Review on different control strategies of LLC series resonant converters

In this paper, different control strategies for both variable-frequency and constant-frequency operations of LLC resonant converters are reviewed. LLC resonant converter provides no-load regulation unlike to a conventional series resonant converter. The main advantages of this converter are its higher power density and efficiency within the wide input and output load range. Considering high power factor for the LLC resonant tank, the output voltage ripple can be reduced. Generally all kinds of resonant converters are operated at high frequencies ranging from kHz to MHZ which results in the reduction in converter efficiency due to increased turn-off and gate charge losses. The review indicates that Pulse- Density-Modulation (PDM) control strategy is advantageous over the other strategies. In PDM control, the switching frequency is made equal to resonant frequency and also skipping gate pulses are made proportional to the load variations. Thus the controller provides a good selectivity of frequency over wider line and load variations.