Capacitive Charging Power Supply Wireless Energy Transfer Architecture

Capacitive energy storage systems, employed in pulsed power applications from medical defibrillators to particle accelerators, traditionally rely on direct electrical connections for charging. However, in scenarios involving rotating machinery, isolated high-voltage chambers, or biomedical implants, these physical connections present significant challenges related to wear, isolation breakdown, and mechanical complexity. The integration of wireless energy transfer (WET) technology into capacitive charging architectures offers a transformative solution, enabling contactless, efficient energy transfer to high-voltage energy storage capacitors. This architecture merges the fields of high-frequency power conversion, resonant magnetic coupling, and high-voltage rectification to create a robust, maintenance-friendly charging system.

The core principle involves inductive power transfer via loosely coupled coils. A primary coil, connected to a high-frequency inverter, generates an alternating magnetic field. A secondary coil, positioned in close proximity but physically isolated, intercepts this field, inducing an AC voltage. For capacitive charging, this induced AC voltage must be rectified to DC and stepped up to the required high voltage, typically several kilovolts. The most efficient method for this is a resonant inductive coupling system, where both the primary and secondary sides incorporate tuning capacitors to form resonant tanks at the operating frequency (typically in the 100 kHz to several MHz range). Resonance dramatically improves the power transfer efficiency and allows for greater coil separation (air gap) while maintaining usable coupling.

The system architecture comprises several key stages. On the transmitter side, a DC power source feeds a high-frequency full-bridge or half-bridge inverter using fast-switching semiconductors like Silicon Carbide (SiC) MOSFETs. This inverter drives the primary resonant tank, which includes the primary coil (Lp) and a series or parallel tuning capacitor (Cp). The frequency of the inverter is controlled to match the resonant frequency of the primary tank or is adjusted via phase-shift modulation to regulate the power flow. On the receiver side, the secondary coil (Ls) and its tuning capacitor (Cs) form the secondary resonant tank. The induced high-frequency AC voltage on the secondary coil can be quite high (hundreds of volts to low kilovolts), but still needs to be rectified and often increased further to reach the final capacitor bank voltage (5-30 kV).

This is where the architecture diverges from low-voltage WET systems. A standard diode bridge rectifier followed by a DC-DC boost converter is inefficient at these voltages and frequencies. Instead, a voltage multiplier circuit is directly integrated onto the secondary side. A Cockcroft-Walton voltage multiplier, constructed using high-voltage, high-frequency diodes and capacitors, is ideally suited. It performs both rectification and voltage multiplication directly from the high-frequency AC source. Each stage of the multiplier adds a voltage approximately equal to the peak AC input voltage. The number of stages is determined by the required output voltage and the induced AC voltage on the secondary coil. The entire multiplier circuit, along with the secondary resonant coil, forms the isolated, contactless receiver module that connects directly to the energy storage capacitor.

Control and regulation present a significant challenge, as there is no direct electrical feedback path from the high-voltage output capacitor back to the primary-side controller. Two primary methods are employed for closed-loop control. The first is load estimation via primary-side measurements. By monitoring the primary coil's current and voltage, and knowing the resonant tank parameters, the controller can estimate the reflected impedance from the secondary side, which changes as the storage capacitor voltage rises. This estimated impedance can be used to infer the output voltage and adjust the transmitter's power accordingly. The second, more precise method uses a wireless telemetry link. A low-power radio or optical transmitter on the high-voltage secondary side sends the measured capacitor voltage data back to the primary controller, which then adjusts the inverter's output to maintain a constant current or constant power charge profile.

Key design considerations include electromagnetic compatibility, efficiency optimization, and safety. The high-frequency, high-power magnetic fields must be shielded to prevent interference with other equipment. The coupling coils are often constructed with Litz wire to minimize skin effect losses at high frequencies. The alignment between primary and secondary coils affects the coupling coefficient (k); the system must be designed to maintain acceptable efficiency over the expected range of misalignment. Isolation and creepage distances for the high-voltage multiplier on the secondary side are critical, especially in atmospheric conditions. This architecture is particularly advantageous for applications like charging the main energy storage capacitor in a rotating computed tomography (CT) X-ray generator or for in-vivo biomedical devices, where it eliminates slip rings or percutaneous wires, thereby enhancing reliability and patient safety.