In wireless power transfer (WPT) system, the information of mutual inductance and load resistance is usually needed for the front-end to regulate power and efficiency. To acquire the values of these two parameters without wireless communication system and without affecting the output power, this article proposes an identification method based on the pulse density modulation (PDM) technique. The underlying principle is to obtain a range of interharmonics by altering the sequence of PDM. First, the PDM strategy brings in interharmonics, which can be identified through fast Fourier transmission (FFT). Then, these interharmonics are used to established multiple sets of equation related to front-end impedance and the two unknowns. Finally, with the least-square approximation, mutual inductance and load resistance can be estimated. This article notices that the sequence of PDM strategy could affect the amplitudes of interharmonics, and selects the sequences to maximize the magnitudes for less sensitivity to measurement errors. Experimental results show that the relative errors of identification are less than 5% when the nominal values of capacitors are the same as actual values, and reach 7.40% when considering the nominal values may deviate from the actual values. Besides, the output power and efficiency are not affected during the identification process. This proposal requires only magnitudes of voltage and current in the transmitter side, no other hardware is needed, and works with the fixed frequency, which is suitable for any frequency range.

Tuning the receiver to match the resonant frequency with the working frequency plays an important role in improving the efficiency. One of the crucial steps in tuning is to determine the value or degree of detuning. In order to obtain the receiver reactance used to determine the detuning value, this article proposes a pulse density modulation (PDM)-based receiver reactance identification method. The parameter identification is performed as follows: implementing PDM technology on the active single-phase rectifier (ASPR) to bring in interharmonics and through Fast Fourier Transformation and least-square approximation, values of inductor and capacitor of the receiver can be identified. This estimation method only requires the magnitudes of voltage and current into ASPR for estimation. No extra hardware, no need to synchronize with the transmitter. In the experimental verification, four cases of resonant, inductive, and capacitive receivers are designed. Results show the maximum relative errors for estimation are within 5%, based on which the ASPR can tune the receiver as resonant. The situation that parameter deviates from the nominal value is also considered, even with deviations of mutual inductance and transmitter-side reactance, the estimation accuracy of receiver-side reactance is still at an average level.

For a traditional 2-coil system outputting constant voltage (CV), the transfer efficiency decreases drastically as transfer distance increases. To solve this problem, this essay proposes a 3-coil system which could achieve CV output and Zero Phase Angle (ZPA) conditions with specific parameter values. This 3-coil system could partly relief transfer efficiency fall at a long transfer distance, without any complicated controls. In order to achieve CV and ZPA condition, this essay devises the parameter values based on the decoupling 3-coil model, and a prototype is designed accordingly to verify these characteristics. With 10 cm transfer distance, output voltage deviation is 5.5% as the load varies from 12 Ω to 200 Ω, proving that the output voltage almost keeps constant with load change. Furthermore, with software simulation, a comparison experiment between the proposed 3-coil system and a Series-Inductor-Capacitor-Inductor (S-LCL) compensated 2-coil system is built to verify the efficiency improvement. The transfer distance change leads to the differentiation of voltage gain for both 2-coil and 3-coil systems. So, the input voltage for both systems and the compensated capacitor in receiver loop of the 3-coil system are manipulated for keeping 60 V output voltage on the 12 Ω load. With distance increasing from 10 cm to 20 cm, transfer efficiency varies from 92.61 to 48.9% for the 2-coil system, and from 92.89 to 84.26% for the 3-coil system, effectively proving the efficiency improvement. The experiment and simulation results prove the effectiveness of the proposed method.