Investigation of resonant dc sources and implementation of a two transformers application

The development in power conversion is in constant demand of high power efficiency and high power density. The DC-DC power conversion is an inevitable part of numerous power supplies and energy related applications. Especially, in PV micro-inverters and front-end converter of power supplies, great challanges affect the power performances of the DC-DC converter stage, which not only require high efficiency and density but also the capability to regulate a wide variation range of input voltage and load conditions. Moreover, with increasing operating frequency, passive components sizes are dramatically reduced which is essential in terms of power density. However, switching loss with higher frequency pose a problem. The resonant DC-DC converters are good candidates to meet these challenges with the advantages of achieving soft switching and low EMI. Among various resonant converter topologies, the LLC resonant converter is very attractive for its wide gain range and providing ZVS for switches from full load to zero load condition. Particularly, low profile TV's became a trend in television industry. The power supplies in OLED TV's which require more power than LCD TV's must be low size as well. This thesis presents high output current high power LLC resonant converter with two transformers which are connected in series-primary parallel-secondary. In terms of series resonant characteristic, power MOSFET's achieve soft switching which is zero voltage switching(ZVS) so that power loss of these semiconducters are reduced. In the proposed converter, the primary windings of two LLC converters are connected in series. Thus two converters have the same currents to ensure that two converters can supply the balance load current. In the output side of two LLC converters are connected in parallel to share the load current and to reduce the current stresses on the secondary windings and the rectifier diodes. In this thesis, the principle of operation, different operating modes and design consideration are provided and discussed and 24V/20A experimental prototype were provided. Keywords: LLC resonant converter, soft switching(ZVS), power density, electromagnetic interferance