MOS-FET RF inverter with auto-tuning function for driving ultrasonic transducer at 1.8 MHz

When an ultrasonic transducer operating at 1.8 MHz for use in cleaning semiconductor wafers or other materials used for industrial purposes is driven from a MOSFET DC-to-RF inverter, the output power severely depends on the frequency of operation, since the quality factor of the transducer is high. In order to tune to the resonating frequency of the ultrasonic transducer, the drive signal frequency of the MOSFET power inverter is automatically pulled in until the frequency is set at the resonating frequency of the ultrasonic transducer. A PLL is used to set the operating frequency at the resonating point so that the output power of the ultrasonic transducer is maximized. The control circuit consists of an output power sensing circuit, a PLL controller, a frequency standard and other peripheral circuits. Although a transducer having many resonating frequencies was used, the operation was satisfactory when the transducer having an output power of 60 W at 1.8 MHz was used.