# Approach-Based Analysis on Wireless Power Transmission for Bio-Implantable Devices
## 3 Analysis of Various WPT Approaches
### WPT Systems Based on Power Control and Regulation Improvement  
In this section, the WPT systems developed based on power control and regulation improvement are being studied theoretically and listed in Table 3.  
A WPT system operates at the frequency of 13.56 MHz with a either 1X or 2X mode resonant regulation rectifier (R\({}^{3}\)) and the power control has been proposed for implants [33] in which the maximum measured power on receiver side is 102 mW and efficiency at receiver is 92.6% whereas regulation or control is pull off by two mechanisms. In the first mechanism, the domestic pulse width modulation (PWM) on the receiver side manages the on and off time of duty cycle to switch the rectifier for the change of modes 1X and 2X as per requirement. Secondly, Backscattering uplink techno \begin{table}
\begin{tabular}{c c c c c} \hline
**Ref.** & **Approach** & **Operating Frequency** & **Performance** & **Remarkable Observation** \\ \hline \multirow{3}{*}{[33]} & WPT with 1X mode or 2X & \multirow{3}{*}{13.56 MHz} & Output power of 10 to 100 mW; max efficiency of 92.6\% & Stable power control attained by using a data backscattering uplink technique \\ \cline{1-1} \cline{3-4}  & & & & \\ \cline{1-1} \cline{3-4}  & System with adjustable compensation network & 570 kHz & Constant output voltage & Components minimized@ Receiver without abating efficiency \\ \hline \multirow{3}{*}{[35]} & Distance and load-insensitive inductive approach & \multirow{3}{*}{Not specified} & Receiver efficiency over 88\%; overall PTE 76\% & Automatic power adjustment attained by closed-loop control scheme; its response time is 69 ms \\ \cline{1-1} \cline{3-4}  & WPT with switched capacitive converter (SCC) & \multirow{3}{*}{13.56 MHz} & Desired output: 5 V (for 1.3–3.3 V input); 57–74\% efficiency & (ii) & Stable output voltage; Reduced switch size; Especially for neuromulator implants. \\ \hline \multirow{3}{*}{[37]} & Receiver (RVM-Rx) with \multi{3}{VPTD technique} & \multirow{3}{*}{13.56 MHz} & \multirow{3}{*}{67.8\% efficiency @Rx} & MDTL and OOTL used \\ \cline{1-1} \cline{3-4}  & & & & \\ \hline \end{tabular}
\end{table} 
Table 3: WPT systems based on power control and regulation improvement. transmission power of the primary coil by having 0.35 \(\upmu\)m CMOS process. Here, the R\({}^{3}\) is a reconfigurable rectifier. An adjustable compensation network has an advantage where the capacitor matrix can be modified [34]. It gives sustained voltage power supply with no regulation loop on the side of receiver, which favors IMDs. Normally this system will work correctly for various load resistances and coupling coefficients. By this approach, with a coil spacing of 32 mm to 40 mm and a load resistance of 100 \(\Omega\) to 0.5 k\(\Omega\), the capacitors arrangement can be spontaneously adapted to provide a sustained voltage of almost 3.5 V to the load. In order to obtain smooth output power, another technique [35] provided automatic power adjustment by using closed loop control scheme. Here, the system is based on inductive power transfer which has a converter (DC to DC) on the primary side and a circuit to track or monitor the load voltage on the secondary side, as well as a communication process using the coils. The tracking circuit of load voltage identifies the value of output voltage and whenever it deviates from the theoretical scale of values, it sends a warning by communicating the transmitter wirelessly. Then, the Transmitter manages the converter output voltage as per the warning received to maintain the charging potential at sustained level. Here, the observed response time in the closed control loop was approximately 69 ms and efficiency at receiver was above 88%. Another WPT system with a step-up Switchable Capacitor Converter (SCC) designed [36] in a CMOS technology environment by reducing the count of outer components to put properly the load and flying capacitors.