High power switching causes surge or pulse conditions in many applications. A high power pulse resistor is often used to suppress the pulse and prevent damage to sensitive components. Applications for pulse resistors include power supplies (inrush current), Defibrillators, welding equipment, medical equipment and medical cabling.
A key design decision is a choice 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 power pulse resistor design.
Resistor Technologies For Pulse Applications
Several resistor technologies are suitable for high power pulse applications. A choice must be made based on the requirements of the application, performance and cost.
Carbon composition resistors are old technology with relatively poor performance. However, they remain one of the best resistor types for pulse applications. Ceramic composition resistors have good high voltage and high energy pulse handling capabilities. Unfortunately, they tend to be expensive and the range of resistance values is limited.
Wirewound resistors can withstand high power pulses for short durations with little resistance shift in resistance. However, they tend to be relatively large devices with high inductance.
Thick film resistors deliver a relatively cheap, non-inductive, solution in a small footprint. Their pulse handling capabilities may be less than other technologies but they deliver good overall performance.
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, 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. The energy curve generated by the surge condition and the application environment must be understood and a resistor selected accordingly.
Thick Film Resistor Pulse Withstand Calculations
Parameters, such as peak power, pulse duration or period need to be identified. These should be and compared to the resistor’s specified pulse load capability.
Pulse load diagrams are generally defined at room temperature. If a resistor is expected to operate at a higher ambient temperature then a resistor with a higher pulse load capability is required.
For short pulses there will be no time for heat dissipation and the heat will remain in the resistive element. The impact on the resistive element temperature tends to be limited, even for high pulse loads. 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 standardized transients. These are often shown on resistor datasheets.
For energy pulses with a long duration, pulse shapes can be converted 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 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, the nature of the application and the pulse condition should be considered. It is important to understand if the pulse condition is repetitive or instantaneous. The average power dissipation over the period of the pulse conditions must not exceed the continuous-power rating of the resistor.
Typical application issues to consider include the board area available for resistor components. The heating impact 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 on the system board area and more effective resistor cooling due to natural convection flow.
Where a fail-safe condition is required thick film power resistors can be designed to fracture (fail) if subjected to unusually high pulse conditions. Scribe lines can be used in the substrate behind potential hot spot areas as a fracture point.
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 device.