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Pulse-width Locked Loop (PWLL) for Automatic Resonant Frequency Tracking in LLC DC%u2013DC Transformer (LLC-DCX)

Fig. 1. Control blocks of the proposed universal adaptive SR driving scheme including frequency tracking.
In recently developed distributed power architecture, the LLC resonant converter has been widely used as a dc - dc transformer (DCX) to provide a semi-regulated or unregulated bus voltage. In order to achieve the highest efficiency, we propose 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 pulse-width locked loop (PWLL) is presented. By minimizing the pulse-width difference between the main switch and the well-tuned SR gate-driving signals, the LLC-DCX always runs at the f0 point to achieve the highest efficiency. For rapid prototyping purposes, the PWLL used to track f0 is realized in a Cyclone III field programmable gate array.

The proposed control strategy for LLC resonant converters, including well-tuned SR latch digital control with PWLL frequency tracking, is shown in Fig. 1. The new control strategy is easy to implement using commercially available FPGAs. Furthermore, key experimental waveforms, which verify operation of the proposed solution, are given in Fig. 2.

By setting the well-tuned SR gate-driving signal as the control reference and using the PWLL to minimize the pulse-width difference between the SR and the main switch-driving signals, the resonant frequency is always tracked (fs= f0).

Fig. 2. Frequency tracking under different load conditions.