New switching control technique to improve the efficiency under light load condition for LLC converter with large magnetizing inductance

The main theme of this paper is to propose a switching control method for LLC converter with larger magnetizing inductance to improve the light load efficiency. The proposed switching technique is to replace conventional frequency modulation method which is with constant duty while regulating the output voltage by varying the switching frequency, with pulse-width modulation while retaining the special feature of zero-voltage switching. The proposed switching technique can further reduce the switching losses under light load condition by reducing the switching frequency as compared with that for frequency modulation. Experimental results are derived from a DSP-based LLC converter. The output voltage = 12V, input voltage = 400 V and output power = 720 W. The test results show that the proposed switching technique can improve the light load efficiency up to 30 % under light load condition.