Electric field coupling wireless power supply system: making wireless charging easy!

Murata Manufacturing Co., Ltd. has developed a wireless power supply system based on the "electric field coupling method". Unlike the "electromagnetic induction method" and "magnetic field resonance method", the wireless power supply technology of the electric field coupling method uses the induced electric field generated between the two electrodes by placing the power transmission side electrode and the power receiving side electrode opposite to each other to supply power, and has the characteristics of strong resistance to horizontal displacement. Murata Manufacturing Co., Ltd. has completed the trial production of a power supply station for wireless power supply for portable terminals such as tablet terminals and e-book terminals . In this article, Murata Manufacturing Co., Ltd.'s new business manager and product planner will introduce the company's strategy and technical details.

As many companies are developing wireless power supply technology that transmits power wirelessly, Murata Manufacturing Co., Ltd. has focused on the technology called "electric field coupling method." Murata Manufacturing Co., Ltd. previously developed wireless power supply technology using "electromagnetic induction method," but around 2008 it decided to switch to promoting the electric field coupling method.

The electric field coupling method has a simple structure and can supply power to the product no matter where it is placed as long as it is within the charging area specified by the power supply station, thus achieving "position-free" power supply. In addition, since the electrode can be made thinner, it has the characteristics that other methods do not have, such as easy embedding in the product.

Murata Manufacturing has spent about three years improving the technology and building a patent network centered on basic patents. It has already completed the trial production of a power supply station that wirelessly supplies power to portable devices such as tablets and e-books.

Murata Manufacturing plans to mass-produce wireless power supply devices for tablet terminals in the fall of 2011, with a 10W output power transmission module and a 10W receiving module. At the same time, standardization work has begun to expand the application of the electric field coupling method.

This article will focus on the technical content and characteristics and introduce the electric field coupling method in detail.

Being a minority in the industry

There are several wireless power supply methods, depending on the principle used (Figure 1). The electromagnetic induction method has a long history of development in wireless power supply technology. It has been adopted in fields such as electric toothbrushes and cordless phones. For the electromagnetic induction method, the industry group "Wireless Power Consortium (WPC)" has been established to carry out standardization work, and has completed the standard specifications for devices below 5W.

Figure 1: Many companies are paying attention to wireless power supply

The figure summarizes the development trends of wireless power supply by power transmission method. While many companies are working on electromagnetic induction and magnetic field resonance methods, Murata Manufacturing is focusing on the electric field coupling method.

The "magnetic field resonance method" is a newly developed technology. Its appeal lies in its ability to transmit power over a distance of several meters, so many companies and research institutions are conducting related research and development.

The electric field coupling method that Murata Manufacturing is committed to developing is a minority in the wireless power supply industry. In addition to Murata Manufacturing, only Takenaka Komu is developing it in Japan.

Mass production in autumn 2011

Murata Manufacturing started full-scale development in April 2010 with the goal of developing an electric field coupling power supply device business. The research itself began around 2008, and the relevant technology has been established since then. In June 2010, Murata Manufacturing officially announced that it had completed the trial production of a power transmission module and a power receiving module with an output power of 3W. In October of the same year, it exhibited at "CEATEC JAPAN 2010" and equipped Murata Manufacturing's bicycle robot "Murata Naughty Boy Type ECO" with this technology. At the same time, an evaluation kit with an output power of 2W was also launched.

Murata Manufacturing Co., Ltd. then decided to mass-produce the power transmission module and the power receiving module (Figure 2). The module is for charging tablet terminals and has an output power of 10W. In addition, it is planned to launch a power supply device equipped with the power transmission module and the power receiving module in 2011 (Figure 3).

Figure 2: 10W products will be mass-produced in the fall of 2011

Murata Manufacturing will mass-produce 10W power transmission and receiving modules for charging cradles used in tablet terminals. The thickest part of the module is the transformer.

Figure 3: Products for tablet terminals will be launched soon

Murata Manufacturing plans to release a wireless power supply station for tablet terminals in 2011. The overall system efficiency is about 70%, and it can handle horizontal misalignment of up to 40mm. The photo shows a prototype when Murata Manufacturing released a press release in 2010.

The device realizes wireless power supply by attaching a jacket with built-in electrodes to the back of the tablet terminal. The power receiving module is stored in the lower part of the jacket. In addition, the electrodes and power transmission module are embedded in the power supply platform, so the tablet terminal can be used by placing the jacket on the power supply platform. The power transmission efficiency can reach 70%.

Transmitting electricity through electric fields

The following is an overview of the electric field coupling wireless power supply technology developed by Murata Manufacturing. Its basic structure is shown in Figure 4. The part called "oscillator" below is the power transmission side, and the part called "load" is the power receiving side. Power is transmitted by using the induced electric field generated by coupling two sets of asymmetric dipoles in the vertical direction.

Figure 4: Power transmission requires the use of two sets of electrodes

Murata's electric field coupling method uses the induced electric field generated by coupling two sets of asymmetric dipoles in the perpendicular direction to transmit power.

The basic principle is that a strong induction electric field is generated in the part marked in light blue in Figure 4, and the electric field is used to transfer power from the power transmission side to the power receiving side. Murata Manufacturing has obtained a patent for this structure (patent number: PCT/FR2006/000614).

The characteristic of Murata's method is that it is an asymmetric dipole and requires two sets of electrodes. Murata calls them active electrode and passive electrode. The passive electrode mainly plays the role of grounding. The system transmits power by combining these electrodes.

In terms of the components of the system, the power supply is the same as other wireless power supply technologies such as electromagnetic induction and magnetic field resonance . The power receiving module is equipped with a step-down circuit and a rectifier circuit to supply power to the rechargeable battery and equipment.

The biggest difference between the electric field coupling method and other methods is the voltage shift change of each part (Figure 5). For example, when an AC adapter with an input voltage of 5V supplies power to the power transmission module, the voltage is first slightly increased by the amplifier, and then increased to about 1.5kV by the boost circuit. After the power is transmitted at this voltage, it is converted into a DC voltage for actual use by the step-down circuit and the rectifier circuit. The switching frequency of the power supply circuit is 200k~400kHz, thus forming a system.

Figure 5: Transmitting power at high voltage

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The figure shows the voltage changes in each part of the electric field coupling method. The most important feature is that the voltage is increased to 1.5kV by the booster circuit to transmit power.

Achieve location freedom

The electric field coupling method has three main characteristics: ① freedom of position during charging, ② thin electrodes, and ③ the temperature of the electrode portion does not rise.

Let's first look at the first feature, which is freedom of position. Although it is not possible to transmit power over a long distance of several meters like the magnetic field resonance method, the horizontal position freedom is high. For example, users can charge their portable terminals by placing them anywhere on the power supply station. This helps improve user convenience.

In fact, if we compare it with the electromagnetic induction method, this difference will be clear (Figure 6). When the misalignment between electrodes (dz/D) is 1, the electrodes and coils on the power transmission side and the power receiving side hardly overlap and are in a state of large misalignment. As can be seen from the figure, the transmission efficiency of the electric field coupling method only decreases by 20% even if the electrodes have a misalignment equivalent to 1. From this, it can be seen that when the misalignment between electrodes is small, the transmission efficiency of this method will only decrease by about 10%.

Figure 6: Achieving excellent positional freedom

The figure compares the efficiency changes of electric field coupling and electromagnetic induction for horizontal misalignment.

The electromagnetic induction method is different. The centers of the power transmission coil and the power receiving coil must be perfectly aligned. If there is a slight misalignment, the transmission efficiency will drop sharply. Therefore, the "Qi" specification formulated by the WPC, an industry group that uses the electromagnetic induction method, has to rack its brains to achieve positional freedom. For example, a large number of power transmission coils are arranged and driven by a motor .

The electric field coupling method can deal with the misalignment problem with a simple structure, so it not only provides convenience but also reduces system costs.

Electrodes with high design freedom

The second feature of the electric field coupling method is that the electrodes are thin. It can be said that they can be made as thin as possible. Therefore, they can be easily embedded in devices and can be supported by a variety of devices.

For example, when it is installed in a smartphone that requires extremely thinness, only a 1.5 cm square and 5 μm thick electrode material (such as copper foil) is required (Figure 7).

Figure 7: Example of embedding into a smartphone back cover

Since extremely thin electrodes can be used, they can be easily embedded in smartphones, etc. The electrodes do not have to be square, and any shape is acceptable.

In addition, it can be made into various shapes. It is not necessary to make it into a square, and it can also be a triangular, circular, or elongated electrode. The material used for the electrode can also be arbitrary, and a conductor is the best. In addition to copper foil, aluminum foil, transparent electrodes, thin films, and gold plating can also be used. Therefore, it can be said that when embedded in machines of various structures and sizes, the design freedom is high.

The third point is that the temperature of the electrode part does not rise, which is also a very important feature (Figure 8). Murata Manufacturing often talks about the importance of thermal countermeasures during discussions with customers. Since the temperature does not rise, the electrode part can be placed close to the battery pack that is easily degraded by heat.

Figure 8: The temperature of the electrode part does not rise

Since the electrode portion does not generate heat, the possibility of thermal degradation of the battery is low even if it is placed close to the battery pack.

As for why the electrode part does not generate heat, as shown in Figure 5, it has a lot to do with increasing the voltage. Since the voltage is increased to about 1.5kV, the current flowing through the electrode part is only about a few mA, so in principle, the cause of heat generation is suppressed.

Of course, since the power transmission module and the power receiving module are equipped with power circuits, the power loss of the power circuit will become heat, so heat of about 10 to 20°C will still be generated (Figure 9). However, countermeasures can be taken in the design stage, such as configuring the power transmission module and the power receiving module away from the battery pack. (To be continued, special contributors: Eiji Ieki, head of the Technology Business Development Headquarters of Murata Manufacturing, Shinji Goma, head of the Product Planning Section of the Application Technology Product Department of the Technology Business Development Headquarters of Murata Manufacturing)

Figure 9: Heat generation in the power receiving part can be counteracted by design

The power receiving module generates heat due to power loss in the step-down circuit and DC-DC converter. However, countermeasures can be taken such as keeping it away from the battery pack or placing it in a place where heat can be easily dissipated.

Three new topics

Taking advantage of these characteristics, the electric field coupling method will be gradually embedded in machines in the future. At that time, three technical aspects will become particularly important: ① wireless interference countermeasures, ② safety countermeasures, and ③ power supply to multiple machines.

First, we need to address the characteristics of wireless interference. This requires the unification of various standards related to the impact on the human body and other machines. Murata Manufacturing will soon begin mass production of 10W power transmission modules and power receiving modules that will comply with relevant standards such as " IC NIRP" and "CISPR 22" (Table 1). Figure 10 shows the test results related to the wireless interference standard "CISPR 22 Class B". The results show that both the noise terminal voltage and the radiated noise meet the standards.

Figure 10: Wireless interference characteristics are below the standard value

The figure shows the situation when wireless transmission of 10W power complies with the international standard for wireless interference "CISPR 22". The values ​​of wireless interference are all below the standard value.

Murata Manufacturing also took corresponding measures when designing the power supply station. The key point is to configure two sets of electrodes, active electrode and passive electrode. Among them, the active electrode generates a very high electric field. The passive electrode plays a grounding role and is configured in the form of surrounding the active electrode.

This configuration is to shield and protect the power transmission module and switches, and prevent the high electric field generated by the active electrode from escaping to the outside. If the design is based on this concept, it can not only fully comply with legal regulations, but also prevent the impact on the human body and surrounding equipment.

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Insulation measures must be taken

Next is ② safety measures. As mentioned many times before, the electric field coupling method uses high voltage. Therefore, Murata Manufacturing itself is also working hard to promote the development of safety measures.

First of all, the electrode part must be insulated. In order to eliminate the possibility of leakage after human contact, the electrode part must be insulated. This alone greatly improves safety.

It is also equipped with a control function that stops power transmission when the device is not placed on the power supply platform and when it is fully charged. In addition, it is equipped with a software processing function that detects load changes and stops power transmission when abnormal situations such as human body approaching or metal foreign objects falling into the device occur.

In addition, measures have been taken to immediately stop power transmission in the event of a current leak to prevent the machine from catching fire or emitting smoke.

Can supply power to multiple machines

Finally, support for multiple devices (③) is expected to be achieved by utilizing the characteristics of the electric field coupling method. This idea is an extension of the characteristic of position freedom, that is, as long as multiple devices are properly placed on the power supply platform, wireless power can be supplied to any device.

When expanding the range of positional freedom, there are two possible means to consider: enlarging the transmission side coupling electrode, or configuring multiple transmission side coupling electrodes for switching. However, enlarging the electrode will cause the induced electric field to expand excessively, which will become a problem in terms of compliance with regulations and ensuring safety.

Therefore, if you want to power multiple devices, it is best to prepare multiple electrodes on the power transmission side. Murata Manufacturing has completed a prototype and exhibited it for the first time at the "2011 International CES" exhibition held in Las Vegas, USA in January 2011. The prototype at that time could power two e-book terminals at the same time.

The power supply station is equipped with four electrodes, and power is supplied only from the electrode closest to the place where the electronic book terminal is placed, which is the object to be powered (Figure 11). In this way, an electric field is generated only in the part where power is to be transmitted. Since no electric field is generated in other parts, there will be no problem even if you touch it with your hands or place metal, and safety can be fully ensured.

Figure 11: Power is only delivered where it is needed

By switching the power transmission electrode, only the part corresponding to the power receiving electrode can be activated. It is also possible to supply power to multiple devices.

Product line expansion and miniaturization

Finally, let me introduce Murata Manufacturing's future development blueprint. The general direction is to expand and miniaturize the product line. First of all, let's look at the expansion of the product line. In addition to the 5W and 10W products that have already been mass-produced, we will also develop products with higher output power such as 50W and 100W.

For 5W and 10W products, miniaturization is the most important (Figure 12). In order to launch small products in fiscal 2012, Murata Manufacturing increased the number of engineers and strengthened the development system.

Figure 12: The goal is to achieve substantial miniaturization

The figure shows the development roadmap. In addition to 10W products, we will expand the variety of various output powers. In addition, we will promote miniaturization and introduce new technologies.

In fact, in order to achieve miniaturization, it is essential to improve the transformer responsible for stepping up and down the voltage. As can be seen from Figure 2, the transformer is the thickest part. Currently, the transformer uses a coil transformer. Now, we are considering reducing the number of coils by increasing the frequency, thereby achieving miniaturization and thinness.

Murata also considered using piezoelectric transformers. Piezoelectric transformers are technically very difficult, but they can produce very good results, so Murata took on the challenge.

A world where charging is easy

In addition to miniaturization, Murata Manufacturing has also made several other technical improvements. Among them, a lot of effort has been put into improving the transmission efficiency, which is of great concern to everyone in wireless power supply. In this regard, the key is to improve the device, especially the Q value.

Of course, Murata has invested a lot of time and manpower in safety measures, but for future practical applications, we plan to push this aspect to a higher level. As a future technology, Murata is also considering equipping it with a communication function. Although it is not equipped with a communication function at present, they often ask for it when negotiating with customers.

Another point that should not be forgotten is cost reduction. When the module was released in 2010, the price of a 3W power transmission module and a power receiving module was less than 1,000 yen. Murata will further reduce costs in the future. Finally, in order to popularize the electric field coupling method in the future, Murata has also considered standardization and carried out related activities.

Murata Manufacturing's goal is to realize a world where wireless charging is truly easy, whether at home or in the office. Murata Manufacturing will first promote the popularization of electric field coupling wireless power supply, focusing on portable terminals.