GaN-based Single-stage Transmitter for Wireless Power Transfer
Wireless power transfer (WPT) is the technology that enables electrical power transfer from source to load without interconnecting. Due to the high flexibility and convenience, it is widely applied in industry in a wide range from low power daily appliances and high power EV chargers . Compared with the inductive coupling, resonant coupling better facilitates concurrent charging of multiple devices with different power and therefore has broad prospects for wireless charging of portable devices. The 6.78 MHz industrial, scientific, and medical (ISM) band is commonly selected as the operating frequency of resonant WPT systems .
The project that designing transmitter side power conversion stage is a partial work for a 100W WPT system which is aiming at multiple portable devices charging application. The responsibility of transmitter side power converter is to process utility AC input to 6.78MHz AC output. There are two design goals for this project: (1) based on the situation that traditional power conversion at transmitter side has low efficiency and bulky size because of cascaded power conversion structure, the first target is to design high efficiency power converter with low components count number via integration. (2) It supports charging multiple consumer electronics loads by a single transmitter.
Experimental results: (a) Picture of the transmitter prototype, (b) rectifier waveforms, (c) Inverter waveforms
A single-stage 6.78 MHz transmitter is proposed which directly converts a utility ac input to a regulated, high frequency (6.78 MHz) ac output for wireless power transmission. The topology integrates a totem-pole rectifier operating in discontinuous conduction mode (DCM), and an asymmetrical voltage cancellation (AVC) controlled full bridge inverter. Compared with the traditional cascaded multi-stage transmitters, this single-stage approach achieves high power efficiency over the full load range, utilizes fewer GaN FETs, and shrinks the size of the converter. The operation and theoretical analysis of the single-stage transmitter are verified using a 100 W, GaN-based prototype .
A simple auxiliary circuit is added in the single-stage transmitter to further improve the light load efficiency by at least 5%. When output power is high, heavy load operation mode (with totem-pole rectifier) provides high PF and low THF of the input current. When the output power decreases to a certain value, light load mode (with voltage doubler) replaces heavy load mode to obtain high efficiency. The smooth transition between the two operation modes is achieved via an auxiliary circuit .
Constant transmitter coil current enables fast response to a sudden load change, so it is preferred in the multiple receivers application where frequent load changes are occurring. With the implementation of AVC modulation and impedance matching network in the single-stage transmitter, constant transmitter coil current is achieved over wide load range. Experimental results verify that multiple consumer electronics loads are charged from a single transmitter .
How WBG Can Help
Considering the ISM band in this MHz WPT system, a constant system operating frequency (fo=6.78 MHz) is preferred. So, all the devices in the single-stage transmitter switch at constant 6.78 MHz. In addition, the power level is 100 W and the bus voltage is over than 300 V. In this case, switching loss dominates the converter loss. To avoid significant switching loss occurs in the converter due to high voltage and high switching frequency, GaN FETs are selected for two considerations.
First, the implemented FETs have to switch very fast. Otherwise large overlap loss during the switching transition would occur. Second, the small output capacitance of devices is preferred. In this high voltage and high frequency application, the implementation of ZVS operation helps reduce switching loss significantly. In this work, ZVS tank is designed to assist ZVS operation. Auxiliary current from ZVS tank needs to charge/discharge output capacitance of devices to be zero during a small deadtime for soft switching purpose. This auxiliary current is a circulating current that results in extra conduction loss in the devices and also brings in extra loss in ZVS tank. Small output capacitance helps reduces the required circulating current while maintaining ZVS, therefore reduce the converter loss.
- Ling Jiang
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 L. Jiang, D. Costinett, A. Fathy and S. Yang, "A single stage AC/RF converter for wireless power transfer applications," 2017 IEEE Applied Power Electronics Conference and Exposition (APEC), 2017, pp. 1682-1688.
 L. Jiang and D. Costinett, "A single-stage 6.78 MHz transmitter with the improved light load efficiency for wireless power transfer applications," 2018 IEEE Applied Power Electronics Conference and Exposition (APEC), San Antonio, TX, USA, 2018, pp. 3160-3166.
 L. Jiang and D. Costinett, "A GaN-Based 6.78 MHz Single-Stage Transmitter with Constant Output Current for Wireless Power Transfer," 2018 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (Wow), Montreal, QC, Canada, 2018, pp. 1-6.