Auto-tuned minimum-deviation digital controller for LLC resonant converters

This paper presents a practical auto-tuned digital controller for LLC resonant dc/dc converters that results in virtually minimal possible output voltage deviation during transients. During transients the controller applies a two-step frequency change algorithm such that the minimum deviation and seamless transition to the new steady state is achieved. The fast voltage regulation is obtained through the output voltage measurements only and fairly simple calculations of the frequency-changing sequence eliminating fast current sensors and complex calculations usually existing in fast controllers for LLC converters. Based on the initial voltage deviation the first frequency step of the controller is determined and adjusted through an auto-tuning process. Smooth transition to the new steady state is achieved through the estimation of the new switching frequency from the ripple amplitude. Experimental results obtained with a 350 W, 400 V to 12 V, isolated LLC converter confirm recovery with practically smallest possible deviation and bump-less transitions to the new steady state.