Abstract
This paper examines the suitability of selected configurations of ultra-low voltage (ULV) oscillators as starters for a voltage boost converter to harvest energy from a thermoelectric generator (TEG). Important properties of particularly promising configurations, suitable for on-chip implementation are compared. On this basis, an improved oscillator with a low startup voltage and a high output voltage swing is proposed. The applicability of n-channel native MOS transistors with negative or near-zero threshold voltage in ULV oscillators is analyzed. The results demonstrate that a near-zero threshold voltage transistor operating in the weak inversion region is most advantageous for the considered application. The obtained results were used as a reference for design of a boost converter starter intended for integration in 180-nm CMOS X-FAB technology. In the selected technology, the most suitable transistor available with a negative threshold voltage was used. Despite using a transistor with a negative threshold voltage, a low startup voltage of 29 mV, a power consumption of 70 µW, and power conversion efficiency of about 1.5% were achieved. A great advantage of the proposed starter is that it eliminates a multistage charge pump necessary to obtain a voltage of sufficient value to supply the boost converter control circuit.
Highlights
Rapid development of body-worn sensors systems integrated on a chip imposes a significant demand on small and efficient power supply circuits
Such a low voltage can be applied to slow circuits based on MOS transistors operating in the subthreshold region
In practical implementations of ultra-low voltage (ULV) boost converters, very significant difficulties are associated with realization of their starters
Summary
Rapid development of body-worn sensors systems integrated on a chip imposes a significant demand on small and efficient power supply circuits. Small TEG size and low temperature difference (typically 1–2 ◦C) limit the upper voltage bound below 100 mV Such a low voltage can be applied to slow circuits based on MOS transistors operating in the subthreshold region. Boost converters can be used to raise voltages as low as 20–40 mV [6,7,8,9,10,11] to the required supply level, they need a voltage within 1 V for efficient energy conversion. In order to minimize the transconductance required to start oscillations in Equation (7), small inductances L1 and L2 and large capacitances C 1, C 2 are advantageous, which is very beneficial for integration of the oscillator
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