Konstantinos Kozalakis

This work proposes a novel technique for the control of modern inductor-based switching voltage converters (SVC) (e.g., boost, or buck converter), which is inspired by an old self-oscillating mechanical converter topology. According to the proposed technique, a relay component replaces both the inductor and the semiconductor switch of the SVC. The magnetic coupling between the switch and the inductor eliminates the need for a separate oscillating control circuit, which is usually required for the operation of an SVC, leading to a very simple converter topology. The electromechanical SVC (EMSVC) is self-oscillating, operating automatically whenever sufficient power is applied at its input. In this work a boost converter EMSVC design, as well as a buck converter EMSVC design are presented. Additionally, a boost converter EMSVC implementation, optimized towards thermoelectric cold start applications, is presented with experimental results. The converter can operate with an input of only 25 mV and 6.5 mA, and if combined with a thermoelectric harvester, it can be used for heat energy harvesting from temperature difference as low as 1 °C.

A large number of autonomous devices is nowadays supported by renewable and green energy sources. A vital sub-circuit in such systems is the power converter circuit, which should efficiently transform and store the available energy. In order to obtain the maximum efficiency under varying energy conditions, various maximum power point tracking (MPPT) methods are used. In this work a complete harvesting module with battery management and MPPT is presented, suitable for a plethora of autonomous applications. A novel, low-complexity and ultra-low power consumption design is proposed, which offers very wide operating voltage and power range with high MPPT efficiency and very low power consumption. It can be combined with different harvesters, such as thermoelectric generators or photovoltaic panels and is able to work under widely varying energy conditions. As supported by experimental results, the proposed module covers a very wide working input power range, from 40 μW up to 4 W, as well as a very wide input voltage range, from 650 mV up to 2.8 V with 96.5% average MPPT efficiency and a total power consumption of 3.9 μW at 3.6 V. The module relies on an embedded ultra-low power microcontroller unit (MCU) to perform the power management and MPPT operations, which can also be used for extra tasks (e.g., sensor reading). Using the proposed module, an autonomous sensor node was built, able to acquire acceleration measurements, and wirelessly communicate with a remote user in order to send an alert or stream the acquired sensor data in real time.

This work presents an ultra-low-power CMOS supercapacitor storage unit suitable for a plethora of low-power autonomous applications. The proposed unit exploits the unregulated voltage output of harvesting circuits (i.e., DC-DC converters) and redirects the power to the storage elements and the working loads. Being able to adapt to the input energy conditions and the connected loads’ supply demands offers extended survival to the system with the self-startup operation and voltage regulation. A low-complexity control unit is implemented which is composed of power switches, comparators and logic gates and is able to supervise two supercapacitors, a small and a larger one, as well as a backup battery. Two separate power outputs are offered for external load connection which can be controlled by a separate unit (e.g., microcontroller). Furthermore, user-controlled parameters such as charging and discharging supercapacitor voltage thresholds, provide increased versatility to the system. The storage unit was designed and fabricated in a 0.18 um standard CMOS process and operates with ultra-low current consumption of 432 nA at 2.3 V. The experimental results validate the proper operation of the overall structure.

In this paper, the challenges of designing a boost converter in standard CMOS technology are discussed. Based on theoretical calculations, which take into consideration the characteristics of the technology used, efficient solutions are proposed to overcome the current-stress limitations as well as the power dissipation issues. Following these guidelines, a test DC-DC converter, designed and fabricated in a standard 0.18um CMOS process is presented, along with experimental results.

This work introduces a light harvesting system with battery management. In contrast to relevant solutions that operate in limited ranges, the proposed system covers a wide operating input power range from 10 uW up to 300 mW. Specifically, experimental results highlight that, combined with a 73 × 94 mm flexible light harvester, it can harness light in a range from 50 LUX (indoor lighting) up to 120,000 LUX (outdoor lighting). The introduced system consists of a boost converter and an ultra-low power microcontroller (MCU). The MCU performs Global Maximum Power Point Tracking (GMPPT), using a resistor-free time-based input power sensing method, to calculate the input power of the converter, which does not interfere with the operation of the boost converter. The efficiency of the GMPPT system was evaluated with detailed experimentation, where we achieved 99.75% average GMPPT tracking efficiency while consuming only 73.5 uW at 4.2 V.

The paper proposes a simple and low-cost technique of a power measuring system for the characterization and evaluation of harvesters, harvesting circuits or switching converters. The system is a lab instrument and it can measure the power that is drawn from or provided to it, by measuring the time duration for a specific voltage difference to occur on a capacitor of known capacitance (V-T). This way, it is easier to measure low power values in comparison with the most straightforward method that depends on voltage and current measurements. The system is using only commercially available discrete components and is optimized for measuring power in the uW range with uW accuracy as supported with experimental results. It was tested to compare different solar harvesting circuits and to measure the available power levels for various light intensities.

The current paper proposes a novel self-oscillating dc-dc boost converter with constant efficiency in a wide power range, for low energy harvesting applications. The circuit is designed for efficient operation in a wide power range and consumes less than 400nW at 5V power supply. The circuit's output is a current pulse which is generated each time the unit collects the required energy. This gives the ability to many identical circuits to be connected at a common output. It also offers input-impedance control capability via an external analog input pin, that must be set by the user. The circuit was constructed using commercially available discrete components for verification of its operation. The operation principle of the converter is supported with simulation and experimental results.