A Novel Design Approach for LLC Resonant Converters in Off-line LED Driving Applications

This work presents an analysis and a design methodology for a DC–DC LLC resonant converter applied to off-line LED driving. An investigation regarding the main design parameters that influence the LED low-frequency (LF) ripple has been carried out. The study was based on a mathematical analysis of the converter and employs numerical algorithms to solve a set of equations that accurately describe the converter behavior. Based on this analysis, a novel design methodology that allows for LF current ripple prediction has been devised. A numerical model has been developed to determine the LF current ripple transmission in DC–DC LLC resonant converters and provide a methodology for designing the bus capacitance in two-stage off-line LED power supplies. The work shows that the design of the LLC resonant converter for off-line LED driving is a trade-off between the converter efficiency and the LF ripple transmission. Experimental results were taken from two 46-W laboratory prototypes considering two different design goals: one for LF ripple reduction and another aiming at the highest converter efficiency and better power quality. Furthermore, the design of a two-stage LED driver based on an LLC converter taking into account the LF ripple transmission is also presented.