Digital control of resonant converters: Enhancing frequency resolution by dithering

Resonant converters and related systems, such as piezoelectric transformers, may require a high-resolution frequency drive when the quality factor of the network is high or to avoid limit cycle oscillations. This high frequency resolution requirement could be beyond the capabilities of low cost microcontrollers. To remedy this problem, a frequency resolution enhancement algorithm was developed, tested by simulations and verified experimentally. The proposed 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 approach on the digital hardware is simple and requires modest additional workload from the CPU. Theoretical analysis was carried out to model the proposed dithering method when applied to drive resonant network in order to identify the causes and to quantify the expected output signal distortion when the signal is used to drive resonant networks. The proposed approach was tested experimentally on two types of resonant converters: a series-resonant parallelloaded converter and a piezoelectric transformer. It was found that the output signal distortion is less than 1% of the peak amplitude of the output drive which would be acceptable in many applications. The experimental results were found to be in excellent agreement with the theoretical predictions, validating the usefulness of the dithering method as a frequency resolution enhancer for resonant network drive.