Driving an ultrasonic transducer with a multicell inverter

Piezoelectric ceramic transducers (PZTs) are widely used in industry for ultrasonic applications such as cleaning, cutting, homogenization, sterilization and welding. Switching inverters are increasingly used to drive these transducers in high power applications. These transducers are non-linear loads that are sensitive to the harmonic content of the driving voltage used. Depending on the inverter topology and modulation method used, the harmonic content of the unfiltered output voltage can vary significantly. It is most desirable to drive tonpilz or horn ultrasonic transducers with a sinusoidal driving signal at the thickness resonance frequency of the ceramic used. Non-sinusoidal driving signals, e.g. the unfiltered square-wave output voltage of a typical switching-inverter contain harmonics at the thickness resonance frequency as well as at and around multiples of the switching frequency. The transducers are sensitive to the harmonic content of the driving signals as unwanted mechanical modes can be excited, leading to the destruction of the transducer. This paper will show that a half-bridge and a 5-cell multicell inverter topology deliver outputs with low harmonic content and are therefore possible switching-converter options for driving an ultrasonic transducer load without activating its destructive modes. In the case of the multicell inverter, the possible voltage unbalance of the cell capacitors will also be discussed.