Graphical analysis of the design principle of electric field coupling wireless charging system

　　The continued popularity of battery-powered consumer electronic devices such as portable media players, smartphones and tablets has resulted in a plethora of different chargers and bundles of wires cluttering homes. The concept of charging devices wirelessly, without any direct wire connection, has been around for some time and is now rapidly gaining interest, making it more flexible and more useful. But what are the different technologies available and what are the design challenges engineers need to address?

　　There are many appealing aspects to wirelessly charging consumer devices, as it eliminates the need for charging cables. Perhaps more clearly, the purpose of wireless charging is to provide a new way to charge device batteries in an innovative way that does not involve wires or connectors.

　　Wireless charging is already popular in many consumer devices such as electric toothbrushes, with the most prominent method being induction based on Maxwell's laws, where a change in magnetic field from one coil induces a current in another coupled coil. While induction using magnetic fields is suitable for many small devices like these, using this method in more modern consumer electronics such as tablets and smartphones presents many engineering challenges.

　　As the power fed to the battery increases, the relative efficiency or flexibility required in placing the coupled coils increases. The main consideration for this inductive approach is how to control the electromagnetic interference (EMI) generated by the signal that generates or "sends" the energy and transmits it to the "receive" device using an induced magnetic field. The receiving device then converts the magnetic field energy into electrical energy to charge the battery. Wi-Fi, Bluetooth, near field communication (NFC), cellular systems and FM radio are some examples of the many wireless voice and data connection methods that can be interfered with by such electromagnetic fields.

　　Of course, another consideration is to make the power transfer efficiency as high as possible, even under challenging constraints such as higher power levels and wider placement tolerances. In the past few years, the industry has proposed many new ideas on how to implement inductive charging technology, but the progress in avoiding EMI effects has not been as smooth as expected, because achieving EMI compliance requires arduous efforts.

　　This challenge has been furthered recently, thanks to the efforts of the Wireless Power Consortium (WPC), an initiative of the Consumer Electronics Association (CEA) of the United States that aims to encourage further research and development to make wireless charging more compelling and therefore more popular with a wider consumer base.

　　Another well-known constraint of the inductive approach is the need to precisely mate the charger and the device being charged, which can be best illustrated with the example of an electric toothbrush. The charger base has a small tower that rises up from the base on which the toothbrush to be charged rests. Using this approach, the two coils are perfectly matched to ensure the transfer of magnetic energy. Any slight misalignment will result in a complete loss of power transfer capability. This approach is obviously inconvenient when using other devices that require slightly higher power levels, such as smartphones or tablets. Finally, there is the issue of how to deal with electro-thermal losses. The higher the charger power, the greater the heat losses. This is even more of a problem for lithium-ion batteries, which are highly temperature sensitive, and can cause component stress in today's highly compact consumer electronic designs.

　　Another alternative to magnetic field wireless charging is to use a capacitive architecture, which works similarly to Maxwell's laws of electric fields. This concept has been adopted by Murata and is widely introduced into new designs. The company's approach is to use a quasi-static electric field and transfer energy through a capacitor, which consists of two electrodes belonging to physically separate devices. Bringing these two devices close together forms a capacitor array and uses it to transfer energy. Figure 1a shows the basic principle of this approach.

　　Figure 1a: Principle of a transmitter-receiver pair for wireless power transfer.

　　Figure 1b: Equivalent circuit of the transmitter-receiver pair shown in Figure 1a.

　　Energy transfer is achieved through electrostatic induction using two sets of electrodes or plates. The charger or "transmitter" and the portable device or "receiver" are used to effectively achieve longitudinal quasi-electrostatic coupling between appropriately sized metal surfaces that form a capacitor. The driving electrode or active electrode is smaller than the other electrode, and a higher voltage is applied to it. The other electrode is a passive electrode, which is longer in size and has a lower voltage on it. Of course, under normal circumstances, the energy transferred by the capacitor is very small, which has a lot to do with the small area of ​​the electrodes. Therefore, in order to meet the power levels required to charge consumer devices (for example, from 5W to 25W), the electrode size and the coupled voltage value need to be increased, depending on the actual configuration.

　　Figure 2a shows a block diagram of an example charger approach using capacitive energy transfer, where the receiver and transmitter modules used are new products recently developed by Murata. This modular approach allows engineers to focus on the design of the electrodes in the coupling zone, which helps to quickly develop wireless charging functions. The amount of energy transferred by the electrostatic method is directly proportional to the frequency used. Therefore, driving the electrode pairs with higher frequencies can enable the design to handle higher powers. However, various countries have restrictions on the frequencies and electric field strengths used. In fact, this configuration can form a very effective antenna structure, so EMI factors usually limit the flexibility of the design. In order to achieve wireless transmission and reception between the coupled electrodes while minimizing the amount of external radiation, it is necessary to design it correctly. Therefore, it is necessary to further understand and determine the correct electrode size, their design, operating voltage, power value, optimal operating frequency and overall size constraints. In general, the ideal frequency range is between 200kHz and 1MHz, and the voltage value of the effective coupling zone is between 800V and 1.52kV.

　　Figure 2a: Capacitive transfer charger block diagram.

　　Figure 2b shows the voltage step-up and step-down during the transmit-to-receive capacitive coupling for a 10W charger that meets EMI compliance requirements. The design concept using a modular architecture allows device manufacturers to use the module as a black box, facilitating the integration of the transmitter and receiver. The transmitter design covers the link to the power supply, the control of the wireless energy transfer, and the control of the active coupling electrode of any form factor based on the location flexibility goal. On the receiver side, the battery interface determines how the design correctly receives power from the active coupling electrode area through the down-conversion module. Due to the wide variety of batteries used in portable devices, the standardization of the circuit interface represents a big step towards very convenient designs, while also considering more challenging concepts such as faster charging speeds. The micro-USB 5V charging interface is becoming the standard for all mobile phones in Europe, mainly due to the continued pressure from the European Commission.

　　Figure 2b: Voltage step-up and step-down as part of the transmit-to-receive capacitive coupling process in a 10W charger.

　　One of the key advantages of using quasi-electrostatic transmission over inductive methods is that the position of the device to be charged on the charging base (or charging tray) is not as critical. Through careful design in the xy (surface) direction, high efficiency and relatively flat energy transfer can be maintained when the receiver is far away from the source, with typical efficiencies of around 80% for any design (even a wired charger), thus having very high position tolerance performance, while z (height) remains the most challenging design parameter.

　　Alternatively, using a flat square or rectangular tabletop tray or a near vertical docking rack allows for placement of the charging device in any orientation, not necessarily with precision. Also, since the main active receiving electrode can be constructed from a simple thin copper foil (on the order of a few microns thick, embedded in a plastic cover material), integration into a consumer device is much simpler than integrating a power sensor.

　　As mentioned before, heat transfer close to the battery is a serious problem for inductive methods. However, the electric field, which is the energy carrier in the capacitive coupling configuration, does not carry any significant current. Since there is no such DC flow, there is no heating problem in the coupling area: all resistive losses are integrated in the module or driver circuit, and none in the coupling area. As a result, device manufacturers have greater design flexibility when integrating micromodules into their devices, and have a lot of design freedom in terms of coupling design, power level and desired positioning tolerance.

　　Considering all these challenges, capacitive coupling wireless energy transfer can achieve higher power transmission, greater positioning flexibility, meet EMC compliance requirements, and provide manufacturers with greater design flexibility. Overall, capacitive coupling wireless energy transfer will greatly encourage manufacturers to integrate the function of wirelessly charging portable devices.