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CU-Boulder ECEN 4517 - Common-Duty-Ratio Control of Input-Series Connected Modular DC–DC Converters

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 42, NO. 4, JULY/AUGUST 2006 1101Common-Duty-Ratio Control of Input-SeriesConnected Modular DC–DC Converters With ActiveInput Voltage and Load-Current SharingRamesh Giri, Member, IEEE, Vijay Choudhary, Raja Ayyanar, Member, IEEE, and Ned Mohan, Fellow, IEEEAbstract—This paper proposes a simple control method toachieve active sharing of input voltage and load current amongmodular converters that are connected in series at the input andin parallel at the output. The input-series connection enables afully modular power-system architecture, where low voltage andlow power modules can be connected in any combination at theinput and/or at the output, to realize any given specifications.Further, the input-series connection enables the use of low-voltageMOSFETs that are optimized for very low RDS ON,thus,re-sulting in lower conduction losses. In the proposed scheme, theduty ratio to all the converter modules connected in input-seriesand output-parallel (ISOP) configuration is made common. Thisscheme does not require a dedicated input-voltage or load-current-share controller. It relies on the inherent self-correcting charac-teristic of the ISOP connection when the duty ratio of all theconverters is the same. The proposed scheme is analyzed using theaverage model of a forward converter. The stability and perfor-mance of the scheme are verified through numerical simulation,both in frequency domain and in time domain. The proposedcontrol method is also validated on an experimental prototypeISOP system comprising of two forward converters.Index Terms—Common duty ratio, fault tolerance, input seriesand output parallel (ISOP), input-series connection, input-voltagesharing, load-current sharing, modular dc–dc converters.I. INTRODUCTIONMODULAR power-system architecture, where low-power“building-block” converter modules are connected inparallel (at input and at output), to meet high load-currentrequirement has been a subject of vigorous research in therecent past [1]–[13]. The modular approach has several ad-vantages [2], [4], [5]: It leads to a significant improvement inthe reliability of the overall system by introducing the desiredPaper IPCSD-06-023, presented at the 2003 IEEE Applied Power ElectronicsConference and Exposition, Miami Beach, FL, February 9–13, and approvedfor publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATION bythe Industrial Power Converter Committee of the IEEE Industry ApplicationsSociety. Manuscript submitted for review November 1, 2003 and released forpublication April 10, 2006. This work was supported in part by the Office ofNaval Research (ONR) under Award N00014-03-1-0802 and Award N00014-05-1-0622.R. Giri is with Maxim Integrated Products, Sunnyvale, CA 94086 USA(e-mail: [email protected]).V. Choudhary and R. Ayyanar are with the Department of ElectricalEngineering, Arizona State University, Tempe, AZ 85287-5706 USA (e-mail:[email protected]).N. Mohan is with the Department of Electrical and Computer Engineer-ing, University of Minnesota, Minneapolis, MN 55455-0170 USA (e-mail:[email protected]).Digital Object Identifier 10.1109/TIA.2006.876064level of redundancy; standardization of components leads toa reduction in the manufacturing time and cost; present trendof very high-current (and low output voltage) power-supplyrequirements can be met with low-current converters that areeasier to design and manufacture. By suitably interleaving theconverter modules, the filter requirement can be reduced lead-ing to a higher power density, and possibly higher efficiency ofthe overall system [6].The challenge in a parallel-connected modular system isto ensure equal sharing of the load current in spite of thedifferences in various converter parameters, controllers, andimpedances of the interconnects. Several control techniques ofvarying complexity and performance, to meet the above chal-lenge, have been proposed, including different types of droopschemes [7], [8], schemes based on current mode control withcommon current reference, automatic master-slave schemes[9], [10], democratic current-share schemes [3], [11], [12], andfrequency-based current-share schemes [13]. A comprehensivereview and comparison of different control methods for parallelconnection is given in [1].However, just the ability to connect converters in paralleldoes not result in a fully modular architecture. Since, the inputvoltages to a system can be from different sources, and thuscan have a wide range of magnitudes, the converter design stillhas to be different depending on the input-voltage level. Hence,low input-voltage modular converters that can be connected inseries at the input, with active input-voltage sharing capability,are needed to realize a fully modular architecture. In suchan architecture, a standardized building-block module can beconnected in any combination–series or parallel–at the input orat the output to meet any input–output specifications.Apart from considerations of modularity, the input-seriesconnection has several other significant advantages. With activevoltage sharing, low-voltage MOSFETs, which are optimizedfor very low RDS ONcan be used. For typical input voltagesof dc–dc converters, the combined RDS ONof N MOSFETs inseries, each with a voltage rating of BVDSS/N , is substantiallylower than the RDS ONof a single MOSFET with a voltagerating of BVDSS[14]. Another advantage is that, for thepresently popular low output-voltage applications, the input-series and output-parallel (ISOP) configuration requires smallerconversion ratios for the individual converters, thus, leading toimproved efficiency.However, in spite of the many advantages, only a very fewpapers have been published on input-series connection of mod-ular converters. The challenge here is to ensure equal sharing of0093-9994/$20.00 © 2006 IEEE1102 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 42, NO. 4, JULY/AUGUST 2006Fig. 1. Four possible combinations of input–output connections. (a) Inputparallel and output series. (b) IPOP. (c) ISOP. (d) Input series and output series.the total input voltage, in the presence of substantial differencesin various converter parameters. In [15], ISOP connection hasbeen implemented for a two-converter system, using a chargecontrol scheme with input-voltage feed forward. In [16], [17], athree-loop control scheme, including a dedicated input-voltagecontroller for ISOP connection has been presented.This


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CU-Boulder ECEN 4517 - Common-Duty-Ratio Control of Input-Series Connected Modular DC–DC Converters

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