Overcoming electric vehicle range anxiety with wireless

By Majid Dadafshar, ON Semiconductor

The automotive industry is going through one of the biggest periods of change in history as powertrains evolve from internal combustion engines (ICE) to electric motors . While technological advances in modern electric vehicles being stuck with a dead battery , known as “range anxiety.”

Most efforts to address this challenge have been focused on making batteries better and vehicles more energy efficient, but other approaches are beginning to emerge. One of the most interesting is the ability to charge EVs wirelessly, allowing the battery to be fully charged while the vehicle is running and without being " hardwired" to a power source . Semiconductor technology plays an important role in the success of wireless electric vehicle charging (WEVC) .

Adopting new technology involves a process of change that can be difficult for many mainstream consumers , unlike early adopters who seem to enjoy the “change” itself . Given the early days of EV development, range anxiety is often cited as a reason for slower-than-expected adoption . Even on a full charge, the typical EV has a much shorter range than a gasoline-powered vehicle for anything other than local commuting . This means charging away from home can become a necessity. Additionally, charging stations are far less common than gas stations, leading to the possibility and fear of being stranded . Finally, while advances in power management technology have allowed charging times to be drastically reduced, they are still much longer than at a traditional gas station.

While charging infrastructure is expanding rapidly, especially with companies like Volkswagen investing $2 billion in clean car infrastructure in the U.S. as part of its efforts to deal with the diesel emissions scandal, many companies are looking for other ways to charge their vehicles more conveniently. One key technology being discussed and evaluated is wireless charging , specifically the ability to eventually charge vehicles dynamically.

Although many people view wireless charging as new technology, it is actually a century old . As early as 1894, Nikolai Tesla proved the feasibility of the technology by powering an entire laboratory of electric lights in New York City. However, there was little progress since then until the recent growth of mobile devices, which brought the technology back to the forefront, mainly because of the convenience it brings to users .

How Wireless Technology Works

In principle, wireless charging works very similarly to wired charging. The mains voltage is converted to direct current (DC) and used to charge the battery. At higher power levels, a power factor correction (PFC) stage is used. Most mains-based chargers use a galvanic isolation transformer, which is the essential difference between wired and wireless chargers.

Figure 1: Typical charger block diagram

In wired applications, the transformer is a unit with a core that ensures that (almost) all flux generated in the primary is coupled to the secondary. This ensures high levels of power transfer, which in turn helps to build an energy-efficient charger.

To create a wireless charger, the transformer is split into a primary and a secondary, with the primary (transmitter) remaining in the charger and the secondary (receiver) located in the device to be charged. The distance between the primary and secondary will vary depending on the application and will have a significant impact on the performance of the charger.

By replacing the core with "air", the flux transfer is reduced. If the coupling coefficient (k) is approximately 1 in a core-based transformer, then in a wireless application the value of k will be closer to 0.25. The actual value will be inversely proportional to the distance between the two coils and will also decrease if the primary and secondary are misaligned.

However, this situation can be improved by introducing magnetic resonance in the primary and secondary. By using two tuned circuits, power is transferred at a specific frequency and the efficiency of power transfer can be nearly doubled compared to non-resonant methods.

Figure 2: Wireless power transfer using the resonant method

Another advantage of this approach is better electromagnetic interference (EMI) performance, which is critical for the large-scale promotion of wireless charging. It also allows the use of technologies such as zero voltage switching (ZVS) or zero current switching (ZCS), both of which play an important role in achieving extremely energy-efficient power transfer.

Current status of WEVC

Even though plugging in a power source will remain the best way to charge a deeply discharged battery for the foreseeable future, the goal of WEVC is to charge the battery while the vehicle is in motion. The ability to charge the vehicle while it is in use could enable longer range or allow for smaller batteries, which in turn increases range by reducing battery/overall vehicle weight.

In recent years, many academic institutions and companies have been involved in developing prototype systems to achieve WEVC. Some systems are designed for static WEVC, such as the system developed by the Fraunhofer Institute for Integrated Systems and Device Technology (IISB), which places the coil close to the front of the vehicle, significantly reducing the coil size.  

In 2017, the Regional Transit Authority of Central Maryland demonstrated another application of the static charging system. They installed an electrostatic charging station along the route, allowing the bus to be fully charged while waiting for passengers to board and disembark. As a result, the electric buses are now able to complete any route within the (transit) network.

The ultimate goal, of course, is to enable vehicles to charge while traveling at high speeds on the highway, and a number of companies are making progress in this regard. Qualcomm’s Dynamic Electric Vehicle Charging (DEVC ) system has been demonstrated to deliver up to 20kW of power at highway speeds of around 60 mph . In other important developments , Japanese automaker Honda published a paper on high-power dynamic charging, describing tests of a system that charged at 180kW (600V DC, 300A) while traveling at speeds of up to 96 mph.

While each approach has made great leaps forward, interoperability to that end, the Society of Automotive Engineers (SAE) recently released SAE J2954, the world’s first specification for wireless power transfer at power levels up to 11kW.

Summarize

Wireless charging is key to overcoming EV development barriers , such as range anxiety , and plays an important role in the technology's global adoption. Early rollouts , such as a bus system in Maryland , have helped, but dynamic charging initiatives like those being tested by companies like Qualcomm and Honda will eventually enable the ultimate goal of EVs , which are to have unlimited range and convenience that surpass gasoline-powered cars .

At the heart of this revolution are semiconductor devices that will eventually provide the required energy efficiency and reliability to make these theoretical solutions a reality and success in mass production. ON Semiconductor is a very active company in this regard, with extensive experience in power management and high-efficiency power conversion . Within its product range, ON Semiconductor offers a comprehensive product range, including discrete switching devices such as high-efficiency IGBTs and MOSFETs, MOSFET drivers, voltage and current management systems, AC-DC controllers and regulators, intelligent power modules and battery management products.