Family of half-bridge LLC resonant converters with auxiliary switches for hold-up operation

Abstract

For applications with hold-up time requirement, the magnetising inductance of the conventional LLC converter must be designed small enough to provide required peak voltage gain. However, the efficiency in normal operation will be decreased when considering the gain requirement. Here, a family of half-bridge (HB) LLC resonant converters is proposed for low-voltage and high-current applications. In order to provide the required voltage gain, auxiliary switches are introduced into parts of the secondary-side rectifiers. In the normal state, the auxiliary switches keep off and high efficiency can be achieved since the proposed converter always operates around the resonant frequency. Without considering the voltage gain requirement, soft-switching is the only consideration for the design of the magnetising inductance. Therefore, circulating current and conduction losses induced by the magnetising inductance can be reduced significantly. During the hold-up time, the auxiliary switches operate with pulse-width modulation (PWM) control to provide sufficient voltage gain. Furthermore, matrix transformers are engaged to simplify the winding structure and increase the output current capability. Operation principles, characteristics, and design considerations of the proposed LLC resonant converter are analysed in detail. Experimental results are given to verify the effectiveness and the advantages of the proposed converter.