An Improved Autonomous Current-Fed Push-Pull Parallel-Resonant Inverter for Inductive Power Transfer System

Abstract

In the inductive power transfer (IPT) system, it is recommended to drive the resonant inverter in zero-voltage switching (ZVS) or zero-current switching (ZCS) operation to reduce switching losses, especially in dynamic applications with variable couplings. This paper proposes an improved autonomous current-fed push-pull parallel-resonant inverter, which not only realizes the ZVS operation by tracking the zero phase angle (ZPA) frequency, but also improves the output power and overall efficiency in a wide range by reducing gate losses and switching losses. The ZPA frequencies characteristic of the parallel-parallel resonant circuit in both bifurcation and bifurcation-free regions is derived and verified by theory and experiments, and the comparative experimental results demonstrate that the improved inverter can significantly increase the output power from 7.68 W to 8.74 W and has an overall efficiency ranging from 63.5% to 72.5% compared with the traditional inverter at a 2 cm coil distance. Furthermore, with a 2-fold input voltage (24 V), the improved inverter can achieve an approximate 4-fold output power of 38.9 W and overall efficiency of 83.6% at a 2 cm coil distance.Keywords: inductive power transfer (IPT); frequency bifurcation; current-fed push-pull; resonant inverter; wireless power transfer IntroductionInductive power transfer (IPT) technology [1], characterized by convenience and safety, has many potential applications in portable devices [2], wireless sensor networks (WSNs) [3], implant medical devices (IMDs) [4][5][6][7], and electrical vehicles [8][9][10]. In general, it is recommended to drive the primary-side inverter of IPT systems at zero phase angle (ZPA) operation in order to minimize the volt-amp (VA) rating of the source supply. Moreover, the resonant inverter can achieve zero-voltage switching (ZVS) or zero-current switching (ZCS) operation with less switching losses and electromagnetic interference (EMI) at ZPA or a similar condition [11][12][13]. However, it is often a great challenge to maintain ZVS or ZCS operation with variable couplings, caused by nonconstant coil distances, misalignment, shape deformation, or metal object proximity. Especially when it occurs to the frequency bifurcation region, the power transfer capability and efficiency can deteriorate rapidly at the original resonant frequency [2,13].Various solutions have been proposed in the previous literature. One is dynamic tuning of reactive elements in either the primary or the secondary compensation network through variable inductors [14][15][16], switchable capacitor bank [9], or transistor-controlled variable capacitor [17], Energies 2018, 11, 2653 2 of 16respectively. This method aims to keep the ZVS frequency fixed under coupling variation, although it needs more switching devices, passive components, and even a complicated control unit. Moreover, another common method is directly adjusting the inverter frequency to the new ZVS frequency as the magnetic coupling changes. For instance, in [18], the authors present a clo...

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