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.

This work proposes a new charge pump design suitable for energy harvesting on small human wearable devices. By using only one system for both cold start and high efficiency harvesting, the required silicon area is heavily reduced. The most fitting energy transducer is determined to be a photovoltaic cell, based on aspects of voltage, power and size. Both indoor and outdoor lighting conditions are suitable. The literature on fully integrated voltage boosting with special attention to cold start shows that the capacitive charge pump is the best solution for high efficiency, low power and small size voltage boosting. For cold start applications, a Dickson charge pump with dynamic charge transfer switches is the best solution. For achieving high efficiency conversion, theMakowski charge pump is the best.
The proposed design combines the advantages of the Dickson charge pump for cold start and the high efficiency aspects of aMakowski charge pump for normal operation in one system with minimal silicon area. Using capacitor splitting, the conversion ratio of the cold start charge pump can be orthogonally chosen from
the conversion ratio of normal operation. Dynamic charge transfer switches are used for the cold start charge pump to increase the performance. An implementation of the proposed design is made in Cadence Virtuoso and is tested for cold start capability and efficiency across five process corners and three temperatures. The results prove that the proposed design can achieve high efficiency energy harvesting that is on average 5% below the efficiency of a normal Makowski charge pump. Cold start was achieved from 100mV that is boosted to a battery voltage of 1.8 V within the micro-watt power range. With these results, the proposed charge pump is suitable for photovoltaic energy harvesting in small devices. The required silicon area is reduced by 50%, compared to systems where cold start and high efficiency harvesting are provided by separate systems.
The proposed design is made into a lay-out, which is currently being tested and will be manufactured in the coming months.