A new LLC converter family with synchronous rectifier to increase voltage gain for hold-up application

For low output voltage application, a half-bridge LLC resonant converter family with synchronous rectifier (SR) that has a boost pulse width modulation (PWM) converter characteristic for hold-up state operation is proposed in this paper. In proposed converters, an auxiliary switch (or an auxiliary switch and a diode) is added at the primary side to provide the charging branch for series resonant inductor. The converter has two different operational characteristics. In the nominal state, the proposed converters show the same operational performance with the conventional LLC converter, which is frequency modulation (FM) method. When ac line is lost and the converters operates at voltage boost operation mode, which needs high voltage gain, the Half-bridge switches keep the minimum switch frequency, and the additional switch operates at PWM mode to charge series inductor to obtain enough voltage gain. To verify the effectiveness of the proposed circuit, operational principle will be explained and simulation and experimental results will be presented with following specification. 250-400 V of input voltage range, 12 V of output voltage, and 300 W output power.