Current-Fed Bidirectional DC-DC Converter Topology for Wireless Charging System Electrical Vehicle Applications

Department of Electrical and Electronics Engineering, Faculty of Engineering and Architecture, Nisantasi University, Istanbul, Turkey Department of Electrical Engineering, Bajaj Institute of Technology, Wardha, Maharashtra, India Department of Electronics and Instrumentation Engineering, Karpagam College of Engineering, Coimbatore, India Department of Electronics and Communication Engineering, Kalasalingam Academy of Research and Education, Srivilliputhur, India Department of Electronics and Communication Engineering, Sri Eshwar College of Engineering, Coimbatore, India Department of Electrical and Electronics Technology, Ethiopian Technical University, Addis Ababa, Ethiopia


Introduction
The revolution in the development of electric vehicles has brought many modifications in charging strategies. Initially, the plug-in electric vehicle battery charging strategy was popular for two, three decades. However, users may get shocked during adverse climatic conditions such as rain and snowfall. The cost of the charging cable also adds more manufacturing cost to the plug-in electric vehicle. These limitations of plug-in vehicles motivated researchers to concentrate on the wireless charging system. Wireless power transfer (WPT) is aimed at making electrical power transfer simpler, reducing complexity, and making it flexible for users and less costly.
Battery charging for electric vehicles (EVs) [1][2][3][4][5][6][7][8][9][10], mobile devices, biomedical implants [11], and lighting applications [6] have been the subject of a recent WPT study. Revolution in WPT technique started with the invention of the microwave power transfer (MPT) technique. In this technique, the user had the capability to transfer power for a wider range [12]. MPT used rectennas that were connected at the sending and receiving end of the power transfer system. The rectenna facilitates conversion of microwave signal to electric power and vice versa. This power transfer technique was very famous among users as it was able to transfer a higher range of power for a larger range of air gap [13]. However, MPT uses a highfrequency power transfer system, which is not safe for living beings. These power transferring paths must be sealed with protective covers. This arrangement increases the system cost and reduces the reliability of the WPT system [14].
Further research in WPT area motivated researchers to develop the inductive power transfer (IPT) technique [15][16][17]. This technique employs a lower range frequency compared to MPT technique for transferring power wirelessly. This WPT technique has been used in a range of applications over short distances. On the other hand, the performance begins to plummet after a certain distance between the coupling coils. With the increase in distance between the power transferring points and the increase in the misalignment between coils, the power transfer efficiency reduces drastically due to the increase in leakage inductance between the coupling coils [18,19].
In order to compensate for the leakage inductance in IPT technique, there are resonating capacitors connected to both sides of the WPT coupling coils.
These resonating capacitors compensate the leakage inductance between the power transfer coils. A basic block diagram of IPT technique is depicted in Figure 1. Generally, IPT technique operates in a frequency range of few kilohertz to 300 MHz to balance converter size, performance, and manufacturing cost of the system [6].
As per the connection of resonating capacitor at both sides of the WPT coils, the WPT technique is divided into two compensation techniques. These techniques are series compensation and parallel compensation technique. WPT using parallel compensation technique is discussed in [8][9][10]. Basically, current source inverters (CSI) are used for parallel compensation of WPT technique. The parallel compensation technique provides various advantages such as it provides necessary reactive power to the coil; the parallel capacitors show lower impedance to higher-order harmonics. This feature reduces the voltage and current across the power transferring terminals of WPT coils. The parallel compensation circuit employs a series inductor connected with the WPT coils [20][21][22][23] This inductor reduces the fault current during inverter fault conditions in CSI. This is a very significant advantage of the WPT technique employing parallel compensation capacitors.
Conversely, this series inductor is of big size, which limits the application of this technique to the area where the source produces a stiff rise in current, such as solar power generation system. The parallel LC tank used in WPT technique increases the impedance of the circuit, which can be solved by connecting a series capacitor in series with the compensating circuit. This series capacitor supplies a fraction of reactive power, which is needed by the coupling network [24,25].
The literature review found that using the CCL compensation technique in the primary side coil and LC compensation technique in the secondary coil increases the reliability of the WPT system. This technique is adopted in this proposed system. This paper proposes a soft-switching nonisolated duplex CCL compensation resonant circuit. Circuit diagram of the proposed system is shown in Figure 2. It is a front-end half-bridge boost convertor followed by an associate in CCL compensation resonant circuit and voltage electronic device at high-voltage facet. The planned convertor has the subsequent merits: (1) zero voltage switch (ZVS) stimulation of all switches in each directions, (2) zero current switching (ZCS) stimulation and turn-off of all diodes in each directions, (3) voltage level decreases across all the components used to fabricate the system, (4) use of extra snubber circuit is avoided in this system, (5) high improvement and step down ratio, and (6) reduced volume of geophysical science.
Contributions of this work are as follows: (i) Soft-switching nonisolated duplex CCL compensation resonant circuit is proposed in this work (ii) The proposed converter uses half-bridge boost converter in the front-end of the network, and CCL resonant circuit is used in this system in order to maximise the power output

Working of the Proposed Bidirectional DC/ DC Converter
The CCL compensation resonant circuit enhances the voltage magnitude at the output of the converter and provides ZVS and ZCS for front-end devices and diodes, respectively. Half-bridge converter connected at the primary side of the compensating network increases the voltage across its output terminals. The CCL compensation technique adopted at the primary coil further increases the voltage level based on the resonant and switching frequency ratio. This voltage at the output of the compensation network is doubled by employing a voltage doubler circuit at the output of the compensation network. This is how the circuit behaves during the boost operation of the system. Circuit behavior during buck operation is provided in the following paragraph. While the user wants to reduce the converter's output voltage, the capacitors of the half-bridge converter are used to divide the input voltage to equal magnitude.  Mode 1 [t 0 < t < t 1 ]: The converter is operating as a boost converter. Switch M 2 is conducting, and inductor L is storing energy. Working of the converter during this mode is depicted in Figure 4(a). Switch M 1 and diodes D 1 and D 3 remain OFF during this mode. Load is fed by output capacitors, C 7 and C 8 .
Mode 2 [t 1 < t < t 2 ]: During this mode, t = t 2 ; switch M 2 is in OFF state. Now, both the switches M 1 and M 2 are OFF. Inductor current i L and resonant current i L r 1 jointly start discharging and charging the parasitic device capacitances C 1 and C 2 , respectively. Working of the converter during this mode is depicted in Figure 4(b). Before the next interval, C 1 discharges the stored charge in it and C 2 is fully charged. This is quick, and the duration is concise. At the end of this Switch M 2 voltage is given by the following: where D = T ON /T s , T ON is the main switch conduction period, and T s is the switching period.
Voltage across diode D 3 is given by the following: Mode 3 ½t 2 < t < t 3 ]: Now, the body diode D 1 starts conducting by i L − i L r 1 causing zero voltage across M 1 . Diode D 4 is forward biased, and current starts flowing through resonant inductor L r 2 and capacitor C P starts charging output capacitor C 8 . Working of the converter during this mode is depicted in Figure 4 Mode 4 [t 3 < t < t 4 ]: At t = t 3 , M 1 starts conducting with ZVS. The equivalent resonant circuit is shown in Figure 5(a). Antiparallel body diode D 4 conducts to charge capacitor C 8 , 3 Wireless Communications and Mobile Computing while diode D 3 is reverse biased. Before the next interval, the diode D 4 turns off with ZCS as the resonant inductor current i L r 2 discontinues to zero. Working of the converter during this mode is shown in Figure 4(d). The equations for this interval are as follows: where Voltage across antiparallel diode D 3 is follows: where Z r is known as characteristic impedance offered by the circuit formed by resonant tank L r 1 , C p , and capacitor C 5 as shown in Figure 5(a). Mode 5 [t 4 < t < t 5 ]: Current continues to flow through switch M 1 , and body diodes do not conduct. Load is fed by energy stored in output capacitor C 7 and C 8 . At t = t 4 , switch M 1 stops conducting. At the end of this mode, v D 3 Working of the converter during this mode is depicted in Figure 4(e).
Mode 6 [t 5 < t < t 6 ]: Parasitic capacitance C 1 is charged, and capacitance C 2 is discharged by i L r 1 − i L . High-side body diodes are reverse biased. Before next interval, capacitance C 2 is discharged completely, and C 1 is charged to V H /1 − D. Working of the converter during this mode is depicted in Figure 4(f). Mode 7 [t 6 < t < t 7 ]: In this interval, diode D 2 starts conducting through a difference of (i L r − i L ) and M 2 can now use for ZVS turn-on. Working of the converter during this  LCL resonance circuit  LCL resonance circuit Mode 8 [t 7 < t < t 8 ]: At t = t 7 , switch M 2 conducts with ZVS. Resonant inductor L r , capacitor C P , and C 4 resonate together as shown in Figure 5(b). Working of the converter during this mode is depicted in Figure 4(h). Before the next interval t = t 8 , body diode D 3 turns off with ZCS. The current through L r is given by the following: for C 5 = C 6 .  LCL resonance circuit  Wireless Communications and Mobile Computing Here, characteristic impedance Z r is offered by the circuit formed by L r 1 , C p , and capacitor C 6 as shown in Figure 5(b). i L is given by the following: i M 2 is given by the following:    Figures 6 and 7, respectively. M 1 and M 2 are not triggered for the entire buck operation.
Interval 1 [t 0 < t < t 1 ]: It is identical to buck or voltagefed operation. Current through switch M 4 flows and power flows to low voltage side through the resonant circuit and body diode D 2 . Resonant current flows in the circuit.
Interval 2 [t 1 < t < t 2 ]: At t = t 1 , switch M 4 is turned off. Parasitic capacitances C 3 and C 4 start charging and discharging, respectively, by resonant current i L r 2 . At the end of this interval, C 3 and C 4 are fully discharged and charged (to V H ), respectively. This is a quick and short interval. Current i L r 1 is given by the following: where C 5 = C 6 ; i L is given by the following: i D 2 is given by the following:  is given by the following: where current i D 3 is given by the following: Interval 6 [t 5 < t < t 6 ]: Antiparallel diode D 4 starts conducting, and M 4 is used for ZVS turn-on. Before next inter-val, antiparallel diode D 2 turns off with ZCS. i L r 2 is given by the following: where current through antiparallel diode D 3 is given by the following: Final values are Interval 7 [t 6 < t < t 7 ]: At t = t 6 , switch M 4 turns on with ZVS. Therefore, i L r 2 flows through switch M 4 . Diode D 1 is forward biased and starts charging capacitor C 5 . At t = t 7 , diode D 1 turns off with ZCS. Then, the next half cycle starts with the same operation cycle in order to complete the other half cycle. gain = 2 × . The overall converter gain is the multiplication of the gains offered by the individual circuits and is given by the following:

Voltage Gain
where D is the duty cycle, f s is the switching frequency, and R ac is effective ac load resistance and is given by R ac = X C p , X L r 1 , X L r 2 , X C 6 which are reactances of C p , L r 1 , L r 2 , and C 6 , respectively. It is straightforward to derive using standard complex ac analysis.
(2) Buck mode: In the buck mode, V H /2 is applied across the coupling circuit due to the half-bridge inverter capacitors. The step down ratio is given by the following: 3.2. ZVS Conditions. Power stored in inductor L r 1 at t = t 1 must be bigger than the power stored in system capacitance of switches M 1 and M 2 to facilitate ZVS of M 1 .
The variance between the power stored in the inductor L r 1 and the power stored in the inductor L must be adequate to charge and discharge system capacitances C 1 and C 2 as shown as follows: In order to adjust the motor speed to the required set point, a controller is used. An error signal is produced to adjust the speed generated by the difference of real motor speed with the reference speed, which facilitates the closed loop control of the motor. The error signal's magnitude and polarity are directly proportional to the real and

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Wireless Communications and Mobile Computing

Simulation Analysis of the Proposed System
Simulation analysis of the proposed system is described in this section. MATLAB software is used in order to carry the simulation of the system. The components used in order to carry out the simulation are listed in Table 1. The simulation model of the proposed system is depicted in Figure 8. Voltage and current waveform at the output terminal of the inverter is depicted in Figure 9.
Waveform of the current passing through the resonant inverter is shown in Figure 10. Voltage across the transmitter coil and receiver coils used in order to transfer power wirelessly is depicted in Figures 11 and 12, respectively.
Simulation model of the proposed system in buck mode is depicted in Figure 12. Output and input voltage waveform of the converter in boost mode is shown in Figure 13.
In boost mode, induction motor drive is used as the load in the proposed system. Simulation model of the system in boost mode is shown in Figure 14. Waveforms of the load current and load voltage during this mode are depicted in Figures 15 and 16, respectively.
The output power of the system is varied and it is compared to the efficiency of the overall system. The comparison is plotted as shown in Figure 17. It is observed that, in this proposed system, when the output power of 400 W is achieved, the efficiency of the system corresponds to 89%.

Future Extension of the Proposed System
(i) The converter can be redesigned in order to reduce power conversion stages (ii) Capacitive power transfer technique can be followed in the improved system in order to reduce manufacturing loss and complexity of the system (iii) New wireless power techniques can be developed in order to increase the power transfer distance and efficiency of the system (iv) New controller and cooling techniques can be developed in order to reduce overall system losses

Conclusion
This paper explains the design and analysis of a nonisolated bidirectional soft-switching current-fed resonant DC/DC converter. The converter is able to achieve high step up/step down ratio, high efficiency, and the low voltage across the components used to manufacture the converter. ZCS and ZVS turn-on of the system is achieved, which is a great achievement of the proposed converter compared to the existing topologies. The output voltage of the converter is clamped without the use of any snubber circuit. The proposed system is linked to an induction motor drive, and the drive's output is controlled. The simulation analysis of the proposed converter resembles the theoretical explanation of the system proving the efficacy of the WPT system.

AC:
Alternating current DC: Direct current WPT: Wireless power transfer MOSFET: Metal oxide semiconductor field effect transistor MPT: Microwave power transfer IPT: Inductive power transfer ICPT: Inductively coupled power transfer EV: Electric vehicle ZVS: Zero voltage switching ZCS: Zero current switching.

Data Availability
The available data will be distributed to readers based on the request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.