Analysis and design of class E-LCCL compensation circuit topology circuit topology for capacitive power transfer system

Khairul Kamarudin Hasan, Shakir Saat, Yusmarnita Yusop, Masmaria Abdul Majid, Mohd Sufian Ramli Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), Cawangan Johor, Kampus Pasir Gudang, Johor, Malaysia Faculty of Electronic and Computer Engineering, Universiti Teknikal Malaysia, Melaka, Malaysia School of Business and Social Sciences, Albukhary International University, Kedah Darul Aman, Malaysia


INTRODUCTION
In this globalisation era, due to the rising popularity of wireless power transfer (WPT), the power cells of electric vehicles and mobile devices can be recharged in the absence of a charging cable or wire. The WPT shown in Figure 1 has lately received recognition as a substitution charging technique. Thus far, there exists five variants of WPT methods with non-radiation power transmission that are suitable for far-field and near-field applications: CPT, inductive power transfer (IPT), microwave power transfer (MTP), optical power transfer (OPT), and acoustic energy transfer (AET) [1]- [13]. Taking into account the weaknesses of AET's sensitivity to frequency variation and IPT's sensitivity to metal barriers [14], [15], CPT was selected in this study. CPT has the added benefits of less electromagnetic interference (EMI) and lower power loss [16], [17]. Fundamentally, CPT transmits power in a near-field application without using cables or wires but utilises an electric field instead [18]- [22]. Although IPT is a rapidly developing WPT method, it is not feasible for low power level digital devices, especially when involving compact physical spots.
Both CPT and IPT have equally excellent galvanic isolation. However, CPT is superior to IPT for high-frequency applications, as it is more cost-effective by not requiring an expensive high-frequency magnetic core. Apart from that, CPT suitability for vehicle application was further reaffirmed when the electric field emissions were significantly decreased after using a 6-plate coupler to replace the coupling capacitive between every two plates [23]. In another study that used applied impedance matching to investigate the best operating frequency for the lowest zero voltage switching (ZVS) error, CPT was able to produce 9.5 W of output power at 95.44% efficiency with a printed circuit board plate with 2.82 nF capacitance operating at 1 MHz [24]. There are four main sections in this paper. Section II presents information about the CPT system. The analysis and design of the Class-E LCCL CPT System are presented in Section III, as well as the design specifications of the system and the experimental and simulation results, a summary of the findings is presented in Section IV.

CAPACITIVE POWER TRANSFER
The CPT system utilises the electric field to transmit energy across two metal plates that behave like a capacitor. The CPT system's capacitor is linked in series to the source. Therefore, the capacitor will have an induced electric field that enables current flow to the next capacitor's section. As depicted in Figure 2, there are two general versions of the CPT structure: bipolar structure and unipolar structure. The unipolar CPT design provides power via two electric field coupling in the absence of a physical return path for the current. The directness of the unipolar CPT structure is beneficial. A study recently showed that single-wire CPT system has remarkable robustness against huge misalignment [25]. Despite being the most direct and fundamental method to create the CPT system, this design is left habitually unnoticed due to its restricted power level and grounding concerns regarding the returning conductive path. Conversely, the bipolar structure appears to be the most widespread method to create the CPT system, where 4 units of metal plates are assembled in a parallel manner to transmit power. Studies in the past confirmed the usefulness of the bipolar design for CPT; thus, it is viable to create commercial products for both high-power and low-power applications [26], [27]. Therefore, the operating principles of the bipolar structure were utilised in this work. With the electric field coupling, the CPT method is able to overcome the limitation of the IPT method, which is the inability to transfer magnetic energy in a metallic medium. In addition, the CPT allows 1267 transmission through metal setting, low energy loss, and excellent anti-interference in the magnetic field. The robust anti-interference trait allows the system to operate in a strong and concentrated magnetic environment; therefore, reducing energy loss [29]. Power transfer between metal obstacles is not possible in IPT since it always seeks the lowest reluctant path in the magnetic environment. Power transfer is disabled once a metal plate is placed in the middle of the power transfer system's primary and secondary coil. This is caused by eddy currents generated in the metal plates, as the magnetic field induces pole-to-pole completion along with the metal plates. This behaviour causes the IPT system to have unusably weak efficiency.

CPT CLASS-E LCCL CPT SYSTEM
This subsection explains the analysis and specifications of every component involved in the construction of LCCL impedance matching and Class-E resonant inverter. The Class-E MOSFET converter design focused on the switching function where the transistor is either fully ON or fully OFF. A minor overlapping in current waveforms and switch voltage will cause power loss as a result of equivalent series resistance (ESR). Losses were ignored in this work. The two main characteristics of a Class-E amplifier are: shunt capacitor installed between the switch, and a net load inductance in series to provide the essential phase shift that facilitates the fundamental wave to work as a harmonic open circuit.
The recommended Class-E LCCL CPT was observed to help to enhance the effectiveness of the overall CPT system by preserving the circumstances of ZVS to be less susceptible to broader load variants. The duty ratio or frequency can be utilised to regulate the output power, but a constant load is needed. Nevertheless, the soft switching sensitivity towards the duty ratio or frequency restricts its application, which emphasises the requirement to investigate other power electronics circuit topologies which can enhance the output power of CPT. Since the rise of output power transfer needs a greater voltage across the metal plates, it was recommended to improve the plate structure in this work while not altering the plate electric field emission or voltage. However, the system was unable to maintain high efficiency while increasing the distance between the capacitive plate. As portrayed in Figure 3, this work recommended compensation circuits for the two sides of the coupling capacitor to reduce the input impedance and magnify the output current to maximise the CPT system's power transfer limit. Essentially, the impedance matching network changes the load impedance or resistance to the required impedance to obtain a particular output power at a specific operating frequency and voltage. Figure 4 shows the circuit conversion impedance matching for Class-E LCCL CPT system. Based on Figure 4 (b), the reactance factor of the series circuit Z A -C P (at the right of point D) is described as shown in (1).
The parallel circuit Z A -C A in Figure 4 (a) can be acquired by modifying the series circuit Z A -C P according to shown in (2), (3), and (4).
With shown in (5), the sum of capacitance for C A and C 2 at location C can be found.
The parallel impedance X ZA -X c can be transformed into the series impedance R L -C 1 in Figure 4 by using as shown in (6), (7), and (8).
As shown in (8) In contrast, the parallel impedance Z A is conveyed in (1)1, while shown in (2) defines the impedance reactance factor at point D's right side: = √ − 1 By performing the calculations above, the reactance of capacitor C P can be conveyed as shown in (14): Next, the design equation for the reactance X C2 is described as shown in (18) In addition, the LC matching network of the series-parallel C m -L m transformation theorem at location E with reactance factor is defined in (19).
As shown in (20) specifies the boosting factor (m) while as shown (21) characterises the conversion-boosting factor and resistance factor by the parallel, and the series factor of the subnetwork: The series reactance and L m are defined in (3) Lastly, the parallel reactance can be formed using shown in (24), and (25) specifies C m : This subsection further examines the integration of the Class-E LCCL matching network and CPT system for low-power applications utilising the single plate methodology, as shown in Figure 5. Initially, the output current, output voltage, and overall efficiency of the simulated Class-E LCCL and Class-E LC compensation designs were contrasted. Next, the study moved on to the design and construction of a 10 W Class-E LCCL CPT prototype to establish an efficient WPT through the load.
With the assumption that every condition of Class-E is greeted with a perfect switch, DC power provided by bias DC source was equivalent to the power dissipated in the load resistor. In theory, the system would achieve 100% efficiency. MATLAB simulation was done to corroborate the results of ZVS, input and output power. The capacitive coupling results at 1mm distance are tabulated in Table 1. Figures 6 and 7 illustrates the LCCL CPT system simulation circuit and experimental setup, respectively. The closest components that match the simulation were used in the experimental work to ensure realistic results, especially when deciding the ZVS, input power, output power, and system efficiency. Results from simulation and experimental work are explained while considering a 1 mm gap across the   In the experimental work as illustrated in Figure 9, the results of ZVS were satisfactory with V DS = 98 V. Both experimental and simulation results documented V DS value above V cc value by three to four times; indicating an outstanding value of V DS switching in theory. While reviewing the efficiency of CPT LCCL system, both input and output power simulation results are as depicted in Figure 10 and Figure 11, while the experimental results are portrayed in Figure 12 and Figure 13. The comparison between simulation and experimental work is tabulated in Table 2. Figures 10 to 13 present the power input and output outcomes; they were used to calculate the system efficiency using as shown in (26).  Table 2 shows the efficiency of the LCCL CPT system during simulation and experiment work which are 97.96% and 96.68%, respectively. Good performance was documented for power transfer to the load at 1 mm gap across capacitance plate couplings; 10 W at 50 Ω load.

CONCLUSION
Based on the investigation outcome, power can be transferred wirelessly through the small air gap between capacitors. This work implemented a Class-E MOSFET converter in its CPT design. Consequently, it is anticipated that WPT over a marginally lengthier gap can be improved using a number of compensation circuit designs to resonate with the capacitive coupler to generate a higher voltage, as explained in the impedance matching theory. Class-E LCCL CPT was implemented to increase the power transfer rate for a small load with low capacitive coupler variant. The experiment outcomes suggested that 9.34 W output power was achieved with 96.68% efficiency utilising the 609 pF capacitive coupling plates at 1 mm working distance. The results documented from the present investigation agrees with the findings recorded by previous literature of CPT research area. Based on the literature, high efficiency is only possible if the operating distance of the capacitive coupler is very near, usually smaller than 1 mm. Nevertheless, the performance of the entire CPT system was influenced by the power loss occurring in the rectifier and transmitter unit as well as the changes in the distance of capacitive coupling.