A harmonic cancellation technique for an ultrasound transducer excited by a switched-mode power converter

The aim of this study is to evaluate the feasibility of using harmonic cancellation for a therapeutic ultrasound transducer excited by a switched-mode power converter without an additional output filter. A switching waveform without the 3^rd harmonic was created by cascading two switched-mode power inverter modules at which their output waveforms were pi/3 phase-shifted from each other. The waveform and harmonic contents of the acoustic pressure generated by a 1-MHz self-focused piezoelectric transducer with and without harmonic cancellation have been evaluated. Measured results indicated that the acoustic 3^rd harmonic-to-fundamental ratio at the focus was small (-48 dB) with harmonic cancellation, compared to that without harmonic cancellation (-20 dB). The measured acoustic levels of the 5^th harmonic for both cases with and without harmonic cancellation were also small (-46 dB) compared to the fundamental. This study shows that it is viable to drive a piezoelectric ultrasound transducer using a switched-mode power converter without the requirement of an additional output filter in many high intensity focused ultrasound (HIFU) applications.