# Wireless Power Transfer Approaches for Medical Implants: A Review
## 2 Different Approaches for a Wireless Power Transfer System
### Hybrid Wireless Power Transfer  
A hybrid wireless power transmission (HWPT) system is a combination of two common methods working as a unit system. The inductive WPT system uses magnetic fields to transfer power, whereas the capacitive WPT system uses electric fields. The capacitive WPT approach has two advantages compared with the inductive one. First, there is no eddy current loss and, second, it uses a lightweight and low-cost coupler. However, the capacitive method is limited to small power transfer and short distance because of the small coupling capacitor. When the transfer distance is in several hundred millimeters, the coupling capacitor is usually in the picofarad range. The voltages across the coupling plates of the coupler, which could be improved with double-sided transformers or various compensation topologies (such as double-sided LC and Z-source), are usually hundreds of times the input voltages to enhance the system power level. Considering that the high voltage stressed in the coils of the inductive unit could be fully used as a driving voltage for the capacitive coupler, combining both systems can be done as a hybrid system. Therefore, it is important to take advantage of the inductive and capacitive hybrid system to achieve higher power for HWPT. A hybrid method includes inductive power transfer and capacitive power transfer, as shown in Figure 9.  
As shown above, the inductive link will generate the alternating magnetic field through coils, which provides the transmission medium. Furthermore, those currents will produce high voltages on the transfer coil because of the self-inductance, whereas the capacitive link system requires high voltages to produce the electric field for the capacitive coupler. Accordingly, the produced voltage on coils of the inductive link system can be used for the capacitive system. A combined system to \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Reference** & **Year** & **Frequency** & **Output** & **Efficiency** & **Active** & **Transmitter** & **Receiver** \\  & & & **Power** & **(\%)** & **Range** & **Dimension** & **Dimension** \\ \hline
[18] & 2014 & 1 MHz & 28 & 1.6 & 105 & 29.6 \(\times\) 72 * & 1 \(\times\) 5 ** \\
[102] & 2013 & 1.07 MHz & - & 45 & - & - & - \\
[103] & 2011 & 1.2 MHz & 100 & 50 & - & d\({}_{\text{ouff}}\) = 44 & - \\
[46] & 2011 & 2.3 MHz & \(\approx\)0.3 & - & 30–400 & d\({}_{\text{ouff}}\) = 8 & - \\
[12] & 2011 & 1 MHz & 23 & - & 120 & - & d\({}_{\text{ouff}}\) = 8 \\
[104] & 2010 & 35 kHz & 1.23 & - & 70 & - & d\({}_{\text{ouff}}\) = 7 \\
[105] & 2010 & 673 kHz & 1000 & 27 & 40 & - & - \\
[106] & 2003 & 100 kHz & 5400 & 36 & 40 & - & - \\
[25] & 2002 & 1 MHz & 2100 & 20 & 40 & - & - \\
[26] & 2001 & 1 MHz & - & 20 & 30 & - & - \\ \hline \hline \end{tabular}  
* Width and total length of 48 symmetric elements of the spherical transducer array. ** Active area of a single element of the flat transducer array.  
\end{table} 
Table 4: _Cont._  
Figure 9: Drawing and diagram of hybrid wireless power transmission (HWPT). construct a hybrid electric power transfer is established in [107]. \(V_{1}\) and \(V_{2}\) present the port voltages as follows:  
\[V_{1} =j\omega L_{1P}I_{P}+j\omega MI_{S} \tag{12a}\] \[V_{2} =j\omega L_{1S}I_{S}+j\omega MI_{P} \tag{12b}\]  
The efficiency of the inductive link is given by the same equation as Equation (4). Whereas the power transfer from the capacitive link of the hybrid system \(P_{CH}\) is given by Equation (13):  
\[P_{CH}=-\omega^{3}CI_{P}I_{S}\Big{(}M^{2}-L_{1P}L_{1S}\Big{)} \tag{13}\]  
where \(I_{P}\) and \(I_{S}\) are the RMS currents in primary and secondary coils. \(L_{1P}\) and \(L_{1S}\) represent the self-inductance of the transmitting and receiving side coils, respectively. M is the mutual inductance. The advantage of a hybrid system is that it occupies less space than separate units. In this case, there is a capability of having different operating bands, more alternatives, and backup.