Energy harvesting: A battle against power losses

Harvesting energy
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 [2], 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,

(1)

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

(2)

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 [2]. 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 C_MEMS, especially when C_MEMS 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