Wireless power transfer coil

Smart charging on the go

Now, wireless charging finally makes it possible to completely free mobile devices from the last wire - much to the delight of smartphone users. TDK has developed an ultra-flat power transfer coil that meets the most stringent Qi specifications of the Wireless Power Consortium (WPC).

Power transmission without the use of wires or connectors has been widely used in various products, such as toothbrushes, LED candles, remote controls, medical devices or induction cookers. This method of power transmission is both reliable and convenient. However, until now, wireless charging systems have only been used for specific products or applications, and it cannot be universally used for other devices of various sizes and shapes. Today, users require charging stations that can universally charge products from different manufacturers and can charge the next generation of product models at the same time.

Future mobile device users will no longer have to worry about battery consumption when using their devices, as universal wireless charging will be available in almost every place they go. More than 120 companies worldwide are now participating in the WPC to create and promote standardized wireless power transfer. Following the WPC's Qi specification, TDK has developed a new ultra-low-profile wireless power transfer coil that enables standardized wireless charging for mobile devices.

TDK's vision is to develop an absolutely flat surface that can charge all WPC-compliant devices, thereby providing an easy-to-use charging platform that can be used anywhere.

Basic principles of wireless power transmission

Electromagnetic induction is the most widely used method for wireless power transmission. The basic principle of this method is to transfer and convert magnetic flux (Figure 1). Here, electromagnetic induction between the primary coil (Tx coil) and the secondary coil (Rx) transfers electromagnetic energy from the primary coil (Tx coil) to the secondary coil (Rx).

Figure 1: Basic principles of wireless charging for mobile devices

Wireless power transfer can wirelessly charge different devices of different sizes and shapes by using electromagnetic induction. A universal system will need to be compatible with the number of windings of the Tx and Rx coils as well as their overall geometry.

One of the technical challenges of wireless charging technology is to align the Tx and Rx coils so that they can achieve the most efficient power transfer. In order to design a universal charging pad for devices of different sizes and shapes, we must avoid the use of mechanical positioning aids such as device holders or similar structures.

The WPC has proposed and specified three positioning methods to optimally align the charging coil:

Use the magnet in the center of the Tx coil and the magnetic material in the center of the Rx coil for guidance and positioning (Figure 2)

Free positioning using a mobile Tx coil that detects the position of the Rx coil

Free positioning using selective activation of one of multiple Tx coils in the coil array closest to the Rx coil

Figure 2: Using magnets for guidance and positioning

In order to achieve the most efficient transfer of magnetic field energy, it is very important to accurately align the Tx and Rx coils. The guided positioning method uses a magnet in the Tx coil to attract the Rx coil to the correct position.

The WPC has adopted a certification system to verify the interoperability of both the Rx and Tx sides for the alignment method described above. Unless the coil can operate correctly for all Tx coils, it is impossible to obtain Rx coil certification, so TDK has designed a true Rx coil assembly to provide the user with the most flexible and easy-to-operate solution. This is achieved by placing a magnet in the center of the Tx coil and magnetic material in the center of the Rx coil.

Reference address of this article: http://www.eepw.com.cn/article/201809/388675.htm

Figure 3: Magnetic flux between Tx and Rx coils

When using the Rx coil for guidance and positioning, magnetic shielding is required to prevent the occurrence of high-frequency magnetic flux. The material and thickness of the magnetic sheet on the Rx side must also be specially designed so that the magnetic sheet does not reach the magnetization saturation point.

Challenges facing magnetic disks

Each method has its own advantages and disadvantages. For example, although the guided positioning method is simpler for the design of the Tx side, it needs to consider the magnetic shielding issue.

In order to prevent the high frequency magnetic flux (over 100 kHz) generated by the Tx coil from reaching the aluminum battery housing and generating unnecessary eddy currents that will be converted into excess heat, it is very important to consider the magnetic shielding issue when designing the Rx coil according to the guided positioning method described above.

Therefore, we must design the material and thickness of the magnetic sheet on the Rx side so that the magnetic sheet does not reach the magnetization saturation point. In this case, the magnetic flux of the magnet will pass through the magnetic sheet on the Rx side (Figure 3). If the magnetic sheet is too thin, it will become a magnetic bias element and be magnetized to the saturation point, causing the coil inductance to be greatly reduced. In this case, it is impossible to supply wireless power normally-Based on TDK's rich experience in this field, TDK has solved a very complex design problem. This includes magnetic material technology and process technology, winding diagram technology expertise obtained from various coils manufactured, and magnetic diagram design technology.

TDK Coil Design

Taking into account the evaluation and design described above, TDK has developed an ultra-low profile wireless charging coil that meets the most stringent specifications set by the WPC (Figure 4). TDK's smartphone receiving coil unit, which is installed in a smartphone to charge the phone by receiving magnetic flux, uses a proprietary flexible thin magnetic sheet that is only 0.57 mm thick, which is the industry's leading position. TDK is also developing a magnetic sheet as thin as 0.50 mm to enable smartphones to have a thinner charging device design. At this point, the output current of the thin magnetic sheet with a thickness of 0.57 mm is approximately 0.5 A to 0.6 A, and a magnetic sheet with a thickness of even thinner 0.50 mm that can provide the same or better output current is already under development and is expected to start mass production in 2013.

These developments reflect TDK's extensive expertise in magnetic material technology and process technology, as well as in creating unique, extremely thin and flexible metal magnetic sheets. As a result, the coil device is not only ultra-thin and lightweight, but also highly shock-resistant because it provides excellent reliability. TDK will continue to contribute to the development and widespread use of wireless charging power transmission technology by providing low-cost, high-performance, high-reliability products and comprehensive solutions that meet all these requirements.

Figure 4: Ultra-low profile TDK charging coil

TDK has developed an ultra-low profile wireless charging coil that meets the most stringent specifications set by WPC. TDK's smartphone receiving coil device (right) is installed in a smartphone to charge the phone by receiving magnetic flux. It uses a proprietary flexible thin magnetic sheet with a thickness of only 0.57 mm, which is the industry's leading position.

Benefits for future designers

Wireless power doesn’t just help devices get rid of power cords and provide convenience; it also changes the way manufacturers design devices. Eliminating the power socket is an important step toward the goal of developing sealed and even thinner smartphones that are waterproof and dustproof. Such phones will also be more reliable and rugged, and no longer need a flat edge to mount a power socket.

Ubiquitous wireless charging will further allow designers to consider smaller batteries, as users will be able to simply top up their phone's battery as needed.