Maximizing the efficiency of wireless power transfer systems with an optimal duty cycle operation
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In a typical high-power inductive wireless power transfer (WPT) system, AC signal, which drives the coupled resonant inductors (couplers), is produced by a half- or full-bridge inverter. However, the output voltage of an inverter is not a pure sinusoidal but a square wave with quite a few harmonics. Harmonics are expected to reduce active power transfer efficiency (APTE) due to high reactive power accumulation at the input to the resonant couplers. In this paper, WPT system efficiency is analyzed by introducing the harmonics of the voltage waveform of a typical inverter circuit in to an analytical model of the WPT system for the first time. In this regard, total harmonic distortion (THD) and harmonic content of the waveform, as well as the APTE of the system are all simulated as a function of duty cycle using an analytic model. Simulation results show that the THD of the source voltage waveform can be minimized with a duty cycle of 75%. Hence the reactive power at the input of the system is also minimized, increasing the APTE of the system in this duty cycle. An experimental wireless power transfer system is implemented to verify the aforementioned simulation based observation. The measured APTE is increased to a maximum of 94.5% from approximately 88.5% by simply reducing the duty cycle from 100% to 75%. The output power delivered to the load, on the other hand, decreases with this reduction in the duty cycle, if DC bus voltage stays constant. This trade of between efficiency and the delivered power to the load is analyzed in two wireless charging scenarios with different power and efficiency levels. We believe that the findings reported in this work would potentially lead researchers develop novel inverter structures to not only use for increasing power but also the efficiency of WPT systems.
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