LLC converter with auxiliary switch for hold up mode operation

Abstract

This paper proposes a new half bridge (HB) LLC resonant converter with pulse width modulated (PWM) auxiliary switch (sLLC converter) for hold-up mode operation. The proposed sLLC converter works with synchronous rectifier (SR), which is suitable for low output voltage applications. The magnetizing inductor can be designed with large value to reduce the circulation loss. In the proposed sLLC converter, the auxiliary switch and diode branch will provide charging path for the resonant inductor. For nominal 400V input, sLLC achieves same performance as conventional LLC converter, and all the good features such as soft switching are naturally retained. For slight input voltage fluctuation, frequency modulation is used to regulate the output voltage. When the input voltage drops further, HB switches will operate at constant minimum frequency, and the auxiliary switch will operate in PWM mode to energize the resonant inductor with 400V bus directly during hold up period, thus the output voltage can be maintained at desired level. To verify the effectiveness of the proposed sLLC converter, operational principle and equivalent circuit will be carefully explained and analyzed in this paper. A 300W prototype is built for 250V-400V input, 12V output application.