Circuits for efficient and secure power delivery in distributed applications

Abstract

The growing number of widely distributed IoT devices presents new challenges in power delivery. Existing frameworks for charging electronic devices, be they wired or wireless, either end up limiting the amount devices can be shrunk due to the size of standardized connectors or end up restricting their free placement due to their reliance on charging mats. The large amounts of vital data IoT devices gather and their heavily distributed nature also makes it important to protect them against unauthorized or malicious charging that might be aimed at damaging and/or stealing data from them. This thesis focuses on developing circuits and architectures to enable a more distributed and secure approach to charging such devices by exploring two topics in particular - energy harvesting and wireless charging. First, a converter for harvesting power from thermoelectric generators (TEGs) is presented. The converter is capable of cold-starting from a bipolar supply as low as 40 mV, making it suitable for applications where the direction of heat flow from which energy is harvested could be bi-directional. Upon cold-starting from such low voltages, the converter transitions to a high-efficiency mode in steady state that achieves up to 60% efficiency while delivering 110 pW of power. With the goal of building a fully integrated transformer in the future, techniques for modeling an integrated transformer for optimal harvester performance have also been presented. Second, a receiver for device-to-device wireless charging is presented. The receiver dynamically measures the end-to-end system efficiency adjusts the ac input impedance it presents to the wireless power transmission system in order to track the maximum efficiency point. It also employs an inverter-inspired resonant rectifier that overall results in a 13.8% total energy saving for the transmitter with when the received power is 650 mW. Next, an adaptive rectifier control scheme for wireless power transfer that integrates linear regulation into the high-side switches of the rectifier is presented. This has the capability to reduce the area of an integrated converter by eliminating an additional LDO at the output of the rectifier. Additionally, the regulation technique presented results in lower system efficiency degradation when the output power is regulated below the maximum value. Finally, a wireless power receiver detuning technique is presented. Detuning allows the receiver to protect itself from harmful transients imposed by counterfeit or malicious chargers, by reducing the received power by up to 16 x. It also allows multiple receivers coupled to the same charger to co-operate in order to balance their received powers. Using this technique, the asymmetry in the output powers of two receivers at 4:1 distance ratios from the transmitter can be reversed.