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==== Capacitive Power Transfer ====

Capacitive Power Transfer (CPT) works by using electric fields to transfer energy between two sets of electrodes separated by a dielectric material, for example air. When a high frequency alternating current signal is applied to the transmitter electrodes, it generates a varying electric field across the dielectric medium. This electric field induces a displacement current between the electrodes, allowing energy to be transferred from the transmitter side to the receiver side without physical connections.

The receiver electrodes capture the energy from the electric field, converting it back into an electrical current. A rectifier circuit then converts the high-frequency AC signal into direct current. To improve efficiency, a compensation network consisting of capacitors and inductors is used on both sides to minimise power loss and enhance the strength of the coupling. The efficiency of a CPT system depends on many factors, the surface area of the electrodes, the distance between them, the properties of the dielectric material and the operating frequency. CPT is not yet commercially available due to the high frequency need and safety reasons (high voltage plates).

The figure below shows a general representation of a four-plate CPT system to explain the basics. The other types of CPT systems will be discussed later in this chapter. The primary side consists of a power source, a high frequency inverter and a primary compensation network. The secondary side consists of the secondary compensation network, a rectifier and a load. P1 and P2 serve as transmitters, P3 and P4 as receiver.

{{ :cpt.png?nolink&600 |}}

Different materials can be used for the capacitive plates, such as zinc, copper and aluminium. The latter is used the most due to its light weight and affordability. Different shapes can be used for different applications. For example, circular shaped plates reduce electric field emissions. The coupling factor k shows how effectively power is transferred between the transmitter and receiver and is defined by the formula below where CM is the mutual capacitance between the transmitter/receiver plates, C1 the self-capacitance of the transmitter (primary circuit) and C2 the self-capacitance of the receiver (secondary circuit).

$$k_E = \frac{C_M}{\sqrt{C_1 C_2}}$$

$$
C_M = \frac{C_{13}C_{24} - C_{14}C_{23}}{C_{13} + C_{14} + C_{23} + C_{24}} \quad [F]
$$

$$
C_1 = C_{12} + \frac{(C_{13} + C_{14})(C_{23} + C_{24})}{C_{13} + C_{14} + C_{23} + C_{24}} \quad [F]
$$

$$
C_2 = C_{34} + \frac{(C_{13} + C_{23})(C_{14} + C_{24})}{C_{13} + C_{14} + C_{23} + C_{24}} \quad [F]
$$

If the distance between the plates increases, the value of k will decrease and vice versa. The switching frequency needs to be increased when k decreases to obtain a sufficient power level. This will drop the system’s total efficiency due to higher switching losses. Also, higher resonant inductors are required which lead to even lower efficiencies due to higher conduction losses. This proves that [[Compensation Networks]] affect the overall efficiency of the system and needs to be properly designed.