Modified Harmonic Reduction Pulse Width Modulation (mHRPWM) for Switched Excitation of Resonant HIFU Transducers

High Intensity Focused Ultrasound (HIFU) is reliant on carefully designed excitation systems capable of high power, and low distortion. The use of array transducers additionally requires amplitude control for apodization and attenuation compensation plus phase control for inter element timing for beamstearing, focusing and correction. Power amplifiers possess all the required characteristics except they can be expensive and bulky thus placing barriers to adoption especially for large channel count arrays. Switched mode excitation systems are capable of fulfilling all requirements except minimizing distortion can be problematic due to the lack of availability of suitably high speed MOSFETs placing restrictions of waveform design. This work presents an modification to the Harmonic Reduction Pulse Width Modulation (HRPWM) method specifically designed to minimize harmonic distortion in systems constrained by MOSFET performance in HIFU excitation systems. The proposed method is of significance as the use of external filtering and transformer coupling of the excitation waveform is not required. The proposed method is demonstrated through experimental hydrophone testing with a 1.1 MHz HIFU transducer and demonstrates the successful generation of excitation waveforms with continuously variable amplitude control and low third harmonic output.