Mainstream wireless charging technology

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Mainstream wireless charging technology [Copy link]

       With the development of the Internet of Things (IoT), wearable and portable devices, consumers are beginning to get tired of messy cables and batteries that need to be charged frequently. The advantages of wireless charging go far beyond getting rid of the constraints of cables. There are various near-field and far-field charging wireless technologies on the market. These technologies need to follow different standards and also need to be integrated to varying degrees. As people's pursuit of getting rid of cable power supply increases day by day, wireless charging is being applied to more and more fields. Wireless charging technology can be roughly divided into four types. The first type is short-range transmission through electromagnetic induction "magnetic coupling". Its characteristics are short transmission distance and relatively fixed use position, but high energy efficiency and simple technology, which is very suitable for use as wireless charging technology. The second type is to transmit electrical energy in the form of electromagnetic wave "radio frequency" or non-radiative resonance "magnetic resonance". It has high efficiency and very good flexibility, and is currently the focus of development in the industry. The third type is the "electric field coupling" method, which has the advantages of small size, low heat generation and high efficiency. The disadvantage is that there are fewer developers and supporters, which is not conducive to popularization. The fourth type is to transmit electric energy wirelessly in the form of microwaves - transmitting it to the remote receiving antenna, and then using it after rectification, modulation and other processing. Before that, let's take a look at their similarities and differences.

1. Electromagnetic induction method Most of the various wireless charging technologies we see today use electromagnetic induction technology, which can be regarded as a separate transformer. We know that the widely used transformer now consists of a magnetic core and two coils (primary coil and secondary coil); when an alternating voltage is applied to both ends of the primary coil, an alternating magnetic field will be generated in the magnetic core, thereby inducing an alternating voltage of the same frequency on the secondary coil, and the electric energy is transmitted from the input circuit to the output circuit. If the coil at the transmitting end and the coil at the receiving end are placed in two separate devices, when the electric energy is input to the coil at the transmitting end, a magnetic field will be generated, and the magnetic field will be induced to the coil at the receiving end, and a current will be generated, so that we have built a wireless power transmission system. The main disadvantage of this technology is that the magnetic field weakens rapidly with the increase of distance, and generally only works within the range of several millimeters to 10mm. In addition, the energy is dispersed in all directions, so the induced current is much smaller than the input current, and the energy efficiency is not high. However, this is not a problem for objects in close contact. The earliest wireless charging product that used this principle was an electric toothbrush. Since electric toothbrushes are often in contact with water, they use a contactless charging method, which can prevent the charging contact points from being exposed, enhance the product's waterproofness, and can also be washed as a whole. There is a coil in the charging socket and the toothbrush. When the toothbrush is placed on the charging stand, there is a magnetic coupling effect, using the principle of electromagnetic induction to transmit electricity. After rectification, the induced voltage can charge the rechargeable battery inside the toothbrush.

The figure shows a schematic diagram of electromagnetic induction wireless charging technology.

2. Magnetic resonance method Compared with the electromagnetic induction method, magnetic resonance technology has a certain tolerance in distance. It can support wireless charging from several centimeters to several meters, and is more flexible in use. Magnetic resonance also uses two coils with completely matched specifications. When one coil is energized, it generates a magnetic field, and the other coil resonates and generates current that can light up a light bulb or charge a device. In addition to the long distance, the magnetic resonance method can also charge multiple devices at the same time, and there is no strict restriction on the location of the devices. The flexibility of use ranks first among all technologies. In terms of transmission efficiency, the magnetic resonance method can reach 40% to 60%. Although it is relatively low, it has no problem entering commercialization.

The figure shows a schematic diagram of magnetic resonance wireless charging technology

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3.电磁耦合方式
       相对于传统的电磁感应式，电场耦合方式有三大优点：充电时设备的位置具备一定的自由度；电极可以做得很薄、更易于嵌入；电极的温度不会显著上升，对嵌入也相当有利。首先在位置方面，虽然它的距离无法像磁共振那样能达到数米的长度，但在水平方向上也同样自由，用户将终端随意放在充电台上就能够正常充电。我们可以看到电场耦合与电磁感应的对比结果，电极或线圈间的错位用dz/D（中心点距离/直径）参数来表示，当该参数为0时，表示两者完全重合，此时能效处于最高状态。当该参数为1时，表示两者完全不重合。我们可以看到，此时电场耦合方式只是降低了20%的能量输入，设备依然是可以正常充电，而电磁感应式稍有错误、能量效率就快速下降，错位超过0.5时就完全无法正常工作，因此，电磁感应式总是需要非常精确的位置匹配。
        电场耦合方式的第二个特点是电极可以做到非常薄，比如它可以使用厚度仅有5微米的铜箔或者铝箔，此外对材料的形状、材料也都不要求，透明电极、薄膜电极都可以使用，除了四方形外，也可以做成其他任何非常规的形状。这些特性决定了电场耦合技术可以被很容易地整合到薄型要求高的智能手机产品中，这也是该技术相对于其他方案最显著的优点。显而易见，若采用电场耦合技术，智能手机厂商在设计产品时就有很宽松的自由度，不会在充电模块设计上遭受制肘。
第三个优点就是电极部分的温度并不会上升——困扰无线充电技术的一个难题就是充电时温度较高，会导致接近电极或线圈的电池组受热劣化，进而影响电池的寿命。电场耦合方式则不存在这种困扰，电极部分的温度并不会上升，因此在内部设计方面不必太刻意。电极部分不发热主要得益于提高电压，如在充电时将电压提升到1.5kv左右，此时流过电极的电流强度只有区区数毫安，电极的发热量就可以控制得很理想。不过美中不足的是，送电模块和受电模块的电源电路仍然会产生一定的热量，一般会导致内部温度提升10～20℃左右，但电路系统可以被配置在较远的位置上，以避免对内部电池产生影响。

图为电磁耦合无线充电技术示意图

4.微波谐振方式
       这项技术采用微波作为能量的传递信号，接收方接受到能量波以后，再经过共振电路和整流电路将其还原为设备可用的直流电。这种方式就相当于我们常用的Wi-Fi无线网络，发收双方都各自拥有一个专门的天线，所不同的是，这一次传递的不是信号而是电能量。微波的频率在300MHz～300GHz之间，波长则在毫米-分米-米级别，微波传输能量的能力非常强大，我们家庭中的微波炉即是用到它的热效应，而英特尔的微波无线充电技术，则是将微波能量转换回电信号。
       微波谐振方式的缺点相当明显，就是能量是四面八方发散的，导致其能量利用效率低得出奇，如英特尔的这套方案，供应电力低至1瓦以下，乍一看起来实用性相当有限。而它的优点，则是位置高度灵活，只要将设备放在充电设备附近即可，对位置的要求很低，是最符合自然的一种充电方式。我们可以看到，当设备收发双方完全重合时，电磁感应和微波谐振方式的能量效率都达到峰值，但电磁感应明显优胜。不过随着X-Y方向发生位移，电磁感应方式出现快速的衰减，而微波谐振则要平缓得多，即便位移较大也具有相当的可用性。

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