High-frequency drive-power and frequency control for ultrasonic transducer operating at 3 MHz

This paper describes RF drive-power and frequency control for the ultrasonic cleaning system operating at a frequency of 3 MHz. The RF output power is maximized by the automatic frequency tuning of the PLL built in the full-bridge MOSFET DC-to-RF power inverter to the series resonating frequency of the ultrasonic transducer operating at 3 MHz. In addition, the full-bridge DC power controller utilizing the pulse-width control of the drive voltage is used to control the drive power for the ultrasonic transducer. Since the ultrasonic transducer has a series impedance of 1 to 2 ohms at the resonating frequency, the quality factor of the ultrasonic transducer is high at the resonating frequency. So, the power conversion efficiency is generally low at 3 MHz. In order to keep the quality factor of the load composed of the sensor and ultrasonic transducer high, a new circuit technique established using an RF current transformer and a digital clipping circuit has been employed to sense the current flowing through the ultrasonic transducer for the PLL circuit. The system provides a total power conversion efficiency of greater than 80% at an RF output power of up to 60 watts at a frequency of 3 MHz when the ultrasonic transducer having a resonating impedance of 1 to 2 ohms is driven.