Contents I. Introduction
There are various techniques to voltage rectification and DC-DC boosting of an AC signal of low magnitude without external bias source. One of the advantage of these techniques is that an AC signal originating from various energy harvesters could be converted and boosted to constant DC source of higher magnitude which can potentially power wearable sensors for real time human health monitoring. Among various energy harvesting technologies such as piezoelectric, electromagnetic, vibration-based, and reverse electrowetting-on-dielectric energy harvesting (REWOD), REWOD has been proved to harvest energy from low frequency motion such as many human notion (walking, running, jogging etc.). Voltage rectification and DC-DC boosting of low magnitude AC voltage from REWOD can be used to reliably self-power the wearable sensors. in REWOD energy harvesting, a mechanical input during the motion causes the electrolyte placed in between two dissimilar electrodes to squeezes back and forth thereby periodically changing the effective interfacial area and hence generating alternating current. In our previous work, an unconditioned REWOD output of 95–240 mV AC is generated using a 50 µL droplet of 0.5M NaCl electrolyte and 2.5 mm of electrode displacement from an oscillation frequency range of 1–3 Hz [1]. This work uses commercial off-the-shelf components(COTS) which involves the integrated circuit design of the CMOS process, making it a highly miniaturized system. 135–240 mV and a forward current of 1 mA convert the generated AC signal to DC voltage. 3 V DC is measured at the boost converter output, proving the system could function as a self-powered motion sensor. For another approach, consider a full-wave rectifier or bridge rectifier consists of p-n junction diodes, which is not suitable for very low input voltage, as they require 0.7 V forward bias voltage to conduct. Similarly, for DC-DC converter the Dickson charge pump is one of the most popular architectures that is used as a DC-DC boost converter but the input voltage needs to be high because of the forward voltage requirement of traditional diodes. Hence, cross-coupled switched-capacitor circuits are better for low input voltage boosting [2]. For a rectifier, the highly efficient voltage-boosting rectifier has a wider frequency operation range of 10 kHz-100 MHz. The designed rectifier by Suri achieved a power conversion efficiency (PCE) of 51% with 100 mV @ 7.25 MHz input signal, which makes it highly applicable for the inductive-coupling based wireless power transfer applications for implantable sensors [3]. Jingmin et al. showed it achieves very high efficiency of 95.5% at 0.2 @ 100 Hz [4], the proposed technique by et al uses DTMOS, for which the body terminal is connected to the gate in a diode-connection form. This factor helps in rectifying the input voltage with wider dynamic control over the threshold voltage. Not only the frequency of the input signal but also the startup voltage plays a major role in the efficient operation of the circuit. Huan Peng proposed a charge pump circuit designed in a standard 0.18 µm CMOS process [5]. It consists of 6 stages, each with a 24 pF pumping capacitor the minimum start-up required is 350 mV output voltage rise from 0 to 2.04 V within 0.1 milliseconds, making it not suitable for input voltage amplitude of 100 mV. A similar problem can be observed for the work present in [6], where the PCE dropped by more than 50% when the input voltage is reduced from 500 mV to 220 mV. But at 500 mV the VCE and PCE are 90% and also for this design, the minimum operating voltage is 380 mV. Similarly, for the DC-DC voltage booster, the charge pump circuit is used to regulate the output DC voltage. Apart from that, Buck boost converters are also a better technique for DC-DC conversion. An inductorless DC-DC converter is used for micro-power harvesting, it provides output regulation at 1.4 V with 58% power conversion efficiency but, the minimum input voltage for this design is 270 mV [7]. When connected to a sinusoidal source of 3.3 V peak amplitude, it allows improving the overall power efficiency by 11% compared to the best recently published results given by a gate cross-coupled-based structure due to its efficiency and input voltage it not suitable for low voltage design [8].
