Nathaniel J. Guilar

Integrated Solar Energy Harvesting and Storage

To explore integrated solar energy harvesting as a power source for low power systems, an array of energy scavenging photodiodes based on a passive-pixel architecture for CMOS imagers has been fabricated together with storage capacitors implemented using on-chip interconnect in a 0.35-mum bulk process. Integrated vertical plate capacitors enable dense energy storage without limiting optical efficiency. Tests were conducted with both a white light source and a green laser. Measurements indicate that 225 muW/mm2 output power may be generated by white light with an intensity of 20 kLUX.

Integrated solar energy harvesting and storage

To explore integrated solar energy harvesting as a power source for low power systems such as wireless sensor nodes, an array of energy scavenging photodiodes based on a passive-pixel architecture for imagers and have been fabricated together with storage capacitors implemented using on-chip interconnect in a 0.35 mum CMOS logic process. Integrated vertical plate capacitors enable dense energy storage without limiting optical efficiency. Measurements show 225 muW/mm2 output power generated by a light intensity of 20k LUX

An energy-aware multiple-input power supply with charge recovery for energy harvesting applications

Energy harvesting systems such as self-powered wireless sensor nodes rely on a diverse set of energy sources in their continuously changing environment for power. A highly flexible power supply with multiple sources is needed to deal with large variation in environmental conditions. This paper describes an integrated power management system for use with multiple energy harvesters, which employs energy awareness and charge recycling. Previous multiple-input power management systems only allow energy to be harvested from a single source at a time [1] (typically the source with the largest voltage) thereby wasting the energy from the other energy harvesters. In order to increase the total energy available, the work presented here allows the system to gather and add together voltages from multiple sources at the same time.

A Full-Wave Rectifier for Interfacing with Multi-Phase Piezoelectric Energy Harvesters

This paper describes an asynchronous full-wave rectifier for use with a multiple-electrode disk-shaped piezoelectric generator. By using quarter-circle shaped electrodes, similar to an ultrasonic motor, multiple output voltage phases are obtained from a single resonating piezoelectric disk. Two-electrode transducer outputs are often rectified using a full-wave diode bridge rectifier, which requires a significant voltage drop between input and output, decreasing the rectifiers voltage efficiency. Previous asynchronous rectifiers use MOS switches to avoid the diode drop, however they lack the ability to efficiently rectify the multiple output phases generated from a disk-shaped transducer. Other CMOS rectifiers need two input waveforms that are equal and opposite, which a piezoelectric transducer may not generate for many input vibrations.

Ultra-low-voltage circuits for sensor applications powered by free-space optics

Advances in photonics have typically been exploited in high performance systems, e.g. high-frequency, low-jitter clocks injected optically [1]. As solid-state lighting and free-space optical (FSO) communication expand, low-performance sensor systems can also benefit from photonics. Sensors can receive power, clock, and data from optical sources at different wavelengths with crosstalk eliminated through narrowband filtering by photonic devices (e.g., ring resonators [2]). Applications include indoor environmental sensing or biomedical devices implanted under the skin (transcutaneous optical power). However, significant challenges to realizing such systems exist. FSO power decreases quadratically (beam divergence) and exponentially (absorption) with distance, thus optically-powered sensors must be extremely low power to maximize operating range. Mixed-signal circuits must support energy-scalable operation with low area overhead (to maximize energy harvesting photodiode area) under variable low voltage (VDD can vary significantly with light intensity near the maximum power point). They must process analog inputs near full-scale to maximize SNR. Energy harvesting photodiodes optimized for conventional CMOS have been developed [3]. In this paper, we describe optically-powered CDR and delta sigma modulator (DSM) circuits. Figure 27.9.1 shows a possible system context. Photodiodes supply power (from ambient light or interrogating beam) and capture optical data, e.g. timing configuration for the DSM clocks. The DSM output bitstream modulates a simple on-off keyed (OOK) transmitter or is saved to non-volatile memory (not shown).

Energy harvesting photodiodes with integrated 2D diffractive storage capacitance

Integrating photodiodes with logic and exploiting on-die interconnect capacitance for energy storage can enable new, low-cost energy harvesting wireless systems. To further explore the tradeoffs between optical efficiency and capacitive energy storage for integrated photodiodes, an array of photovoltaics with various diffractive storage capacitors was designed in TSMCs 90 nm CMOS technology. Transient effects from interfacing the photodiodes with switching regulators were examined. A quantitative comparison between 90 nm and 0.35 μm CMOS logic processes for energy harvesting capabilities was carried out. Measurements show an increase in power generation for the newer CMOS technology, however at the cost of reduced output voltage.

A Passive Switched-Capacitor Finite-Impulse-Response Equalizer

A passive CMOS switched-capacitor finite-impulse-response equalizer is described. A sampling rate of 200 MS/s is achieved by six time-interleaved channels. Nonlinear parasitic capacitance scales the equalized output but does not affect the zero locations of the equalizer for a binary or ternary data signal. The 4-tap equalizer prototype is fully differential. At 200 MS/s, the equalizer dissipates 19.5 mW, which is virtually all consumed by clock drivers, and occupies an active area of 1.3 mm2 in a 0.35 mum CMOS process

Analysis of DC-DC Conversion for Energy Harvesting Systems Using a Mixed-Signal Sliding-Mode Controller

Increasing efficiencies in electronic devices have opened the door for energy harvesters to power wireless systems by scavenging energy from solar, thermal, or mechanical sources. The design of an efficient power supply that can regulate scavenged energy to produce stable voltages for multiple loads is presented here. To limit energy dissipation and increase the flexibility of the regulator, a discrete-time sliding-mode controller is presented. The performance of sliding-mode controllers is hard to predict because their switching frequency and output voltage ripple are dependent on duty cycle and load conditions. An analytical approach for the closed-loop steady- state behavior of a discrete-time sliding-mode controller is presented here. The proposed analytical solution matches measured results to within 4% for duty cycles ranging between 0.1 and 0.9.

A 200 MS/s passive switched-capacitor FIR equalizer using a time-interleaved topology

A low-power passive switched-capacitor finite-impulse response equalizer with six time-interleaved channels has been fabricated in 0.35/spl mu/m CMOS. Nonlinear parasitic capacitance scales the equalized output but does not affect the zero locations of the equalizer for a binary or ternary data signal. The equalizer is fully differential with a 4-tap transfer function. The equalizer consumes 19.5 mW at 200 MS/s and occupies an active area of 1.3mm/sup 2/.

Energy harvesting and limits of low power mixed-signal circuit design

Wireless sensors and implantable medical devices have driven IC design to extremes of low power consumption to maximize system operating lifetimes from fixed energy stores or from energy harvested from the environment. Reaching the limits of miniaturization will require approaching the limits of power dissipation. We describe three key sensor subsystems: integrated diodes for solar energy harvesting, efficient microwatt power conversion circuits, and supply-voltage-ripple-tolerant digital circuits. We then extrapolate from these examples to find the minimum surface area and volume required for energy harvesting sensors.