# Recent Progress of Switching Power Management for Triboelectric Nanogenerators

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## Abstract

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## 1. Introduction

## 2. Fundamentals of Switching Power Management for TENG

#### 2.1. The V-Q-x Relationship for TENGs

_{oc}is the open-circuit voltage. The right side of the equation consists of two voltage terms: the V

_{oc}term is generated by the polarized triboelectric charge, and the −Q/C(x) term is the contribution of the transferred charge to V.

#### 2.2. Output Characteristics under Resistive Load

_{0}is the sum of the thickness to relative permittivity ratios of all the dielectric materials, x

_{max}is the maximum separation distance, S is the contact area, v is the average separation speed, and ε

_{0}is the permittivity of vacuum. It can be seen from the formula that the optimal impedance is not related to the surface charge density. Figure 2c shows the dependence of the power on external resistor and speeds.

#### 2.3. Charging Characteristics under Capacitive Load

_{sat}only has a relationship with the maximum value of short-circuit transferred charges, minimum and maximum capacitance.

#### 2.4. The V-Q Curve for TENG

_{c}is less than the maximum short-circuit transfer charges amount Q

_{sc,max}. When the travel switches are connected in parallel and short-circuited at the extreme displacement, the area of the V-Q curve will be expanded (Figure 2h). As shown in Figure 2i, when the load is infinite, the curve enclosing area is close to a trapezoid, which is larger than the area under any impedance, so this cycle is called ‘cycles for maximized energy output’ (CMEO). The energy output per cycle is the enclosing area of the trapezoid and could be calculated by Equation (7) [61]:

_{OC,max}is the maximum open-circuit voltage. V

^{′}

_{max}is the absolute value of maximum voltage in the condition that Q = Q

_{max}.

## 3. Power Management with Mechanical Switch

#### 3.1. Travel Switch

#### 3.1.1. Series Switch

^{5}W/m

^{2}under 500 Ω, which is more than 1100 times of ordinary TENG (Figure 3c).

_{2}. The energy stored in the inductor is 3.14 μJ. Compared with the single cycle output energy under resistive load (3.33 μJ) (Figure 3f), the inductance loss energy is about 5.7%. The overall energy storage efficiency can eventually reach to 48% (Figure 3g).

^{2}(Impedance, 22 Ω to 120 Ω) (Figure 3j). The average power density is as high as 790 mWm

^{−2}Hz

^{−1}and does not change with the external impedance (22 Ω to 10 MΩ), as shown in Figure 3k. This output has set the highest record of TENG output.

#### 3.1.2. Parallel Switch

#### 3.1.3. Switch Capacitor Convertor

#### 3.2. Voltage-Triggered Switch

#### 3.2.1. Spark Switch

_{buf}) through the Bennet circuit. Secondly, when the voltage in C

_{buf}reaches the pull-down voltage of the MEMS switch or the breakdown voltage of the plasma, the hysteresis switch can connect the circuit through direct contact between the anode and cathode or the air breakdown discharge, and the energy is converted by the buck. Finally, the energy is stored in the energy storage capacitor C

_{store}, which is stabilized at DC 3.3 V by a commercial regulator chip.

_{in}through half-wave rectification. When the voltage of C

_{in}reaches the breakdown voltage of the air switch (about 7.5 kV), the switch discharges. The V-Q curve shows that the energy output by C

_{in}reaches the maximum of 1.42 mJ when the air gap is 2.4 mm (Figure 6i). The transformer can convert the impedance through electromagnetic conversion. (The efficiency of the transformer is 86.7% which can be calculated by the ratio of load consumption energy to the output energy from C

_{in}) (Figure 6j). In pulse mode, the load is one parallel resistor, and 11.13 kW/m

^{2}pulse power (1 Hz, 22 Ω) can be output through power management (Figure 6k). In constant mode, the load is a resistor parallel to a filter capacitor, and the average output power under 200 KΩ load reaches 1.102 mW. Compared with the matching resistance of 35 GΩ before management, 78.5% of the output power is retained (Figure 6l).

#### 3.2.2. Electrostatic Switch

## 4. Power Management with Electronic Switch

#### 4.1. Discrete Transistor Switch

#### 4.1.1. Silicon-Controlled Rectifier

_{in}through the rectifier bridge. In the second stage, when the voltage in C

_{in}reaches the reverse bias voltage threshold of the regulator D

_{5}, the current passing through can turn on the SCR, and the energy flows from C

_{in}into the inductor L, the output capacitor C

_{out}, and the resistor R. In the third stage, after all the energy in C

_{in}is transmitted to the back end, the SCR cuts off, and the inductive energy continues to transfer to C

_{out}and R. In the final stage, the energy in C

_{out}is continuously consumed by the resistor R.

_{in}and C

_{out}voltages are shown in Figure 8c,d. The voltage of C

_{in}decreases once per cycle, which means that the energy of C

_{in}is released to the back end once per cycle. The C

_{out}voltage presents a ripple shape, and the ripple value decreases with the increase of capacitance. Due to the appropriate turn-on time and very low energy loss of SCR, this switching circuit can reduce the output matching impedance of contact separation TENG from 150 MΩ to 2 MΩ, while maintaining 84.3% of AC peak power under 150 MΩ load, which is shown in Figure 8e.

_{1}, C

_{1}, and voltage comparator Amp is used to detect the voltage peak of TENG. The delay circuit that is composed of R

_{2}, C

_{2}, AND gate circuit and inverter circuit is used to accurately adjust the pulse duration of the switch control signal. This work adopts a design idea that is similar to that reported by Cheng et al. [75]. The difference is that the author not only verified the feasibility of using triode as a switch to manage the output power of TENG through simulation and experiment, but also studied the influence of resistance in both differential and delay circuit on power management performance in detail.

_{1}in the differential circuit, the closer the pulse voltage at point h to the voltage peak of TENG. However, the practical tests show that when the resistance increases to more than 600 kΩ, multiple pulses are generated (Figure 8h). Therefore, R

_{1}is set at 600 kΩ as the optimal value of the differential circuit. R

_{2}has an important influence on the ON time of the switch in the time-delay circuit. By comparing the energy optimization in the end energy storage capacitor C after charging for 10 s, the authors obtained the optimal value of R

_{2}(Figure 8i). Through the above optimization, the authors obtained a power management efficiency of 37.8%.

#### 4.1.2. MOSFET

_{IN}through rectification, and the back end of C

_{IN}is a standard buck conversion circuit. In particular, the MOSFET control signal is output by an electronic MPPT controller, which can detect the C

_{IN}voltage in real time and maintain V

_{IN}at half of the generator’s open-circuit voltage. The author follows the idea of tracking the maximum power point in piezoelectric and believes that when the output voltage is controlled to half of the open-circuit voltage, energy can be harvested with the highest efficiency. The voltage waveforms are shown in Figure 9l. The blue line shows that V

_{IN}oscillates around 100 V. Each pulse peak of Φ

_{N1}(pink line) corresponds to a turn-on of the MOSFET, and it also corresponds to a steep drop in V

_{IN}, which means energy is transferred once.

#### 4.2. Integrated Circuit

_{bp}, and a capacitor C

_{s}. When the energy in C

_{s}is not enough to power the control circuit, the K

_{bp}is turned on so that the current bypasses the flyback circuit and flows directly into C

_{s}. When the energy in C

_{s}can power the control circuit, the two switches K

_{p}and K

_{s}in the flyback circuit are closed at the voltage near the generator voltage peak to extract the generator voltage with maximum efficiency. The voltages of C

_{b}and C

_{s}in the start-up phase are shown in Figure 10b. The voltage waveform proves that the control circuit works intermittently in the start-up phase until the rising slopes of the two capacitor voltages are the same, and the start-up process is completed. The integrated circuit is manufactured in an AMS 350 nm complementary metal–oxide–semiconductor (CMOS) process. The off-chip components include a flyback circuit, a rectifier bridge, two buffer capacitors, derivative capacitors, and dMOS. The power consumption of the peak detection circuit is 150 nW@3 V.

_{in,P}and C

_{in,M}, respectively, by the dual-output rectifier. Through reasonable control circuit design, the above two capacitors are accurately controlled to release energy to the buck circuit when the maximum output power is reached. This method could extract TENG energy with the highest efficiency. Based on the maximum power point tracking analysis of TENG and the fractional open-circuit voltage method (FOCV), the authors experimentally obtained the ratio of the output voltage to the open-circuit voltage at the maximum average output power of TENG. The above-mentioned integrated circuit adopts the 180 nm Bipolar-CMOS-DMOS (BCD) process with an effective area of 2.482 mm

^{2}. The off-chip components include an inductor, five input capacitors, one output capacitor, and four resistors. The total power consumption is 754.6 nW. After connecting the TENG and the load, the author measured the accuracy and efficiency of the MPPT. The accuracy is higher than 96.39%, and the MPPT efficiency reaches the highest of 94.86% when the input is 17.13 μW. When the input power is 20.9 μW, the overall end-to-end efficiency is 52.9%. This work proves the feasibility and effectiveness of the maximum power point tracking method for triboelectric energy harvesting.

_{rect}reaches 70 V, the step-down conversion is started, the voltage of C

_{out}is stabilized at 10 V, and the load voltage is stabilized at 2 V. Similar to the idea of piezoelectric management, when the switch is closed at the time of zero-current the voltage can be reversed instantaneously, so as to avoid energy loss.

## 5. Conclusions and Prospects

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**Brief timeline of the switching power management methods for triboelectric nanogenerator (TENG). Reproduced with permission [59]. Copyright 2013, American Chemical Society. Reproduced with permission [60]. Copyright 2015, IOP science. Reproduced with permission [61]. Copyright 2015, Nature. Reproduced with permission [62]. Copyright 2015, Nature. Reproduced with permission [63]. Copyright 2017, Elsevier. Reproduced with permission [64]. Copyright 2020, Nature. Reproduced with permission [65]. Copyright 2020, Nature. Reproduced with permission [66]. Copyright 2020, Elsevier. Reproduced with permission [67]. Copyright 2021, Nature. Reproduced with permission [68]. Copyright 2021, Elsevier.

**Figure 2.**Fundamentals of switching power management for TENG. (

**a**) TENG connected with resistances load. (

**b**) ‘Three-region’ characteristics under resistive load. (

**c**) Dependence of the power on external load and velocity. (

**d**) Full-wave rectifier circuit. Reproduced with permission [33]. Copyright 2014, Elsevier. (

**e**) Saturation curve of capacitor voltage. (

**f**) Relationship between optimal matching capacitance and numbers of charging cycle [39]. (

**g**) Cycles for energy output under 100 MΩ. (

**h**) Four steps of cycles for maximized energy output (CMEO) under 100 MΩ. (

**i**) CMEO under infinite load. Reproduced with permission [61]. Copyright 2015, Nature.

**Figure 3.**Power management with series travel switch. (

**a**) Structure of the travel switch. (

**b**) Pulse current at the switch-on moment. (

**c**) Dependence of instantaneous power on resistor with/without travel switch. Reproduced with permission [59]. Copyright 2013, American Chemical Society. (

**d**) Structure and working process of the rectified travel switch. (

**e**) The energy storage circuit and energy transmission process. (

**f**) Dependence of the output energy on the resistance. (

**g**) The overall energy storage efficiency with/without travel switch. Reproduced with permission [69]. Copyright 2018, John Wiley & Sons. (

**h**) The structure, electrical connection and working process of the opposite-charge-enhanced transistor-like TENG (OCT-TENG). (

**i**) Electron cloud-potential well model. (

**j**) Pulsed power comparison. (

**k**) The dependence of average power and current density on resistance. Reproduced with permission [67]. Copyright 2021, Nature.

**Figure 4.**Power management with parallel travel switch. (

**a**) Electrical connection and closing position of the parallel travel switch. (

**b**) V-Q curve of the charging circuit with the switch. (

**c**) The dependence of energy per cycle-on-cycle number with/without management. Reproduced with permission [70]. Copyright 2016, Nature.

**Figure 5.**Switch capacitor convertor with travel switch. (

**a**) The switching process and charge transfer of the first switch capacitor convertor for TENG. (

**b**) Output energy on different load with/without power management. (

**c**) Output energy on load at different speed with/without power management. (

**d**) Electrical connection of transformed and non-transformed circuit. (

**e**) Transformed and non-transformed energy output [60]. (

**f**) Structure of the inductor-free triboelectric management method. (

**g**) Structure of the travel switch. (

**h**) V-Q curve of the managed energy output. (

**i**) Comparison of the charged voltage between rectifier circuit and the power management circuit. Reproduced with permission [71]. Copyright 2017, Elsevier. (

**j**) Fractal design. (

**k**) Comparison of output charge between short-circuit and fractal design based switched-capacitor-convertors (FSCC) circuit. (

**l**) Input V-Q curve and (

**m**) output V-Q curve. (

**n**) Electrical connection of constant mode and pulse mode with FSCC. (

**o**) Comparison of dependence of average power density on load between FSCC and standard circuit. Reproduced with permission [64]. Copyright 2020, Nature.

**Figure 6.**Power management with spark switch. (

**a**) Arc discharge. (

**b**) Corona discharge. (

**c**) The relationship between the discharge energy per cycle and the electrode spacing. Reproduced with permission [72]. Copyright 2018, Elsevier. (

**d**) The circuit architecture of the energy harvesting and conditioning. (

**e**) Fixed switch. (

**f**) Movable switch. (

**g**) Comparison of charging rates for different switch configurations. Reproduced with permission [65]. Copyright 2020, Nature. (

**h**) The circuit architecture of power management with spark switch. (

**i**) Comparison of output energy by C

_{in}under different air gaps. (

**j**) The dependence of output energy and transformer primary coil number. (

**k**) The dependence of the voltage and power density on load resistance in pulse mode. (

**l**) Comparison of average power between power management and standard circuit. Reproduced with permission [68]. Copyright 2021, Elsevier.

**Figure 7.**Power management with electrostatic switch. (

**a**) Motion of cantilever vibration switch under electrostatic attraction. (

**b**) Turntable TENG with the electrostatic switch. (

**c**) The dependence of output energy on rotation rate. Reproduced with permission [73]. Copyright 2018, Elsevier.

**Figure 8.**Power management with silicon-controlled rectifier (SCR) and triode. (

**a**) Power management circuit with SCR. (

**b**) Energy flow in four stages. (

**c**) The voltage waveforms of C

_{in}and C

_{out}. (

**d**) The dependence of C

_{in}and C

_{out}voltage on frequency. (

**e**) Average output power under different loads with/without power management. Reproduced with permission [66]. Copyright 2020, Elsevier. (

**f**) Power management circuit with triode. (

**g**) The dependence of V

_{b}and V

_{h}on different value of R

_{1}. (

**h**) The optimum value of R

_{1}. (

**i**) The optimum value of R

_{2}[74].

**Figure 9.**Power management with MOSFET switch. (

**a**) The two-stage energy releasing power management circuit. (

**b**) Relationship between average power and resistance before management. (

**c**) The optimum DC power is obtained at 180 kΩ load. Reproduced with permission [62]. Copyright 2015, Nature. (

**d**) Power management circuit combining TENG maximized energy output cycle (CMEO) and buck convertor. (

**e**) Basic configuration of the tribotronic energy extractor. (

**f**) Theoretical and measured V-Q curve. (

**g**) The output matching impedance of TENG before and after the power management. Reproduced with permission [63]. Copyright 2017, Elsevier. (

**h**) The circuit of power management method and working process of the switch. (

**i**) Basic circuit topology. (

**j**) Output impedance-power diagram under different PMMs and standard circuit. Reproduced with permission [76]. Copyright 2019, Elsevier. (

**k**) A triboelectric power management circuit based on maximum power point tracking (MPPT). (

**l**) Output waveforms of TENG open-circuit voltage, battery voltage, input capacitor voltage and switching control signal. Reproduced with permission [77]. Copyright 2021, Elsevier. (

**m**) Circuit topology of P-SSHI, S-SSHI and FWR. (

**n**) The dependence of energy per cycle on voltage of battery load [78].

**Figure 10.**Power management with integrated circuit. (

**a**) A self-starting power management integrated circuit for triboelectric energy harvesting. (

**b**) Voltage waveform of capacitor C

_{b}and C

_{s}[79]. (

**c**) A high-voltage dual-input integrated circuit converter for power management [80]. (

**d**) A triboelectric energy harvesting circuit based on synchronous inductor parallel switch and DC step-down conversion [81].

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## Share and Cite

**MDPI and ACS Style**

Zhou, H.; Liu, G.; Zeng, J.; Dai, Y.; Zhou, W.; Xiao, C.; Dang, T.; Yu, W.; Chen, Y.; Zhang, C.
Recent Progress of Switching Power Management for Triboelectric Nanogenerators. *Sensors* **2022**, *22*, 1668.
https://doi.org/10.3390/s22041668

**AMA Style**

Zhou H, Liu G, Zeng J, Dai Y, Zhou W, Xiao C, Dang T, Yu W, Chen Y, Zhang C.
Recent Progress of Switching Power Management for Triboelectric Nanogenerators. *Sensors*. 2022; 22(4):1668.
https://doi.org/10.3390/s22041668

**Chicago/Turabian Style**

Zhou, Han, Guoxu Liu, Jianhua Zeng, Yiming Dai, Weilin Zhou, Chongyong Xiao, Tianrui Dang, Wenbo Yu, Yuanfen Chen, and Chi Zhang.
2022. "Recent Progress of Switching Power Management for Triboelectric Nanogenerators" *Sensors* 22, no. 4: 1668.
https://doi.org/10.3390/s22041668