Digital implementation of optimal SR ON-time control and asymmetric duty control in LLC resonant converter

In this paper, a half-bridge (HB) LLC resonant converter which has been used for server power supply, is controlled by digital controller. In order to increase the entire load efficiency of HB LLC converter, a synchronous rectifier (SR) should has a robust control at any condition such as PFC noise or the lack of current sensing. For these reason, digital controller with LLC SR control is preferred in server power supply market. In this paper, the optimal SR ON-time control which operates as a constant ON-time at below region, and the same ON-time of primary switch at above region, is implemented. However, the problems of optimal SR ON-time control still exist. Because of senseless operation, when the ON-time of SR is larger than the half period of LLC resonant tank, the current of converter flows reversely and it will make the big surge current which damage to active devices. From this result, the ON-time of SR should be shorter than the half period of LLC resonant tank. Moreover, in the case of current unbalance in center-tapped rectifier, the conducting time of rectifiers is different due to the parasitic components. Since the powering periods of two SR are different, the SR ON-time of HB LLC resonant converter should be determined by minimum conducting time of rectifiers. This result causes a large conduction loss of body diodes which makes the low efficiency or the reliability problem of LLC resonant converter. Therefore, HB LLC resonant converter with optimal SR ON-time control is implemented and the asymmetric duty control is proposed in this paper, which solves the current unbalance problem in center-tapped rectifier. Therefore, the LLC resonant converter which has wide input variation, can achieve the high efficiency with optimal SR ON-time control and eliminate the current unbalance problem with asymmetric duty control.