GaN-based Frequency Synchronous Rectifier with On-board Control for 6.78MHz Wireless Power Transfer
In the last decade, the wireless power transfer (WPT) technology has been a popular topic in power electronics research and increasingly adopted by consumers. Several WPT standards are established to regulate wireless charging devices and ensure their interoperation across different products. The AirFuel standard utilizes resonant coils to transfer energy at 6.78 MHz, introducing many benefits such as longer charging distance, multi-device charging, and high power transfer efficiency. In recent studies, the active full-bridge rectifier is employed on the power receive unit (PRU) to improve the system efficiency because of its superior characteristics. However, the rectifier switch actions have to be synchronized with the magnetic field. In the literature, there are many solutions for synchronizing the rectifier, but most of them are bulky and not reliable, which is not suitable for a small PRU. Therefore, a frequency synchronous rectifier with an on-board control is proposed in this project. The rectifier power stage is designed to achieved full ZVS while delivering 40W to the load. Also, the synchronization control is accomplished and stabilized using an analog compensator, which eliminates the need for bulky control and external sensing hardware. The power stage and control network are implemented on the same board with a small footprint, allowing the PRU to be integrated into daily consumer electronics such as laptops and monitors.
Experimental setup for synchronization tests.
The research is divided into 2 parts: power stage design and synchronization control design. For the power stage, the modified full-bridge rectifier circuit with a resonant tank at the input is used to achieve full ZVS, which is a key feature to reduce the switching loss at 6.78 MHz. The power loss, including transistor conduction loss, inductor conduction loss, and core loss, is modeled. The rectifier steady-state operating point is computed using the discrete-time state-space modeling techniques. An optimization process is proposed to find the optimal operating point and facilitate the power component selection.
For the synchronization control design, the phase-locked-loop based synchronization control is implemented using commercial components, including a low-cost microcontroller. A local sinusoidal signal depending on the magnetic field is sensed as a reference signal for the synchronization control. However, there is inherent feedback from the rectifier dynamics to the sensed signal complicating the control design. The discrete-time small-signal model with the incorporated deadtime is employed to analyze synchronization control input behaviors under the effect of the power stage. Finally, an analog compensator is designed to stabilize the control loop.
How WBG Can Help
Using GaN FETs for switches allows the rectifier to operate at a high frequency of 6.78 MHz with minimal losses due to the absence of the instinct body diode and small parasitic capacitance Coss. Also, small Coss requires a small resonant tank inductance to achieve full ZVS. Furthermore, for switches with similar ratings, GaN FETs have a much smaller footprint, compared with silicon counterparts. As a result, the power receiving unit overall volume is reduced significantly, only about 10% of that of the previous attempt made by another student, enabling it to be integrated into consumer electronics, i.e., laptops and monitors.
- Peter Pham
- Spencer Cochran