![]() ![]() But again, this should not be construed as a predictor of overall system efficiency. Measuring efficiency into a dc load at the output of the rectified receiver can also result in a high reading (greater than 85% in the closely-coupled system and 75% in the loosely-coupled system). This does not imply that the overall system efficiency is 90%, however, so this data point can be misleading. We have seen claims of coil-to-coil or “dc-in” to “dc-out” efficiency numbers, but these do not predict overall system performance.įor example, both closely-coupled (not operating in resonance) and loosely-coupled (operating in resonance) systems can hit coil-to-coil efficiencies in excess of 90%. One difficulty is the lack of an agreed measurement method. With these two charging approaches in mind, consider the problem of useable efficiency data. It is also more difficult to contain the EMI/RFI/EMF in a loosely-coupled system that is operating at high frequency. The cost of the approach is lower efficiency and a higher bill of materials. Of course, a loosely-coupled system has a greater charging distance. Loose coupling approaches are useful in after-market installations of existing furniture and represent about 5 to 10% of the volume demand for wireless chargers. The device-to-be-charged sits somewhere on the table surface. Loosely-coupled configurations are those characterized by a power transmitter mounting under, say, the surface of a desk, countertop or other furniture. This mode results in maximum power transfer efficiency, lowest EMI/RFI/EMF and lowest cost. The classic example of this approach is that of a smartphone sitting in its wireless charging stand on a desktop. Claims of Rezence benefits are derived from publically-available marketing materials.Ĭlosely-coupled configurations make up the lion’s share of the demand for wireless chargers. As of the writing of this article, there were no Rezence systems available in the market. Here are the tradeoffs of the various magnetic induction-based wireless charging approaches. Without an industry consensus on how to measure efficiency, a basic design tradeoff cannot be assessed. The key tradeoff is efficiency versus a purported benefit. As designers consider incorporating wireless charging technology into their products, they must assess the benefits and tradeoffs of the available or emerging standards. Unfortunately, none of the various approaches are compatible nor are they interoperable. Wireless charging for mobile consumer electronics has reached mainstream adoption, and a host of approaches in various stages of development and deployment have emerged that offer a range of benefits. It is possible, however, to specify the efficiency of a wireless charger more accurately by comparing the total energy used by the battery over a complete charge cycle, to the energy into the wireless power transmitter over the same charge cycle. This industry convention results in misleading and inaccurate characterizations of the performance of a power supply that is designed specifically as a wireless charging system. Consequently, wireless charging systems are often characterized by their efficiency at a given load current. Wireless chargers are essentially power supplies, and power supply engineers generally assess the quality of a power supply by its efficiency over a load range. That said, it is true that there’s limited public data comparing the real-world power-transfer efficiency of different wireless power standards. Additionally, efficiency data helps engineers and product designers determine which user benefits are worth the “cost” in terms of efficiency. ![]() ![]() It serves important purposes by helping decision makers assess different standards, and it enables informed choices regarding the deployment of a given standard. After all, it is hard to envision how two induction coils sitting some distance away from each other could engage in energy efficient charging.įortunately, there is meaningful data available about wireless charging efficiency. T here are a lot of claims made about the energy efficiency of wireless charging schemes. ![]()
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