The LLC series resonant converter was largely overlooked and is recently considered suitable for isolated DC/DC conversion in high power SMPS, improving light load efficiency and the aspect of cost .However, whereas the LLC resonant converter works fine for high voltage applications, its applicability for low
voltage and high power applications, such as server or telecom power supplies, requires an enhancement of the first order capacitive output filter. Because the output of the LLC resonant circuit injects a sine wave of current into the rectifier and output filter capacitor, there will be a significant ac component at double switching frequency
superimposed on the dc output voltage due to the p.d. at the ESR of the filter capacitor. As a common workaround, adding an additional second order filter to the output may eliminate the ac component of the output voltage but efficiency and power density will suffer