Digital control of resonant converters: Enhancing frequency resolution by dithering

This article presents a digital algorithm for frequency resolution enhancement to accurately drive resonant converters and related frequency controlled systems. The approach is based on a modification of the fractional-N dithering concept and includes an adaptive dithering period and smooth DPWM frequency transitions. The implementation of the solution on digital hardware is simple and requires modest additional workload from the CPU. The method is demonstrated on two types of resonant converters: a series-resonant parallel-loaded converter and a piezoelectric transformer. The resultant output signal distortion is less than 1% of the peak amplitude of the output drive which would be acceptable in many applications. Performance comparison of the proposed method against other popular dithering techniques demonstrates the superiority of the proposed method in terms of lowest distortion to the output signal. The experimental results are in excellent agreement with the theoretical predictions, validating the usefulness of the dithering method as a frequency resolution enhancer for resonant network drives.