What are the major technical challenges for EVs?

The majority of new electronic systems in conventional vehicles, besides active safety, autonomous driving, and infotainment systems, are being used to enable energy savings with direct injection, start-stop systems and BLDC motor drives in body and chassis electronics. Carbon dioxide legislation, with the request of 95g/km CO², is driving the push for increased fuel efficiency and more vehicle electrification, especially in urban centers and mega cities with heavy traffic. CO² and particle emissions need to be significantly reduced to maintain air quality.  

Several factors are responsible for the future trends and successes of EVs, from battery technology - energy density, size, and price; driving range and efficiency; charging performance, time, and infrastructure to price, reliability and maintenance costs; and safety.

Power allocation

In a crash situation, the electronic system needs to disconnect from all existing energy storage elements, such as batteries, capacitors, and inductive components. Direct contact with high voltages can cause significant bodily harm to drivers, passengers and first responders. To discharge the energy of such storage elements, resistive dummy loads need to be connected immediately.

Intelligent energy management is important in ensuring all safety-related applications, such as braking, steering, wiper, lighting, and passive safety systems, are available over long driving distances. In addition to safety electronics, which have the highest priority for power consumption, comfort electronics also have to be considered. Air conditioning during the summer and compartment heating and window defrosting during the winter are must-have features in a modern vehicle. In EV designs the big challenge is to reduce the electrical consumption of such high-power loads.

The next most important task is to provide enough charging stations for the area in which vehicles are moving, and especially parking. Fast charging is of great importance for the end user, who usually does not like to wait more than two hours for a fully charged battery. During work hours, business visits, or shopping times, the modern EV has to be fully charged.

The EV’s battery charging system’s main functions are to convert AC to DC, perform PFC functions and match the charging profile of the battery system.

Battery charging options

There are two different major methods of battery charging, each with their own advantages. The first is on-board charging, i.e. one- or three-phase AC charging from the electric grid line. Advantages are that it is easy to connect to grid power and no major charging infrastructure is required. The second option is off-board charging, i.e fast and high-DC off-board charging. Advantages are short times, high power and fast charging performance. It requires a charger infrastructure with universal high-power DC chargers.

A key part of an on-board charging system is the AC/DC inverter, which is integrated into the vehicle network. It connects the car to the AC electric grid and converts to DC power. Due to high voltage usage, safety is a concern and standards need to be applied. All electronics have to fulfill these automotive-grade quality standards.

Another option is to have an off-board DC/DC charger with high-voltage DC input to the EV instead of AC input. This option offers very high-power charging functionality and allows weight and space savings for the on-board charger function, which remains as the battery charging stage control and communicates with the off-board charger. It keeps AC voltage and its related safety concerns away from the vehicle and reduces the transient level for ECUs. Such industrial chargers with power up to 50 kW are available and will be part of infrastructure installations such as parking areas and bus stops as part of the automotive segment.

Contactless charging

A third method, just on the horizon, concerns contactless inductive charging. It aims to provide a charging infrastructure almost everywhere to reduce the stop time for charging, and offers almost instant charging possibilities.

The semiconductor and passive component industry has to design new components to reduce the cost of EV controllers and actuators. Mechatronic and high-voltage driver solutions are the key components in optimising reliability and increasing efficiency.

Multi-phase converters and inverters are strong focuses for designs. Component manufacturers are developing cost-performance-optimised components and new topologies for high-power and high-energy applications.

Figure 1 - Contactless inductive charging aims to make charging ubiquitous to reduce the stop time for charging.

Major components for EVs are IGBT modules for motor drives and inverters, high-voltage MOSFETs, high-current filter inductors, planar transformers, optocouplers, solid state relays, high-voltage resistor dividers, PTC current limiters, high-voltage diodes for power drives and rectification modules.

Passive components need more space and have higher costs. Their design is also more critical than the design of semiconductor modules. New topologies work on higher switching frequencies to reduce the size of passive parts, such as transformers, filters and energy storage components. These topologies include film capacitors for DC-linking or filtering, aluminum capacitors for DC-linking or buffering, and shunt resistors for high- voltage and high-current sensing. Planar transformers are for high switching frequencies and offer the best efficiency in high-voltage DC/DC converters.

Electronic drivers

The electronic drivers for EVs are split into two high-voltage applications (150 to 550VDC battery line) and low-voltage applications (12V loads). The DC/DC buck converter, to generate the 12V out of the high-voltage Li-Ion battery, is responsible for low power loads up to 100W. The efficiency for such converters has to be as high as possible.

One of the biggest challenges for EVs is ensuring the efficiency of motor drives using high-voltage semiconductor drivers. To avoid sparks in high-voltage switches, it is necessary to discharge the energy stored in the battery and other elements by using dummy energy resistors, which quickly eliminate energy (Joules) to avoid a fire. The emergency battery disconnect is another area to be optimised by redesigning the existing big and heavy profiles.

As with conventional automobiles, system designers are trying to reduce the amount of parts. One way to do this is a series of matched resistor dividers, accurate for applications with voltages up to 3kV. Such surface-mount, high-voltage dividers can replace 20-40 single resistors. They are currently used as floating dividers to detect voltage stability in the board system and support the voltage drop regulation for increased efficiency.

Various parts of the EV present their own particular challenges. For example, motor drives for air conditioning compressors require highly efficient galvanic isolated DC/DC converters. For this application, discrete components with extremely low profiles play a key role.

Voltages above 30VAC and 60VDC require enhanced protection against human body electric shock. Galvanic isolation between low-voltage (12V) digital / analogue parts and the high-voltage terminal is essential.

EVs will be of interest for short-distance driving (average distance of 50km/day and maximum of 100km) and cannot yet satisfy long-distance driving (i.e. over 150km). Due to higher end user prices for EVs compared to conventional cars, investment for charging infrastructure, and development towards alternative energies, mainly government regulations and incentives can enhance the quantitative development of Battery Electric Vehicles (BEVs).