Research On Start-Up And Light Load Control Of LLC Resonant Converter

LLC resonant converter features simple structure,zero-voltage switching for primary switches and zero-current switching for secondary diodes,low switching loss and high conversion efficiency.Also,the transformer leakage inductor can be utilized as resonant inductor,which can further the overall size and improve the power density.These advantages make LLC resonant converter popular for low to medium power applications.Although there are already many kinds of control method and ASICs(Application Specific Integrated Circuit)for LLC resonant converter,there are still some problems are to be improved and solved.Under certain operation conditions,such as start-up,load transient,and input voltage step.half-bridge LLC resonant converter may lose zero-voltage-switching for the primary side switches and cause capacitive switching,threatening the reliability of the converter.Through analyzing the conditions for capacitive switching in high-frequency start-up method,the relationship between switching frequency and duty cycle for safe start-up are clarified.Based on this,the inrush current is optimized by theoretical analysis and simulation to derive an optimal high-frequency start-up method.Experimental results verify the proposed control scheme and theoretical analysis.Due to the stringent requirement for light-load efficiency,usually LLC resonant converter implements burst mode to reduce the equivalent switching frequency and power loss at light load.However,due to the discontinuity of switching,magnetic components may cause audible noise.Since LLC resonant converter is widely used in portable devices and usually works near human body,the problem of audible noise must be suppressed.Though there are quite a few methods to reduce audible noise in real products,most of them would cause extra cost.In this dissertation,the audible noise in burst mode is divided into two categories,burst frequency in audible frequency range(20Hz?20kHz),and burst frequency below 20Hz.Spectrum analysis and simulation are used to analyze the audible noise source and the related solutions are proposed and compared according to the power loss and audible noise level to get the optimal control methods.Simulations and experiments on a prototype verify the validity of the proposed control methods.