A Constant Voltage/Current Converter Based On Buck+LLC Cascaded Topology

The switching power supply is being challenged by the development trend ofsmall volume, high frequency, low EMI level, high efficiency and wide changingrange of input voltage and load. To satisfy the aforementioned requirements, cascadedconverters cause more attention. Non-isolated converters such as Buck, Boost,Buck-Boost and isolated converters such as flyback and bridge type circuits arepriorities in most applications. The procedure of choosing source&load converter andthe method to improve the cascaded converter’s efficiency are illustrated in detail.The main contents of this paper include the following aspects:Buck, Boost, Buck-Boost converters are comprehensively compared with eachother to find their respective advantages and disadvantages. Series resonant converter,parallel resonant converter and LLC resonant converter are inter-compared in the sameway. Buck and LLC converter are ultimately chosen to be the source and loadconverter.Buck converter’s dynamic model is analyzed. The distortion of relativewaveforms in the light load situation is explained. The arising resonant phenomenonbetween junction capacitors and filter inductor are resolved by the application ofquasi-synchronous Buck converter.FHA and time domain analysis are used to obtain the steady state model of LLCconverter； The transmission gain is corrected considering the secondary side’sleakage inductor of the transformer； Dynamic model of LLC converter is derived byusing EDF; Realizations of synchronous rectification and over-current protection arealso introduced later.The estimation method to calculate IGBT and MOSFET’s power losses areprovided to guide the design of improving efficiency. Input impedance andinput-output dynamic model are listed to help the design of close-loop regulator.Finally, a960W prototype was built in laboratory. Constant voltage/currentmodes were tested. The experimental results verify the validity of the above analysis.