A −8 mV/+15 mV Double Polarity Piezoelectric Transformer-Based Step-Up Oscillator for Energy Harvesting Applications

Abstract

This paper presents two circuit topologies of battery-less integrated boost oscillators suitable for kick-starting electronic systems in fully discharged states with ultra-low input voltages, in the context of energy harvesting applications based on thermoelectric generators, by coupling a piezoelectric transformer in a feedback loop. With respect to the prior work, the first presented solution is a double polarity circuit designed in a $0.18~\mu \text{m}$ CMOS technology able to boost ultra-low positive and negative voltages without using switching matrixes. The circuit exploits a CMOS inverter made up of low threshold transistors, and also includes a hysteretic voltage monitor consuming only ~15 nW to enable an external circuit. The minimum achieved positive and negative oscillation voltages are +15 and −8 mV, which to the best of the authors’ knowledge, are among the lowest start-up voltages achieved in literature up to now without using magnetic components. Moreover, the input impedance in the range of several $\text{k}\Omega $ makes the presented solution suitable also for high impedances sources, such as rectennas. The second presented circuit, designed in a $0.32~\mu \text{m}$ CMOS technology, exploits an input stage based on depletion-mode MOSFETs in a common source stage configuration and achieves a maximum step ratio of ~60.