Dominic Maurath

A closed-loop wide-range tunable mechanical resonator for energy harvesting systems

This paper presents a novel electrically tunable structure which can be used as a resonator for vibration-based energy harvesters. The adjustment of the resonance frequency is provided by mechanical stiffening of the structure using piezoelectric actuators. This concept can easily be stand-alone integrated to realize an autonomous, tunable harvester. The resonator was simulated using ANSYS to find the optimum operation point concerning tuning range. The scalability of this tuning concept is also investigated in this work. A phase shift control circuit was developed for very efficient autonomous closed-loop control of the resonance frequency. Prototypes of the resonators were fabricated and measured with and without the control circuit. The tuning voltage can be kept as low as ±5 V leading to a measured resonance shift of ±15% for the larger resonator (40 mm) and around ±8% for the smaller resonator (27 mm). This tuning range can be simply enhanced by increasing the tuning voltage.

A Sub-500 mV Highly Efficient Active Rectifier for Energy Harvesting Applications

This paper presents a highly efficient, ultra-low-voltage active full wave rectifier. A two-stage concept is used including a first passive stage and only one active diode as second stage. A bulk-input comparator working in the subthreshold region is used to drive the switch of the active diode. The voltage drop over the rectifier is some tens of millivolt, which results in voltage and power efficiencies of over 90%. The design was successfully implemented in an 0.35 μm CMOS technology. The measured power consumption of the comparator is 266 nW@500 mV and the minimum operating voltage is 380 mV. Input voltages with frequencies up to 10 kHz can be rectified.

Efficient Energy Harvesting With Electromagnetic Energy Transducers Using Active Low-Voltage Rectification and Maximum Power Point Tracking

This paper reports on efficient interfacing of typical vibration-driven electromagnetic transducers for micro energy harvesting. For this reason, an adaptive charge pump for dynamic maximum power point tracking is compared with a novel active full-wave rectifier design. For efficient ultra-low voltage rectification, the introduced active diode design uses a common-gate stage in conjunction with supply-independent biasing. While this active rectifier offers low voltage drops, low complexity and ultra-low power consumption, the adaptive charge pump allows dynamic maximum power point tracking with implicit voltage up-conversion. Hence, efficient energy harvesting with high-resistive transducers, e.g., electromagnetic generators, becomes possible even at buffer voltage levels far above actual transducer output voltages. Both interfaces are fully-integrated in a standard 0.35 μm twin-well CMOS process. The designs are optimized for sub-mW transducer power levels and wide supply voltage ranges. Thus, these presented transducer interfaces are particularly suitable for compact micro energy harvesting systems, such as wireless sensor nodes or medical implants. The active diode rectifier achieves efficiencies over 90% at a wide range of input voltage amplitudes of 0.48 V up to 3.3 V. The adaptive charge pump can harvest with a total efficiency of close to 50%, but very independent of the actual buffer voltage. This charge pump starts operating at a supply voltage of 0.8 V, and has an input voltage range of 0.5 V-2.5 V . Finally, results of harvesting from an actual electromagnetic generator prototype are presented.

A Fully Autonomous Integrated Interface Circuit for Piezoelectric Harvesters

This paper presents a fully autonomous, adaptive pulsed synchronous charge extractor (PSCE) circuit optimized for piezoelectric harvesters (PEHs) which have a wide output voltage range 1.3-20 V. The PSCE chip fabricated in a 0.35 μm CMOS process is supplied exclusively by the buffer capacitor where the harvested energy is stored in. Due to the low power consumption, the chip can handle a minimum PEH output power of 5.7 μW. The system performs a startup from an uncharged buffer capacitor and operates in the adaptive mode at storage buffer voltages from 1.4 V to 5 V. By reducing the series resistance losses, the implementation of an improved switching technique increases the extracted power by up to 20% compared to the formerly presented Synchronous Electric Charge Extraction (SECE) technique and enables the chip efficiency to reach values of up to 85%. Compared to a low-voltage-drop passive full-wave rectifier, the PSCE chip increases the extracted power to 123% when the PEH is driven at resonance and to 206% at off-resonance.

A self-adaptive switched-capacitor voltage converter with dynamic input load control for energy harvesting

This paper presents an implementation of a fully-integrated switched capacitor voltage converter with self-adjusting source loading. A charge control unit ensures improved load matching to the power source, which can be an RFID antenna or vibration energy harvesting generator. In conjunction with an adaptive stacking scheme voltage-up conversion is also realized leading to capacitor storage voltages which are higher than the generator voltage amplitudes. This and the improved load matching result in a higher generator output power. In addition, due to the switched capacitor charge pump no diode for rectification is necessary. The converter design is completely implemented and fully-integrated in a standard 0.35 µm twin-well CMOS process. Generator powers of up to 780 µW can be treated, and maximum conversion efficiency is close to 48%. Input voltage amplitudes are possible between 0.5 V – 2.5 V , while the supply voltage range is 0.9 V – 3.6 V.

An ultra-low-voltage active rectifier for energy harvesting applications

This paper presents an ultra-low-voltage active rectifier for micro energy harvesting. A two stage concept is used including a first passive stage and an active diode as second stage. A bulk-input comparator design is used which is well suited for low voltage applications. The power consumption is 200 nW and the minimum operation voltage is 350 mV using a 0.35 µm low VTh CMOS technology. The voltage drop over the rectifier is some tens of millivolt which results in voltage and power efficiencies of over 90 %. Input voltages with frequencies in the range of mHz to low kHz can be rectified.

High efficiency, low-voltage and self-adjusting charge pump with enhanced impedance matching

This paper presents a novel fully integrated charge pump voltage up-converter designed in 0.35 mum standard CMOS process. This proposed charge pump is composed for dynamic input impedance adaptation for optimal loading of micro-energy harvesting generators. The ac generator voltage is oversampled with two flying capacitor arrays. Thus, conversion of alternating input voltages into a dc output-voltage by automatically adapting the conversion ratio is achieved. This yields improved harvesting and conversion efficiency of over 85% in average with a given minimum generator voltage of 330 mV at a nominal generator power of 100 muW.

A fully autonomous pulsed synchronous charge extractor for high-voltage piezoelectric harvesters

This paper presents a fully autonomous, self-adjusting pulsed synchronous charge extractor chip optimized for piezoelectric harvesters with an output voltage from 3V to 18V. The chip which has been fabricated in a 0.35 μm CMOS process is supplied exclusively by the buffer capacitor where the harvested energy is stored in. Due to the low power consumption, the chip can handle a minimum piezo output power of 30μW. The system performs a startup from an uncharged buffer capacitor and operates in the adaptive mode at storage buffer voltages from 1.4 V to 5V. The implementation of the improved switching technique increases the chip efficiency by up to 15% compared to the commonly used Synchronous Electric Charge Extraction technique and enables the chip efficiency to reach values of up to 90%.

Autonomous and self-starting efficient micro energy harvesting interface with adaptive MPPT, buffer monitoring, and voltage stabilization

This paper presents an efficient autonomous micro energy harvesting interface optimized for high-resistive vibration-driven electromagnetic energy transducers. A novel active voltage doubler and an energy processing scheme with adaptive maximum power point tracking (MPPT) is implemented. The interface enables wide voltage range harvesting for amplitudes between 0.44 V and 4.15V. Harvesting with tracking efficiencies of up to 93% and total efficiencies of up to 72% is enabled. In order to supply energy harvesting applications such as low power wireless sensor nodes a programmable voltage stabilization is implemented. The prototype chip is fabricated in a 0.35 μm CMOS process and is self-supplied needing no start-up help.

20.6 Electromagnetic vibration energy harvester interface IC with conduction-angle-controlled maximum-power-point tracking and harvesting efficiencies of up to 90%

An electromagnetic vibration energy harvester (EMH) is an electromechanical mass-spring-damper system transducing electrical energy out of ambient vibrations. Resistive load matching [1] as well as maximum power point (MPP) AC-DC conversion [2] are highly suitable techniques for enhancing the electrical energy output of an EMH. The presented interface IC (Fig. 20.6.1) enables MPP AC-DC conversion by tracking the optimum conduction angle and by employing a hysteretic input voltage controlled inductive DC-DC boost converter. All control signals are derived from the harvester voltage itself. Thus, no additional sensor, harvester disconnection, or DC-DC converter duty-cycle control are needed. Additionally, the implemented voltage conditioning provides over-voltage protection (OVP) and application voltage regulation (VR).