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Resonant Converter with Coupling Independent Resonance for Wireless Power Transfer Application

Image of LCCL-LC resonant converter circuit topology
Fig. 1. LCCL-LC resonant converter.
One crucial challenge of resonant converter design in wireless power transfer (WPT) applications is the variable coupling between transmitter coil and receiver coil. The most efficient operating point of the resonant converter is normally at the system resonant frequency. If the resonant frequency is dependent on coupling, it is difficult to stay at optimal point when the coupling between transmitter and receiver is variable. Therefore, the resonant converter with coupling independent resonance is highly attractive in the WPT application. Another challenge of the resonant converter design is a zero voltage switching (ZVS) operation of the switching device. The switching frequency recommended by Rezence, a wireless power transfer standard, is 6.78MHz. In such high switching frequency, it is detrimental for a switching device to operate without ZVS operation.

The LCCL-LC resonant converter, as shown in Fig. 1 is formed by adding two capacitors (Cp and Cs) in LCL resonant converter. Cs is added first to form a coupling independent resonant frequency fo. The formula of resonant frequency is shown in Eq.1 in which Ls is the self inductance of the receiver coil. Self inductance is independent of coupling, therefore, the resonant frequency is independent of coupling between the transmitter and receiver coils. Cp is added to make sure that input impedance at fo is inductive, which is necessary for the ZVS operation of the primary switching device. From a gain characteristics point of view, Cp creates a parallel resonant frequency (f1) lower than fo. Then, f1 improves the ZVS region of this converter so it is similar to the LLC resonant converter.

The gain characteristics of the LCCL-LC resonant converter in different coupling conditions are shown in Fig. 2. In this figure, it's easy to see that fo is independent of the coupling condition, which means that this converter can always operate at an optimal point in variable coupling cases. Besides, ZVS operation in the blue region can be guaranteed due to existence of f1. Therefore, the LCCL-LC resonant converter is very suitable for WPT applications and more topologies with similar characteristics will be provided in the full paper.

fo=1/(2(Lr Cr ))=1/(2(Ls Cs ))
Eq. 1.

Image of Fig. voltage gain characteristics when k=0.4
Fig. 2. Voltage gain characteristics in different coupling coefficient (k). (a) k=0.4
Image of Fig. voltage gain characteristics when k=0.2
Fig. 2. (b) k=0.2
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