ENMOS: Energy Module for Self-Sustainable Wearable Sensors

Abstract

This work focuses on addressing the fundamental limitation on the minimum cold start-up voltage that could be harvested from a thermoelectric element (TEG) for human-body wearable applications. For ultra-low DC voltages, the challenge translates to generating a timed-signal to amplify it up to a value that could be used to drive, say, a boost converter which can then start-up the entire energy module. Contemporary works have, thus, strived to accomplish this using a charge-pump-based or a transformer-based approach, which in turn imposes a limit on the minimum TEG voltage that can be harvested. The solution this work proposes is to decouple the Cold Start-up system from the TEG altogether and instead, use a piezoelectric element (PEH). This element being capable of producing a well-timed (AC) signal for free, based on human body vibrations, can potentially drive a boost converter. To this end, an integrated circuit (IC) is designed that can utilize the voltage from the PEH, amplifying it up to generate a well-controlled signal that could operate the boost converter. At the heart of this IC, is a self-reconfigurable charge pump that arranges its stages in different boosting ratios (without any complex logic or DSP) based on the input voltage, to allow for a maximum harvested power. The proposed self-reconfigurable architecture can potentially lead the charge pump to be load-variation-resistant. It achieves this by providing an almost constant voltage while increasing the power for higher load demands, at the same time maintaining a constant efficiency. Thus, the fully on-chip implementation in TSMC 0.18 um CMOS, can cold start-up the system from 25 mV of thermoelectric voltage to deliver an output voltage of 1 V at 56.5 % converter efficiency, consuming only 240.5 pW of dynamic power (simulation). The minimum Cold Start-up voltage and dynamic power were found to be 18 mV (ΔT = 0.1 K) and 231.6 pW respectively, to supply 1 V at 44 % converter efficiency. Moreover, in order to prove that the fundamental limitation on the Cold Start-up voltage has been addressed successfully, the IC was also simulated to check whether it can further be lowered. In this case, by providing a piezoelectric excitation voltage 73.3 mVRMS higher, the Cold Start-up voltage was found to be reduced to 15 mV to supply a constant 1 V at the load. It was also found that increasing the inductor value in this case, can also allow the energy module to support even lower Cold Start-up voltages.