Is it possible, are indefinite operational life and wireless power grids possible? Maybe not for every application, but how about for micro-scale devices? The fact is in situ energy sources like MEMS vibrational and thermoelectric generators can potentially achieve these goals for small footprint system-in-package (SiP) solutions like bio-implantable devices and wireless sensor transceiver network nodes. The key objective is to scavenge sufficient energy from the environment to sustain the micro-power system indefinitely, or at least extend life to practical levels. The problem, however, is micro-scale harvesters can only generate low-to-moderate power, and the energy-storage and power-delivery processes of the system inherently consume a portion of that, which is why the various functions of a loading application must be power-moded, that is, multiplexed, duty-cycled, and turned off when not needed. Fortunately, low frequency ambient vibrations are relatively abundant, stable, and predictable, and tuned MEMS- and CMOS-compatible electrostatic harvesters, for instance, can generate moderate power levels , but only if they prevail over the power losses associated with energy storage and power delivery. The focus of this article is to therefore identify, quantify, and discuss the power-consuming mechanisms present in a harvester circuit.
Before attempting to discern the relevant power losses in a harvester, the process and circuit must be understood. For the purposes of this study, a voltage-constrained electrostatic scavenger that harnesses some of the kinetic energy present in vibrations is considered because it is both MEMS and CMOS compatible; in other words, it can all be co-packaged into a single chip. Its operation, as presented in , is divided into three distinct phases: pre-charge, harvesting, and recovery. First, when the capacitance of a variable-plate MEMS capacitor is at its peak, energy is invested into the system by pre-charging the capacitor to the battery voltage. The MEMS capacitor is then connected directly to a rechargeable battery (for example, Li-Ion battery), driving charge and energy into the battery when the capacitance drops (that is, the parallel plates separate) in response to ambient vibrations,
When minimum capacitance is reached, harvesting ends and the remaining energy in the capacitor is recovered. The end result, assuming no power is lost in the process, is a net energy-per-cycle gain in the battery of
To transfer energy back and forth between the battery and the harvesting capacitor, an inductor is used, as shown in Figure 1, because of its low power-consuming properties. The pre-charge phase is therefore decomposed into a sequence of cycles that alternately energize the inductor (Step 1) and exhaust it in charging the MEMS capacitor (Step 2). In the recovery phase, the energy remaining in the capacitor is transferred back into the battery by reversing the Step 1-2 sequence . Harvesting is achieved by short-circuiting the battery to the capacitor when its capacitance decreases, as illustrated with Step 3 in Figure 1.
Figure 1. Energy harvester with inductor-based pre-charge and recovery circuit
A more complete and practical realization of the circuit is shown in Figure 2 where ideal switches are replaced with CMOS transistors and their respective body diodes, the battery is replaced with a capacitor-resistor model, and other parasitic capacitors and resistors are included. The 2.7 to 4.2 V Li-Ion battery is, on first order, a fixed-charge energy source with a parasitic load-dependent voltage drop and can therefore be modeled with a large pre-charged capacitor and an equivalent series resistor (ESR). MEMS device CMEMS also has a parasitic ESR in addition to a parasitic capacitor across its terminals. The pre-charge and recovery circuitry features an inductor with its own ESR; CMOS switches MP1, MN2, and MN3; and CMOS transmission gate MN4-MP4. The purpose of MN4 in the transmission gate is to help MP4 short-circuit the inductor to CMEMS, especially when CMEMS is discharged, which reduces MP4's gate-drive low enough to increase MP4's resistance beyond acceptable values. Two back-to-back transistors are used in place of S5 in Figure 1 to prevent body-diode conduction during the pre-charge and recovery phases, which would have otherwise resulted with a single PMOS switching device.
Figure 2. (a) Energy harvester circuit with non-ideal components and (b) control signals