Time-Domain Analysis of APWM-Frequency Modulated Low-Q LLC Resonant Converter for Wide Input and Load Range Applications

A novel time-domain analysis of an LLC resonant converter employing a hybrid modulation technique for light load efficiency improvement is presented. The proposed technique is a combination of frequency modulation (FM) and asymmetrical pulse-width modulation (APWM) that allows a wide range of output voltage control with zero-voltage-switching (ZVS) of low quality-factor (Q) resonant circuit, with a narrow switching-frequency variation. This ensures optimized magnetics design which has a significant impact on light load efficiency. Two modes of operation are identified and analyzed using time-domain modelling. Characteristic curves for voltage gain, switch turn-off current and ZVS angle are presented and design issues are discussed. The proposed hybrid modulation scheme is demonstrated on an experimental LLC converter prototype of 200W, 380V output voltage with an input voltage range of 20-40V, typical of photovoltaic (PV) applications.