Universal and Scalable Multi-Source Energy Harvesting Power Supply with Dynamic Load Adaptation

Abstract

Autonomous powering of electronic systems in applications spanning biomedical, automation, transportation and space industries relies on harvesting energy from the abundant ambient sources such as vibration, thermal, solar, RF and biofuel cell. The energy harvesting system consists of a transducer that converts energy from the ambient physical form into electrical form, followed by a power conversion system consisting of electronic circuits that extracts the maximum available power and converts it to a user-compatible DC power supply. Harvested energy not only allows the remote charging of batteries or supercapacitors, but also reduces the demand on storage size. Nevertheless, an energy buffer is inevitable for providing a stable source of power amongst the variations and vagaries of the ambient. A step towards further reducing the battery dependence is tapping energy simultaneously from multiple ambient sources. Multi-source energy harvesting systems strive to ensure a perpetual supply of harvested energy from one or more of the available sources while also providing a higher amount of total energy and an increased system reliability. A well-designed power conversion system is the key to satisfy the load demand with the available ambient power.

This thesis explores and proposes the key constructs for designing an efficient, monolithic integrated circuit-based end-to-end power conversion system for multi-source energy harvesting. It consists of an extractor that extracts the maximum power from a heterogeneity of transducer sources with dynamically varying power-impedance inputs and directly delivers it to a voltage-regulated DC output. A second DC output is derived from the extractor output by a regulator that provides fine regulation and dynamic voltage attributes to power a wide dynamic range of load. For the extractor, we propose an optimized architecture and control scheme that improves its efficiency beyond the state-of-the-art. For the regulator, we propose a new pulse-frequency modulation-based DC-DC buck converter that dynamically scales its losses with load while responding to load transients using a synchronous control loop. One key component of any power conversion system is a diode. A conventional PN junction diode has a forward voltage drop of •700mV which is too high for low voltage applications of •1V. We propose a new all-CMOS bias-free fast-switching analog active diode implementation and deploy it extensively in the system to provide diode drop less than 100 mV. The thesis develops on the three stated constructs and the key contributions are summarized as follows.

Active diode uses a MOSFET-switch which is controlled based on the polarity of the diode voltage or current, to either conduct in the deep triode region with a low voltage drop or go into subthreshold region (off state). The key innovation is a bias-free analog controller that senses voltage polarity from the discrete conduction of the respective sensing transistor, presenting inherently offset-free operation. It also achieves fast-switching by providing a no-overlap-drive circuit that aids to trigger a regenerative feedback. Thus, the active diode achieves the unique trait of fast-switching with bias-free operation for minimal control power. The active diode can be engineered for a specified switching speed and forward voltage at a rated forward current and control voltage. We fabricated and evaluated the performance of active diodes with different specifications in 180 nm CMOS technology. Bias-free operation provides an active diode-efficiency of 97.0% even with respect to sub-mW power level (500 µW). While its switching transition time as low as 10 ns preserves the same efficiency till 500 kHz of switching frequency, it also gives efficiencies of 95.5 % at 1 MHz and 85.3 % at 4 MHz. The measured efficiency exceeds the state-of-the-art under similar conditions.

We develop a universal and scalable platform for the multi-source energy extractor. The unique architecture with shared inductor reduces source-end conduction loss by 50% with a single-switch-path. Efficiency is improved by operating it at near-maximum duty cycle in discontinuous conduction mode and by dynamically optimizing switch-size, switching frequency, mode, and duty cycle allocation across sources, based on power and impedance. Bridge rectifier for piezoelectric device is realized using the proposed active diodes. Active diode is also used for zero-current switching within 10 ns in the flyback converter and for limiting the diode-drop to 50 mV in a switched-capacitor voltage inverter that provides the negative voltage drive of input-side switches of the extractor. The extractor was fabricated in 180 nm CMOS technology. The efficiency of 10 Ω-thermoelectric channel was 62.2-77.7% in the available input power range of 90-2250 µW while simultaneously harvesting from photovoltaic and piezoelectric sources at efficiency of 83.6% and 85.0%, respectively. As compared to prior-art of multi-source extractors, a 10-20% improvement in efficiency is seen for the low impedance thermoelectric channel.

The regulator is a DC-DC buck converter that works for a load span of 1:4096 in the sub-mW range. It sustains the efficiency at light loads along with the voltage regulation and ripple characteristics. A dynamic clock controller is used to adapt the operating frequency of the controller with the dynamically varying load. The state-of-the-art load transient response rate is enhanced to 2.6-657 µA in <1 ms within the synchronous mode, which helps to preserve the output voltage attributes at the transients. An active diode was used for zero current switching in ≤10 ns. The regulator fabricated in 180 nm CMOS technology shows a peak efficiency of 94.8% at 788 µW output, while also demonstrating the specified characteristics.

We conclude by studying a potential application of such energy harvesting power supplies in the space industry