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The efficiency issue of wireless charging is often attributed to "high loss" or "alignment deviation". But in the scenario of high temperature environment in summer or long-term charging of equipment, a more hidden failure path is: the magnetic permeability of the magnetic separator drifts with temperature, the inductance of the receiving coil changes accordingly, the resonant frequency deviates from the working point of the IC, and the system efficiency decreases without a significant increase in losses.
The reason why this path is concealed is that the ambient temperature μ 'data of the magnetic separator is usually qualified. The problem is not at room temperature, but after a temperature change.
The premise of a resonator is that μ 'is not a constant, but a curve that varies with temperature
The wireless charging receiver operates on an LC resonant circuit with a resonant frequency of f=1/(2 π√ LC). The inductance L of the receiving coil depends on the geometric structure of the coil and the effective μ 'of the surrounding magnetic material. The magnetic separator is attached to the back of the coil, and its μ 'directly participates in the formation of L. When the ambient temperature or the coil's own heating causes a change in the temperature of the magnetic separator, μ 'drifts with the temperature, L drifts accordingly, and the resonant frequency shifts.
Modern Qi receiving ICs typically have built-in resonance tracking or tuning mechanisms, which adjust the equivalent capacitance of the matching network to bring the resonance point back to the operating frequency. But the tracking range is limited. If the drift of μ 'causes the change in L to exceed the tracking margin, the resonance point cannot be pulled back and the system enters a' detuning 'state. Disharmony does not manifest as damage to a particular component, but rather as a decrease in efficiency, an increase in heat generation, and active limitation of charging power by the IC.
Temperature curves of ferrite and nanocrystals: same index, two behaviors
The difference in temperature characteristics between ferrite and nanocrystalline magnetic separators is a key variable in the design of resonance tracking margins.
The μ 'of MnZn ferrite usually drops sharply near the Curie temperature (about 100-250 ℃). Within the working range of -40 ℃ to 125 ℃, the μ 'of ferrite is not a straight line, but exhibits different slopes in different temperature ranges. More importantly, some ferrite formulations have a 'negative temperature coefficient' range in their μ '- when the temperature increases, μ' decreases, which means that L decreases and the resonant frequency increases. If the tracking direction or margin of the IC is set according to normal temperature data, the offset at high temperatures may exceed expectations.
The Curie temperature of nanocrystalline magnetic separators can reach over 500 ℃, and within the working range of -40 ℃ to 125 ℃, they are far from the Curie point. The temperature drift amplitude of μ 'is usually smaller than that of ferrite. This is not to say that the μ 'of nanocrystals is completely unaffected by temperature, but rather that their drift is smoother in the conventional operating temperature range, leaving more room for resonance tracking. From a system design perspective, the temperature drift of ferrite may not necessarily be a "disadvantage" - if the drift direction is consistent with the tracking direction of the IC and the amplitude is within the tracking range, the system can still operate stably. The problem is that if the selection only considers the normal temperature μ 'without confirming the change curve of μ' within the target temperature range, the design of tracking margin will lack basis.
The failure chain of "meeting the standard at room temperature and losing harmony at high temperature"
The typical development process of this failed chain is:
at room temperatureThe magnetic isolation plate μ 'meets the design value, the receiving coil L is within the target range, and the resonance point is tuned to the operating frequency of the IC. The system efficiency meets the standard and the temperature rise is controllable.
The device starts high-power charging or is in a high-temperature environmentThe temperature of the magnetic separator rises. As the temperature of ferrite decreases (or changes at a specific slope), L decreases and the resonant frequency increases. The tracking mechanism of the IC begins to adjust the matching capacitor in an attempt to bring the resonant point back to the operating frequency.
If the temperature rise amplitude is within the tracking marginThe system can maintain resonance. If the temperature rise causes the drift of μ 'to exceed the tracking range, the resonance point cannot be pulled back and the system enters a detuned state. Disharmony leads to a decrease in efficiency, with more energy dissipating in the form of heat at the receiving end, further increasing the temperature. The temperature rise further drives the drift of μ '. This positive feedback may cause the system to rapidly evolve from a "slight decrease in efficiency" to "power limitation" or "intermittent charging interruption" in high-temperature environments.
In this chain, the ambient temperature μ 'and loss data of the magnetic separator are both' qualified ', and failure occurs at the system level after temperature changes. This is also why it is impossible to predict the charging performance in high temperature environments based solely on room temperature specifications.
Selection judgment based on "temperature inductance tracking margin"
Firstly, suppliers are required to provide the temperature curve of the target temperature range, rather than just providing single point data at room temperature
For in car, outdoor, or high-power fast charging scenarios, the amplitude and direction of the change in the μ 'of the magnetic separator should be confirmed within the range of -40 ℃ to 125 ℃ (or actual working temperature zone). If the supplier can only provide μ 'at 25 ℃, their ability to predict the risk of high-temperature detuning is limited.
Secondly, match the temperature coefficient of μ 'with the resonance tracking range of the IC for calculation, rather than evaluating the material in isolation
The tracking margin varies among different receiving ICs. When selecting, it should be confirmed whether the change in L caused by the drift of the magnetic isolation plate μ 'at the upper temperature limit of the target temperature range is still within the tracking range of the IC. If the margin is insufficient, priority should be given to selecting material systems with smoother temperature drift, or reducing the operating temperature of the magnetic separator through structural design.
Thirdly, in high-temperature scenarios, the operating temperature of the magnetic separator itself should be managed as a design variable
The root cause of μ 'drift is temperature. If the magnetic separator is mounted with low thermal conductivity adhesive or adjacent to the heating element, its actual operating temperature may be much higher than the ambient temperature. Choosing a thermal conductive adhesive, optimizing the thermal path, or adjusting the relative position of the magnetic separator and the heat source can all reduce the amplitude of the μ 'drift. For high-power scenarios of 25W and above, thermal management and magnetic stability are two sides of the same issue.
Fourth, distinguish between "intrinsic temperature characteristics of materials" and "system temperature characteristics after finished product bonding"
The temperature curve of the magnetic separator is the intrinsic data of the material, but the actual temperature of the finished product after being mounted on the back of the coil depends on the overall thermal environment of the machine. When selecting, it is necessary to confirm under what conditions the supplier's μ 'temperature data was measured (free state or bonded state), and to verify the actual offset of the resonance point during the sample stage in conjunction with the whole machine thermal test.
Advanced Institute (Shenzhen) Technology Co., LtdThe wireless charging magnetic isolation sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, providing temperature curve data within the target temperature range.
The Curie temperature of the nanocrystalline series is ≥ 500 ℃, and the magnetic permeability stability is better than that of conventional ferrite solutions in the range of -40 ℃ to 150 ℃. The effective magnetic permeability of the finished product can be customized from 150-2000, with a thickness coverage of 0.02-0.20 mm. The product supports the option of thermal conductive backing adhesive, which can synergistically match the thermal path and magnetic performance stability according to the overall thermal environment of the machine. For more technical information, please visithttp://www.avanzado.cn/.