## 1 Introduction  
Since the 1890s, efforts have focused on the wireless transfer of power [1], and the most successful approach has been inductive power transfer (IPT), which is based on magnetic field coupling. Inductive power transfer has been widely used for powering industrial machines [2, 3], charging mobile devices [2, 4] and electric vehicles [5, 6], and powering biomedical implants [7]. However, IPT has clear limitations with regard to high eddy current losses to the surroundings especially with metals, high standing losses, and complicated design, requiring bulky and expensive magnetic materials and electromagnetic interference (EMI) shielding [8, 9]. Also, given its nature, IPT is unable to transfer power across metal barriers. Capacitive power transfer (CPT), a technology that uses a varying electric field to transfer power, has been introduced as a good alternative to IPT for near field power transfer due to its unique features, such as design flexibility of coupling structure, ability to transfer power across electrically isolated metal barriers, low standing losses and EMI, and low cost and weight [10; 11; 12; 13; 14]. Capacitive power transfer can be used in various applications, such as integrated circuits (IC) [15], biomedical devices [16], consumer electronics [11; 12; 17; 18; 19], and electric vehicles (EVs) [20]. The conventional CPT system is illustrated in Figure 1, where the capacitive interface is typically in the range of hundreds of picofarads (pF) when the plates are almost touching with an insulation layer in between and 1-2 mm air gap, and few tens of picofarads when there is a wide air gap, e.g., 150 mm to 300 mm air gap is usually considered for EV charging applications [21]. This results in a high-quality factor and voltage stress across the capacitive interface. For example, if the operating frequency \(f\) is 1 MHz, the load resistance \(R_{L}\) is 12.3 \(\Omega\), the capacitive interface \(C_{c1}=C_{c2}=2C_{c}=1\) nF and the tuning inductor \(L\) is 50.7 uH, which provides the quality factor [22]\(Q_{conv}\) of \(8/\pi^{2}R_{L}\) (sqrt(\(L/C_{c}\)) \(\cong\) 32 and provides the maximum voltage stress across the single pair of the capacitive interface \(V_{c\_conv}\) of 320 V for an AC input voltage \(V_{in\_conv}\) of just 20 V. Consequently, practically attaining tuning is challenging, and the system is highly sensitive to coupling variation and load change. The high voltage stress across the coupling interface increases the probability of dielectric breakdown and sparks occurrence. In addition, system safety becomes an issue as the related leakage electric field emissions around the plates increase beyond the safety margins as will be detailed in Section 4. These matters result in lower efficiency, decreased ability to deliver power, and increased difficulty in offering commercially accepted systems. Previous studies introduced a partial solution for the previously mentioned issues [10; 21; 23; 24; 25]; however, no comprehensive design overcomes both issues simultaneously. For instance, one pulse switching active capacitor (OPSAC) was proposed to overcome the sensitivity issue [24]; however, the system is complicated and the voltage stress across the coupling interface is high. Tuning the operating frequency [25] is an effective method to mitigate the sensitivity issue and even with a high-quality factor, the system can still transfer the power with high efficiency, but by changing the frequency, following the regulations becomes challenging and the design is complex, costly, and cannot avoid the high voltage stress. In this paper, a CPT system with a DC-DC buck converter on the secondary side is introduced to simultaneously reduce the system sensitivity and the voltage stress across the capacitive interface. The proposed system can also keep the leakage electric field low, which improves the safety of CPT systems. Figure 1: The conventional capacitive power transfer (CPT) system. The rest of the paper is arranged as follows.