Kory Schroeder, Director of Marketing and Product Engineering, Stackpole Electronics
With the rapid growth of electronic content, devices, and circuitry pervading so many market segments, the need for current sense resistors has exploded. Because of the wide range of end products and applications, there is an equally wide range of sense resistor requirements. While there are several viable technologies that can provide accurate sensing results for some applications, those that require the highest precision, highest stability, best pulse handling, and best overall performance utilize all metal sense resistors. For example, automotive electronics may need to function under a wide range of temperatures and have robust surge handling. For hybrid and electric vehicle power control, UPS systems, and appliances, they must handle high levels of current and high surge energy reliably with no performance degradation. Let’s examine the different technologies, their materials, construction and characteristics, to demonstrate the benefits of all metal shunt resistors.
Film Element Technology
Film resistors typically are thick film screen printed ruthenium oxide, but recent developments in thin film technology have made thin film nichrome or copper sense resistors a viable solution. Thick film sense resistors are excellent at providing low cost performance in a wide range of sizes. Thick film resistors are typically used for resistance values above 10 milliohm, therefore they are not normally used for high current applications, those that require high stability, or those that require sensing stability over a wide temperature range. Film resistors will have the lowest surge handling capability of the technologies and will have difficulty providing low TCR options in lower values. But for high volume commercial applications with limited environmental stresses and efficiency needs, thick film resistors offer an ideal blend of performance and low cost. Recently, thin film current sense resistors have been developed that provide improved TCR over thick film. However, thin film current sense resistors are not able to offer extremely low resistance values and still suffer from poorer pulse handling compared to all metal shunt resistors.
Foil on Ceramic Carrier Technology
Foil on ceramic carrier technology utilizes a metal alloy foil laminated to a ceramic substrate. This construction process provides the electrical benefits of a metal alloy while minimizing cost and provides a rigid, robust package. This process yields a sense resistor with excellent performance characteristics, improved TCR over film type elements, as well as better pulse and overload handling compared to film technology. Foil on ceramic carrier technology in larger sizes are not susceptible to board flex failure because unlike film resistors, the foil element will remain intact and essentially unchanged even if the ceramic carrier substrate becomes cracked. The manufacturing processes allow for easy downsizing, making them ideal for the next generation of portable consumer electronic devices. While the resistance range for this technology is good, it is not capable of extremely low resistance values, like all metal current sense resistors, nor is it capable of higher resistance values, as you may find in thick film resistive elements. Surge handling for foil on ceramic falls between that of film resistors and all metal type shunts. From a cost standpoint, foil on ceramic is slightly more than thick film technology and less than typical all metal current sense resistors. Overall, foil on ceramic sense resistors fill an important performance space between the inexpensive film resistor types, and the high performance all metal shunt resistors.
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Figure 1b: Welded terminations
All Metal Current Sense Resistors
Current sense resistors that are entirely composed of a metal alloy or multiple alloys, will typically be constructed in one of two ways. One method uses a high precision element alloy such as Manganin with copper terminations welded to the ends to provide the PCB connection point. This welding process of dissimilar metals is not trivial and requires precise process control maintain product integrity and may be done by either electron beam welding or percussive welding. Both welding types produce a robust resistor with excellent environmental and thermal characteristics, however electron beam welding typically means lower EMF. Another metal element manufacturing method utilizes an entire plate of the sense alloy material and adds plated terminations directly onto the ends of the element itself. Eliminating the dissimilar metals weld is advantageous, however, the plating process to the sense alloy requires higher precision than is needed for copper terminated product. Regardless of which is used, these processes provide the lowest possible resistance values and will typically offer the best TCR performance for these low values as well. All metal elements are limited in their resistance values as the metal alloys used are only available in low resistance values. Therefore, applications with lower current requirements, or where sensing accuracy is more important than overall power efficiency, film or foil resistors typically will be chosen instead. All metal shunt resistors technology also have difficulty in providing smaller chip sizes due to process and material limitations and may be higher in cost, especially those with extremely low values, tight tolerance, or high continuous power ratings.
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Figure 2: 1206 size current sense resistors specification table
1206 size 10 milliohm spec comparison
The table shown in Fig. 2 demonstrates the difficulty in doing a spec comparison of these technologies. There are differences in TCR and the environmental performance, especially comparing the film vs. foil and all metal options. From a thermal resistance standpoint, all three technologies will behave similarly, however it should be noted that foil on ceramic and all metal resistors will typically be more stable at higher temperatures which is important for high power monitoring and control. The most notable difference in performance between these technologies lies in the pulse performance. Pulse energy handling in any component is a function of the materials used and the amount of material present. Because the element thickness for all metal resistors ranges between 3 to 6 times thicker than foil and 15 to 300 times thicker than film elements (depending on if comparing to thick or thin film), the material mass is proportionally larger as well. Fig. 3 below shows how much better the all metal technology is compared to a similar sized foil on ceramic carrier and film sense resistors. For longer pulse durations, the pulse performance of these technologies converges, which is logical since their continuous power ratings are the same. However, as the pulse duration decreases, the benefit of all metal construction becomes more apparent.
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Figure 3: Pulse power vs. duration comparison
The various technologies capable of producing current sense resistors may have comparable performance from a specification standpoint, but their performance may vary significantly under high current or pulse conditions. Design engineers with applications that have critical current handling or pulse handling requirements should consult with the resistor manufacturer to ensure adequate safety margin for handling their circuit conditions. However, clearly all metal shunt resistors provide the best pulse handling, thermal stability, TCR, electrical and environmental stability due to their superior materials and construction.