The levels of miniaturization typically demanded by these end user groups place a premium on every square millimeter of board space. Designers need to minimize the real estate devoted to secondary over-current protection. Conventional wire fuses in SMD packages display a number of strong features: they are robust, have high breaking capacity, are available in ratings up to 10 A, and the technology also supports fast acting or time delay type fuse operation. They can address a wide range of applications, including over-current protection of power lines. On the other hand, package sizes are not likely to reduce below the industrystandard 2410 SMT outline. In consumer applications where low rated currents and breaking capacities are required, Chip Fuses are emerging to satisfy designers’ demands for the next level of component miniaturization.

Chip fuses feature a conductive fuse element that is typically deposited as a thick-film, electroplated, or thin-film layer onto a ceramic substrate. Using these basic technologies, secondary over-current protection is able to migrate into smaller SMT packages including 1206, 0603, and even 0402. However, two further imperatives are the need for long-term stability of the fusing characteristic and a low unit price to enable a cost-effective solution. Stability is heavily dependent on the accuracy of the fabrication technique used to create the fuse element. Traditionally, a thick-film element for a chip fuse is deposited using a screen printing process, while most fuse elements are electroplated. Both of these techniques enable quite accurate control over the dimensions of the fuse element in order to achieve the desired fusing characteristic. However, the homogeneous crystal structure of the metal layer has an important influence over the long-term stability, due to aging factors such as power dissipation or external high temperatures in combination with thermal cycles. To simultaneously improve control over the dimensions and crystal structure of the fuse element, Vishay Beyschlag MFU-series chip fuses are created using a thin film sputtering process in place of screen printing or electro-plating.

This process leverages precision chip resistor manufacturing knowledge and assembly capacity. In addition, a special protective coating comprising layers of glass and epoxy lacquer also now raises the capability of chip fuses to withstand harsh thermal shocks and wide-ranging humidity requirements. Benchmark tests on MFU series chip fuses demonstrate superior performance in this respect due to this special protective system.

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- Mount 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.

The Basics: How Does a Fuse Work?

A fuse is a simple and highly effective way to protect a device from dangerous levels of current:

• Current flowing through a conductor’s nonzero resistance leads to power dissipation.

• Power is dissipated in the form of heat.

• Heat raises the temperature of the conductor.

• If the combination of current amplitude and duration is sufficient to raise the temperature above the fuse’s melting point, the fuse becomes an open circuit and current flow ceases.

Though the fundamental operation of an Axial Lead & Cartridge Fuse is not complicated, there are subtle points to keep in mind. The rest of this article will help you to understand some important details related to the behavior and use of fuses.

How a Fuse Is Tripped: Heat, Not Current

A fuse is not tripped directly by current; rather, the current creates heat, and heat trips the fuse. This is actually a rather important distinction because it means that the Automotive Fuse operation is influenced by ambient temperature and by the temporal characteristics of the current.

The specified current rating of a fuse is relevant only to a specific ambient temperature (usually, or maybe always, 25°C), and consequently you need to adjust your fuse selection if you’re designing a device that will operate outdoors in, say, Antarctica or Death Valley. The following plot shows how ambient temperature affects the actual current rating—relative to the nominal 25°C current rating—of three types of fuses.

Regarding the temporal characteristics of the current passing through the Power Fuse, we all know that the effect of heat accumulates over time (momentarily touching a hot skillet is nothing compared to picking it up and realizing that it’s hot when you’re halfway between the stove and the dining table). Consequently, the current rating of a fuse is a simplification of its real behavior. We can’t expect a fuse to respond to high-amplitude transients because the short duration of the higher power dissipation doesn’t increase the temperature enough to cause tripping.

The following plot shows the time-current characteristics for a group of surface-mount fuses made by Panasonic. The rated current is on top, and the curve represents the amount of time required to trip the fuse in relation to the amount of current flowing through the fuse.

Fuse Design Best Practices: Rated Current vs. Operating Current

It would be perfectly reasonable to assume that a fuse rated for 6 amps could be used in a circuit that might need 5 amps of steady-state current. It turns out, though, that this is not good design practice.

The current rating of a Resettable Fuse is not a high-precision specification, and furthermore (as discussed above) the actual tripping current is influenced by ambient temperature. Consequently, to avoid “nuisance tripping,” you should have a fairly generous gap between your expected steady-state current and your fuse’s rated current.

This document from Littelfuse suggests a “rerating” of 25% (for operation at room temperature); thus, a fuse with a rating of 10 amps would be used only if the circuit’s steady-state current will stay below 7.5 amps.