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When metallic objects are exposed to an alternating magnetic field, eddy currents cause such objects to heat up. The amount of heating depends on the amplitude and frequency of the magnetic field, as well as on the characteristics of the object such, as its resistivity, size, and shape. In a wireless power transfer system, such heating is undesired as it manifests itself as a power loss, and therefore a reduced power tranfer efficiency. Moreover, if no appropriate measures are taken, such heating could lead to unsafe situations if the heated objects would reach high temperatures.

This Annex D discusses the recommended mechanism for Power Transmitters and Power Receivers to deal with the power loss due to parasitic metals—i.e. metals that are neither part of the Power Transmitter, nor of the Power Receiver, but which are dissipating power from the magnetic field during power transfer. Examples of such parasitic metals are coins, keys, paperclips, etc.

It is recommended that a Power Receiver performs metal object detection by monitoring the temperature near its Interface Surface. If the measured temperature exceeds an internal safety threshold, the Power Receiver should terminate power transfer by communicating an End Power Transfer Packet—with End Power Transfer Code set to 0x03—to the Power Transmitter.

In addition, it is recommended that a Power Transmitter estimates the power that is transmitted through its Interface Surface, and monitors the Rectified Power Values that are communicated by the Power Receiver. If the combination of the estimate and these values indicate an unexpected power loss, the Power Transmitter should terminate the power transfer.

仅供参考：

    当金属物体接触到一个交变的电磁场时，涡流会使这个物体加热。加热的幅度取决于电磁场的幅度和频率，同时也取决于该物体的特性，例如电阻率、尺寸以及形状。在一个无线功率传输系统，这样的加热不是我们所希望的，因为这表现为功率的损耗，因此功率传输效率降低。还有，如果不采取合适的措施的话，被加热的物体若达到比较高的温度的话，这样的热效应可能会引起不安全的情况。

    附录D讨论了功率传输器和接收器推荐的结构来处理由于寄生金属导致的功率损耗-即金属物体既不是功率传输器也不是功率接收器的组成部分，但是他在功率传输过程中从磁场消耗功率。这样的寄生金属例子有硬币，钥匙，回形针等。

    因此，建议一个功率接收器去通过监测靠近他的接口表面的温度来检测金属物体。如果测量的温度超出内部安全阈值，功率接收器通过向功率发射器发送“终止功率传输包”终止功率传输-把终止功率传输包”代码设为0x03。

    还有，建议功率发射器要估计通过其接口表面已发射的功率以及监控功率接收器所通知的整流功率值。如果这个估计值和整流功率值的组合表明出现了一个不期望的功率损耗，功率发射器应终止功率传输。
 

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