Input-Series-Output-Parallel Converter System With Natural Voltage-Sharing Characteristics

In the power systems for high-speed train and subway,the input voltage of the auxiliary power supply is 750 V or 1500 V.In these applications with high input voltage,the input-series-outputparallel(ISOP)converter system,in which multiple converter modules are connected in series at the input side and in parallel at the output side,could be adopted for reducing the voltage stress of the power devices.The primary control objective for ISOP converter system is to ensure input-voltage sharing(IVS)and output-current sharing(OCS)among the constituent modules.This thesis focuses on a natural voltage-sharing ISOP converter system.An ISOP converter system based on the boost+LLC resonant converter is proposed in this thesis.In the boost+LLC resonant converter,the boost converter is used to regulate the output voltage,while the LLC resonant converter is operated with constant switching frequency for achieving electrical isolation and voltage matching.For the input-series boost converters,the input current of the boost converter is continuous,thus the input dividing capacitors can be removed.Furthermore,the interleaving control is employed for greatly reducing the boost inductors.The operating principle of the LLC resonant converter is analyzed,and the LLC resonant is intentionally operated as a dc transformer(DCX)with appropriate parameters design.The mechanism of IVS and OCS of the proposed ISOP system is analyzed in this thesis.By applying a common duty ratio to all the boost converters and designing the LLC resonant converter as a DCX,the ISOP converter system achieves natural voltage sharing without any additional control.Thanks to the DCX characteristic of the LLC resonant converter,its voltage gain remains nearly unchanged even when the parameters drift.Thus,a good voltage sharing is achieved when the parameters of the LLC resonant converter are not well matched.For the boost converters,the mismatches between all the boost inductors do not affect IVS and OCS since they are connected in series and can be combined into one inductor.So,compared with a single-stage isolated dc-dc converter,the parameters inconsistency in the proposed boost+LLC resonant converters has less influence on IVS.The parameters design for the LLC resonant converter are discussed,and the characteristic of DCX is kept even when the parameters have relatively large variation range.The proposed boost+LLC resonant converter is extended such that the two-stage converter could be in the form of pre-regulator + DCX and DCX + post-regulator modules.With these two basic modules,both the ISOP converter system and the input-parallel-output-series(IPOS)converter system are able to achieve natural input/output voltage sharing when the pre-regulator or post-regulator are controlled with common duty cycle,while the operation of the input-parallel-output-parallel(IPOP)and input-series-output-series(ISOS)converter systems are unstable.Finally,a 12-kW ISOP converter system consisting of two boost+LLC resonant converters is built and tested in the lab.The experimental results verify the effectiveness of the proposed natural voltage-sharing ISOP converter system.