[Engineering Essentials]
Bolster Overcurrent Protection With Chip Fuses
The accuracy, stability, and repeatability of the fusing characteristic are key to selecting the best fuse for the application.
Fuse elements incorporating a mix of different materials, such as copper-tin alloys, are designed to achieve a high value of I2T. Yet they’re particularly sensitive to shorter opening times after successive thermal stresses because the stresses induce migration of the constituent materials.
Figure 4 illustrates the ongoing migration process of Cu-Sn after pulse-load stress. Depending on the magnitude and duration of the power load, these types of fuses change their fusing characteristics to faster melting times. Techniques to preserve the stability of chip fuse resistance value will prevent such drifting of the fusing characteristic.
REPEATABILITY During the design-in process, electronic engineers face high variations of fusing characteristics. Generically, chip fuses are resistors of low ohmic value, having resistances down to the milliohm range. As explained above, the fusing characteristic is related to the resistance value. If there’s a wide variation of resistance value, there will be a corresponding wide variation of fusing characteristic.
Due to this variation in resistance value, a chip fuse may open during normal inrush current. Or, conversely, it may fail to open when necessary during an overload condition. This, of course, is the worst-case situation, which engineers must avoid. Figure 5 illustrates the typical spread of fusing characteristics for printed thickfilm fuses.
SOLVING STABILITY AND PRECISENESS OF FUSING CHARACTERISTIC Thin-film technology can meet all of the requirements in relation to the advanced stability and preciseness and narrow spread of the fusing characteristic. Thin-film sputtering technology has been used to produce highly stable and precise thin-film resistors since the end of the 1960s, and several billions of these devices are now deployed in harsh environmental conditions in all fields of electronics.
Current sputtering techniques benefit from key advantages, such as tight control over the deposit thickness, and achieve a homogenous crystalline structure in the resulting metal layer. When using thinfilm technology to create chip fuses, these attributes directly influence the stability and narrow spread of fusing parameters.
However, tight control over the geometry of the fuse element is also necessary to control the rated current of chip fuses. Structuring of the fuse element using a photolithographic process offers the ability to produce precise geometric contours and dissolve unused conductive material between the terminations.
Using photolithography, the length and width of the fusing element can be controlled with the same accuracy and precision as the thickness of the sputtered thin-film layer. Figure 6 shows how the photolithographic process used to produce the Vishay MFU series thin-film chip fuses creates a fuse element with a clean and clear shape.
SHAPE OF AN MFU FUSE ELEMENT By combining thin-film sputtering technology with photolithography, component manufacturers can achieve tight tolerances on fuse element geometries. At the same time, they can ensure a homogeneous crystalline structure of the fuse element.
This delivers the twin benefits of minimizing stress-induced deviations in the resistance value as well as promoting repeatability in manufacturing. Figure 7 illustrates the resulting close correlation between minimum and maximum blow times for MFU series chip fuses produced using this combination of techniques.
SUMMARY Thin-film technology is an established technology for high-grade passive components, and it has been proven and refined over decades. Its advantages in terms of accuracy, repeatability, and stability are appreciated in mass production for billions of thin-film resistors every year.
Chip fuses produced in thin-film technology now deliver similarly predictable properties in terms of the stability and narrow spread of the fusing characteristic. With this proven technology embodied in next-generation safety devices for overcurrent protection, power electronics designers can achieve higher levels of safety and performance in new product designs.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
