Extraction of small signal characteristics and loop compensation of digitally controlled LLC resonant converter with period modulation

The LLC resonant converter has been studied with direct frequency modulation, but seldom with direct period modulation that introduces a non-linear gain. A low cost and high speed digital controller (hardware implementation of the digital filter) will present moving zeroes and poles when the sampling rate changes along with the switching period. The moving zero (in the case of digital PI controller) will cancel a part of the non-linear gain induced by the period modulation leaving a residual that can be ignored in sense of engineering practice. Small signal characteristics of the period modulated LLC resonant converter is extracted from an evaluation converter. The expanded DCX load trap behavior of the double tuned LLC is also observed in certain transformer structures with an integrated asymmetrical resonant inductor.