1 Introduction

Electromagnetic interference (EMI) is an important topic within medical imaging applications (especially ultrasound scanners), as these devices are sensitive to noise within the ultrasound probe frequency range. EMI at these frequencies can reduce image quality, so minimizing the known causes of this noise in your design is critical for measurement accuracy and quality results.

While designing any ultrasound application, the probe frequency range (typically between 2MHz and 20MHz) and the third harmonic are important parameters to consider. The third harmonic can be calculated as three times the frequency of interest; for this article we are particularly interested in the switching frequency (fsw) which does not interfere with the ultrasound frequency range. To avoid any interference from the third harmonic of the switching frequency, calculate as: Equation 1 or Equation 2. Therefore, keeping the switching frequency at or less than 600kHz makes the design flexible and allows the power supply to be used with any probes ranging from 2MHz – 20MHz. For this purpose, all buck converters in this application note are operated at a switching frequency of 500kHz.