Impedance characteristic of ultrasonic transducer operated as load of MOSFET DC-to-RF power inverter

To construct a MOSFET power inverter operating at high frequency in the order of 1-5 MHz, the load impedance characteristic, especially, among various loads, for the ultrasonic transducer used for cleaning the surfaces of semiconductor wafers or other industrial materials has rather low impedance at the resonating frequency. However, it is impossible for us to directly measure the impedance of the ultrasonic transducer in water. So, at first, the impedance of the ultrasonic transducer set in air is measured, and then the impedance of the ultrasonic transducer put in water is estimated based on the impedance characteristics measured and calculated in air. The obtained impedance of the ultrasonic transducer was 0.65 Ω in air and 2.7 Ω in water at the resonating frequency. The load impedance measurement is very useful for designing and constructing the MOSFET DC-to-RF inverter. The primary to secondary winding ratio on an output transformer of the inverter is determined to be 3:1 according to the impedance of the ultrasonic transducer. Lowermost and uppermost frequencies for the AFC using the PLL are determined to be 2 MHz and 3.3 MHz, respectively