Multi-load Multi-frequency Wireless Power Transfer
There are several WPT alliances or consortiums, each of which provide their respective charging specifications to different products. For example, the wireless power consortium and Qi specify a transmission frequency in the 87 kHz to 205 kHz range. The technology from AirFuel Alliance limits their transmission frequency to 6.78MHz ± 15 kHz. As a result, multi-frequency wireless power transfer (WPT) system is considered advantageous to be compatible with different standards, and to regulate individual loads. However, multiple inverters are often used in state of art devices and thereby the volume and cost of the system are increased. In this project, a single-inverter based dual frequency WPT system, which employs a dual-frequency selective harmonic elimination modulation scheme, is proposed. This dual-frequency WPT system can simultaneously generate and regulate 100 kHz and 6.78 MHz loads, which facilitates the development of multi-standard WPT technology for consumer electronics. Also multi-frequency operation can support multi-load applications within the same standard as well, and individual regulation can be achieved without designing complex matching network. This multi-frequency WPT technology is compatible with conventional design procedures, which eases the application of such method for multi-load applications.
(a) Multi-frequency WPT Prototype. (b) Output voltage waveforms of multi-frequency WPT system. (c) Spectrum of the inverter output.
In this research, the dual-frequency selective harmonic elimination (DFSHE) modulation scheme, which can simultaneously generate two different ac frequencies while suppressing undesired harmonics in between, is employed on a GaN-based WPT inverter, and then is verified on a 5 W prototype. Considering the wide difference between the fundamental output and its 67th harmonic, initial guess for numeric iteration algorithm is very critical and thus some investigation and modification are conducted for DFSHE to accommodate its application for WPT. Then, the circuit model of dual-frequency WPT system is studied and assumptions are made to achieve desired performance. In such model, different frequency path should show high impedance other than its own resonant frequency to minimize circulation current. High-order harmonics that out of the control range may lead to voltage pulsation, and some techniques will be investigated to alleviate such issues. In addition, multi-load regulation and operation within the same standard will be accomplished using the proposed modulation scheme. By fully utilizing available frequency band in one standard, individual load could be well regulated and controlled on its own, while only one inverter is involved. The selection of certain frequencies, and resonant network design will be considered and discussed to achieve better regulation performance.
How WBG Can Help
Since the operating frequency of such WPT system increases to several megahertz range, many concerns that originally lies in radio-frequency (RF) fields will influence power conversions. The power stage PCB layout and the package of employed GaN devices will inevitably influence the prototype effectiveness and efficiency, which is critical for power electronics, but not fully addressed in previous literatures due to such high frequency operation. Compared with common single-switch resonant power amplifier in RF field, a full bridge inverter is used in this project. More devices will bring about issues like dead time configuration, output capacitance impact, and reasonable PCB layout and decoupling component selection. By using GaN devices, above issues can be partially alleviated but challenges still remain to achieve ideal operation.
- Chongwen Zhao