Research On A Wide-Input Range Buck-Boost+LLC Cascaded Converter

Recently, in order to cope with the energy crisis and environmental pollution, electric vehicles(EVs) and hybrid electric vehicles(HEVs) have been developed rapidly. The power batteries with a wide output voltage range supply the energy to the vehicle equipment or the spare batteries via the on-board DC/DC converter. Therefore the on-board DC/DC converter should be capable of operating under constant-voltage or constant-current modes with a wide input voltage variation. To meet the requirements above, this paper studies an on-board Buck-Boost + LLC cascaded converter with a wide input range.The operation principle, DC gain characteristic and control strategy for both the half bridge LLC resonant converter and four-switch Buck-Boost(FSBB) converter are described in detail. On the basis of these analyses, the DC gain characteristic, constant-voltage/-current control scheme, protection circuit and parameters of the four-switch Buck-Boost+LLC(FSBB+LLC) cascaded converter are presented.Based on the FSBB+LLC converter, by making the boost unit and LLC unit share the same switches, the integrated Buck-Boost LLC(IBBLLC) converter is obtained. The DC gain characteristic is discussed. Both the dual and single frequency control strategies are investigated, respectively. With the dual frequency control scheme, the Buck unit operates in the hard switching state with a lower frequency and the shared switches work in the soft switching state with a higher frequency. The modulation strategy, control scheme and ZVS conditions of the dual frequency control scheme are discussed. All switches can achieve ZVS under the single frequency control scheme by applying phase shift control. The different operating modes are analyzed and summarized. The influence of the front-stage inductor Lb on both the power transfer and the Lb’s RMS current is discussed. The ZVS area is given through the research of ZVS conditions. The parameters are designed based on the theoretical analysis.Finally, an experimental prototype with 200~450V input and 14V/80 A output is designed and built to demonstrate the effectiveness and feasibility of both the FSBB+LLC converter and IBBLLC converter. The efficiency curves of all schemes are provided.