Ying-Khai Teh

Design of Transformer-Based Boost Converter for High Internal Resistance Energy Harvesting Sources With 21 mV Self-Startup Voltage and 74% Power Efficiency

Thin-film thermoelectric generators (TEG) or graphene-based microbial fuel cells (MFC) are emerging energy harvesting sources with promising power density and sustainability. Nevertheless, conventional transformer-based boost converters commonly used to achieve autonomous low voltage startup encounter low efficiency and potential startup problems with these novel power sources due to their high internal resistance. In this paper, an improved design of transformer-based boost converter addressing these issues is demonstrated with prototype chip fabricated using a standard 0.13 μm CMOS process. The self-start oscillation does not rely on the conventional LC resonant principle, but instead is dependent on the MOS transistors active-over-leakage current ratio and the mutual coupling between the two identical transformer coils. Circuit design techniques to regulate output voltage and to track systems maximum power point (MPP) of this boost converter are presented. Measurement results confirmed that the proposed circuit works with either low threshold voltage or native MOS transistors. It needs minimum self-startup voltage of 21 mV (at 5.8 μW input power) and minimum startup power of 1.3 μW (at 35 mV input voltage) respectively. The maximum output power is 2 mW and peak power conversion efficiency is 74% at a regulated output voltage of 1 V.

Improved Designs for an Electrothermal In-Plane Microactuator

Reported presently are two design approaches to improve the performance of an electrothermal in-plane microactuator with “chevron” beams. One incorporates beams with uniform cross sections but nonuniform lengths or tilt angles to accommodate the thermally induced expansion of the “shuttle”; the other incorporates beams with nonuniform cross sections to widen the high-temperature “expansion” zones. It is derived analytically, verified using finite-element simulations, and tested by microfabricating actuators occupying a constrained device area that the incorporation of one or the other proposed features leads to an improved performance figure-of-merit, defined to be the product of the actuation displacement and force. An increase in the figure-of-merit by up to 65% per beam has been measured.

Design of coupled inductor-based boost converter for ultra low power thermoelectric energy harvesting using pulse transformer with 75mV start-up voltage

This paper presents a coupled inductor-based converter capable of self-starting and has consistent conversion efficiency independent of input voltage and source resistance. Instead of using a customized high turns-ratio miniature transformer, existing compact profile pulse transformer commonly available for telecommunication circuits can be used. Simulation results with a standard CMOS 0.13-μm technology confirmed that using the proposed topology, power conversion efficiency can be improved to over 60% and is sustainable over wide input voltage range.

A low-voltage high-efficiency voltage doubler for thermoelectric energy harvesting

This paper presents a low-voltage high-efficiency voltage doubler for thermoelectric energy harvesting. A negative charge pump is used to reduce on-resistance of PMOSFET load switches. Additional circuitry for achieving high efficiency just requires small chip-area and low-cost. The voltage doubler is implemented in 0.13-μm 1.2V CMOS process. The proposed doubler improves the low-voltage power efficiency by 17%, compared to conventional voltage doublers. Moreover, the proposed doubler can support wider load range. The maximum power efficiency reaches 52% at the input voltage of 0.2V.

A Stacked Capacitor Multi-Microwatts Source Energy Harvesting Scheme With 86 mV Minimum Input Voltage and

A multi-source energy harvesting scheme that emulates the principle of a water bucket fountain using series-stacked storage capacitors to power a multi-VDD mixed-signal wireless sensor node is presented. Feasible miniaturized microwatts energy sources include photovoltaic cell, piezoelectric harvester and thermoelectric generator. A digital logic based shunt regulator with 1 V clamping output is first designed using multi-threshold transistors to minimize static leakage current. Three shunt regulator blocks on separate dies are subsequently paired up with the series-stacked storage capacitors to increase the system output voltage to 3 V. The stacked shunt regulators not only provide voltage regulation and over-voltage protection, but also balance each individual storage capacitor to improve system reliability. By using the mirroring property of pulse transformer-based boost converter and one additional storage capacitor, the system is capable of generating bipolar ± 3 V output voltage at minimum input voltage of 86 mV (boost conversion ratio up to 70) and input power of 26 μW only. The increased supply voltage headroom is desirable for sensitive and high power blocks i.e., sensors, analog and radio-frequency circuits in terms of achieving better analog-to-digital-converter resolution, amplifier gain and linearity. The remaining branches of the stacked capacitors simultaneously provide 2 V and 1 V outputs which can be used by ultra-low power digital blocks of less supply voltage sensitive. The circuit prototypes are implemented on a standard 0.13- μm complementary metal-oxide-semiconductor process.

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Recent advancement in commercial MEMS-based piezoelectric energy harvesters has enabled further reduction of both system cost and size as the device dimension scales down. A nature inspired interface circuit by mimicking water bucket fountain to harvest piezoelectric charge is presented. The proposed architecture offers built-in input voltage protection, harvests electrical energy using minimal switching activity and does not cause mechanical dampening to the piezoelectric cantilever. System simulation and measurement using a standard CMOS 0.13μm process verified the proposed architecture. The pulse transformer boost converter has self-start voltage as low as 45mV and peak efficiency up to 75%, whereas the remaining digital control and voltage processing circuits require less than 1.5μW to operate.

V Bipolar Output Voltage

Reported presently are two designs to improve the performance of a “chevron” electro-thermal in-plane actuator. One incorporates beams with uniform cross-sections but nonuniform lengths and tilt angles to accommodate the thermally induced expansion of the “shuttle”; the other incorporates beams with non-uniform cross-sections to achieve a wider spread of the high temperature “expansion” regions of the beams. With the product of the actuation force and displacement defined as a figure-of-merit, it is verified using finite-element simulation that the incorporation of non-uniform lengths and tilt angles, and non-uniform beam cross-sections leads to respective improvement of 10 and 65% in the figure-of-merit. The effectiveness of these designs was also tested by micro-fabricating actuators occupying fixed device areas.

A piezoelectric energy harvesting interface circuit using one-shot pulse transformer boost converter based on water bucket fountain strategy

An energy-harvesting system that accommodates both discrete-time or continuous-time energy sources simultaneously is presented. The core dc-dc converter is a pulse transformer boost converter using dynamic threshold MOS transistor that self-starts at 36 mV-input voltage and generates bipolar output voltages up to ±2.5 V. Employing the dynamic body bias technique to the boost converter power transistor improves power efficiency at sub-300-mV input voltages up to two times over the identical transistor in a conventional configuration. At large harvested power, the source-to-body diode of the power transistor functions as an input voltage limiter. Dynamic threshold MOSFET (DTMOS) also increases transistor saturation current and output power compared to conventional transistors at similar input voltage. Instead of using a linear shunt regulator, excess power is dissipated by internal loss of a bipolar-clocked cross-coupled charge pump. An on-chip voltage-controlled oscillator, which generates clock frequency in proportion to the harvested power, and a unipolar-to-bipolar level shifter are included to drive the charge pump designed. The circuit prototypes are fabricated by using the UMC 0.13-μm CMOS process with the triple-well option.

Designs for improving the performance of an electro-thermal in-plane actuator

Boost converter designed using miniaturized high turn-ratios transformer is an excellent circuit to achieve fully electrical autonomous start up at low input voltage. Nevertheless, at regulated output voltage, its power conversion loss at high input voltage is significant. A 2× cross-coupled charge pump is added to assist energy harvesting at high input voltage. The charge pump is turned on whenever the input voltage exceeds programmed threshold. Digital-only technique which guarantees minimal quiescent current is used to realize the shunt regulator implementation and power monitoring circuit, where the input voltage (power) is determined by measuring the transformer self-oscillation period. Time-based digital control eliminates the need of conventionally power hungry components i.e. amplifiers and voltage domain ADC. Simulation results using a standard CMOS 0.13-μm technology and thermoelectric generator equivalent circuit model validated the proposed architecture, which can generate output voltage levels at 1 V and 2 V, respectively. Additional voltage headroom provided by the charge pump is instrumental for sensor, analog and RF circuits. The system requires a minimum input voltage of 34 mV and input power of 110 μW to start up.