1. What is FOD?
FOD is the abbreviation of Foreign Object Detection. It is a technical detection method used in the wireless charging process to prevent unexpected objects from causing excessive temperature rise in the system.
Wireless charging is different from wired charging. The latter can only work after a physical hard connection, while wireless charging connects two independent objects through a magnetic field, so there may be foreign objects in the path. If the foreign object is a conductor such as metal, then an induced electromotive force will inevitably be generated in the alternating magnetic field, forming an induced current inside the conductor. At this time, the metal is equivalent to a resistor, resulting in high heat and causing harm.
Therefore, FOD technology is essential and very important in wireless charging applications.
For example.
Figure 1: With and without FOD detection protection
In the left picture, the charging transmitter (TX) has the FOD function. When a coin is placed, the TX detects the presence of foreign objects through FOD detection and exits the charging mode.
In the right picture, TX does not have FOD function. When a coin is placed, TX will not stop emitting magnetic field due to the presence of foreign objects. The coin will generate eddy current heating in the alternating magnetic field continuously established by TX, which can easily cause accidents.
2. Several detection methods of FOD
There are many ways to detect foreign objects, such as by detecting Power loss and Q value.
The Qi protocol only stipulates the temperature rise of the receiver (hereinafter referred to as RX) in wireless charging. The certification laboratory test stipulates that the RX temperature rise must not exceed 12° of the ambient temperature. However, it does not stipulate what method must be used to achieve it. The current common solutions are listed in the following table:
Table 1: FOD detection methods
2.1 What is Power Loss Detection?
Figure 2: PLOSS detection working principle diagram
*Note:
PLOSS = 300 mW is an appropriate threshold value for limiting heating of Foreign Objects
As shown in the figure, TX determines whether FOD occurs by calculating P LOSS .
PLOSS=PPT-PPTLOSS
in:
①P PT = P IN - P PTLOSS , P IN is equal to the input power, P PTLOSS is all the necessary transmission loss power at the TX end, including but not limited to the equivalent impedance of the primary coil and capacitor, the power consumption generated by the inverter circuit and PCB routing, the eddy current loss generated by the metal devices at the TX end, and the system power consumption.
②P PR = P OUT + P PRLOSS , P OUT is equal to the output power, P PRLOSS is all the necessary transmission loss power at the RX end, including but not limited to the equivalent impedance of the secondary coil and capacitor, the power consumption generated by the rectifier circuit and PCB routing, the eddy current loss generated by the metal devices at the RX end, and the system power consumption.
The reliability of using Power Loss to detect the presence of foreign objects mainly depends on whether the power reported by TX and RX is accurate. Considering some unmeasurable factors, such as the influence of degrees of freedom, the power reported by TX and RX under different loads has an offset, and the energy is partially dissipated into the air. In order to prevent TX from falsely reporting FOD, the Preceived reported by the RX end is usually P PR + PΔ, where PΔ is used as compensation for the above factors.
This shows that when there is no foreign matter on the TX surface, the power reported by RX is always equal to or greater than the transmission power P PT of TX . Depending on the output power, PΔ is also different, and its value is generally 5% of the maximum output power. The recommended PΔ value in Qi is as follows:
Table 2: Recommended values of PΔ
The advantage of this solution is that FOD can be detected in real time during power transmission, but it also has disadvantages. Since PPT and PPR are estimated by the TX and RX ends respectively, there may be systematic deviations even in the absence of foreign matter, affecting the FOD accuracy of TX.
For example, when there is no foreign object, the transmit power P PT calculated by the TX end is 5W, and the receive power P PR calculated by the RX end is also equal to 5W. However, due to detection and estimation errors, the actual P PT of the TX end is 5.1W (or 4.9W), and the actual P PR of the RX end is equal to 4.9W (or 5.1W). Then P LOSS is deviated from the standard 0 to 200mW (-200mW), and the original 300mW margin becomes 100mW (500mW), which reduces the tolerance threshold of the system. At this time, if the TX relaxes its threshold, it will lead to insensitive foreign object detection, and shrinking the threshold may cause false detection.
2.1.1 Power Calibration
In order to improve the effectiveness of the power loss detection method, the TX end can use power correction to solve its system deviation problem. However, the premise of power correction is that there is no foreign matter on the TX surface. This problem can be determined by the Q value detection in the next section.
According to the previous section, the system deviation is related to the transmission power level. Ideally, it should be calibrated in segments over the full range of output power. However, since this method is difficult to implement, a compromise solution is adopted: in the calibration stage, TX and RX will determine their own output and receive power according to the "light" load and "connected" load respectively. Based on these two load states, TX can use the following linear interpolation method to calibrate the output or receive power.
2.2 Q value detection
Q value is the main parameter to measure inductor components. It refers to the ratio of inductive reactance to equivalent loss resistance when the inductor works under an AC voltage of a certain frequency. The higher the Q value, the smaller the inductor loss. The Q value of the TX coil will be affected by the external environment. For example, when there are other metals on its surface, the inductance of the TX coil will decrease, while the equivalent impedance will increase, so the Q value will decrease.
Qi's EPP protocol stipulates that TX must include a Q value detection function in the negotiation phase to determine whether there is foreign matter on the surface. In order to ensure that TX can correctly determine whether the reason for the Q value reduction is the RX coil or foreign matter, RX should provide a reference Q value to TX through the 0x22 package. TX then determines a reasonable Q value margin based on this value, and finally compares it with the measured value to determine whether there is foreign matter.
Figure 3: Q value judgment
There are many ways to detect the Q value. Qi provides a measurement scheme, but does not limit it. The figure below is a reference scheme for Qi. As shown in the figure, the left side is the schematic diagram of the detection circuit. In addition to the coil, it also includes a sinusoidal voltage source and a resonant capacitor. When selecting the resonant capacitor, the reasonable operating range of the circuit should be combined, and the resonant frequency should be determined before selection. In this example, the resonant frequency is 100kHz, and the Q value of the coil is equal to the ratio of the effective voltage value at both ends of the coil to the effective voltage value of the driving power supply, that is, Q=V2/V1.
Figure 4: Qi protocol Q value detection circuit diagram
The figure below is a Q value detection scheme adopted by Volta for reference. When detecting the Q value, the oscillation circuit is pre-charged first, that is, S5 and Q4 are turned on at the same time. When the voltage reaches the preset value Initial, S5 is disconnected and Q3 is turned on. At this time, the current in the capacitor and inductor circuit will oscillate and gradually decay to 0. The presence or absence of foreign matter will cause the current decay time to change. By taking fixed thresholds Ith1 and Ith2 and measuring ΔT, the Q value can be calculated.
Figure 5: A Q value detection circuit diagram of Volta
Without FOD 50us/div
With FOD 20us/div
Compared with the above figure, it can be clearly seen that when there is foreign matter, its Q value is significantly reduced.
