Elimination of Circulating Current in Wide Range LLC Resonant Converter with a Hybrid Bridge and Simultaneous PWM and PFM Control

A hybrid bridge LLC-type converter is proposed for wide range dc-dc resonant conversion, to eliminate circulating current with the simultaneous use of pulsewidth modulation and pulse frequency modulation. The exact model of the converter is derived based on time domain analysis to aid design and control. Secondary side and output capacitor rms currents are significantly reduced, benefitting low output voltage/high power designs in terms of lower cooling and output filtering requirements. Absence of the L-m-L-r-C-r resonance interval, also suppresses secondary side parasitic ringing and can help to mitigate EMI issues. A GaN-based prototype of the converter is built and tested at 400V/18-36V, 500W. Analytical and experimental comparisons made with a standard LLC converter as well as with an alternative fixed frequency modulation scheme, validate the reduction of rms currents enabled by the proposed method and its potential for increased efficiency at higher power levels.