Systematic analysis of advanced wireless charging technology for electric vehicles

With the development of electronic technology, more and more electronic devices in life are converted from wireless to wired. Electric toothbrushes, mobile phones around, and new energy vehicles on the road have all changed from traditional wired charging to wireless charging. Traditional charging methods have many disadvantages. Some of them may be mechanically aged, some may leak electricity, and some take up space. But wireless charging can optimize most problems. And it has higher safety and functionality. This paper expends on Capacitive Power Transfer (CPT), Microwave Power Transfer(MPT), Electromagnetic induction that the three mainstream wireless charging methods. The article introduces the working principles of the three wireless charging methods and their respective advantages and disadvantages. And through comparison, to provide a strategy for the method of choosing a wireless charging method.


Introduction
With the climate changing, nowadays, the methods to protect our environment and to find a more effective way to transition power became popular topics in public.Electricity is an efficiency secondary energy, which can turn one form of energy to another easily and quickly.
In the modern society, the car pollution has already become one of the most significant parts of the air pollution [1].To decrease the consequences of these vehicles, the electric vehicle (EV), including electric buses and electric private cars, should be promoted around the world [2].In the nearly years, EV can replace traditional vehicle and become the most popular and important part of modern society [3].
Comparing EV and traditional vehicle, EV has powerful electric motors and large capacity mobile batteries, which can supply electric power for motors for a long journey.The distance of the journey is limited by the capacity of batteries, because when the battery run out of power people should stop their vehicle near a power supply to charge the batteries.The vehicle has to connect with the power supply with a wire, so people cannot drive their vehicle freely when the batteries are charging.
It can be easily noticed that the EV needs plenty of modern technology and some of them still have some problems.The key-tech of EV are as follows.

High quality motors
When people drive electric car on the road, they usually want to drive more quickly.With a high speed, the current go through the electric motors can become very high and it need large power motors to trans electric power to speed of the car.On the other hand, vehicles have to face many different situations and surroundings may be very tough.The motors should be very strong that can run well in some tough conditions to finish difficult tasks [4] .

Large capacity battery groups
It is not enough to just have high quality electric motors on EV.The capacity of battery groups also matters.If the capacity of batteries is very low, the vehicle may not keep running for a long way, and have to stop to charge the batteries.So, it is of great necessity for EV to has large capacity battery groups, in order to run for a long way with once charge.
Another key-tech of the battery is the safety [5].It is a common point that the high-density battery is not safer than traditional fuel, and even more dangerous when the battery being pierced by some metal sticks in traffic accidents.So, the safety of vehicle batteries needs to be studied and improved.

Convenience charge method
The battery on EV needs to be charged when the electric power running out.A convenience method is one of the key-tech of EV.The traditional way to charge vehicle batteries is that connect the vehicles and chargers with wires.However, in this way, vehicles limited by the power wire and not convenience for the drivers.To find a method that can make battery-charge easer is a significant problem people have to face.
To face the battery-charging problem, the necessity of wireless power transfer (WPT) can be easily noticed [6].With WPT, people can charge their vehicle when they are driving, without the limit of power wires.Besides, WPT devices can be easily placed on some road devices, like well covers, so it nearly not occupies road resources to charge the needed vehicles.
The classifications of WPT are showed in Fig. 1.It can be noticed that, in Fig. 1, the wireless power transfer is divided into two parts, which are acoustic field power transfer and electromagnetic field power transfer.The acoustic field power transfer lead to acoustic power transfer, and the electromagnetic field power transfer also divided into two parts, which are electric field power transfer and magnetic field power transfer.The electric field power transfer lead to capacitive power transfer, and the magnetic field power transfer divided into two parts, which are radiative power transfer and non-radiative power transfer.The non-radiative power transfer lead to inductive power transfer, and the radiative power transfer divided into two parts, which are optical power transfer and microwave power transfer.
This paper introduces the basic information of three classifications of WPT, which are capacitive power transfer, microwave power transfer and inductive power transfer, and analyze the principal of these three classifications of WPT.The rest of the paper lists out the advantages and disadvantages of each technologies, and find out a typical way to reduce the drawbacks of each technologies.

Introduction
Capacitive power transfer (CPT) is one of the wireless power transfers (WPT) technology, which can transfer electric power without limited by the powerline.CPT can also avoid the negative impact of metal friction between the transmitting terminal and the receive terminal [7].
Recently, the technology of CPT is widely studied by many electric engineers and researchers, who want to find out an efficient way to trans power freely.It is one of the typical models of CPT, which is shown in Fig. 2  One of the typical models of capacitive power transfer.It can be easily seen in Fig. 2 that CPT is combined by 7 parts, including a DC supply, an inverter, two compensation networks, a capacitive coupler, a rectifier and an equivalent R-load.The DC supply offers DC voltage, which may connect with long-distance powerline or power storage.After that, a DC-AC inverter turns the DC voltage into AC, followed by a compensation network, which is including inductors and capacitors.And all these parts play the primary side role.As for secondary side, another compensation network with inductors and capacitors follows the capacitive coupler, which connects the primary side and the secondary side.After this compensation network, there is a rectifier that turns AC voltage into DC, and offer electric power to the R-load followed.
About the capacitive coupler, different kind of CPT has different style of capacitive coupler.For example, in this research, the capacitive coupler is made by two metal plates with wires.One of them connects with primary side, which can be placed on the road or some parking places.Another one connects with the secondary side, which is placed on an electric car in order to charge the car battery.The value of the coupling capacitance is decided by the size of two metal plates and the distance between them.

Topology of compensation network
The compensation network has many topologies during studying before and three of them are most popular, including double-side L-mode, double-side LC-mode and double-side LCLC-mode.The study results are as follows.

Double-side L-mode.
As we can see in Fig. 3(a), two inductors named L1 and L2 are located in the primary side and secondary side to compensate the capacitors in the middle of the circuit.The DC supply and DC-AC inverter are equivalent to an AC source named Vi, and similarly, the AC-DC rectifier and R-load are equivalent to Vo on the secondary side.
The frequency of the input voltage usually is very high in order to reduce the volume of the inductor.To make the secondary side concise, in many situations, two inductors are combined into one, located on the primary side.

Double-side LC-mode.
As we can see in Fig. 3(b), two inductors named L1 and L2 are located in the primary side and secondary side to compensate the capacitors in the middle of the circuit.However, the coupling capacitance is too low for the system power needs.Two capacitors named C1 and C2 are added into the compensation network in order to increase the volume of the coupling capacitance.Similar as the double-side L-mode, the DC supply and DC-AC inverter are equivalent to an AC source named Vi, and similarly, the AC-DC rectifier and R-load are equivalent to Vo on the secondary side.
With the increase of the volume of capacitance, the need of inductance shows a downward trend, so both volume and size of L1 and L2 are reduced.

Double-side LCLC-mode.
As we can see in Fig. 3(c), L2, L3, C2 and C3 create a LC-mode.And also, in order to increase more volume of the coupling capacitance, both primary side and secondary side add one more inductor and capacitor, and, finally, the double-side LCLC-mode is created.It can be easily noticed that, multiple resonances are created in this circuit to make the power transfer more efficiently.
However, with more components added in, the power loss cannot be ignored.To deal with this drawback, we can exchange the position of L1 and C1, and also exchange that of L4 and C4, creating CLLC-mode.This mode can decrease the inductance need of L2 and L3, so it can decrease the volume of the system.

Advantages and disadvantages of capacitive power transfer
The advantages of CPT are that it is easy to place the transmission part.The transmitter can be created very small and light, placing on the road behind the metal plate or even combining with a well lid.The receiver can also be a part of the new electric cars easily, which is just place a receive plate at the bottom of the car with a rectifier, connecting with the car battery.Another advantage is that the CPT can runs stably.The transmitter is just a metal plate instead of coils or some other complex devices, so it can run more stably in complex situations.And also, compared with inductive power transfer (IPT), CPT can neglect eddy-current loss, because CPT trans power with electric fields instead of magnetic fields.
The disadvantage of CPT is that, according to some researches, compared with IPT, CPT has much lower power density [7].

Introduction
We use the principle of electromagnetic induction to realize electromagnetic induction wireless charging.This technology converts electrical energy into magnetic field, and the coil converts the field energy back into electrical energy.This type of wireless charging technology requires small equipment, only two coils or coil arrays are required.At the same time, it has high reliability, relatively and high charging efficiency.It can also meet the communication requirements of the sending end and the receiving end, and has high flexibility.

Principle of electromagnetic induction
In 1831, Michael Faraday discovered the interconnection and transformation relationship between magnetism and electricity, as shown in Fig. 4. The electromagnetic induction wireless charging system has two subsystems, as shown in Fig. 5.They are the sending end system and the receiving end system respectively 5. The charging principle of the whole system is similar to that of a transformer, which converts electrical energy into magnetic field energy through the primary side coil [8].Then converts magnetic field energy into electrical energy through the secondary side coil.In this way, it can realize the purpose of energy exchange in the two subsystems.The DC current input is converted into the required high-frequency AC power after passing through the high-frequency inverter circuit.Then the energy is transmitted through the coupler and the compensation capacitor, and the transmitting coil emits an alternating electromagnetic field, propagates in the air, and is coupled to the receiving coil to transmit energy.Finally, the rectification circuit processes and reuses the received electric energy. .Schematic diagram of wireless charging system.The current wireless charging equipment includes a "charging base", which is actually a coil.After the charging base is powered, the coil will generate a magnetic field around it due to the current magnetic effect.When putting the charging device on the charging base, the coil of the charging device will generate an induced current due to the magnetic field.Thus, completing the charging.We all know that the magnetic field must be in motion to generate electromagnetic induction.To complete wireless charging for a non-moving charging device, it must use alternating current to charge it.That is to say, the direction of the current is constantly changing alternately, flowing along for a while, and flowing backwards for a while.Because of this, the magnetic field generated by the coil of the charging stand is constantly changing direction and does not remain unchanged, which meets the requirements of electromagnetic induction.
As demonstrated in Fig. 6, electromagnetic induction wireless charging consists of two parts: a power transmitting coil and a power receiving coil.When the power is turned on, the coil converts electrical energy into magnetic energy and alternates into a magnetic field.

Advantages and disadvantages
Although the electromagnetic induction wireless charging technology is simple, its transmission efficiency is very high in short distances.When the transmission distance increases, the resulting current drops drastically, resulting in lower efficiency.
The technology of this is the most mature among the three wireless power transfer technologies.There are already many related products on the market.Compared with the other two wireless charging methods, Electromagnetic induction has high transmission efficiency and strong penetrating power.But it is extremely strict on transmission distance and coil position.In the charging mode, moving the charging position slightly will cause the charging efficiency to be greatly reduced.
Besides, Qi and Power2.0, the mainstream wireless charging standards in the industry, are based on the electromagnetic induction.[6].The basic principles of MPT will be described next.

Introduction Microwave Power Transfer (MPT) is a far-field WPT technique working with microwaves, which has a longer transmission distance than other wireless charging technologies
After obtaining electricity from the grid, the AC is converted to DC through the rectifier circuit.And the HVDC generator powers the magnetron to generate a microwave, which passes through a waveguide and then radiates outward by the transmitting antenna.Then, through a long-distance transmission, received by the receiving antenna, the microwave into electric energy through the demodulation circuit, and then through the rectifier circuit, the power regulation circuit module, the electric energy into the required current to charge the electric vehicle battery.
Microwave wireless charging system consists of microwave signal source, microwave power amplifier, transceiver antenna, microwave rectifier circuit and other parts.Its architecture is as shown in the Fig. 7 as shown.The system can be roughly divided into transmitting end and receiving end, the transmitter end contains microwave signal source, power amplifier, and transmitting antenna, the receiving end includes receiving antenna, rectifier circuit, and load.Among them, the microwave signal source and power amplifier coincide with the requirements of the traditional communication system, and the devices in the traditional electronic communication field can be directly used.The requirements of the transceiver antenna and the rectifier circuit are different from the traditional communication system, and there is a lot of room for improvement in the energy transmission system [10].

Microwave signal source and power amplifier.
Low power (milliwatt level) microwave signals are produced using the microwave signal source.The microwave signal will be output as a high power (watt level) signal after being amplified by the power amplifier.Both play the duty of converting DC energy into microwave energy.In microwave wireless charging system, transmission efficiency is an important index, so the overall efficiency of signal source and power amplifier in the system is relatively high.In order to avoid interfering with the existing communication frequency band, the bandwidth is also an important indicator.For other indicators of noise, such as phase noise, noise coefficient, etc., the requirements can be appropriately reduced.With the development of semiconductor device technology, devices manufactured by the third generation of semiconductor materials silicon such as SiC and GaN materials show the environment tolerance, high frequency and high-power characteristics, which make microwave semiconductor solid state devices gradually replace microwave tubes, become an important part of microwave signal source and power amplifier.Microwave source and power amplifier based on the third-generation semiconductor solid state device have the characteristics of wide frequency band and high-power capacity and high efficiency, which is very suitable for microwave wireless charging system.

Antenna.
The receiving antenna transforms electromagnetic waves of the corresponding frequency band in the area into microwave signals in the circuit into the rectifier circuit, and the transmitting antenna transforms the fed high-power microwave signal into electromagnetic waves and radiates it into free space.Considering the efficiency and bandwidth index of the microwave wireless charging system, the energy emitted by the transmitting antenna needs to have a strong directionality, a high gain, and has the corresponding bandwidth.

Microwave rectifier circuit.
The microwave rectifier circuit transforms the incoming microwave energy into DC energy and supplies power to the load by taking advantage of the rectifier diode's nonlinear effect.The bandwidth expansion of the rectifier circuit is conducive to the improvement of system compatibility.In the field of energy collection, it is conducive to receiving more abundant microwave energy in the frequency spectrum and improving the system performance.However, due to the limited bandwidth of the matching circuit, the bandwidth of the rectifier circuit is difficult to widen, even if the broadening, it will reduce the efficiency, which is similar to the process of power amplifier bandwidth broadening, and the bandwidth is widened at the cost of slight mismatch in the band.There are some solutions to the problem of bandwidth broadening, most of which change the circuit structure, such as introducing couplers, coupling lines, etc.But these schemes still have shortcomings, such as the bandwidth is not wide enough, the frequency cannot be high.

Advantage and disadvantage
The technique used for microwave wireless energy transfer, which transmits energy as electromagnetic waves from one end to the other, is used for microwave wireless charging.The transmission loss is very low, only atmospheric (including atmosphere, raindrops, dust, etc.) loss and shielding loss.Radio wave transmission has the characteristics of "directional, penetrating monolayer" and so on, so its transmission distance can be very far.Because microwave wireless energy transmission is widely applicable, with small transmission attenuation and large power capacity, it is one of the important ways of wireless power transmission in the future.
There are also examples of wireless radio-wave wireless charging technology for electric vehicles: Mitsubishi Heavy Industries has used the radio-wave energy transfer principle to develop a wireless charging system for microwave electric vehicles.The same 2.45 GH 2 electric wave generator as the microwave oven can transmit 1k W of power to the vehicle through the rectifier antenna ", but the transmission efficiency is only 38%.
Due to the microwave wireless charging technology has high working frequency range and strict characteristics of equipment requirements, and microwave is omnidirectional radiation, and the system emission frequency is very high, the loss of the system is very large, unable to achieve high efficiency of energy transmission.Bulky antennas are needed for high power levels, which may preclude their use in particular circumstances.So the microwave technology in the electric car this short transmission is not applicable.

Comparison
WPT can be implemented through a variety of technologies.The previous sections have introduced the basic principles, advantages and disadvantages of capacitive microwave and electromagnetic methods applied in electric vehicle charging.Table 1.summarizes and compares these techniques [11].
While some wireless power transmission technologies have matured, given their respective characteristics, it is expected that future wireless chargers for electric vehicles will consider a hybrid use of these several technologies to provide the possibility of using each charger in the most convenient way and situation.For this kind of application, a number of hybrid chargers have been suggested.

Expectations
In the past ten years, the new energy vehicle industry has developed rapidly.As a result, wireless power transmission technology will also develop quickly.It is expected that within the next ten years, the new energy vehicle industry will gradually mature, and the market of wireless charging will further expand.
At present, the research on all aspects of wireless charging technology in electric vehicles has achieved good results, but the shortcomings are also obvious.It is mainly manifested in the following aspects:

Different charging standards
For the moment, an important reason for restricting the development of wireless charging is that the charging standards are not unified.At present, the three international mainstream wireless charging technology standards are Alliance for Wireless Power standard (A4WP), Power Matters Alliance standard (PMA) and Qi standard developed by Wireless Power Consortium(WPC).Unified charging standards is an essential process for the development of wireless charging.It is beneficial to the standardized growth of global electric vehicles to hasten the formulation of unified standards for wireless charging of electric vehicles.

Higher cost
Wireless charging of electric cars requires a more difficult technology.Therefore, the capital investment in the early stage of research and development is relatively high, and before the popularization and mass production, the cost is higher than wired charging.Currently, a fully after-sale home wireless charging system costs about $2,500 to $3,000, however, 7 kW wired charging piles for only $300 to $500.As a result, high costs still hinder the adoption and mass production of wireless charging technology, and technological advances should gradually reduce its costs in the future.

Charging stability and safety issues
After the electric energy is converted into electromagnetic waves, it will produce high-frequency radiation, and if there are small animals or metal foreign bodies involved when charging, it will also cause faults, which will affect the stability of charging and have certain safety risks.In addition, some transmission modes are also harmful to human body safety.To lessen the effects of electromagnetic radiation on the human body and investigate safe and effective anti-radiation techniques, it is therefore necessary to continue studying the radiation levels and effects of various charging techniques on the human body.

Lower transmission efficiency
It is mainly reflected in the form of wireless charging compared with the form of wired charging, the overall energy transmission efficiency of wireless charging is not high, and is greatly affected by the transmission distance, so the transmission power and efficiency can be improved from the following perspectives:

Loop construction.
There is a mutual coupling between the three-phase coils, which makes it simple to produce the three-phase system unbalance phenomenon.This will impact the system's ability to transmit power and lower its efficiency.The energy transmission efficiency between the coils directly affects the benefit of the whole charging system, and seeks the best distance between the two coils, as well as the winding method, arrangement and shape of the coils, so as to improve the transmission efficiency.
6.4.2.Topological structure and the control algorithm.The improvement of transmission efficiency of charging system is inseparable from the optimization of compensation network.The standard "rectifyhigh frequency inverter" structure is typically used in three phase wireless charging systems, although this two-stage transformation topology decreases the system's power density and efficiency.Exploring the power electron topological structure, which has low positive resistance and simple matching, plays a crucial role in lowering the system offset, assuring the stability of the circuit, and improving transmission efficiency.

Resonance frequency.
During the process of wireless charging, sometimes the instability of the system makes the resonant frequency fluctuate, resulting in detuning and reducing the transmission efficiency.Closed-loop control is added to the receiving end and sending end of the coil for adjustment and control, and then a monitoring module can be added to monitor and adjust the resonance frequency in real time, which can boost transmission efficiency and significantly minimize loss.

Conclusion
Three wireless charging methods have their own advantages and disadvantages.The transmission distance of capacitive power transfer and electromagnetic induction are short.But they are very efficient and low cost.They are relatively mature wireless charging methods at present.They are widely used in new energy vehicle charging piles.Microwave power transfer has a longer transmission distance but the safety cannot be guaranteed.If the cost of wireless charging can be further reduced and potential safety hazards can be solved, wireless charging technology will get another breakthrough.In addition, charging standards may also change in the future.The transfer with long transmission distance may be popular in future.
In summary, the wireless charging of new energy vehicles is still in the development stage.There are still many security issues and transmission distance issues to be resolved.In the future, wireless charging will be combined with dynamic charging and unlimited driving technology.

Figure 1 .
Figure 1.Classifications of wireless power transfer systems.It can be noticed that, in Fig.1, the wireless power transfer is divided into two parts, which are acoustic field power transfer and electromagnetic field power transfer.The acoustic field power transfer lead to acoustic power transfer, and the electromagnetic field power transfer also divided into two parts, which are electric field power transfer and magnetic field power transfer.The electric field power

Figure 2 .
Figure 2. One of the typical models of capacitive power transfer.It can be easily seen in Fig.2that CPT is combined by 7 parts, including a DC supply, an inverter, two compensation networks, a capacitive coupler, a rectifier and an equivalent R-load.The DC supply offers DC voltage, which may connect with long-distance powerline or power storage.After that, a DC-AC inverter turns the DC voltage into AC, followed by a compensation network, which is including inductors and capacitors.And all these parts play the primary side role.As for secondary side, another compensation network with inductors and capacitors follows the capacitive coupler, which connects the primary side and the secondary side.After this compensation network, there is a rectifier that turns AC voltage into DC, and offer electric power to the R-load followed.About the capacitive coupler, different kind of CPT has different style of capacitive coupler.For example, in this research, the capacitive coupler is made by two metal plates with wires.One of them connects with primary side, which can be placed on the road or some parking places.Another one connects with the secondary side, which is placed on an electric car in order to charge the car battery.The value of the coupling capacitance is decided by the size of two metal plates and the distance between them.

Figure 4 .
Figure 4. Electromagnetic induction schematic.The DC current input is converted into the required high-frequency AC power after passing through the high-frequency inverter circuit.Then the energy is transmitted through the coupler and the compensation capacitor, and the transmitting coil emits an alternating electromagnetic field, propagates in the air, and is coupled to the receiving coil to transmit energy.Finally, the rectification circuit processes and reuses the received electric energy.

Figure 5
Figure 5. Schematic diagram of wireless charging system.The current wireless charging equipment includes a "charging base", which is actually a coil.After the charging base is powered, the coil will generate a magnetic field around it due to the current magnetic effect.When putting the charging device on the charging base, the coil of the charging device will generate an induced current due to the magnetic field.Thus, completing the charging.

Figure 7 .
Figure 7. Generic diagram of a MPT system. .

Table 1 .
Comparative study of the WPT technologies applied to EVs.