Date

2013

Author

Uluşan, Hasan

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Energy harvesters use environmental energy sources such as heat, light, and vibration, and convert them to electrical energy. The development of micro-scale energy harvesters together with the decreasing power demand of new generation integrated circuits, allow the use of these harvesters as energy source in microsystems as an alternative for batteries. Energy harvesting can then be used for supplying power to the sensors which operate in inaccessible environments, as an enabling technology. However, the generated electrical energy from vibration is in AC form, and design of an efficient and low power interface electronic circuit is crucial especially if the generated voltage and power of these harvesters are at low levels. The aim of this study is to design a fully-integrated and battery-free interface circuit for electromagnetic (EM) energy harvester with low voltage output. The voltage generated by EM energy harvesters is considerably low (<1 V) at low vibration levels (< 10Hz). Hence a low voltage and highly efficient interface circuit must be designed in order to efficiently transfer the power generated by the harvester to the load at a standard system voltage. The interface circuit requires various blocks for efficient power transfer from the harvester to the electrical load. Firstly, there should be an AC to DC converter or a rectifier for converting the generated AC voltage to a DC voltage. Another important block is the DC to DC converter which enables the step-up of the converted voltage to a desired value. Finally, a voltage regulator block should be used for obtaining a stable DC voltage as of a typical battery. The amount of power dissipated by the interface electronics must be kept as low as possible compared to the generated power since the available output power from the harvester is typically low. The designed interface circuit includes all necessary blocks for a full system solution and shows good performance especially at low input AC voltages (<0.5 V peak). The circuit includes a novel rectifier circuit which uses an AC/DC doubler structure to benefit from the full cycle of the input signal. The proposed rectifier uses active diode structure to minimize the effective voltage drop at the output. The comparator circuits utilized at the active diodes are powered internally by another passive rectifier and external power requirement of the circuit is eliminated. The DC/DC converter block of the interface circuit includes a low voltage charge pump and a low power on- chip oscillator design. The final block of the system is a low power voltage regulator which regulates the output to 1 V DC voltage. The proposed interface electronic is designed and fabricated by using TSMC 90 nm CMOS technology. The designed system is merged with an in-house EM energy harvester module and tested with various vibration conditions. The maximum power conversion efficiency of the rectifier stage of the circuit is 67%, while providing 0.61 V for 40 µA load, and it is able to operate down to 100 mV input peak voltages. When the rectifier block is combined with the DC/DC converter, the circuit is able to generate more than 1 V output voltage on 1 MΩ load resistance for input peak voltages higher than 0.4 V. During the tests of the fabricated IC, it is observed that the voltage regulator block does not operate as expected. The reason for this is investigated through tests and simulations, and it is concluded that the fabricated transistors were slower than the typical transistors in this specific IC run. The regulator block is then re-designed to be more robust to fabrication variations for another IC run. In the frame of this study, a low voltage CMOS interface electronics have been designed and tested together with a vibration based electromagnetic energy harvester module. The results show that the designed rectifier has the lowest operation voltage among the fully-integrated and self-powered interface circuits for low vibration range, to the best of our knowledge. Furthermore, the rectified voltage is stepped up with the DC/DC converter, so that the output of the circuit reaches to typical battery voltages even with very low AC input signals, enabling the use of the system as an alternative energy source for wireless microsystems.

Subject Keywords

Electric current rectifiers., Energy harvesting., Electromagnetic theory., Analog-to-digital converters.

URI

http://etd.lib.metu.edu.tr/upload/12616251/index.pdf
https://hdl.handle.net/11511/23016

Collections

Graduate School of Natural and Applied Sciences, Thesis