Optimization Of Hybrid Controlled Full-bridge LLC Resonant Converter

In distributed power generation system, renewable energy generation units are stronglydependent on the climates and weather, and thus the output voltage of these units has a wide variationrange. In order to provide a stable dc voltage for the downstream converters, a dc-dc converter isnecessary to interface the renewable energy generation units and the downstream converters. Thefull-bridge LLC resonant converter has the advantages of achieving zero-voltage-switching for all thepower switches and zero-current-switching for the output rectifier diodes in the whole load range, andit is a promosing topology for the interfaced dc-dc converter. This thesis is dedicated to optimizationof the full-bridge LLC resonant converter to achieve a high efficiency over the entire wide inputvoltage range.The full-bridge LLC resonant converter can be operated with either variable frequency control orfixed frequency control. Each control method has its own advantages and disadvantages. The twocontrol methods can be integrated to form a hybrid control, where the converter is operated withvariable frequency control at lower input voltage and phase-shift control at high input voltage. Thisthe converter efficiency can be optimized in a wide input voltage range. This thesis analyzes theoperation principle of the full-bridge LLC resonant converter under the two control methods, and thevoltage gain characteristics of the converter are analyzed by time domain analysis method. In order tofurther increase the efficiency, the thesis proposes a optimization solution for the switching point ofthe input voltage between the variable-frequency control and phase-shifted control, and theparameters design for the resonant tank by fundamental harmonic approximation method is presented.The realization and design of the control circuit is presented. The small-signal characteristics of theconverter are obtained by simulation method, and the parameter design for control loop is provided.Two3kW prototypes adopting hybrid control and variable frequency control, respectively, arefabricated and tested in the lab. The measured efficiencies under the two control methods show thatthe hybrid control can achieve a better efficiency over the entire wide input voltage range compared tothe variable frequency control.