Optimization of Capacitor for Piezoelectric Energy Harvesting

Miniaturized version of Micro-electro-mechanical system (MEMS) based Piezoelectric (PZ) a.c. energy harvester is becoming a potential solution to power today's low power electronic systems. This a.c. electrical output energy needs to be converted into d.c. to power the systems. This conversion efficiency directly depends on the magnitude of flipping the voltage across the internal capacitance (C
_p
) of the PZ device during the zero crossing of the internal current source (i
_p
(t)) of the PZ device. Recently, the synchronized switch harvesting on capacitance (SSHC) is becoming an efficient method of flipping the voltage across the C
_p
. The effectiveness of such circuits highly depends on the values of the parameters of the circuit primitives. For optimal flipping, the theoretical value of switching capacitance (C
₁
) of SSHC is equal to C
_p
. However, considering the practical issues in implementation, the flipping efficiency increases with C
₁
. In this paper optimal value of the C
₁
is obtained to increase the flipping efficiency. The circuit is implemented using 0.18 μm technology. The value of the Cr is considered as 25 nF. Using theoretical optimum value of C
₁
as 25 nF, the improvement in the a.c. to d.c. conversion efficiency is 12.16% with respect to the normal FBR. The conversion efficiency increases to 14.86 % with C1 as 50 nF. This shows a direct improvement of the conversion efficiency by 2.7 %.