Wireless charging evolution can help speed EV acceptance & viability
01 October 2020
Range anxiety is a familiar term in the context of battery-powered electric vehicles (EVs) – and for some, presents a barrier to more widespread adoption. Two things that can overcome resistance to emerging technologies are reason and, of course, engineering improvements that address valid concerns.
This article was originally featured in the October 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
As far as reason is concerned, EV proponents have pointed out that typical daily mileage, involving commuting to and from a place of work and doing household duties, is actually quite low for most drivers – and well within the capabilities of many EVs currently on the market. Although this may convince buyers already keen to make the switch, others may take more persuading. Here, Patrik Kalbermatten, Senior Manager – Distribution Promotion & Product Management for Magnetic, Sensor & Actuator at KEMET Electronics Corporation explores how manufacturers such as KEMET are developing passive component technology to deliver the engineering improvements that will help ‘unlock’ the full potential of systems, including wireless charging, to help drive accelerated adoption of EVs by consumers.
For those exceptional days that involve extra errands or a longer trip, or to be ready for an emergency, it’s good to have some miles in reserve. Faster charging could provide a solution. Any EV can charge from a basic Level 1 charger that plugs into the standard wall outlet and operates up to about 1kW, although adding 100 miles’ range could take almost 20 hours (see Figure 1).
Level 2 chargers, up to about 20kW, are installed at roadside and city centre parking spaces, and can be optionally installed at home by domestic users. They can transfer comparable energy in typically one-quarter of the time or less. Level 3 chargers, or DC Fast Chargers (DCFC) are the highest powered, and hence the fastest, charger type. However, they are not available at all public charging stations and are not suitable for all EVs. For drivers to ‘top up’ their vehicle battery from any of these types of outlets – from Level 1 to Level 3 – involves finding an equipped parking space, retrieving cables from the car, plugging in and waiting.
Adding wireless to the mix
As part of the ‘charging mix’ that can be made available to drivers while on the go, wireless charging could make topping up more convenient – and so help overcome range anxiety. Wireless charging of EVs relies on resonant magnetic induction to transfer energy between a pad on the ground and another under the floor of a compatible EV. A typical charging pad is around a metre square, while the receiving pad on the underside of the car with which it couples is enclosed in a smaller enclosure under the car. Once the two are aligned, charging can take place at rates from 3.3kW up to 20kW.
A suitable wireless power transmitter can be embedded in the road surface and detect when a vehicle is stationed above. The charging process can be setup when the vehicle arrives and terminated upon departure without intervention from the driver. Building out a suitable Wireless Electric Vehicle Charging System (WEVCS) infrastructure then gives EV drivers the opportunity to recharge incrementally any time the vehicle stops for a short period, such as when quickly visiting a convenience store or waiting at a venue to collect a friend or family member.
There may also be the prospect of dynamic WEVCS (D-WEVCS), comprising areas of highway that contain a series of embedded charging transmitters to recharge batteries of passing vehicles while in motion. Dynamic charging cold be most effective for taxis or buses, allowing recharging at specific locations on a route or while waiting at stops or pickup/drop-off points.
Figure 2 describes the main functional blocks of a static or dynamic WEVCS. The transmitter is likely to be in a fixed position and takes power from an AC supply at 50-60Hz. After rectification and power-factor correction, an inverter produces an AC output at 80-160kHz, which is transferred into the power-transmitter coil. The diagram shows the locations of the input noise filter for conducted emissions, PFC reactor, and normal-mode and common-mode choke coils for conducted and radiated emissions. The radiated power from the transmitter is coupled into the receiver coil attached to the underside of the vehicle. Normal-mode and common-mode filters, and an AC/DC converter generate a stable DC power supply to charge the battery, controlled with a battery-management system (BMS).
Optimising the WEVCS
Reliability and energy efficiency are two of the most important challenges for equipment designers. Both the fixed transmitter and vehicle-mounted receiver can be exposed to extremes of temperature, humidity and physical forces. KEMET helps address these challenges by offering environmentally hardened chokes and filters that are created expressly for EV-charging applications. In addition, new materials have been developed to enable downsizing of components, which is particularly needed to minimise the size and weight of the vehicle-mounted equipment.
In the inverter and AC/DC converter of the transmitter and receiver respectively, wide-bandgap power semiconductors enable efficient power conversion at the wireless power-transfer frequency, with an adequate voltage rating to provide a suitable safety margin above the nominal operating voltage.
To accompany these wide-bandgap semiconductors, improved ceramic capacitors are needed that offer superior capacitance stability over temperature and voltage, allowing the devices to withstand very high ripple currents. An enhanced dielectric system that results in very low equivalent series resistance (ESR) and thermal resistance allow the capacitors to operate close to the fast-switching semiconductors, allowing WEVCS modules to deliver high power density and reliable performance. KEMET’s KC-Link multi-layer ceramic capacitors (MLCCs) feature a robust and proprietary C0G/NP0 base metal electrode (BME) dielectric system that ensure low ESR with high thermal stability. Moreover, high mechanical robustness allows the capacitors to be constructed without a leadframe, thereby minimising the effective series inductance (ESL) and so allowing a broad operating-frequency range and further miniaturisation.
Careful attention must also be paid to managing the inductive coupling of transmitter and receiver coils to maximise the power-transfer efficiency. The coil design and layout can be optimised to reduce dependence on alignment of the transmitter and receiver. Other traffic or road furniture may prevent either a human driver or autonomous driving system positioning the vehicle perfectly over the transmitter. Alternatively, research has shown how the position of a moveable receiver coil can be fine-tuned to optimise alignment, enabling charging up to 5kW at over 90% efficiency. An air gap of about 150-300mm is usually considered optimal. A raising or lowering mechanism may be considered to accommodate significant differences in vehicle ride height that can exist between small cars and larger vehicles such as SUVs.
In addition, applying shielding materials around the power transmitter and the receiver on-board the vehicle helps to shape the magnetic field and hence, further increases power-transfer efficiency. KEMET has developed high-permeability sintered ferrite tiles that effectively minimise magnetic flux losses and are AEC-Q200 qualified for automotive applications.
Wireless charging is convenient and easy to use, requiring no driver involvement to instigate charging. Wireless charging can be provided in locations such as car parks and roadside waiting areas, or in bus stops or pickup/drop-off areas for charging taxis. Dynamically charging vehicles on the move is also possible. Easy access to wireless charging ‘top ups’, as a supplement to wired charging, could become a decisive factor in the mission to achieve widespread acceptance of EVs.
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