Ultrasonic wireless power transfer (WPT) has been proved to be a promising approach to power biomedical implants. To extract the energy generated from the transducer, a rectifier is typically required. Previous inductor-based rectifiers (SSHI and SECE) require a large off-chip inductor to achieve good performance, which is not desired for miniaturization and safety reasons. Synchronized switch harvesting on capacitors (SSHC) rectifiers have been proved to achieve high performance without inductors; however, they are mainly designed for low-frequency kinetic energy harvesting. In this paper, an improved SSHC rectifier is designed to achieve a fully integrated design with all flying capacitors implemented on-chip. The proposed SSHC rectifier can properly operate at ultrasonic excitation frequency (100 KHz) with precise switching time control and ultrafast voltage flipping techniques. In addition, an on-chip ultralow-power LDO allows the system to be self-sustained. The system is designed in a TSMC 180nm BCD technology and post-layout simulation results are presented.

Synchronized switch harvesting on inductor (SSHI) is an efficient active rectifier to extract energy generated from piezoelectric transducer in piezoelectric energy harvesting system. Unlike passive rectifiers, SSHI rectifiers require a power supply to drive synchronized switches. Unfortunately, there is no stable supply when the system starts from the cold state. Most designs let the system work as a passive full bridge rectifier (FBR) to charge power capacitor until a supply is available. However, a FBR requires high open-circuit voltage (VOC) and the FBR’s output voltage cannot go over VOC. This prevents the system from starting the SSHI rectifier if VOCis low. This paper proposes a new transducer reconfiguration design to lower the required VOCby 4 $\times$ to start up the SSHI system from the cold state. The proposed system is designed in a 0.18$-\mu$m BCD process and post-layout simulations show that the successful cold-startup under low VOCvoltage.

A nanopower highly efficient low-dropout (LDO) regulator for energy harvesting (EH) applications is presented in this paper. The LDO is fully autonomous with a bandgap reference (BGR) featuring a novel bandgap supply-switching (SS) topology, an over-voltage protection (OVP), a under-voltage lockout (UVLO) and control block to obtain stable output and robust cold-start. The system provides configurable voltage supply (1.1 $\sim$2V) for potential loads, while consuming as low as 66 nW power. The entire system achieves a peak power efficiency of 95.6% at Vout=2V and $I_{\iota_{oad}}$=100$\mu$A.

Harvesting energy from environments has been a promising approach to power ubiquitously distributed Internet of Things (IoT) devices. For harvesting kinetic energy with piezoelectric transducers, synchronized switch harvesting on capacitors (SSHC) rectifiers have been demonstrated to achieve high energy extraction performance without using any inductor. In previous studies, the switched capacitors in SSHC rectifiers are chosen equal to the inherent capacitance of the piezoelectric transducer (PT) to achieve good voltage flip efficiencies at 33.3%, 50% and 66.7% for 1-, 2- and 4-stage SSHC rectifiers, respectively. However, with much larger switched capacitors and the same SMD package size, this paper finds that the voltage flip efficiency can be further increased to 50%, 66.7% and 80% for 1-, 2- and 4-stage SSHC rectifiers, respectively; as a result, the output power can be greatly increased. This paper also finds that the proposed design only requires half number of capacitors to achieve the same voltage flip efficiency for conventional SSHC rectifiers, which significantly reduces the system form factor for miniaturization.