Burst Mode and Phase Shift Hybrid Control Method of LLC Converters for Wide Output Range Applications

LLC converter is difficult to realize the wide output range with conventional PFM (pulse frequency modulation) control because of the limited operating frequency. This paper proposes a new hybrid control method. The proposed strategy combines the burst mode control and the phase shift control methods for the LLC converter. The burst mode control can regulate the output voltage from zero to its maximum value, and the phase shift control can reduce the increased resonant current during burst-on time. By using this method, the wide output voltage range is realized with the low resonant current during burst-on time and ZVS (Zero Voltage Switch) for all switches. The reliability and efficiency of the converter can be improved accordingly. The burst duty ratio is regulated by the reference output voltage, and the phase shift angle is derived when realizing ZVS for lagging-leg switches. Moreover, the resonant parameters and the maximum phase shift angle are optimized under the constraints of current stress and ZVS condition. This method is easy to be implemented in low-cost micro controller because high frequency real-time calculations are not necessary. A 1.5-kW prototype is built to verify the feasibility and validity of the proposed control method.