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

This work 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 prior work, the first presented solution is a double polarity integrated circuit designed in a 0.18 µm CMOS technology able to boost ultra-low positive and negative voltages without the need of 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 mV 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 kΩ makes the presented solution suitable also for high impedances sources such as rectennas. The second presented circuit, designed in a 0.32 µm 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, and a minimum activation voltage of 35 mV.