Panel Builder System Integrator

January 4, 2021

By Bill Schweber, Editor, Digi-Key North America division

The thermally activated fuse is the oldest circuit protection device and is still in widespread use. It is well understood, reliable, consistent, and approved by regulatory standards. However, with end products increasing in complexity and shrinking in size, designers need an alternative to the user-replaceable fuse and fuse holder in order to reduce the form factor, simplify assembly, improve ruggedness, and further enhance safety.

Instead, designers can use surface mount devices (SMDs) without a performance compromise. SMD fuses employ diverse technologies to provide thermal-based fusing along with the full range of necessary fuse characteristics, such as fast acting and slow blow.

This article will provide a quick primer on fusing, circuit protection, and design considerations. It will then introduce and describe SMD fuses from Bourns, including their primary characteristics and how they’re applied.

The basic fuse endures

The traditional fuse with its thermally activated fusible link is about 150 years old and is the best-known, most direct type of circuit protection device. It is reliable and simple to understand, with consistency of performance in its single function of providing protection against overcurrent occurrences. It does this by unambiguously and irrevocably opening the circuit path and interrupting current flow when the current level exceeds the amount determined by the fuse’s design.

A traditional fuse is represented by various schematic symbols depending on the graphic standard and consists of a metal wire which is precisely engineered in design, dimensions, and materials. When the current passing through this fusible link exceeds a pre-set limit for enough time, the link will melt as it self-heats. This self-heating is a direct consequence of ohmic I2R power loss due to current flow through the link’s resistance.

Fuses are available in many package types such as the well-known 3AG style small glass cartridge which measures ¼ inch in diameter and 1¼ inches long. For each fuse and current rating, vendors provide detailed graphs showing the relationship between overcurrent value and the accumulated time required for the fuse element to melt and thus stop the current flow through the fuse. This is referred to as the I2t rating indicating available thermal energy resulting from current flow and has units of amperes2-seconds (A2s).

The fuse is not the only circuit protection device used by designers. There are other passive devices which provide other forms of protection by limiting, blocking, shunting, or “crowbarring” excessive surges of current or voltage. None of these, however, offer the clear-cut and irreversible current cutoff of the fuse. They do not replace the function of the fuse, but they may be used where a fuse is not the appropriate protection option, or to supplement the fuse’s action where it makes technical sense. Among the other well-known circuit protection devices are the:

  •    -   Metal oxide varistor (MOV)
  •    -   Positive temperature coefficient (PTC) thermistor
  •    -   Transient voltage suppressor (TVS) diode
  •    -   Gas discharge tube (GDT)
  •    -   Polymer PTC resettable fuse

Like fuses, each of these has a role in providing circuit protection, yet the basic circuit breaking, fusible link retains its role and function in many designs due to its combination of characteristics including consistency, direct action, and irreversibility.

Beyond the replaceable fusible link

Thermal fuses are often assumed to be field-replaceable units when paired with a suitable fuse holder or socket. That said, enabling field replacement by the user is often unnecessary, and can be undesirable for many products. This applies to lower power products such as cell phones, set-top boxes, small battery chargers, AC/DC wall adapters, and toys; mid-range devices including power tools, industrial controllers, and consumer generators; and even higher power systems such as electric vehicle (EV) chargers. Consider these scenarios:

  •    -   Fuses of different ratings may be needed to protect different sub-circuits of a larger circuit including those with sensitive signal paths, rather than the entire product.
  •    -   The unit being fused may be a tiny, sealed product such as a smartphone where the fuse is needed primarily to protect the battery and its charging circuitry, and there is no option for end-user access to the inside.
  •    -   From a safety perspective, unless the actual reason for the blown fuse is known, such as a mechanic having inadvertently touched a power rail and connected it to the car chassis, replacing it just because it is easy to do so is a waste of time at best, and risky at worst. For example, if a fuse is part of a protection circuit for a lithium-based battery and its charging circuit, it is a critical element of that function. Therefore, it is essential to find the root cause of the fuse “blowing” rather than just blindly replacing it.
  •    -   A fuse holder and its contacts add to reliability concerns due to corrosion, vibration, and other operating environment factors.
  •    -   Finally, there’s the issue of size: a fuse that is soldered in place without a holder will have a smaller footprint and lower profile on the pc board.

To implement tiny, holder-free fuses as SMDs and thus use standard board population and soldering equipment, it is necessary to look beyond the traditional fuse with its wire-like fusible link, while still retaining the self-heating principle which melts and thus opens the current path.

Wide range of SMD fuses meet challenges of modern design

By using combinations of materials, technologies, formulations, and manufacturing techniques, Bourns has developed a family of SMD fuses which can provide the thermally-based fuse function over a wide range of currents and operating voltages. The Bourns SinglFuse SMD fuses product portfolio employs seven different fuse construction technologies: thin film sputtering, thin film pc board, ceramic multilayer, ceramic cavity laminate, wire core, ceramic tube, and ceramic cube.

This variety of technologies and construction approaches enables the broad SinglFuse portfolio to offer fuses with a wide range of specifications across key parameters such as rated current, rated voltage, breaking capacity, I2t, and operating temperature. In addition, the SinglFuse products are UL, TUV and VDE compliant, and conform with UL 248 and IEC 60127 standards, which eases the path to overall product certification. For automotive applications where full specifications and reliable operation over a wide temperature are required, one of many automotive mandates, or for other harsh operating environments, fuses that are compliant with AEC-Q200 are available.

Small SMD size does not limit capabilities

There are situations where the imperative for smaller components, especially in SMD packages, imposes limits on their features or capabilities. This is not the case with the SinglFuse devices, which are available in package sizes from an almost invisible 0402 (0.040 inch × 0.020 inch; 1.0 × 0.5 millimeter (mm)) at the lower current ranges to 3812 (0.150 inch × 0.100 inch; 3.81 × 2.54 mm) for the higher capacity fuses, still a diminutive size.

Over the years, fuse vendors have developed specialized versions of fusible link devices with unique attributes to meet circuit needs. Recognizing this situation, the SinglFuse devices are available with different response characteristics, including:

  •    -   Fast acting
  •    -   Fast acting precision: with tighter tolerance on key specifications
  •    -   Slow blow: to handle a temporary surge current which exceeds the current rating of the fuse
  •    -   Time lag: allows electrical surge for a short time before it actually blows
  •    -   High inrush current: for excessive start-up currents

SMDFusesFigure4.JPG

Note that the specifics of the current-versus-time profiles for these different fuse “personalities” are defined on their respective data sheets and should be studied by the designer to get the best match to the application.

Current rating extremes show range of performance

Designers can use SMD fuses across a wide range of nominal current values. For example, the SF-2410FP0062T-2 fast acting precision SMD fuse is housed in a ceramic tube with an EIA 2410 (6125 metric) footprint, and measures about 6 mm long and 2.1 × 2.6 mm on the rectangular end.

This fuse is specified for 125 volts AC/DC operation and has a 62 milliamp (mA) nominal rating along with a typical I2t rating of 0.0012 A2s. While the top-level summary specification is that it opens within five seconds at 200% of rated current, users will likely want to study its performance graphs which quantify pre-arcing time (Figure 4) and I2t rating (Figure 5), key indicators of fuse response time. Designers should be aware of the IR voltage drop through the fuse when operating within its current rating due to its resistance of about 6 Ohms (Ω); this drop is under 40 millivolts (mV) maximum.

A very different range and performance profile is offered by the SF-1206S700 slow blow fuse, a 7 A device specified to open within five seconds at 250% of its maximum rated current.SMDFusesFigure5.JPG

The SF-1206S700 uses a different package and technology than the SF-2410FP-T, and comes in a 3216 (EIA 1206, 1.55 × 3.1 mm) flat package measuring just 0.6 mm high due to its thin film construction. Its resistance of just 7 milliohms (mΩ) ensures a low IR drop of just under 50 mV at maximum current.

While the data sheet for this fuse has graphs similar to those for the SF-2410FP-T 62 mA fuse, as a slow blow fuse it also needs an “I2T Derating Curve vs Repeater Rush Current” curve which further defines slow blow fuse performance with repeated full on/off circuit operation.

It’s useful for designers to hands-on evaluate different fuses with respect to their type, ratings, and size, but there’s a challenge in doing so. Unlike active components (op amps, for example) or passive ones (resistors, indicators, capacitors), a fuse is a one-time device and can only be fully tested by pushing it to effectively self-destruct. As a consequence, it is useful to have multiples of various fuse values and types in hand for evaluation.

To facilitate this process, Bourns offers the SF-SP-LAB1 SinglFuse SMD FuseLab Kit for quick-turn prototype testing. It contains five each of 18 slow blow fuses (total of 90 pieces) in 0402, 0603, and 1206 size fuses ((1608 to 3216 metric); the similar SF-FP-LAB1 kit has 160 pieces of fast acting precision fuses (five each of 32 values) in 0402 to 1206 (1005 to 3216 metric) packages.

Conclusion

Despite their conceptual simplicity, thermal fuses are sophisticated, passive electrical and mechanical components based on advanced thermal, material, and fabrication considerations. As circuits and products shrink, making user replacement of a fuse increasingly impractical, unwise, or even dangerous, the need for surface mount device (SMD) fuses, which are handled just like any other SMD device, is apparent. In addition, SMD fuses simplify the assembly and manufacturing process and reduce a design’s susceptibility to vibration and corrosion.

As shown, the Bourns SinglFuse Series of SMD fuses offers designers a wide range of overcurrent protection ranges and types to meet the needs of today’s products and pc board fabrication processes.

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