Phase Control and Power Optimization of LLC Converter

LLC resonant converters offer high efficiency at high frequencies because of their ability to achieve Zero Voltage Switching (ZVS) or Zero Current Switching (ZCS). However, they require a variable frequency control, which generates a wide range of harmonics and causes electromagnetic compatibility issues. The frequency control reduces the converter efficiency at lighter loads and fails to maintain monotonicity of the control power to output transfer function. These limitations are dramatically improved by phase shift modulation (PSM) technique, which is studied in this paper. PSM controlled technique in full-bridge converter, not only improve the monotonicity of the power supply control to output transfer function, but also improves the power supply efficiency at light loads. A systematic analysis and characteristics of a PSM LLC resonant converter is proposed in this paper along with a comparison with conventional frequency modulated (FM) controlled LLC converter. The simulated PSM controlled LLC topology exhibited a maximum peak efficiency of 97.47%. At lighter load conditions, it records an efficiency of 91.66% while the FM controlled converter recorded only 82.8 %. In addition, PSM has a better control of leakage, magnetizing, and resonant currents, which results in a superior behavior than conventional FM converters.