On-line DC-link voltage control of LLC resonant converter for server power applications

The main theme of this paper is to propose an online DC-link voltage control technique of LLC converter for server power applications. The proposed technique on-line controls the DC-link bus voltage of power factor corrector (PFC) to limit the switching frequency of LLC resonant converter in order to reduce the conduction loss and switching loss (turn off loss) as the switching frequency is less and higher than the resonant frequency, respectively. To achieve voltage regulation of server power, the output voltage of PFC is increased as load increases and vice versa. Comparing to previous approaches, neither additional current sensor nor SR MOSFET voltage sensing circuit is required for the proposed technique. The specifications of the server power supply include: AC input voltage = 110/220 V, DC output voltage = 12 V and output power = 480 W. Experimental results show that the efficiency can be improved from 30% load to 100% load for both low line and high line inputs. These results confirm the effectiveness of the proposed on-line DC-link voltage control technique.