Bidirectional Thermoelectric Assisted Synchronous Charge Extraction Circuit Based on Buck Structure for Piezoelectric Energy Harvesting

Energy harvesting technology is currently a research hotspot, which can effectively solve energy supply problems in fields such as wireless sensor networks, wearable devices, and smart homes. An energy harvesting system using a thermoelectric generator (TEG) for energy assistance of a piezoelectric transducer (PZT) is proposed. It employs a synchronous charge extraction circuit based on a buck structure (BUCK-SECE), which uses thermoelectric energy to inject energy into the piezoelectric element in both directions to boost the initial voltage of the BUCK-SECE, increasing its output power. The experimental results demonstrate that the utilization of thermoelectric energy can increase the output power of BUCK-SECE by a factor of 3.3 under low-vibration conditions and by a factor of 0.8 under high-vibration conditions. Therefore, the proposed energy harvesting system in this article can effectively enhance the output power of piezoelectric energy transducers and yield desirable performance in real-world applications.


Introduction
The emergence of 5G technology has led to an accelerated growth of the Internet of Things (IoT).However, IoT is composed of a large number of wireless sensor nodes (WSN).The way to power these nodes is still mainly battery-powered.Although battery technology is advancing, the capacity of the battery is still limited and there are problems such as the need for regular replacement.Based on the new IoT technology standards released by the Third Generation Partnership Project (3GPP) organization, the battery-powered life of WSN nodes should exceed 10 years during standard operational conditions [1].
To address this challenge, a promising approach is to obtain energy from the environment.Energy harvesters have been explored as an alternative to conventional batteries and have shown great potential for practical applications.As a result, there is a growing interest in increasing the output power of energy harvesters, making it a hot research topic in the field.
Many interface circuits have been proposed for piezoelectric energy because of its high energy density in the environment.Although the standard energy harvesting circuit (SEH) was a breakthrough in interface circuit technology, its high conduction voltage often results in a significant loss of energy.This has prompted a need to pursue more efficient energy conversion methods to improve performance.Research is currently focused on identifying innovative solutions that address SEH's limitations and enable further advancements in the field of energy harvesting.Furthermore, the carrying capacity of this interface circuit is poor.To improve the efficiency of piezoelectric transducer (PZT), several circuits have been proposed in [2][3][4], including the parallel synchronized switch harvesting on inductor (P-SSHI) circuit, synchronous electric charge extraction (SECE) circuit, and series synchronized switch harvesting on inductor (S-SSHI) circuit.While these circuits can effectively increase the harvesting efficiency, additional circuits are required to control the switches.In [5], a self-powered synchronized switch harvesting on an inductor (SP-SSHI) circuit is proposed.Additionally, in [6], scholars presented a self-powered efficient SECE (SP-ESECE) circuit to address the self-powering issue.In [7], an energy injection method was proposed to enhance the conversion efficiency and output power of piezoelectric energy.Using injection energy from the energy storage capacitor is not an economically feasible solution, as it is not a sustainable way for providing power to the system.A more cost-effective and efficient alternative needs to be explored to improve the longterm functionality of the system.A buck-based synchronous charge extraction (BUCK-SECE) interface for the PZT was proposed in [8] and can be easily extended to multiple piezoelectric inputs.But there is still a limit to the output power of a single PZT.Wang et al. [9] used weak thermoelectric energy from the environment as an aid to improve the SECE extraction efficiency.However, only half of the SECE cycle is assisted and no energy boost is obtained over the whole cycle.
To solve these problems, an energy harvesting circuit with bidirectional thermoelectric energy assistance for BUCK-SECE circuits is proposed in this paper.It can increase the output power of PZT under different operating conditions of BUCK-SECE.

Proposed circuit
As shown in Figure 1, a structure for bidirectional energy assistance of BUCK-SECE is presented using thermoelectric energy.The thermoelectric equivalent model consists of a voltage source V T , R T , and C T , while the piezoelectric equivalent model consists of AC sources I P and C P .Two inductors L 1 and L 2 are used to transmit energy, C T is used to temporarily store thermal electric energy, C STO is used as a storage capacitor, and the switches are composed of MOS transistors and tertiary tubes.The control module comprises various functional components, including a peak detector (PKD), a zerocrossing detector (ZCD), a level shifter [10], a pulse generator, and a logic controller.The PKD and ZCD work together to detect and extract piezoelectric energy signals, which are then processed by the level shifter to ensure the correct operation of the piezoelectric charging switch.The thermoelectric energy is injected into the PZT after each half cycle of energy extraction by the BUCK-SECE so that the PZT generates more power and increases its conversion rate.The energy E H harvested by the BUCK-SECE in one cycle can be expressed as follows [9].V max is the maximum voltage on the C P and can be expressed as After the PZT obtains the thermoelectric energy, an initial voltage V 0 is obtained on C P , and the maximum voltage V max0 on C P can be expressed as At this point, the energy E H0 harvested by the BUCK-SECE in one cycle can be expressed as It can be seen that the larger the V 0 , the more energy is extracted in one cycle and the higher the output power is.
Figure 2 shows that the proposed bidirectional thermoelectric energy-assisted piezoelectric energy harvesting circuit consists of five operating loops.In the first stage, as shown in Figure 2(a), the BUCK-SECE positive half-cycle is used as an example, and at the end of the BUCK-SECE negative half-cycle, as the PZT deforms, it constantly produces electrical energy, which is then accumulated in the capacitor C p .Meanwhile, the thermoelectric energy is captured and stored in the storage capacitor C T .After the energy on C P reaches its peak, switch S 1 closes, forming the CLC loop shown in blue in Figure 2(b).The energy in C P is gradually released and transferred to L 2 and C STO , while, switch S LP closes, forming the CL resonant loop in Figure 2(c), transferring the thermoelectric energy stored in C T to inductor L 1 , which continues until the start of the thermoelectric energy assist stage.After the charge in CP is completely transferred, it enters the thermoelectric energy assist phase and the inductor freewheeling phase, where switch S 1 is disconnected and L 2 transfers the remaining energy to C STO through the freewheeling diode D 3 .Switches S P1 and S P2 close but S LP breaks, forming the loop in Figure 2(d), which injects the energy in inductor L 1 into the clamping capacitor C P of the PZT, boosting the initial energy of the negative half-cycle of the PZT.The negative half-cycle works in the same way as the positive half-cycle, which is not discussed here.

Simulation results and Analysis
LT-spice was used to simulate the operation of this work.Table 1 presents the primary simulation parameters used in the study.In regards to the thermoelectric generator (TEG), specifically model SP1848, it is noted that the device generates a voltage of 0.25 V when operating under a temperature differential of 5℃.In the simulation, a 10 MΩ resistor was connected in parallel to the model of the PZT.In this paper, the signals generated by PKD and ZCD are refined to generate signals for controlling the thermoelectric energy management switches (SLP, SLN, SP1, SP2, SN1, SN2) to achieve bidirectional energy injection to BUCK-SECE.
Table 1.Parameters of main components Symbol Value Figure 3 shows representative current and analog voltage waveforms of the proposed circuit operating at a thermoelectric voltage of 0.2 V.As shown in Figure 3, when V P reaches its peak, switch S LP will close and the thermoelectric storage capacitor C T charges the inductor L 1 , gradually transferring the charge in C T to L 1 , and the voltage of C T gradually decreases, while the current of inductor L 1 gradually rises as shown in Figure 3  Figure 4(a) shows the simulation results of the output power of the harvester at different amplitude conditions by varying IP for an output voltage of 2 V.A 10 μF storage capacitor is used for the simulation.Figure 4(a) shows that the output power of this paper under weak vibration conditions is significantly higher than that of the conventional BUCK-SECE under the same amplitude conditions.For example, at an IP of 70 μA, the output power is 4.3 times that of a conventional BUCK-SECE when using 200 mV thermoelectric energy assistance.And it also reaches 1.8 times under strongly vibrating conditions.From the figure, it can be seen that as the output voltage becomes larger, the output power first becomes larger with the output voltage.Once a particular threshold is surpassed, the output power experiences a decline.This can be attributed to the use of a BUCK-SECE design in the study, whereby the BUCK circuit is only capable of reducing voltage.When the output voltage surpasses a certain level, the circuit power is reduced accordingly [9].However, it can be seen that the output power of the BUCK-SECE is higher than that of the conventional BUCK-SECE after obtaining thermoelectric assistance.

Conclusion
Vibrational energy is often abundant in the environment.But the thermoelectric energy is weak.In this paper, thermoelectric energy is used as an auxiliary energy to inject bi-directional energy into the PZT to increase the output power of vibration energy.Under the same vibration conditions, the work improves the output power by 330% under weak vibration conditions and 80% under strong vibration conditions compared to a conventional energy harvester.

Figure 2 .
Figure 2. Operating switches of the proposed circuit.a) Natural energy storage b) PZT energy extraction and inductor freewheeling c) Thermoelectric energy extraction d) Thermoelectric energy assist.
Figure 3 shows representative current and analog voltage waveforms of the proposed circuit operating at a thermoelectric voltage of 0.2 V.As shown in Figure 3, when V P reaches its peak, switch S LP will close and the thermoelectric storage capacitor C T charges the inductor L 1 , gradually transferring the charge in C T to L 1 , and the voltage of C T gradually decreases, while the current of inductor L 1 gradually rises as shown in Figure 3(d).At the same time, S 1 closes, transferring the energy in C P to L 2 , and Figure 3(b) shows the rapid drop of C P open circuit voltage V OC .After L 2 completely transfers the charge to C STO , S LP breaks, S P1 and S P2 close, inductor L 1 injects energy into PZT, and the voltage V N of the negative half-cycle of the piezo rises rapidly, achieving the function of boosting the initial voltage.

Figure 3 .
Figure 3. Simulation waveforms.a) Voltage of capacitive V CT b) Open circuit voltage V OC of PZT c) Current of inductor L 1 d) Current of inductor L 1 and voltage of capacitor C T during energy injection.

Figure 4 .
Figure 4. Relationships between Output Power and PZT Current (a) & Output Voltage (b).

Figure 4 (
Figure 4(b) illustrates the output power at different output voltage V DC .From the figure, it can be seen that as the output voltage becomes larger, the output power first becomes larger with the output voltage.Once a particular threshold is surpassed, the output power experiences a decline.This can be attributed to the use of a BUCK-SECE design in the study, whereby the BUCK circuit is only capable of reducing voltage.When the output voltage surpasses a certain level, the circuit power is reduced accordingly[9].However, it can be seen that the output power of the BUCK-SECE is higher than that of the conventional BUCK-SECE after obtaining thermoelectric assistance.