The market for higher-voltage automotive systems is expanding. The number of electric vehicles that use battery-powered drive systems having voltage equal to or higher than 48V is expected to grow by double digits in the coming years. As both consumers and governments press toward less reliance on fossil fuels and demand improved air quality via lower or zero emissions, electric vehicles are moving to the forefront. At the same time, electric systems comprise a greater percentage of overall automotive function. Components that used to be driven mechanically or hydraulically now require higher voltage and more powerful batteries. Onboard entertainment, safety, and convenience features add to the electric draw.
Lithium-ion batteries in a closed system have emerged as the technology solution for electric vehicles. The advantages include power density and charging cycle life; however, to operate safely, lithium-ion batteries require attention in monitoring and protection. Reliable battery operation is only possible with a state of charge maintained between ~20% to ~90% of the maximum. High discharging and overcharging reduce the efficiency and lifespan of lithium-ion batteries. Excessive current flow causes shorts and dendritic lithium plating, which destroys the cell. Under-voltage may cause shorts or breakdown in electrode materials. Excessive temperature can trigger a chain reaction of events, from shorts to outgassing of flammable gases from organic solvents. For all these reasons, designers employ battery management systems that prevent costly and potentially dangerous battery failures.
The challenge for vehicle designers is that a single best practice has not emerged. There is no standard for battery management systems, and it is difficult to maximize economies of scale with the multitude of approaches from which to choose. Companies attempt to find the best way by doing it a different way. This generation of design engineers introduces new voltage classes, new architectures, and new ways to adapt their performance from mechanical metrics to electric. To meet these new challenges for business customers, suppliers are also learning new approaches.
Designers and suppliers agree that circuit protection is mission-critical in electric vehicles employing a very high-energy system operating at hundreds of volts and amps. In the case of an accident, a damaged, crumpled, or punctured battery can lead to a thermal incident or contact with the metal chassis of the car. Design engineers seek to eliminate the possibility of such events, and to do so requires knowing which fuses to use and where to put them. Protection goes beyond performance and to the safety of the consumer, as well as the company's image and reputation.
The fuses used in these applications must meet special performance criteria. Fuses should have a low-temperature derating. Unlike “mobile” batteries in cellphones or tablets, these fuses must adapt to temperature cycles and vibrations. The small form factor is essential. The design needs to incorporate a lifecycle of 15 years, more than 150,000 miles of road vibrations, and 8,000 hours of operation. For example, fuses for use in battery management system sense line protection need to be selected carefully and pass automotive-specific reliability testing.
The battery management system maintains the safe operation of the high-voltage battery, and can relay information about the battery to power and energy management systems. The battery management system requires protection from threats such as overcurrents, surges, and electrostatic discharge (ESD). Fuses, TVS diodes, and diode arrays keep this system reliable and safe under all conditions, from the assembly to maintenance and normal operations.
The battery management system's voltage range and interrupt rating requirements depend on the battery configuration. Within the system, each module of batteries has cell monitoring systems. These subsystems monitor the voltage for proper balance. Microcontrollers then oversee each of these modules to provide the highest energy efficiency and longest life of a battery.
Fundamental Building Blocks
For the creators of a high-voltage battery management system, there are fundamental building blocks in the design. The cells are connected in series, making up a module. As modules connect in series, the total voltage of the system increases. The series of modules in quantity and connected in parallel increases the voltage of the battery and the energy capacity. As cells and modules are added to a battery management system, the complexity and cost increase. The challenge is to find the correct balance of cells/modules, complexity, and cost to create a safe and efficient system.
The designer's selected architecture for the battery management system determines the protection parameters. In a decentralized architecture, sense balancing ICs connect by long wires between the cells and the slave boards. High-voltage fuses are used to reduce the risk of a short circuit under high-voltage conditions, both in the module and between the cells in an accident. If a component is damaged in this decentralized architecture, it can be replaced separately, making it a less expensive and simpler option.
In centralized architecture, all the components integrate into single modules. The distance between the cells and slave boards is much smaller. In this case, an accident is unlikely to cause short circuit under high-voltage conditions; however, low/medium-voltage fuses can be considered to protect against component failure and contamination on a BMS board at a lower cost. Two types of failure must be taken into account: short circuits and overload conditions. The small form factor is important, especially when hybrid space is shared with a combustion engine; however, if one component fails, the entire module has to be replaced, at a higher cost.
Vehicle Charging Systems
SiC MOSFETs improve power density without loss of system efficiency.
Car buyers view the operating distance range per charge as a drawback in hybrid and electric vehicle adoption. A rapid battery charge system alleviates this objection, but demands the car manufacturer improve onboard charging and power density in the finite space available.
Power electronic converter losses challenge electric vehicles. The reduction in size of inductors and transformers with silicon semiconductors would lower system size, weight, and cost, and increase operating distance range. Unfortunately, silicon (Si) semiconductors prove impractical in miniaturization because the semiconductors in the same circuit must be controlled at a high switching frequency. High junction temperatures and thermal load at high switching frequency are not possible at an on-off switch rate of 50 to 100 ns. In contrast, silicon carbide (SiC) MOSFETs can operate efficiently with higher thermal loads, switching up to five to ten times faster than silicon MOSFETs, at 10 ns.
Once difficult to manufacture, SiC MOSFETs are moving mainstream. Their efficiency makes them well suited for high-voltage and high-power electric vehicle applications. SiC MOSFETs used in the design of inverters and other power converters reduce size and weight of other components, without loss of power density. Their ruggedness. reliability, and reasonable cost prove advantageous to green energy manufacturing interests.
The Application is Key
The battery management system is critical for circuit protection both in safety and reliability. Having robust protection throughout the system is essential. Because of the sensing lines at each cell, there is the potential for a short circuit in any cell. The cell monitor block or direct line must also be fused to avoid overcurrent damage. An overview of protection locations for electric vehicles and the devices used include:
ICs used in cell monitoring to protect against overvoltage: TVS diodes
Communication lines between units to protect against ESD: TVS diode arrays
Battery IC protection in case of voltage transients: High-voltage TVS diodes
Microcontrollers: TVS diode arrays
Final protection barrier: High-voltage and high-current fuse in series with the main switch
Other applications (e.g., inverters, DC/DC converters): High-voltage TVS diodes
The application defines the solution. Compared to stationary fuse and battery applications that are inherently vibration-free, cars are a very different environment in which the mass of the fuse matters. Battery voltages have been increased since the first generations of hybrid and electric cars. This requires the fuse to be larger in physical mass. The addition of vibration, inertia, and momentum creates a mechanical as well as an electrical design challenge. Vehicle designers need to be confident that every component performs at or exceeds specified levels. Automotive design engineers should expect to work with a partner who is willing to customize testing to their product level. Suppliers need to offer expertise in simulation testing along many different metrics to serve the automotive market and show certification in automotive-grade standards.
Design engineers are cognizant of the critical safety role of the battery management system in electric vehicle automotive development. Testing is being moved from a final-phase step to early in the process. Partnering with suppliers who embrace the application parameter requirements, provide innovative testing solutions, and support advanced problem-solving with the customer is the level of service expected. In this ever-evolving market, teamwork between customer and supplier develops expertise together.
To reach optimal protection selection within automotive safety standards, automotive designers need to be educated in current safety standards and their requirements, and seek suppliers who are equally committed to knowing the best devices available for an application. Drawing on expertise from innovators in battery management systems and electric vehicles assists with circuit protection device selection or creation of custom solutions.
Regulations and end-user demands will rule the market of hybrid and pure electric vehicles. Safe, efficient, and affordable vehicles of the future depend on designers applying circuit protection solutions able to satisfy those requirements.
This article was written by Carlos Castro, Global Director of Marketing, Automotive Electronics, at Littelfuse, Chicago, IL. For more information, Click Here.