31.11 A Capacitor-Based Bias-Flip Rectifier with Electrostatic Charge Boosting for Triboelectric Energy Harvesting Achieving Auto-MPPT at Breakdown Voltage and 14× Power Extraction Improvement

With the advantages of flexibility and power density, triboelectric nanogenerators (TENGs) are promising in harvesting ambient mechanical energy via triboelectrification and electrostatic induction to power lower-power Internet-of-Things (IOT) and biomedical devices. The equivalent circuit model of a TENG contains an AC current source, $\mathrm{I}_{\mathrm{T}}$, and a variable capacitor, $\mathrm{C}_{\mathrm{T}}$, with a capacitance-varying ratio of $\beta$, equal to $\mathrm{C}_{\mathrm{T}, \max } / \mathrm{C}_{\mathrm{T}, \min}$ (Fig. 31.11.1 top). When harvesting energy with a passive rectifier, e.g., a dual-output full-bridge rectifier (FBR) [1, 2], the energy extraction performance is typically low due to the energy loss when the TENG voltage is under the FBR threshold. To improve the performance, the parallel-synchronized switch harvesting on inductor (P-SSHI) rectifier was employed to synchronously flip the charge across $C_{T}$ to reduce energy losses [3]. However, the voltage across the TENG $\left({\mathrm {V}}_{\mathrm {T}}\right)$ may decrease in the negative- ${\mathrm {I}}_{\mathrm {T}}$ half-period, due to the increasing $\mathrm{C}_{\mathrm{T}}$, resulting in electrostatic energy loss. In [4], a multi-chip-stacked bias-flip (MSC-BF) rectifier was proposed for a wind-driven TENG to extract energy at higher voltage levels. Different from typical TENGs, the wind-driven TENG has a constant $\mathrm{C}_{\mathrm {T}}$; thus, it does not introduce electrostatic energy loss, and rectification topologies used in piezoelectric energy harvesting can work well with this TENG. However, an efficient energy extraction technique for varying-$\mathrm{C}_{\mathrm{T}}$ TENGs is urgently needed. Besides the energy extraction performance, large inductors (>1mH) are typically required for bias-flip operations in prior designs to achieve good voltage flip efficiencies.