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LCCL-LC Resonant Converter and Its Soft Switching Realization for Omnidirectional Wireless Power Transfer Systems

Fig 1
Fig. 1. The LCCL-LC Resonant Converter.
  Wireless power transfer (WPT) with loosely coupled coils is a promising solution to deliver power to a battery in con- sumer electronics applications. The LCCL-LC resonant converter, as shown in Fig. 1, is a promising topology for such systems, due to the following merits: 1) coupled independent resonant frequency; 2) load-independent output voltage; 3) load-independent transmitter coil current; 4) maximal efficiency power transfer; and 5) soft switching of active devices. To increase the spatial charging freedom, the system frequency is pushed to the megahertz (MHz) range. In a MHz system, zero voltage switching (ZVS) of the switching devices is essential in reducing the switching loss and switching associated noise. In this paper, a design methodology to achieve ZVS operation is proposed for the LCCL-LC resonant converter for wireless power transfer applications.
  To achieve zero voltage switching (ZVS) for the LCCL-LC converter, the turnoff current (Ioff) of the primary devices must be high enough to discharge the devicesÂ’ junction capacitor during the dead-time period. In a MHz WPT system, the reactance of the full bridge rectifier can no longer be neglected; therefore, an analytical model of the full bridge rectifier input impedance is proposed. Then, the total impedance of the converter is derived, and the input current is calculated by dividing the input voltage by the input impedance. The turnoff current is thereby determined, and found to be dependent on the load and coupling condition. The turnoff current is smallest at the heavy load, strong coupling condition; therefore, the heavy load, strong coupling case is identified as the worst case for ZVS conditions. If the turn-off current is designed to be high enough to achieve ZVS operation at the heavy load, strong coupling case, the ZVS operation can be guaranteed in different load and coupling conditions. The experimental waveforms validate the ZVS operation in different conditions in Fig. 2. Peak system efficiency of 82% at 5 W output power is achieved.
  Recently, omnidirectional wireless power transfer (WPT) systems have been studied intensely, due to their improved flexibility as compared to their planar counterparts. In an omnidirectional WPT system, there are multiple transmitter coils, and in the full paper the previous ZVS analysis is extended to the case of multiple transmitter coils.
Fig 2
Fig. 2. Experimental switching node voltage and current waveform. (a). k = 0.24, Po = 5 W (worst case). (b). k = 0.12, Po = 5 W. (c). k = 0.24, Po = 2 W.
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