Pulsed Self-Oscillating Nonlinear Systems for Robust Wireless Power Transfer

While wired-power-transfer devices ensure robust power delivery even if the receiver position or load impedance changes, achieving the robustness of wireless power transfer (WPT) is challenging. Conventional solutions are based on additional control circuits for dynamic tuning. Here we propose a robust WPT system in which no additional tuning circuitry is required for robust operation. This is achieved by our systematically designing the load and the coupling link to be parts of the feedback circuit. Therefore, the WPT operation is automatically adjusted to the optimal working condition under a wide range of load and receiver positions. In addition, pulsed oscillations instead of single-harmonic oscillation are adopted to increase the overall efficiency. An example system is designed with use of a capacitive coupling link. It realizes a virtual, nearly-ideal oscillating voltage source at the load site, giving efficient power transfer comparable to that of the ideal wired-connection scenario. We numerically and experimentally verify the robustness of the WPT system under the variations of load and coupling, where coupling is changing by our varying the alignment of aluminum plates. The working frequency and the transferred power agree well with analytical models. The proposed paradigm can have a significant impact on future high-performance WPT devices. The designed system can also work as a smart table supporting multiple receivers with robust and efficient operation.

Figure 1

Comparison between the conventional and proposed WPT systems. (a) In conventional systems, the high-frequency ac oscillation is formed in the oscillator (generator) and then transmitted to the load through a capacitive wireless link. (b) In the proposed WPT system, the wireless link and the load are parts of the feedback circuit. The whole system forms a unified oscillator, and the oscillating power is directly generated at the load.

Figure 2

The proposed WPT system based on a self-oscillating circuit and its theoretical performance. (a) The WPT system with variable definitions. The feedback loops at inverting ($-$) and noninverting (+) terminals of the operational amplifier make the output oscillate, driving the energy wirelessly to the load. (b) Voltage oscillations from simulations (solid lines) and analytical solutions of $V_1$ and $V_3$ in Eqs. (3) and (4) (dashed lines). (c) Analytical power ratio $P_r$ as a function of $C_p/C_0$ and $\beta$. The dashed yellow line indicates the boundary of the oscillation regime. The system operates closer to the ideal wired-connection case when the configuration is further away from the boundary. The horizontal dashed black line is for $\beta =0.2$.

Figure 3

Analytical plot of the generalized power ratio as a function of $C_{p,\mathrm{left}}/C_0$ and $C_{p,\mathrm{right}}/C_0$ when $\beta =0.2$. The dashed yellow line indicates the boundary of the oscillation; that is, $1/\gamma =0.2$ from Eq. (6). The solid black line shows the position where the two plate capacitors have the same capacitance.

Figure 4

(a) The conventional WPT system for comparison. (b) Comparison of the two power ratios against coupling $C_p$ for the same system parameters. Simulation results are from the circuits made of a nearly ideal operational amplifier.

Figure 5

Experimental demonstration of the robustness against load variation. (a) Experimental setup. The capacitive wireless links are made of aluminum plates. (b) The working frequency is inversely proportional to the load resistance. (c) Power ratio versus load resistance. The measured value drops when the load resistance decreases. The inset shows the experimental voltage oscillations when $R_L=1.5\mathrm{k}\mathrm{\Omega }$.

Figure 6

Experimental demonstration of the robustness against receiver position (overlap distance). (a) The plate capacitance is linearly proportional to the overlap distance $l$ of the aluminum plates. The inset shows how the plates are moved. (b),(c) Measured (symbols) and analytical (lines) working frequency and power ratio versus the overlap distance. Five load values are measured.

Figure 7

Efficiency of the proposed WPT system (both simulation and experiment results) and the conventional WPT system (simulation results) without counting the loss in the selected operational amplifier.