Multi-mode design for wireless charging that takes into account both efficiency and convenience

Multi-mode solutions that are compatible with the advantages of different wireless charging technologies will become the mainstream in the future. As there is no unified standard for current wireless charging technology, and each standard camp has its own technical advantages and disadvantages, the industry has adopted a multi-mode solution to solve the compatibility issues between different standards and allow wireless charging products to provide a user experience that combines charging efficiency and spatial freedom.

Wireless charging technology solutions include magnetic induction (MI) and magnetic resonance (MR). Regardless of the direction of the consumer market, wireless charging has become an inevitable trend. In the next few years, wireless charging will be mainly driven by mobile phone manufacturers and begin to penetrate the mobile phone market; then, the computer market with a sound ecosystem will follow and bring about the growth of wireless charging technology. From then on, wireless charging will develop into a solution that supports mobile phones and computers. There are currently many reports and studies on the adoption rate and potential total available market (TAM) of wireless power, but it is not easy to provide accurate market information because adoption rate and technology selection are key parameters in these forecasts. There are two main standards for magnetic induction technology: the Wireless Power Consortium (WPC) and the Power Management Alliance (PMA). Both standards are quite mature, and there are many products in use in the consumer market.

The Alliance for Wireless Power (A4WP) is the first standard for magnetic resonance. It is worth noting that Intel's magnetic resonance wireless charging technology is designed for its own ultra-thin laptops and ecosystem; others such as PowerbyProxi and WiTricity, which have established their position in the industrial and military fields, are also beginning to enter the consumer market.

To understand the impact of standards and solutions on the future direction of wireless charging technology, we must first understand the differences between MI and MR technologies. Once we fully understand and are familiar with the requirements of the application/system, we can choose a solution for a specific application.

Solving the bottleneck of battery capacity and gaining popularity in wireless charging applications

Mobile solutions are the first to adopt wireless charging technology in the consumer market. Because of the Long Range Evolution (LTE) technology, communication speed and bandwidth will not encounter bottlenecks for at least the next few years. Convenience is one of the key factors driving mobile solutions in the consumer market. Different mobile solutions, such as mobile phones, tablet computers, multimedia players and mobile TVs, require different transformers and connector interfaces. Therefore, to charge mobile devices, you need to carry a lot of connectors and transformers. If there is a universal wireless transformer plus a complete infrastructure and ecosystem, this demand can be met. Wireless charging can be carried out at any time in cars, coffee shops, libraries, restaurants, trains, airplanes, offices, conference halls and other places, which can bring the convenience that everyone expects.

Every two years, the appearance, performance and various functions of mobile solutions are upgraded, and these upgrades force changes in power requirements, connectors and interfaces, thus requiring new transformers. These changes and upgrades also cause waste due to the elimination and disposal of existing transformers. If various transformers and connectors can be eliminated and standard wireless charging is adopted, it will help reduce electronic waste and improve the "green credentials" of mobile devices.

Another important factor is the technology upgrade of mobile solutions, such as the use of 1,080p and 3D display technologies. Mobile solutions will increase the use of high-resolution display technology, which is supported by high-performance graphics controllers and multi-core central processing units (CPUs); in addition, the integration of an increasing number of mobile solution technologies, including 3D global satellite positioning system (GPS) solutions, high-performance audio and video technology, near-field wireless communication (NFC) technology, portable TVs and high-performance games, will increase the demand for device battery power.

Mobile solutions are usually powered by lithium-ion (Li-ion) polymer batteries, whose energy density has reached saturation for several years. The performance and life of lithium batteries, which have been improved by technology upgrades and the transition to different metals, can no longer meet the increased power demand, and the battery must remain small in size to meet the application requirements of mobile solutions. Because the battery capacity per unit volume has reached its limit, the solution will need to achieve a higher battery capacity or increase the charging frequency.

As mobile solutions shrink in size, higher capacity batteries will impact the overall solution size and cost; it should also be noted that higher capacity batteries require faster charging efficiency, and chemical changes occur when maintaining the battery life cycle and required life conditions. Therefore, improving charging efficiency seems to be a more obvious solution.

Technical principles affect MI/MR application areas

Any application that requires power may use wireless charging solutions. However, to choose between MI or MR wireless charging technology, it is necessary to first examine the basic principles of the two.

MI and MR have many similarities in their technical architectures. For example, both use magnetic fields as a bridge for power transmission, and current is induced in the resonant circuit to generate a magnetic field that transmits power. The magnetic parameters have a profound impact on how the electromagnetic field is formed; the magnetic flux can be controlled by directly using electromagnetic shielding and/or changing the actual shape of the magnetic core. The density and capacity of the magnetic flux can be increased by improving the penetration of the electromagnetic field shield (Figure 1).

Figure 1 Wireless charging magnetic field

Cost and thickness are key factors in choosing the appropriate electromagnetic shielding. The arrangement of the current field receiving and transmitting coils, and the distance between them, will determine the efficiency of power transmission; the greater the distance between the transmitting and receiving coils, the lower the efficiency of power transmission. Other factors that have a significant impact on energy transfer efficiency include resonant frequency, ratio of transmission and receiving coil sizes, coupling coefficient, coil impedance, skin effect, AC and DC components and coil parasitics.

When x, y and z are separated and the ratio angle of the transmitting and receiving coils increases, it will have a great impact on energy loss and efficiency. In the WPC specification, there are specific requirements for the position of the receiver (Rx) coil on the transmitter (Tx) to maintain its efficiency and achieve the highest coupling coefficient between the two coils. But in MR technology, there is freedom in placement and single or multiple devices can be placed in the magnetic field, which can make it more convenient for users; however, when the spacing between the coupling devices increases, it will also affect the transmission efficiency.

All wireless charging technologies can use single or multiple coil solutions, depending on different requirements, including cost and size considerations. MI technology based on WPC and PMA specifications can transmit power over a wide frequency range. The resonant frequency of power transmission is selected based on the load impedance, because this variable has a relatively low Q factor compared to MR solutions, and can only achieve optimal efficiency at a specified frequency and load impedance.

For MR technology, because power can only be transmitted at a specific resonant frequency, the Q factor is large and a very close resonant impedance network matching is required between the receiver and the transmitter. In MR and MI technologies, the variation of the matching network parameters must be strictly controlled because it directly affects power transmission.

In the WPC 1.1 standard, the resonant frequency can be selected in the range of 100k-205kHz. In the case of PMA, the frequency range is similar, with a frequency range of 277k-357kHz. However, recently the frequency range has been changed and now depends on the input supply voltage. Typical Q factors for these solutions range from 30-50 (Figure 2).

Figure 2 Q coefficient percentage

In the A4WP specification solution, because the frequency is fixed, the resonant frequency and impedance network between the transmitter and the receiver need to be more accurately matched. The typical MR solution requires a higher Q factor (50-100) compared to the MI solution.

Power management affects wireless charging performance

The development of high-performance power management architectures has a significant impact on the successful implementation of MR and MI solutions. For the transmitter, in order to induce current in the resonant circuit, a DC to AC conversion must be performed. In MI technology, a half-bridge or full-bridge inverter is used for this conversion; in MR technology, the current is induced through a power amplifier (PA).

The architecture and classification of power amplifiers vary depending on the frequency, quiescent current, efficiency, size, cost and integration requirements of each application. Careful consideration must be given to reducing gate driver losses, switching, conduction, biasing, internal diode losses, and external component equivalent series resistance (ESR) and equivalent series inductance parasitics (ESL). These are some of the major challenges encountered in developing high-performance integrated solutions.