Comparison tests of real-world applications demonstrate the superior performance of latest surface-mount fuses

BY MIKE ROACH, Technical
Sales Manager
AEM Components
www.aemcomponents.com

Automotive
applications for electronics are subjected to among the harshest of
environments — wide temperature variations, shock and vibration, and exposure
to humidity, water, and salt. Traditionally, blade-type automotive fuses,
located under the dashboard in a fuse box, provided the necessary fault
protection. But as cars get “smart” and “connected,” more and more embedded and
distributed electronics have required pc board-mounted circuit protection. And
with the rapid emergence of electric (EV) and hybrid electric (HEV) vehicles —
most with high-energy lithium battery systems — the demand for reliable circuit
protection devices to protect against catastrophic failures is critical. These
applications have placed emphasis on improving surface-mount fuse technology.

This
article describes the common types of one-time, surface-mount fuses and
compares alternative fuse structures for each type. It presents the results of
comparison tests simulating real-world application scenarios to demonstrate the
dramatically better performance offered by the latest generation of
surface-mount fuses.

The
need for surface-mount fuses
Historically,
it is typically an industry’s need for technology advances that ultimately
drives adoption in a wide range of applications. For example, brighter, more efficient,
and lighter displays — first developed by the television industry — are now an
integral part of consumer, commercial, industrial, military, and aerospace
applications. In a similar way, it would appear that the demands of the
automotive industry are driving advances in circuit protection technology.

Resettable
devices are the ideal choice in which overcurrent conditions are the result of
a transitory fault condition. But in many applications, particularly where
fault currents can result in serious damage to other circuits or systems, the
venerable fuse is still the best choice for protection. While traditional
clip-mounted glass-tube fuses are found in a host of applications and
blade-type fuses are ubiquitous in automotive applications, the move toward
smaller, distributed, and embedded electronic functionality has elevated the
demand for high-performance, space-saving surface-mount fuses. In a similar
manner as chip inductors and multi-layer ceramic capacitors (MLCCs), surface-mount
fuses are packaged in a variety of EIA standard sizes determined by the
technology uses and the rating.

While
others exist, this article focuses on common surface-mount fuse types: solid-body,
or chip, fuses and wire-in-air fuses.

Solid-body
(chip) fuses
Solid-body,
or chip, fuses are used in a very wide range of space-constrained applications,
including portable electronics, entertainment systems, disk drives, and many
others. Current ratings typically range from as low as 125 mA to several amperes.
Devices are offered in both slow-blow and fast-acting configurations.

The
two most common structures for solid-body fuses are the multi-layer ceramic
type and printed-circuit style. The ceramic fuse has a co-fired monolithic
structure with up to four layers of fusible material embedded in the structure.
In the printed circuit structure, the device is mainly comprised of the epoxy
substrate and glass fiber (FR4) structure. The fuse element is bonded to the
surface of the pc board and coated with a protective polymer.

While
the printed-circuit style is most common, the ceramic type offers several
distinct advantages. Because of its monolithic structure, it is capable of
higher current ratings in a smaller package, has a wider operating temperature, and has stable operating characteristics in extreme conditions. Additionally, the structure
is less susceptible to mechanical damage.

A
new advancement in ceramic fuse technology is the SolidMatrix ceramic fuse.
This solid-body ceramic fuse’s patented, multi-layer construction provides
excellent mechanical and thermal stability over a wide temperature range (–55°C
to 150°C).

Fig. 1
shows a how a SolidMatrix ceramic fuse and a conventional printed-circuit-type
fuse might behave when subjected to an overcurrent fault condition. The fuse
element of the conventional printed-circuit-board-type fuse on the right opened
as intended, but the high overcurrent fault condition led to surface melting,
cracking, and compromised mechanical integrity. As a result, arcing and surface
damage are very apparent in the image on the right.

The
SolidMatrix ceramic fuse, as shown in the left-hand image in Fig. 1, fared significantly better. The proprietary
construction of this device allows for the metal fusing elements to be defused
into the ceramic; plus, their central location ensures that the energy is
contained within the body. As a result, mechanical integrity is maintained and
there is no external change in the device’s appearance.

AEM01_CircProt_jun2017

Fig.
1: Monolithic structure of a SolidMatrix ceramic fuse (left) absorbs fault
current and shows no external damage — compared to the visible damage of the
conventional printed-circuit-type chip fuse (right).

Wire-in-air fuses
Wire-in-air
fuses are typically found in higher-operating-current applications in which
fast-acting and superior arc suppression are required. Applications include
battery chargers, battery packs, and circuits subject to very high fault
currents and higher voltages. The common construction for this type of fuse has
the fusible wire element housed inside a ceramic tube and connected to the
endcaps with solder beads.

There
are several disadvantages associated with conventional wire-in-air fuses.
Endcap detachment is a common failure mode in the conventional construction.
There is also a lack of uniformity in performance due to the variability in the
placement of the wire element inside the ceramic tube. Additionally, under
worst-case, high-current-stress conditions, the solder in the ceramic tube can
vaporize and build up pressure to the point where the fuse explodes. If this
occurs, solder is redeposited across the trace, which can result in a secondary
conductive path with potentially serious consequences.

In
comparison, the fuse element of advanced AirMatrix wire-in-air fuses uses a
proprietary, hermetically sealed wire-in-air structure that ensures consistent
electrical performance. The AirMatrix’s fuse element is uniformly straight
across the cavity and externally bonded to the endcap. Unlike the conventional
square nano-type fuse, with its ceramic body and solder connect design, the
AirMatrix fuse uses a fiberglass-enforced body and solderless direct connect
construction.

Fig. 2 shows two conventional wire-in-air fuses
subjected to an EV short-circuit condition. Sample A at 250 V/250 A (left
image) and Sample B at 450 V/450 A exhibited significant damage to the fuse and
collateral damage to the surrounding circuitry. In the waveforms, the current
flow (yellow trace) through the fuses each displays secondary current flow that
ultimately resulted in pc board damage.

AEM02ab_CircProt_jun2017

Fig.
2: Damage resulting from two conventional wire-in-air fuses subjected to
extreme overload conditions — simulating a
catastrophic EV battery short-circuit.

When
subjected to the same EV battery short-circuit as the square nanotube fuses,
the AirMatrix fuse’s advanced construction withstood 450-V/450 A conditions
without experiencing any external damage (see Fig. 3). Note how in the waveforms, the current flow (yellow
trace) through the AirMatrix fuse drops to zero. The voltage (green trace)
shows an open circuit for the AirMatrix fuse with no secondary conduction.

AEM03_CircProt_jun2017

Fig.
3: AirMatrix fuse sustains no damage after being subjected to extreme overload
conditions — simulating a
catastrophic EV battery short-circuit.

Meeting
automotive standards

Automotive
applications engineers need to qualify their devices for the AEC-Q200
automotive standard. As demonstrated by reliability testing, new structures
being utilized by SolidMatrix multi-layer ceramic chip fuse and the AirMatrix
wire-in-air fuse offer significant advantages over typical fuse approaches.
These fuses are manufactured in a TS16949-certified facility and are
specifically designed for reliable operation in high-stress automotive
applications.