Pulsewidth Locked Loop (PWLL) for Automatic Resonant Frequency Tracking in LLC DC–DC Transformer (LLC -DCX)

In recently developed distributed power architecture, the LLC resonant converter has been widely used as a dc–dc transformer (DCX) to provide a semiregulated or unregulated bus voltage. In order to achieve the highest efficiency, this paper proposes a novel switching frequency control scheme for the LLC-DCX. First, the synchronous rectifier (SR) gate-driving signals are tuned by eliminating the body diode conduction. Then, the pulsewidth locked loop (PWLL) is presented, by minimizing the pulsewidth difference between the main switch and the well-tuned SR gate-driving signals, the LLC-DCX always runs at the $f_0$ point to achieve the highest efficiency. For rapid prototyping purposes, the PWLL used to track $f_{0}$ is realized in a Cyclone III field programmable gate array.