Mike Roach, AEM Components (USA), Inc.
Electronic systems for automotive applications are subjected to among the harshest of environments – wide temperature variations, shock and vibration, exposure to humidity, water chemicals and salt. Traditionally, blade-type automotive fuses have provided the necessary fault protection, but as cars get “smart” and “connected,” more and more embedded and distributed electronics have started to turn to pc board-mounted circuit protection. The rapid emergence of electric (EV) and hybrid electric vehicles (HEV) in particular, have underscored the necessity for highly reliable circuit protection to help protect against catastrophic failures and this has in turn placed increased emphasis on improving surface-mount fuse technology.
This article describes the common types of one-time, wire-in-air surface-mount fuses, and compares the 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 latest generation of these type surface-mount fuses.
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 – now are integral part of consumer, commercial, industrial, military and aerospace applications. In a similar way, it would appear the demands of the automotive industry are driving advances in circuit protection technology.
Resettable devices are the ideal choice where 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 traditional one-time 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 other surface mount, solid body fuses exist, this article focuses primarily on higher voltage applications that would generally utilize surface mount, wire-in-air fuses. In addition, information related to etched fuses is also presented, as they have garnered interest due of their low cost and limited use of board real estate.
Wire-in-Air fuses are typically found in higher operating current applications where 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.
There are several disadvantages associated with these conventional ceramic wire-in-air fuses. Endcap detachment can be a common failure mode in the conventional construction. In addition, here is also a lack of uniformity in performance due to the variability in the placement of the wire element inside the ceramic tube. Even worse is the fact that under worst case, high current-fault conditions, the solder in the ceramic tube can vaporize and build up pressure to the point where the fuse explodes. If this happens, 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 use a proprietary, hermetically-sealed structure that assures repeatable and 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.
Figure 1 shows two conventional wire-in-air fuses subjected to an EV short circuit condition. Sample A at 250V/250A (left image) and Sample B at 450V/450A 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 display secondary arcing that ultimately resulted in pc board damage.
When subjected to the same EV battery short circuit conditions as the square nano tube fuses, the AirMatrix fuse’s advanced construction withstood 450V/450A conditions without experiencing any external damage (Figure 2). Note how in the waveforms, the current flow (yellow trace) through the AirMatrix fuse drops to zero, with no secondary conduction and the voltage potential across the fuse (green trace) shows an open circuit.
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Figure 2: AirMatrix fuse sustains no damage after being subjected to extreme overload conditions – simulating a catastrophic EV battery short circuit
One other approach being given consideration as an option for circuit protection in
BMS sensing lines is etched fuses on flexible printed circuit boards. Given the lack of board space needed for these fuses and their comparative low cost, it is easy to see why they would be attractive, but a lack in consistency in response time from fuse to fuse, and their open-air construction, can present a considerably safety risk, especially in worst case overload conditions.
To determine the suitability of etched fuse technology for this highly demanding automotive application, samples of a 1 amp etched fuse and 1 amp advanced wire-in-air AirMatrix fuse were subjected to overload test conditions of between 7 & 50 amps and 50 & 125 Vdc and monitored closely throughout the test for performance.
In all cases, the etched fuse exhibited some degree of overheating, with the degree of damage being further pronounced at more extreme conditions. At higher current values, the amount of energy present was sufficient enough to cause combustion.
As with earlier testing performed on the AirMatrix fuses, these samples exhibited no external damage and no evidence of overheating, while maintaining their mechanical integrity, even at the highest test level of 50 amps and 125 Vdc. (See Figure 3)
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Figure 3: All etched fuse samples exhibited some degree of overheating, burning or fire after being subjected to overload conditions, whereas all the AirMatrix fuse samples passed the overload conditions with no external damage or damage or indication of overheating
Meeting Automotive Standards for Performance and Safety
As demonstrated by reliability testing, new structures being utilized in both solid body and wire-in-air surface-mount fuses offer significant advantages in performance and safety over the more traditional fuse approaches. The SolidMatrix multi-layer ceramic chip fuse and the AirMatrix wire-in-air fuse are specifically designed to help automotive applications engineers qualify their devices for the AEC-Q200 automotive standard. These advanced surface-mount fuses are manufactured in aIATF16949-certified facility and are specifically designed for reliable performance in highly demanding automotive applications.