An energy-efficient floating NRESO and programming-free resonant current-mode wireless receiver with real-time energy tracking and wide input range

Abstract

Implantable medical device (IMD) is evolving towards lighter and low-power systems while simultaneously advancing to incorporate more features, thereby increasing preference among people who require them. Similarly, the ability to recharge the battery outside the human body is essential to ensure the stable operation of the IMD systems. Wireless power transfer (WPT) technology has emerged as a promising solution to such issues. However, conventional voltage-mode (VM) WPT technologies used in applications such as mobile phones and electric vehicle charging face limitations due to their multi-stage structure. Issues arise from drawbacks such as the threshold voltage of rectifiers for AC-DC conversion, minimum input power for DC-DC regulators, making it necessary to receive a certain level of power to function effectively. Furthermore, power delivery limited by stringent safety regulations, related to heat generation in human tissues, has posed limitations. These challenges complicate the design of IMDs and their charging systems, hindering them from being more preferred options among patients. In Chapter 1 of this dissertation, the discussion revolves around resonant current-mode (RCM) WPT methods that can overcome the limitations of traditional VM approaches, and the existing battery charging integrated circuits (ICs) using this technology. In Chapter 2, a controller capable of autonomously determining the number of resonance cycles, NRESO, the most critical parameter in RCM WPT is introduced. A technique that the controller can dynamically detect the optimal NRESO, Nopt, even when the input power changes with periodic resets of a whole system. In Chapter 3, energy-efficiency of a floating NRESO is discussed, while previous works have been able to have only integer NRESO, as well as the controller that can find NRESO by itself. Each chapter is described for the detailed information as follows: In the beginning, Chapter 2 introduces a wireless battery charging IC that can determine the number of resonance cycles NRESO and operate according to the decided NRESO. This system finds the most energy-efficient NRESO with the sampling capacitors that model the battery. The amount of energy accumulated in the LC tank is determined as the capacitor voltage. It periodically repeats sampling phases so that it can respond to changes in the received energy via a receiver (Rx) coil. In addition, Non-Residual VC1 Feedback Loop, which prevents residue energy in the LC tank every charging phase, is adapted to enable energy-efficient charging. The proposed self-NRESO wireless receiver IC is fabricated in a 180nm BCDMOS process, achieving its measured maximum efficiency of 70% when input power is 16.8mW and up to 30.6% energy delivery improvement thanks to the feedback loop. Second, Chapter 3 presents the energy efficiency of floating NRESO developed from the NRESO that could only be integer in previous works. Based on the equations and simulations, the results are analyzed and show that it is more efficient from an energy perspective to charge the battery while leaving energy in the LC tank without Non-Residual VC1 Feedback Loop discussed in Chapter 2. Keywords: Power management IC (PMIC), implantable medical device (IMD), wireless power transmission (WPT), resonant current-mode (RCM), battery charger