LCLC resonant converter for hold up mode operation

In computer and telecommunication power supplies, the front end DC-DC stage is required to operate with a wide input voltage range to provide hold up time when AC voltage fails. Conventional LLC converter serving as the DC-DC stage is not suitable for this task, as the normal operation efficiency at 400Vin will be penalized once the converter is designed to achieve high peak gain, i.e. the wide input voltage range. This paper examines the operation of the LCLC converter and proposes a design methodology of LCLC converter to increase the peak gain without sacrificing the efficiency at normal input voltage condition. In normal operation, LCLC converter behaves like an LLC converter with large magnetizing inductor L_m, hence the magnetizing inductor current is reduced. During the hold-up condition, by reducing the switching frequency, the equivalent magnetizing inductive reactance of the LCLC converter will reduce more than that of the LLC converter, thus the converter enjoys higher peak gain. To verify the effectiveness of the LCLC converter for hold up operation, analysis will be presented, a design method based on capacitor voltage stress will be introduced, and experimental results from a 250V-400V input and 12V/500W prototype will be demonstrated.