Power maximizing of ultrasonic transducer driven by MOSFET inverter operating at 1 MHz

Since the ultrasonic transducer for cleaning the semiconductor wafer or other industrial materials is driven at its own resonating frequency, the operating frequency of the MOSFET DC-to-RF inverter should be tuned to the resonating frequency. As one method to make the operating frequency of the inverter tune to the resonating frequency of the ultrasonic transducer, automatic frequency control (AFC) using the phase-locked loop (PLL) is presented. The ultrasonic transducer has the minimum impedance and maximum power supply at its own resonating frequency. From the view point of the safety operation of the inverter, the power supply system should be operated at the resonating frequency of the load. Since the ultrasonic transducer has a complicated frequency dependent impedance, the RF power fed to the load can be maximized by new type of PLL tuning circuit even if the resonating frequency of the ultrasonic transducer changes due to change of the load impedance. The ultrasonic transducer has poles at 1.04 MHz, 1.8 MHz, and so on. The generating frequency range of VCO was to cover 0.8 MHz to 2.7 MHz so that the resonating frequency of the ultrasonic transducer was set at 1 MHz which was the center frequency of the VCO output. The operation of the system for feeding an output power of 60 W at 1 MHz was satisfactory