp pulses common pulse shapes can be described by standardised transients. These are often shown on resistor datasheets.
For high energy pulses with a long duration, convert pulse shapes to rectangular shapes. These can then be compared with standardised pulse load diagrams shown on most resistor datasheets.
Selecting A Pulse Resistor
Some design engineers take a cautious approach and over design circuits and components to withstand a pulse event. The alternative is to take the time to consider pulse conditions before selecting an appropriate resistor. This approach can reduce the size and weight of the resistor component and thereby save on system board area and cost.
The first step in choosing an appropriate thick film pulse resistor is to determine the maximum voltage of the pulse. This should be within the rated maximum voltage of the resistor. Depending on the operating temperature the rated pulse power of the resistor should be derated accordingly.
Finally, consider the nature of the application and the pulse condition. It is important to understand if the pulse condition is repetitive or instantaneous. The average power dissipation over the pulse duration must not exceed the continuous-power rating of the resistor.
Typical application issues to consider include the board area available for resistor components. Heating effects on adjacent components and solder joint issues caused by repeated temperature cycling must also be evaluated.
The solution, in some cases, can be to use a vertical inline component. This approach delivers a large surface area without impacting the system board area and more effective resistor cooling due to natural convection flow.
Where the application demands a fail-safe condition, thick film power resistors can be designed to fracture (fail) if subjected to unusually high pulse conditions. Scribe lines placed in the substrate behind potential hotspot areas act as a fracture point. Sometimes the element itself is designed to ensure open circuit at overload.
By understanding the application, the nature of the pulse condition and the environment it is possible to use a manufacturer’s data to select an appropriate thick film pulse resistor device. If no standard device is available then the TSEC engineering team is always available to discuss the design and manufacture of an application-specific pulse resistor device.