Mark Patrick, Mouser Electronics
The recently released Bloomberg Electric Vehicle Outlook 2020 report highlights that new EV sales will continue to grow to an estimated 26 million vehicles by 2030. The increasing choice of available models, lower battery costs, and broader market adoption contribute to sales growth. The availability of a widespread EV charging infrastructure will further contribute to consumer confidence and reduce the range anxiety concerns experienced by many potential EV purchasers.
The deployment of electric vehicle charging stations is a costly business. New automotive manufacturers that lead the way with EV vehicles, such as Tesla, deemed the need to develop their charging infrastructure critical to establishing significant market share at an early stage. Established automobile brands, never having had to concern themselves with building a network of filling stations, rely on the growing number of commercial organisations deploying charging infrastructure. The establishment of industry-standard connector configurations is crucial to growing the number of wired EV charging stations.
Initially, the automotive industry focused on both in-car chargers and car park based rapid charging stations. EV owners are now investing in having a charger installed within their garage or accessible on their driveway as a convenient way of charging their vehicles overnight. Charging stations can be found in shopping centre car parks, and outside fast-food outlets and coffee shops. In all cases, a lead connects the charging column to the vehicle.
An alternative approach to charging a suitably equipped electric vehicle is to use a wireless charging method. The concept of wirelessly charging electric vehicles is not new, but adoption has been slow because of several untrue myths propagating the thought that it was not suitable. These include a lack of EV wireless charging standards, low power transfer efficiencies, and slow charging times. Wireless charging yields a more straightforward and convenient charging method, with the driver having to drive into a parking bay with a charging mat. Such an approach also opens up installing charging infrastructure on motorways, allowing vehicles to top up their charge and extend their range while continuing on the journey.
Wireless Charging Technologies
There are several ways of transferring energy wirelessly, each with a different set of implementation criteria, transfer efficiency, and transfer distance. Some consumer devices such as smartphones have been capable of using compact desktop wireless charging pads for several years. However, the electrical requirements for charging a smartphone are considerably different from charging an EV. For electric vehicle charging, the amount of power to be transferred in a given time is crucial. There are two fundamental approaches; inductive or capacitive. An inductive approach can incorporate a near field electromagnetic technique to transfer energy using the magnetic flux between two coils. The capacitive method uses an electric field coupling between two plates, essentially forming a large capacitor. Both forms of wireless power transfer (WPT) are relatively efficient and similar to a wired charging station efficiency performance of 85 - 95 %, but suffer the same practical limitations that the reasonable transfer distance is measured in centimetres. To achieve these small distances between a coil or plate on the parking bay's surface and a similar one mounted on the vehicle's underside is impractical. Firstly, the degree of vertical movement in a vehicle's suspension from fully laden to unladen can involve tens of cms. Also, the positional accuracy when parking in the charging bay would probably require parking sensors to guide the driver to achieve optimal coupling.
However, another approach uses magnetic resonance to transfer energy between two resonant coils using the magnetic field. The magnetically-coupled resonant (MCR) technique requires both the sending and receiving coils to operate at the same resonant frequency, achieving higher efficiency of energy transfer. Impedance matching components such as capacitors and resistors are used to compensate for physical differences in each resonator's construction and to tune the circuit to achieve optimum power transfer. See Figure 1. Most importantly, the realistic transfer distance is significantly better than near field methods.
MCR-WPT is gaining widespread adoption across the EV industry with significant players including companies like WiTricity. The approach offers many benefits compared to other wireless charging technologies - see Figure 2.
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Figure 2: The benefits of using magnetic coupled resonant frequency for wireless power transfer for electric vehicles. (Source: WiTricity)
Implementing a Magnetically Coupled Resonant EV Charger
An example functional block diagram of an MCR-WPT system is illustrated in Figure 3. The power amplifier and inverter drive blocks can benefit from the recent advances in wide bandgap (WBG) power semiconductors using GaN and SiC process technologies. WBG semiconductors are less impacted by high operating temperatures, exhibit up to a 10 x higher breakdown voltage characteristic, and are capable of operating at high switching frequencies than their silicon semiconductor counterparts. Since the first WBG devices have come to market, most semiconductor vendors active in the automotive market now offer WBG power transistors for gate drive and power conversion applications. WBG devices exhibit much lower conduction losses (Rdson) than a silicon MOSFET, up to a factor of 100x less, and consequently have lower switching loss characteristics. They are typically smaller, so occupy less board space. Since they can operate at higher switching frequencies, the size of the associated passive components used is correspondingly smaller, further reducing the overall PCB footprint. As conduction losses reduce, conversion efficiency increases and the amount of waste heat to be dissipated reduces, saving additional space. Examples of WBG devices suitable for use in EV power conversion applications include SiC FETs from UnitedSiC and the CoolFET series of SiC MOSFET devices from Infineon.
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Figure 3: A wireless charging lane on a motorway (Source: Mouser)
A Bluetooth communication link would also be established between the charging system and the vehicle's internal systems to report on the EV state of charge, charge voltage and current, to feedback charging progress and maximise the charging efficiency for a specific charging pad location. Signal processing functions between the charger and the vehicle maintain the system operating at the desired resonant frequency and the point of maximum power transmission efficiency.
EV Wireless Charging Moving Forward
The concept of charging an electric vehicle without wires is not new. University collaboration with leading automotive manufacturers and on-going projects led to the University of Auckland, New Zealand, showcasing a resonant charging pilot project that could deliver 220 Watt with a 95% transmission efficiency across 30 cm in 2011. This project used the HaloIPT technology initially developed by Qualcomm and subsequently sold to WiTricity in 2019. In 2018, BMW incorporated WiTricity MCR-WPT technology in a custom version of the BMW530e, achieving 85 % across an 8 cm air gap. The wireless charging pad delivered a 3.2 kW capability, which charged the car's 9.2 kWh battery in roughly 3.5 hours.
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Figure 4: BMW wireless charging and charging mat. (Source: BMW)
As interest in wireless charging grew and magnetic resonance proved to be a viable candidate, the automotive industry looked to the standardisation body SAE International to steer future developments. Standards formation commenced in 2012 and was based on WiTricity's magnetic resonant coupling approach. Now in an advanced development stage, the SAE J2954 standard documents wireless power transfer for electric vehicles and is expected to be finalised before the end of 2020. J2954 stipulates three classes of charging speed; WPT 1, 2, and 3, with a maximum charge capability of 3.7 kW, 7.7 kW, and 11 kW. To accommodate the need for higher capacity energy transfer for commercial vehicles such as trucks and public transit vehicles, standard J2954/2 accommodates 500 kW charging.
Government initiatives have also led research and development into wireless charging to boost EV adoption by tackling consumer concerns such as range anxiety and the higher price point EVs demand. The European Green Vehicles Initiative, UNPLUGGED, commenced in 2014, has been working with industry partners to establish two test and development centres in Europe and investigate the potential for dynamic en-route charging.
Magnetic resonance coupling looks set to change the EV charging landscape significantly. Despite the industry adoption of the term wireless, magnetic resonance is a wire-less charging method but does not use any radio frequency methods for transferring energy. The capability to charge an EV using charging lanes on major roads opens up significant benefits for EV adoption and development, including reducing range anxiety for owners and the potential for EV manufacturers to reduce battery capacity, saving cost and weight.