High-Performance and High-Reliability DC-DC Converter for Energy Harvesting Systems in IoT Applications

Abstract

With the rapid development of energy harvesting systems in the Internet of Things (IoT) applications, which use harvesters as inputs to convert ambient energy into electrical energy, the power management integrated circuits (PMICs) that can accurately and efficiently deliver the harvested energies to storage devices or IoT devices have been increasingly demanded. To meet the above requirements, these PMICs need to have various capabilities including high peak output power, high power conversion efficiency, high reliability, high accuracy, fast transient response, small form fact, and so on. Therefore, this dissertation presents a single-inductor multi-input multi-output (SIMIMO) converter with high peak output power and high power conversion efficiency, a single-inductor multioutput (SIMO) converter with high reliability, and a fully integrated hybrid low dropout (LDO) regulator with fast transient response and high DC accuracy, all of which are suitable for energy harvesting systems in IoT applications. First, a SIMIMO converter, which consists of inputs with four harvesters and a battery, and outputs with 1.8 V output, 3.3 V output, and a battery, is proposed in an attempt to achieve high peak output power and high power conversion efficiency. The proposed SIMIMO converter controlled by the power management controller (PMC) employs the optimal on-time control method that optimally adjusts the on-time required to energize the inductor, thus increasing the peak output power, while even covering a wide distance range for IoT applications. Moreover, it adopts the hybrid zero current switching (ZCS) method that adaptively calibrates the offset of the ZCS comparator, thus increasing the power conversion efficiency. The proposed SIMIMO converter was fabricated using a 0.18-μm BCD process technology. The measurement results show that the proposed SIMIMO converter achieves a peak output power of 462 mW, the peak power conversion efficiencies of 84.5%, 89.2%, and 86.4% at 3.3 V output, 1.8 V output, and battery, respectively, and a quiescent current of the PMC of only 1.6 μA. Therefore, the proposed SIMIMO converter is suitable for energy harvesting systems in various IoT applications requiring high peak output power and high power conversion efficiency. Second, a SIMO converter controlled by the PMC including the hybrid starter, overcharging protector (OCP), and switch controller (SWC) is proposed in an attempt to achieve high reliability. The proposed hybrid starter adopts the power-on-reset (POR) that accurately determines the activated and deactivated moments of the SWC, thus producing a system supply voltage (VSYS) within the stable operating voltage range. In addition, the proposed OCP adopts the diode-connection and Schmitt trigger schemes that keep track of the voltage of a storage device (VSTG) in real time, leading to the protection of the IC from being damaged, while eliminating the use of either a capacitor or Zener diode, which causes the additional area or reverse leakage current, respectively. The proposed SIMO converter with the PMC was fabricated using a 0.18-μm BCD process technology. The measured VSYS values of the proposed SIMO converter are 1.510 V and 1.798 V at the deactivated and activated moments of the SWC, respectively, both of which are within the stable system voltage range for 1.8 V devices. In addition, the maximum VSTG is measured to be 5.892 V, which is much lower than an absolute maximum rated voltage of 9.2 V for 5 V devices. Moreover, the measurement results indicate that the hybrid starter with the POR enables the proposed SIMO converter to properly restart immediately after VSYS is shorted to ground, while the OCP safely protects the proposed SIMO converter even when the storage device is suddenly removed from the IC while charging. Therefore, the proposed SIMO converter with the PMC is suitable for energy harvesting systems in various IoT applications requiring high reliability. Finally, a hybrid LDO, which consists of the analog and digital LDOs (ALDO and DLDO), and a hybrid controller, is proposed in an attempt to achieve fast transient response and high DC voltage accuracy. The proposed hybrid LDO adopts a hybrid algorithm to generate its accurately regulated output voltage by controlling the ALDO and DLDO. The proposed hybrid algorithm with the countable bidirectional binary search (CBBS) and soft swap switching (SSS) is implemented in the hybrid controller through the logic synthesis. The CBBS adjusts the number of turned-on switches in the DLDO to quickly determine the coarse-tuning current of the DLDO, thus achieving a fast transient response. In addition, the SSS controls the moment when the ALDO is activated to precisely adjust the fine-tuning current of the ALDO, thus achieving a highly accurate DC voltage. The proposed hybrid LDO was fabricated using a 0.18-μm CMOS process technology. The measurement results reveal that the output of the hybrid LDO is regulated at 0.9 V when the input voltage ranges from 1.2 V to 1.8 V. In addition, the overshoot and undershoot voltages are measured to be 56 mV and 34 mV with the settling times of 344 ns and 68 ns, respectively, when the load current step is about 40 mA. Therefore, the proposed hybrid LDO is suitable for system-on-chips in IoT applications requiring fast transient response and high DC voltage accuracy.