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The selection of magnetic separators for wireless charging receivers usually revolves around efficiency, temperature rise, and thickness. These three indicators are intuitive, quantifiable, and easy to compare differences during the sample stage. But there is a dimension directly related to safety that is rarely included on the first page of the selection review: the impact of magnetic separators on FOD (foreign object detection).
The engineering logic of FOD is: while transmitting power, the transmitting end continuously compares the transmitting power with the power reported by the receiving end. If the difference between the two exceeds a preset threshold, it is determined that there is a metal foreign object and the power supply is stopped. The upper limit of foreign object loss allowed by the WPC compliance system is 350 mW, which does not include the known system losses at the transmitting and receiving ends. In other words, the determination of FOD is not about finding a state of "absolute zero loss", but rather about assessing whether the loss has "added extra" on a baseline that already has system loss. Magnetic separators are precisely one of the main components of this baseline.
The reason why the threshold of FOD is narrow is that the "budget" for system loss itself is not abundant
What is the magnitude of 350 mW? The power of a coin's eddy current heating in an alternating magnetic field may approach or even exceed this value. The hysteresis loss and eddy current loss of the magnetic separator itself, as well as the eddy current excited by unconstrained leakage magnetic flux in adjacent metal parts, will be included in the total loss of the system. If the loss of the magnetic separator is high or fluctuates greatly between batches, the baseline of system loss will be raised, and the "net foreign matter loss" judgment window of FOD will be compressed. Conversely, if the loss of the magnetic separator is abnormally low (such as switching to a thinner or lower loss solution), the baseline may decrease, and the small foreign object loss that was originally "masked" by the baseline may suddenly become significant, triggering a stop in charging. The design of FOD threshold is essentially a balance between "tolerating normal system losses" and "capturing dangerous foreign objects". The performance change of the magnetic separator moves precisely at this equilibrium point.
Path 1: Q-value baseline drift, causing FOD's "reference frame" to be misaligned
The FOD of Qi standard usually relies on Q-value measurement to determine whether there are foreign objects on the surface of the transmitting end before power transmission. The combination of the receiving coil and the magnetic separator defines the reference Q value of the system. FOD IC will write this benchmark Q value into the threshold parameter during production line calibration.
The loss characteristics of the magnetic separator directly contribute to the composition of the Q value. A low loss magnetic separator can keep the Q value of the receiving coil at a high level; A magnetic separator with high loss or insufficient batch consistency will lower the Q value. The problem is that the Q-value threshold of FOD is calibrated under a specific magnetic isolation scheme. If there is a material replacement or batch switch in the mass production of magnetic separators, the Q-value baseline will shift accordingly, and the original FOD threshold will no longer match the new system state. Manifesting as: Normal charging combinations are judged as "foreign object presence", and the charger repeatedly stops charging; Or real metal foreign objects may be overlooked due to insufficient significant differences in Q values. The stability of the Q value of the magnetic separator is not a "icing on the cake" performance indicator, but a part of the FOD judgment reference frame. If suppliers can only provide initial Q values and cannot provide discrete data of Q values between batches, the consistency of FOD in the mass production stage will lack a predictable basis.
Path 2: Unconstrained magnetic leakage, creating "invisible losses" outside the magnetic separator
One of the functions of the magnetic separator is to confine the magnetic flux within the target area. If the coverage of the magnetic separator is insufficient, the edge magnetic flux overflows, or the magnetic permeability of the material itself is not sufficient to "take over" the magnetic field lines outside the antenna projection area, this leakage magnetic flux will enter adjacent metal structures (battery shielding layer, middle frame, camera decoration) and excite eddy currents therein. These eddy current losses are not included in the reported power at the receiving end, but are included in the total loss difference observed at the transmitting end.
From the perspective of FOD, leakage loss and foreign object loss cannot be distinguished in terms of power difference: both are manifested as "the energy emitted does not reach the receiving end". The leakage caused by improper design of the magnetic separator may be misinterpreted by FOD as the presence of foreign objects. A more covert situation is that the leakage magnetic loss is at the threshold edge, and the system barely passes through during normal operation. However, when the temperature rises or the coil deviates slightly, the leakage magnetic loss increases, and the loss difference exceeds the threshold, triggering intermittent FOD stop charging. The engineering boundary with a coverage rate of 90% and a margin requirement of 1.0-1.5 mm on each side have gained another layer of meaning here: they are not only requirements for resonance stability, but also requirements for FOD stability. The leakage caused by insufficient coverage will ultimately be feedback to the user experience in the form of FOD false triggering.
Path 3: Temperature drift changes the baseline loss, FOD threshold is calibrated on the "moving target"
The loss and permeability of the magnetic separator vary with temperature. During the charging process, the heat generated by the magnetic separator itself, the copper loss of the coil, and the heat generated by the battery are combined, and the internal temperature of the receiving end is not constant during the charging cycle. If the loss temperature coefficient of the magnetic separator is large, the temperature rise will cause the system loss baseline to drift continuously during the charging process.
The threshold of FOD is usually calibrated at room temperature or a specific thermal equilibrium state. When the actual working temperature deviates from the calibration conditions, the change in the loss of the magnetic separator will be included in the power difference and mixed with the loss of foreign objects. For material systems sensitive to temperature drift, this confusion may lead to FOD performing normally in the early stages of charging (at lower temperatures), but frequently triggering incorrectly in the later stages of charging (at higher temperatures). The impact of temperature drift on FOD and the impact of temperature drift on resonance tracking are two sides of the same coin: resonance tracking concerns how changes in μ 'move the resonance point, while FOD concerns how changes in μ' and loss move the loss baseline. Both require the loss characteristics of the magnetic separator to be sufficiently stable within the target temperature range, rather than just meeting the standard at a single point at room temperature.
Selection judgment based on FOD compatibility
Firstly, consider FOD compatibility as an independent verification item for selecting magnetic separators, rather than an accessory for efficiency verification
High efficiency does not mean stable FOD. A magnetic separator with extremely low loss may increase system efficiency, but if its batch to batch Q value dispersion is large, the FOD false triggering rate during mass production may actually increase. When selecting, suppliers should be required to provide Q-values or loss dispersion data between batches, and magnetic separators should be installed into the actual FOD system for threshold recalibration during the sample stage, rather than assuming that "low loss means compatibility".
Secondly, inquire about the quantitative data of leakage magnetic control, rather than just looking at the nominal value of magnetic permeability
Magnetic leakage is one of the main sources of "hidden losses" caused by FOD false triggering. Require suppliers to provide data on the leakage or shielding effectiveness of magnetic separators under actual coverage schemes, rather than just providing the material's own μ '. For schemes with coverage near the 90% boundary, it should be confirmed whether the contribution of edge flux overflow to adjacent metal losses is within the margin allowed by the FOD threshold.
Thirdly, verify the thermal stability of FOD threshold in temperature rise scenarios, rather than just conducting FOD testing at room temperature
For applications above 15W, the temperature rise during charging cannot be ignored. When selecting, it is necessary to confirm the magnitude of the loss change of the magnetic separator within the target operating temperature range, and evaluate whether this change will narrow the net loss judgment window of FOD to an unacceptable level. If the loss temperature drift of the magnetic separator is large, a larger margin may need to be reserved for the FOD threshold, which in turn reduces the detection ability for small-sized foreign objects.
Advanced Institute (Shenzhen) Technology Co., LtdThe wireless charging magnetic isolation sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, providing discrete Q-value data between batches and loss stability data within the target temperature range, and incorporating FOD compatibility into the selection verification process.
The effective magnetic permeability of the nanocrystalline series products can be customized to 150-2000, with a saturation magnetic inductance of about 1.2 T and a thickness coverage of 0.02-0.20 mm. It supports joint debugging verification with FOD systems. For more technical information, please visithttp://www.avanzado.cn/.