The traditional fuse, contactor, or relay solution is being reevaluated in favor of faster-response, more robust, and more reliable solutions, such as the pyrofuse and the electronic fuse, or E-Fuse. A leading solution is the E-Fuse, based on silicon carbide (SiC) technology. SiC offers high operating voltage, high operating temperature, low on-state resistance, low off-state leakage current, and durability against overvoltage transients. The E-Fuse's solid-state design eliminates reliability issues related to arcing, mechanical wear, contact bounce, and spot welding. Economizer hardware is no longer required to actuate contactor coils. The E-Fuse improves system performance through its configurability, controlled on/off operation, integrated diagnostics, and resistance to high-voltage transients.
The resettable design eliminates the need for maintenance.
With a fully SiC-based design, E-Fuse has an unparalleled response time to short circuits, reacting several hundred times faster than even a fuse-based ignition coil. This makes E-Fuse a natural complement to an ignition coil-based protection solution. While an ignition coil offers robust and reliable circuit protection, it is not resettable. It is a single-use device, like the ignition coil in an airbag.
An ignition coil is used as a safety measure to deactivate the system in critical situations. Once detonated, it must be replaced. Replacing a component in a high-voltage system is not as simple as in a 12V system. With system voltages of 400V or 800V, well above the 60V limit that the automotive industry generally considers safe, repairs can only be safely performed by qualified service technicians. Fortunately, with a configurable tripping profile, an E-Fuse, as a complementary system-level solution, can be more sensitive to overcurrents than a fire fuse, ensuring it trips first and preventing the fire fuse from activating. One advantage of the electronic fuse over current solutions is that it can be reset, saving the electric vehicle owner the time, expense, and hassle associated with taking the vehicle to a service center.
Robust DC Circuit Protection.
Protecting circuits in high-voltage DC systems presents a unique challenge. Unlike AC systems, where zero crossing helps extinguish an arc, DC systems lack this zero crossing. To mitigate this, high-voltage VE relays and contactors incorporate sophisticated additional features to safely extinguish electrical arcs. However, arcs still erode contacts and cause reliability issues, such as high contact resistance or spot welding. On the other hand
, the E-Fuse safely disconnects DC circuits without generating an arc. The same type of inductive energy responsible for arcing in a relay-based solution is present in a circuit protected by an E-Fuse, so an E-Fuse solution must absorb that energy when interrupting a current.
The key difference is that an E-Fuse has a fast response time, reducing the peak current to orders of magnitude lower than that of a traditional solution. Since inductive energy is proportional to the square of the current, a decrease in peak short-circuit current results in a significant decrease in step energy. This also means lower voltage on the wiring and a potential downstream fault load.
Electronic Fuse Demonstrator with Configurable Firing Profile.
The Microchip E-Fuse Auxiliary Technology Demonstrator shown in Figure 1 is available to designers developing high-voltage E-Fuses for automotive or solid-state relays. The six hardware variants provide 400 V and 800 V options and current ratings of 10 A, 20 A, and 30 A, enabling the evaluation of single or parallel SiC MOSFETs with RDS(on) ratings from 15 mΩ to 40 mΩ.
Figure 1 - Microchip E-Fuse Auxiliary Technology Demonstrator
The E-Fuse's control and protection circuits are powered by the 12V system. Equipped with a LIN communication interface, it supports direct connection to a 12V battery, waking from sleep mode via LIN activity, or from the switched battery output of a control module.
The E-Fuse incorporates three overcurrent detection methods, ranging from slight overcurrents to very high short-circuit currents, as illustrated by the time-current characteristic (TCC) curve in Figure 2. The TCC curve defines the E-Fuse's fuse-like behavior, with a slow response time to low overcurrents and a fast response to high overcurrents.
It can be easily adjusted to protect wiring and loads. All three detection methods are easily configurable via software or through the LIN interface. The leftmost detection method, shown in blue, characterizes the tripping behavior using a junction temperature estimation algorithm. This algorithm uses current measurement, ambient temperature measurement, the RDS(on) of the SiC MOSFET, and thermal design characteristics to estimate the SiC MOSFET's junction temperature.
The response time varies with the magnitude of the overcurrent. The center line segment represents a detection method that uses only current measurement with a fixed response time. The rightmost line segment represents a hardware-based, though software-configurable, detection method. This method leverages the core-independent peripherals (CIPs) of the PIC® MCU, specifically a comparator, a fixed voltage reference, a digital-to-analog converter, and configurable logic cells configured as SR latches. This allows for rapid signal propagation in under a few hundred nanoseconds, enabling immediate detection of a short circuit and protection of the high-voltage system.
Figure 2 - Time-current characteristic curve for a 400 V, 20 A fuse variant E
In addition to its fuse-like behavior, an E-Fuse can function as an electromechanical relay. Just as a relay coil and its high-voltage contacts are galvanically isolated, a high-voltage E-Fuse also includes an isolation barrier between the control signals and the high-voltage terminals. Like a relay, an E-Fuse can be connected in the system as a high-voltage output supplying power from the high-voltage positive battery to a load, or as a low-voltage output providing a path for the load to the negative return of the high-voltage battery, as illustrated in Figure 3.
Figure 3 - E-Fuse system-level configurations
High-Voltage Short-Circuit Performance
To truly appreciate the difference in response time between an E-Fuse and a conventional automotive high-voltage fuse, each was subjected to a short circuit under similar test conditions of 450 V and approximately 3 µH of line inductance. The resulting waveforms are shown in Figure 4. The black waveform represents the current flowing through the high-voltage fuse under test. In 30 µs, the current reaches 3800 A, which is the limit of the measuring equipment, and melts the high-voltage fuse 50 µs later. Based on the test parameters, the peak current is estimated to have exceeded 6000 A. However, with an E-Fuse, as shown in the blue waveform, the current reaches only 128 A before tripping. This is a significant reduction in the through current, minimizing stress on the wiring and downstream loads.
It offers system designers the option to optimize wiring in terms of weight and cost. In some cases, the low current throughput of the E-Fuse will be the difference between a towing condition, where a fault causing a high voltage surge results in permanent hardware damage, and a recoverable fault that allows the system to automatically reset and the driver to continue operating the vehicle.
Figure 4 - Current waveforms for the E-Fuse versus the high voltage fuse
Beyond the electric vehicle itself, supporting infrastructure such as DC fast-charging stations or the microgrids that supply those stations can also benefit from E-Fuse. The advantages of an E-Fuse are not limited to automotive applications.
Applications using fuses and contactors benefit from some of the topics discussed, as well as other advantages, such as onboard current sensing, which enables greater system-level integration and optimization. Off-board applications use a common-source anti-series SiC MOSFET configuration and may require a higher current capacity than the demonstrator offers. Fortunately, the design is easily scalable and can be adapted for use with readily available common-source SiC power modules.
As more and more attention is paid to performance, safety, and reliability, E-Fuse as a circuit protection solution will continue to grow as the preferred method, just as we have seen the shift in the 12V system from fuses and relays to protected solid-state controllers and, more recently, to low-voltage E-Fuse.
AUTHOR: By Ehab Tarmoom, Technical Application Engineer, Silicon Carbide Business Unit, Microchip Technology
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