From Murmann Mixed-Signal Group
MSEE, Stanford University, 2009
Admitted to Ph.D. Candidacy: 2011-2012
Research Project: An Energy-Harvesting, Power-Aware Sensor Platform for Wireless Data Acquisition Applications
Email: mahmoud [at] stanford [dot] edu
Conventional wireless sensor devices utilize batteries as the main power source for the sensing circuits and wireless transceiver. However, it is often desired to eliminate the battery from a system due to its size, cost (mainly maintenance cost), environmental impact, operating limits, and limited capacity and lifetime. Ambient energy harvesting is a viable alternative source for low power, low data-rate wireless sensing applications; where the sensor monitors a physical quantity that changes slowly and needs to be transmitted infrequently. Depending on the application, a sensor node can harvest solar, thermal, vibrational, RF, or other forms of ambient energy.
This project involves design and implementation of high performance Power Management Integrated Circuits (PMIC) for ambient energy harvesting. The core of the PMIC is a high-efficiency, ultra-low voltage, low-quiescent current, DC-DC converter which can harvest energy from one or more energy transducers and provide multiple supply rails to power the subsequent blocks and charge a temporary storage component.
The current phase of this project is the design of a reconfigurable closed-loop switched-capacitor DC-DC converter for sub-mW energy harvesting applications. This design has the following characteristics: First, to support a wide input voltage range, the switched-capacitor converter is designed to be reconfigurable: It adjusts its voltage gain and frequency based on the operating condition of the converter. Second, our design is targeted to operate with DC voltages from 0.5 V to 2.5 V. This range covers operation with solar cells, high voltage thermo-electric generators and rectified AC sources. Finally, since a wireless sensor typically consumes tens to hundreds microwatts of power, we aim at delivering at least one hundred microwatts of power to the load (comprised of sensor circuits). The target output voltage is 1.2 V and the regulation window is from 1 V to 1.4 V.
The following figure shows the block diagram of the reconfigurable closed-loop switched-capacitor DC-DC converter. The power conversion stage is a reconfigurable series-parallel converter. Two gain control and frequency control loops run simulataneously to regulate the output voltage and guarantee high-efficiency operation. This power management circuit has been simulated in a 0.25-μm CMOS process and simulation results confirm that with an input voltage ranging from 0.5 V to 2.5 V, the converter can generate a regulated 1.2 V output rail and deliver a maximum load current of 100 μA. The power conversion efficiency is higher than 80% across a wide range of the input voltage with a maximum efficiency of 88%.