Integrated circuits design and control solutions for wireless power transfer applications

Abstract

In the past years, extensive research has been carried out on wireless power transfer (WPT), since it offers customers a convenient charging technique compared to wires which can be inconvenient and allows charging of portable devices in a wide power range. WPT technology has been observed to have some remarkable technological advantages due to its spatial freedom. Owing to modern power electronics technology and high-power semiconductors, WPT technology is highly integrated, and many smart WPT devices have been developed for different applications such as biomedical devices, electric vehicles (EV), and portable consumer electronic devices. A burst in the WPT market for consumer electronics is expected in the coming five years, due to the ubiquitous usage of 3G/4G wireless networks and continually evolving technologies for media and entertainment. WPT systems for consumer devices can be classified into two main types: inductive power transfer (IPT) and magnetic resonant coupling (MRC). The former works at 100-200 kHz, whereas the latter works at 6.78 or 13.56 MHz. The bottleneck of the IPT technology is to design a low cost but high performance WPT system since the IPT mainly targets on the application with less requirement of distance freedom like home appliance or some industrial applications with short charging distance. The main targets of MRC technology is to provide consumers a flexible WPT solution with higher distance freedom. The main concern of this technology is that 1) it is challenging to design high efficiency circuits for MHz application, especially for rectifier; 2) the control of the MRC is challenging since the output power would be effected remarkably by distance, which is known as splitting frequency phenomenon.

This research develops circuits and control algorithms for both types of WPT systems. Firstly, a single-stage AC-DC voltage regulator is developed. A noncharging configuration is introduced to regulate the output voltage at a desired level with a pulse width modulation (PWM) control algorithm. In this way, there is no need for the DC-DC stage after the rectifier and additional wireless communication circuit blocks. In addition, a full-active cross-coupled structure is utilized to reduce the power loss of the regulator. Experimental results of the proposed regulator based on a 0.35 µm technology and PCB circuit demonstrate a maximum output power of 15 W and peak efficiency of 92 %. Another contribution of this dissertation is a design of a full active rectifier working at 6.78 MHz. An adaptive time delay (ATD) circuit is used to maximize the conduction interval of the gallium nitride (GaN) switch in order to minimize the power loss due to the forward voltage drop of the diode. The proposed control algorithm also eliminates the reverse leakage current of the rectifier. Except the power devices, all the other circuits have been taped out with a 0.18 µ m CMOS process. The experimental results of the proposed rectifier shows that it can output a maximum output current of 3 A at 5 V with a 6.78-MHz AC input voltage with a peak power efficiency over 90%. The last contribution is a novel control method for a 6.78-MHz MRC system. In this part, detailed analysis of MRC, especially on the relationship between operating frequency, transfer efficiency, output power, and coupling coefficient are presented. Contrary to the traditional research, this research proves that the MRC system is able to work with sufficient output power and efficiency within a wide coupling coefficient range, relaxing the system from the bound of the key coupling coefficient limitation. A hybrid control method with frequency/phaseshift tuning with zero voltage switching (ZVS) Class-D amplifier is proposed to provide the load with a constant output voltage. The testing results shows that it is able to transfer 10 W over a 5 cm with an overall efficiency (end to end) of 71.8 %, and over a 1 cm with an overall efficiency of 73.1%.