A high efficiency half-bridge LLC converter with simple hold-up compensation circuit

The half-bridge (HB) LLC converter has been widely used in medium power applications because of low component count, no transformer dc-offset current, and wide zero-voltage-switching (ZVS) range. However, the HB LLC converter should be designed with small transformer magnetizing inductance to obtain high voltage gain in wide-input-voltage applications with the hold-up time conditions. It causes the HB LLC converter to have large conduction and switch turn-off losses in the primary side at nominal input where a high efficiency is required. To overcome these problems, a simple control method of the HB LLC converter with one additional switch and capacitor in the primary side is proposed in this letter. At nominal input, since the proposed method enables the HB LLC converter to operate with large transformer magnetizing inductance, it can reduce the conduction and switch turn-off losses in the primary side, which makes a high efficiency. On the other hand, during the hold-up time, since the proposed method increases the resonant capacitance by turning on one additional switch, the HB LLC converter can obtain high voltage gain. To confirm the validity of the HB LLC converter with the proposed method, a 325-385V input and 56V/350W output laboratory prototype is built and tested.