## 3 Control of the WPT System
which leads to the following:  
Figure 3:0  
As shown in (27), the LCC-LCC-compensated WPT system has a constant optimized voltage ratio for the optimal efficiency of the analog path at the design frequency. It is worth mentioning that the control variables need to be adjusted correspondingly during charging to satisfy the optimal voltage ratio \(G_{\text{21,opt}}\) to achieve higher AC efficiency. Especially during the CC charging stage, due to the variation in battery voltage \(U_{\text{bat}}\) (or the secondary DC voltage \(U_{\text{2 DC}}\)), the duty cycles of both sides should be matched constantly to follow the optimal voltage ratio, as well as to provide the required charging power. Figure 5 shows the curves of both duty cycles \(D_{1}\) and \(D_{2}\) using the proposed optimal voltage ratio to optimize the CC-stage efficiency. The duty cycles in the fast charging mode are illustrated with the two solid lines, which provide a constant charging current of 18.75 A. At the beginning, the battery voltage \(U_{\text{bat}}\) is too low to satisfy the conditions simultaneously. 1. The required charging current should be reached \(I_{\text{bat}}=18.75\)A;
2. The voltage ratio equals the optimal value \(G_{\text{21,opt}}\). Therefore, the secondary duty cycle \(D_{2}\) is selected at the maximum to ensure high efficiency, and the primary duty cycle \(D_{1}\) is tuned to match the required current. In the middle, both conditions can be met simultaneously, so both curves of the duty cycles are adjusted continuously, according to the battery voltage. After that, the optimal voltage ratio condition cannot be met anymore since the battery voltage is increased. Therefore, applying the maximum \(D_{1}\) and tuning \(D_{2}\) to match the charging current is the best solution. As shown in the right side of Figure 5, the maximal efficiency is only achieved in the middle. For the slow charging mode, both the duty cycles are illustrated with dashed lines. Since the required charging current is lower than the fast charging current, the tuning range of both duty cycles is wider. For the illustrated 2 kW charging, the optimal voltage ratio can be achieved for the entire CC stage, and the optimal efficiency is reached all the time, as shown in the right side of Figure 3:1 However, due to narrow duty cycles, the switching losses of the power electronics will increase, and these increases can be reduced by introducing more reactive power via a tuning phase difference, \(\varphi_{\text{21}}\). This proposed control algorithm requires information on the charging profile and the co-control of full bridges on both sides. In this work, a near-field-based communication between a primary and secondary WPT system is introduced, which enables the synchronization of both full bridges, the information transfer between both sides, and the co-control of the inverter and the rectifier. Based on near-field communication, the switching frequency of both full bridges is synchronized periodically, and the phase difference \(\varphi_{\text{21}}\) is set  
Figure 3:2 The proposed voltage ratio tuning method for optimal AC efficiency. The fast charging mode is shown with solid lines, and the slow charging mode is shown with dashed lines. directly. Complex current sensing and current-based synchronization are, therefore, not required.