Design Considerations for Coils in Wireless Charging Systems

We have all heard of TI, Linear Technology, IDT, Toshiba and many other companies developing wireless power transmitter/receiver ICs for the fast-growing market of wireless power transmission. A key component that is often overlooked but in any case a wireless power transmission solution is the transmit/receive antenna coil. Designers can design their own coils, but there are two very outstanding and capable companies that design a wide variety of coils for the user.

At this year's DesignCon 2015, the author visited the Würth Elektronik booth and was amazed at the company's depth of design in terms of wireless charging and transmitting coils.

First of all, I am very optimistic about the standards and interoperability that a wide range of technologies such as wireless power transmission follow. Würth Elektronik eiSos (Passive and Mechanical Components) is a member of the WPC Wireless Charging Alliance and the A4WP (also known as Rezence) Wireless Charging Alliance. In order to further consolidate and unify the standards, A4WP has agreed to merge with the PMA Wireless Charging Alliance (PMA). This will help the industry adopt a common standard and is expected to adopt the same standards in the near future.

The next most important thing for designers is to get a good performance board. Würth Elektronik eiSos has a 5W design kit that uses optimized components to meet the Qi standard. Designers need tools like this to ease their design burden and speed time to market. Digi-Key offers this kit on their website. Mouser also has this kit for sale on their website.

Figure 1: The Würth 5W kit is important for designers who use the Qi standard to perfect wireless power transmission inventions. These boards use TI's ICs and a variety of charging and transmitting coils that users can choose from.

Figure 1: The Würth 5W kit is important for designers who use the Qi standard to perfect wireless power transmission inventions. These boards use TI's ICs and a variety of charging and transmitting coils that users can choose from.

Figure 2: Available in a variety of coil sizes and shapes for designers to choose from. Table 1 lists the various sizes available in this kit.

Figure 2: Available in a variety of coil sizes and shapes for designers to choose from. Table 1 lists the various sizes available in this kit.

Table 1, the various wireless power coils available in the 5W demo kit.

Table 1, the various wireless power coils available in the 5W demo kit.

Principle of wireless power transmission

Figure 3: Principles of wireless power transmission

Figure 3: Principles of wireless power transmission

NuCurrent coil

NuCurrent has a wireless charging antenna technology that is compatible with A4WP, Qi and PMA standards. Users don't have to think too much about standard issues after using the company's solutions because they simplify standard issues. By using their multimode solution, the receive coils work with a variety of transmitters including Qi, PMA and A4WP. In fact, NuCurrent's high-efficiency resonators have been selected for the first commercial A4WP certified product and WPC standard antenna for on-board charging (ie, the phone is placed on the car's central armrest for charging). NuCurrent also announced a strategic partnership and investment from Molex, whose products were selected as reference antennas in systems developed by other wireless charging industry leaders such as Broadcom and Epson.

Well, if EPC chooses NuCurrent as a partner, then I am convinced that they definitely have something special. EPC develops rugged high-speed, high-power eGaN MOSFETs that are suitable for use as high-power drivers for wireless charging coils, and EPC's application engineers are a highly talented group.

Therefore, the author specifically investigated the design company that invented the printed coil technology and recently interviewed the company's CEO, Jacob Babcock. What is the difference between NuCurrent coils? Babcock tells that these coils have the highest quality factor on the market.

What is the quality factor Q?

WPC defines Q as:

For coils of the same volume and shape but with different winding arrangements, the ratio of inductance L to resistance R remains the same. In order to distinguish between different coil configurations, it is meaningful to define this ratio as the quality factor Q.

The voltage induced by the same current in the inductor varies with frequency f and apparent power in the device. The general definition of the quality factor is based on the ratio of apparent power to power loss in the device. According to this definition, the quality factor of a coil can be derived:

Q = ωL/R

Where ω = 2πf:

The quality factor Q ranges from 0 to infinity, although it is difficult for coils to obtain values ​​well above 1000. For batch products, the quality factor that can be expected is about 100. A quality factor below 10 is not very useful. These values ​​must be considered on a typical order of magnitude.

For a fixed operating frequency, the quality factor Q depends primarily on the shape and size of the coil and the materials used. Standard coil technology (such as wire wound coils, PCB coils) are generally specified with quality factor values.

Compared to traditional wireless charging antennas, NuCurrent devices are smaller, extend battery life, cover longer distances, have lower heat generation, and provide greater flexibility in alignment.

The two most common antenna frequencies used by WPC and A4WP are 150 kHz and 6.78 MHz, respectively.

Figure 4: NuCurrent technology can be used to create efficient and flexible wireless charging antennas (specially provided by NuCurrent)

Figure 4: NuCurrent technology can be used to create efficient and flexible wireless charging antennas (specially provided by NuCurrent)

At low frequencies (Qi compatible), the system typically operates at close proximity of approximately 2 cm. The benefits of printed coils are durability, flexibility, scalability, and ease of integration.

At higher frequencies (A4WP compatible), the potential benefits are farther power transmission distances, better directional flexibility, and smaller size. Such antennas are referred to herein as "resonators", and by virtue of their small size and low price, they can provide the best possible Q values ​​because they help to implement designs into standard manufacturing processes like PCBs and flexible circuits. in.

Two breakthroughs in coil technology

Babcock told me that two fundamental breakthroughs have enabled his technology to surface in the wireless power transmission field. His first effort was the wireless charging implanter.

One is the inherent “skin effect” and the other is the “proximity effect”.

The high-frequency "skin effect" and "proximity effect" are two prominent issues in Babcock's wireless charging implant design. He studied how to minimize these effects and found that if the current in the wire is shunted to multiple smaller diameter wires, the resistance can be significantly reduced, and the efficiency is higher than other printing methods and magnetic wires. many. In a way, this is like placing a distracting, inconsistent but low-resistance Litz wire in a rugged, scalable printed substrate.

effectiveness

Efficiency is specific to the application, but in general, NuCurrent's inductor efficiency is higher at high frequencies (1MHz+) than any other type of inductor. Their Q values ​​are increased by 20% and 60%, respectively, at 150 kHz and 6.78 MHz.

The basic premise of NuCurrent's proprietary technology is to mitigate the typical high-frequency "skin effect" that occurs in wires at higher frequencies. Their design provides more surface area for current flow, which helps designers achieve higher efficiency, smaller size, better durability and lower cost.

NuCurrent's design tools and proprietary algorithms use multilayer (ML) wire technology to convert designs into inductive structures with lower resistance and higher Q than most other solutions. These designs support multi-layer multi-turn (MLMT) wires of different shapes on FR-4 or flexible polyimide.

NuCurrent structures are more consistent from units 1 to 1000; more robust in harsh environments; more cost effective in high volume; can be integrated directly into PCBs or other substrates without any soldering (the latest development is Integrated into the interior of the furniture, such as wardrobes, dining tables, etc.).

Figure 5: NuCurrent's patented technology decomposes frequency and resistance; it mitigates high frequency effects in the inductor, resulting in higher efficiency/higher Q inductance. (Special provided by NuCurrent)

Figure 5: NuCurrent's patented technology decomposes frequency and resistance; it mitigates high frequency effects in the inductor, resulting in higher efficiency/higher Q inductance. (Special provided by NuCurrent)

Please keep a close track on the development of this technology in 2015, as more companies will find new applications and improve this technology to win more of this growing market, and more technical results will be shown to everyone. before.

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