Stabilizing the Operating Frequency of a Resonant Converter for Wireless Power Transfer to Implantable Biomedical Sensors

Resonant converters have been applied in wireless power supplies for implantable sensors due to their inherent advantages of low cost, high frequency and high reliability. However, the operating frequency of the resonant converter varies with the load and circuit parameters changes, which can significantly reduce the maximum power that the system can transfer. As a result the implanted sensors may not work properly because of insufficient power delivery. Uncertainty in frequency associated with system instability can contribute to electromagnetic interference for the implanted sensors, wireless communication networks, and other peripheral electronic devices. This paper proposes a new method to stabilize the system operating frequency using a fixed capacitor whose equivalent capacitance is controlled by semiconductor switches. Two control strategies based on Zero Voltage Switching techniques are analysed in details and practically implemented. The simulation and experimental results have demonstrated that the proposed method performs well in stabilizing the operating frequency of a wireless power supply system while maintaining the complete soft switching operation of the resonant converter.