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This paper presents RX-Sensorless technologies required in a self-induced type of low power wireless power transfer system. Recently, apart from the fields of smartphone and electrical vehicles which have been arduously studied for application of wireless power transfer, wireless power transfer technologies are being applied to various fields. Among them, there is a very rapidly increasing demand for low power systems in displays, body implant medical devices, drones, electrical kickboards and so on. In order to acquire portability of RX side of low power systems, the most critical factors are power density and efficiency. For this purpose, there have been very much ongoing studies to improve the power density on the RX side. Among various research, most frequently used types of methods involve derivation of output voltage using the physical quantity on the TX side while removing sensors on the RX side. However, in the modeling process to derive output voltage, they fail to properly reflect the parasite components of wireless power transfer and also due to the lack of measures in case of being out of the range of resonance frequency, there tends to be a high error in the output voltage which raises an issue. Therefore, this paper aims to suggest a model which reflects the parasite resistance components and the permeability rate of medium for the TX/RX pads of wireless power transfer systems. By reflecting winding losses and eddy current losses of TX/RX pads to a model, it is possible to reflect the parasite resistance components and using the Neumann formula, it is possible to reflect it to the medium between RX/TX pads. As a result, based on the derived model, it is possible to control the output voltage. Prototypes of hardware are designed and implemented for verifications. According to the computer simulation results and test results, output voltage estimates yield an error in the range of less than or equal to 8% and model-based output voltage control also shows the control range with errors of less than or equal to 5%.

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