Automatic frequency control for maximizing RF power fed to ultrasonic transducer operating at 1 MHz

For driving an ultrasonic transducer for cleaning semiconductor wafers or other industrial materials, the ultrasonic transducer having an input power of 60 W and operating at 1 MHz is driven from a prototype of the DC-to-RF inverter with an automatic frequency tuning function. Though the automatic frequency tuning circuit using a PLL circuit is not easy to build since the ultrasonic transducer has a complicated frequency dependent impedance, the RF power fed to the load can be maximized by a 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 a pole at 1 MHz. The frequency range of a VCO was designed to cover 0.8 MHz to 1.2 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 RF power of 60 W to the ultrasonic transducer at 1 MHz was satisfactory