State-of-the-Art Power Transfer Methods in Triboelectric Energy Harvesters.

Triboelectric nanogenerators (TENGs) are attracting increasing attention as viable power sources for self-powered systems, due to advantages such as material and form-factor versatility, compatibility with low-frequency mechanical stimuli, and scalable, low-cost fabrication. Unlike conventional harvesters, TENGs exhibit two device-level characteristics that critically shape interface-circuit design: (i) a time-varying internal capacitance that induces a strongly dynamic source impedance, and (ii) a significantly high effective source impedance that yields very high open-circuit voltages at low currents. These characteristics directly affect impedance matching, rectification, voltage conversion, and maximum power point tracking (MPPT) strategies for maximum power transfer. While TENGs and piezoelectric energy harvesters (PEHs) share similar lumped electrical models, the time variance and voltage/current operating regime of TENGs fundamentally limit the portability of PEH-oriented power transfer methods. This paper provides two contributions. First, we introduce a figure-of-merit (FoM) that serves as an energy-extraction coefficient: the fraction of the ideal maximum power of the TENG device (under instantaneous impedance tracking) that appears at the rectifier input, typically the first stage of a power management unit (PMU). The FoM exposes losses arising from mismatch at the device-PMU boundary, thereby helping circuit designers localize dominant loss mechanisms (e.g., impedance mismatch, rectifier topology or suboptimal MPPT policies) and guiding device researchers to prioritize physical parameters (e.g., dielectric thickness, displacement, electrode area) with explicit awareness of interface constraints. Secondly, we conduct a thorough evaluation of the suitability of advanced PEH-derived methods, such as rectifiers (both passive and active configurations), DC-DC conversion, and MPPT, for application to TENGs. We derive theoretical upper bounds on extractable power for representative rectifier families under TENG-specific operating conditions, and we analyze technology-imposed voltage limits and their implications for architecture and control. We also survey recent TENG demonstrations together with their PMU interfaces and interpret reported performance through the proposed FoM. Overall, the analysis highlights that the unique characteristics of TENGs and technology limits for the voltages must be explicitly accounted for while developing interface circuits to realize maximum power extraction. This process can significantly benefit from coordinated device-circuit co-design via system-level metrics such as the proposed FoM.