Improved CPT system with less voltage stress and sensitivity using a step-down transformer on receiving side

This paper proposes a capacitive power transfer (CPT) system with a step-down transformer on the secondary side to reduce the circuit quality factor (Q), and thereby reduce the sensitivity to parameters variations, as well as the voltage stress across the coupling interface. The system operating principle is analyzed mathematically, and the focus is given to understand the effect of the leakage inductance (Llk) of the non-ideal transformer on the system performance. The analytical and simulation results show that at a given constant output power, the voltage across the plates is significantly reduced, and the system becomes less sensitive to the coupling variations by increasing the turns ratio of the step-down transformer. It is found that Llk can be advantageously utilized as a tuning inductor (L) or part of it. The proposed method is verified by building a prototype CPT system that delivered 25 W at an operation frequency of 1 MHz, and an efficiency of more than 70%. Simultaneously, the voltage stress across the single pair of the coupling plates is reduced from 252 V of a conventional CPT system without a step-down transformer, to 50.4 V using a high-frequency transformer with a turns ratio of 5.