A Cold-Startup SSHI Rectifier for Piezoelectric Energy Harvesters With Increased Open-Circuit Voltage

Piezoelectric vibration energy harvesting has drawn much research interest over the last decade toward the goal of enabling self-sustained wireless sensor nodes. In order to make use of the harvested energy, interface circuits are needed to rectify and manage the energy. Among all active interface circuits, synchronized switch harvesting on inductor (SSHI) and synchronous electric charge extraction are widely employed due to their high energy efficiencies. However, the cold-startup issue still remains, since an interface circuit needs a stable dc supply and the whole system is completely out of charge at the beginning of implementations or after a certain period of time without input vibration excitation. In this paper, a new cold-startup SSHI interface circuit is presented, which dynamically increases the open-circuit voltage generated from the piezoelectric transducer in the cold state to start the system under much lower excitation levels. The proposed circuit is designed and fabricated in a 0.18-
$\mu$
m CMOS process and experimentally validated together with a custom microelectromechanical system harvester, which is designed with split electrodes to work with the proposed power extraction circuit. The experiments were performed to start the system from the cold state under variable excitation levels. The results show that the proposed system lowers the required excitation level by at least 50% in order to perform a cold startup. This aids restarting of the energy harvesting system under low excitation levels each time it enters the cold state.