Design and Implementation of Z Source Resonant Converter for EV Wire Less Charging Application

Wireless Power Transfer (WPT) technology is an emerging research area due to its safety and convenience. A Conventional WPT system has Front End PFC and DC-DC Boost Converter which makes the system bulkier. The Z Source Inverter (ZSI) was introduced into WPT systems to improve the system performance. The ZSI regulates the input voltage in WPT systems without Front End Converters and makes the inverter bridge immune to Shoot Through (ST) states. The results are through simulations and Finally, the designed system is implemented experimentally


I. INTRODUCTION
WPT technology deliver power through an electromagnetic field without any physical connection between the transmitter and receiver [1]- [3]. Recent advancements in this field have led to more stringent design requirements being proposed and studied by researchers, such as efficiency improvement [4]- [8], coupling variation [9], [10], foreign object detection [11], [12], and output regulation [13]- [15]. Electronic technology plays a crucial role in these research studies and spurs WPT technology development. The voltage source inverter (VSI) is an essential part of a WPT system that generates highfrequency ac power for transmission across a wireless media. Unfortunately, the output voltage of conventional VSI is always is equal to or lower than the input voltage, which limits this inverter's application in small-voltage or wide input situations. To address this barrier, front-end converters, such as boost converter or buck-boost converter are inserted between a dc source and a VSI to boost the dc-rail voltage [16], [17]. However, this require more space and increase the cost of the system. To add one additional IGBT/MOSFET one extra heat sink and associated drive circuitry is to be accommodated. Considering the incremental cost and design complications, the ZSI presents a better alternative to frontend converters in WPT system. When compared with a conventional VSI, the ZSI has an input diode DS and a Z-source network added between the dc voltage source and the VSI [18]. The Z-source network consists of two identical inductors (L1 and L2) and capacitors (C1 and C2) to boost the output voltage by shorting one or two legs of the rear-end inverter bridge. This is also referred to as the shoot-through (ST) state. Guidelines to design Z-source network based on steady state parameters were presented in [19]. Since the network is connected to a three-phase VSI, the output current of the network is regarded as a constant current. Meanwhile, the current in the network of WPT system is approximately sinusoidal over part of one switching period due to a sinusoidal resonant current. The mathematical analysis in [19] is modified when applied to a WPT system. In [20], the benefits of a ZSI in resonant converters are analyzed. These benefits include improved robustness and reliability, buck/boost function, and high efficiency over wide input and load ranges. The ZSN in the proposed ZSRI provides the unique feature of inherent power factor correction (PFC) without adding extra switching devices. It is possible because it adds the unique features of immunity to the H-bridge inverter during shootthrough states. This characteristic makes the input current as a sinusoidal waveform and in phase with the ac input voltage. This variable also provides a boost factor to the system. However, to regulate the output voltage, the proposed ZSNbased inverter uses the active state duty cycle, which is a common control variable used in series resonant inverters. Both control variables are used in the series resonant Hbridge inverter and the ZSRI does not require additional control circuitry to provide power factor correction. In other words, because of the ZSN, the ZSRI can perform power factor correction as well as dc/ac conversion in single stage.

II. OPERATION OF Z SOURCE RESONANT CONVERTER
The ZSRC has more states in one switching cycle compared with other DC-AC systems. It is important to clarify all these states to understand the ZSRC. The boost ratio of ZSN is still related to the total shoot-through state duty cycle among these states. The operation principle of the ZSRC is described based here with an example of the phase-shift control method. Assuming that the ZSN is symmetrical (C 1 = C 2 = C, and L 1 = L 2 = L) therefore, V C1 = V C2 = VC, and V C1 = V C2 = V C . Also, the resonant frequency of L and C in ZSN is at least ten times smaller than the switching frequency. Hence, the ZSN inductor current and the ZSN capacitor voltage are considered to be constant in one switching cycle. Fig. 2 shows the conducting devices in different states-active state, shoot through state, and zero state.

Simulation Parameters
Simulation parameters are shown in table 1.

TABLE 1
The input voltage of the ZSRC is Vs=33V and the output voltage and current is Vo=88v and Io=2.28A with the resonant frequency of 20kHZ. Assuming that the ZSN is symmetrical (C 1 = C 2 = C, and L 1 = L 2 = L) therefore, V C1 = V C2 = V C , and V L1 = V L2 = V L . Also, the resonant frequency of L and C in ZSN is at least ten times smaller than the switching frequency. Hence, the ZSN inductor current and the ZSN capacitor voltage are constant in one switching cycle. The significant part of the design is choosing the inductor and capacitor values and operating frequency. On the contrary, use of the low frequency leads to increases both on the size and also the cost of inductors and capacitors. Thus, there is a trade-off between the size and efficiency in determining the operating frequency of the converter. The frequency is selected as 20kHz.

Simulation Result
The complete model of the developed WPT is shown in Fig.4   Even if there is any fluctuations in the input the output will be maintained constant by the program stored in the microcontroller. Microcontroller sends the corresponding signals which will be too feable to drive the MOSFET switches, so a driver circuit consisting of TLP250 Driver are used which boosts the amplitude of the signals enough to drive the switches. A new circuit topology for high power WPT applications using a full bridge Z-source resonant inverter has been developed and simulated. The control methods in the system with the insertion of shoot-through modes of the Z-source inverter have been investigated. In simulation, ideal MOSFETs and diodes were used. The output voltage of 27 V and output current of 0.9 A, i.e output of 23.5 W is obtained for the coupling coefficient k of 0.2. In the experiment, an output current of 0.9 A, and output voltage of 16 V was obtained. The slight difference between the simulation and experimental results is due to the following reasons: Core losses, simulation-ideal diode and MOSFET switches were used. Nevertheless, in both the simulation and experiment, the results obtained are very close to the analytical design value. This, in turn, validates the designed parameters.