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1、http:/-1-An Improved Large-Signal Model of Downstream DC/DC Converters for Front-End PFC Rectifier Design and Its Verification by Computer Simulation1 Runxin Wang,Jinjun Liu School of Electrical Engineering,Xian Jiaotong University,Xian,CHINA Abstract This paper applied the whole modeling process of
2、 the large-signal model to an UCC3802-controlled forward DC/DC converter,which was fed by an UC3854-controlled PFC rectifier,and also contributed some improvements to the definition of this model.Simulation results showed that the improved large-signal model was accurate enough in treating the large
3、-signal transient process and it was also more efficient in computer simulation.Keywords:DC/DC converters,PFC,Computer simulation,Models 1 Introduction To investigate the large-signal interactions among cascaded or paralleled converters in a distributed power system(DPS)and avoid using time-consumin
4、g switching models,paper 1 proposed a large-signal model of downstream DC/DC converters,which was expected to be used in analyzing and designing the upstream power factor correction(PFC)rectifiers,and gave out some experimental results as proofs for the key ideas in the proposed modeling process.Pap
5、er 2 and 3 made further efforts and implemented the above model in SABER environment.Also,by feeding a commercial DC/DC converter with a self-designed PFC rectifier,some detailed waveforms were obtained to show validity of the model in specified situations.But for a complete application of this mode
6、l and an intuitive verification of its merits,we still lack a concrete example so far.This paper applies the whole modeling process to an UCC3802-controlled forward DC/DC converter,which was fed by an UC3854-controlled PFC rectifier.Simulation results for both circumstances,that is,using switching m
7、odel for both PFC rectifier and DC/DC converter and using switching model and the large-signal model for them respectively,are given,providing an effective way for comparison.Some improvements to the large-signal model definition are contributed when implementing the DC/DC converter,in order to emul
8、ate new features found in current mode control and UCC3802 controller.2 Simulated system and its new features Fig.1 shows the system been simulated in this paper.The upstream single-phase PFC front-end rectifier was controlled by UC3854 while Ucc3802,which made the converter work in current mode,1 T
9、his work was support by the Specialized Research Fund for the Doctoral Program of Higher Education(20040698061).http:/-2-controls the downstream forward DC/DC converter.The power rating was designed to 500 watts,and the maximum and minimum input voltages of DC/DC converter for normal operation are 5
10、00V and 250V respectively.Figure 1:The system composed of a front-end PFC rectifier and a downstream DC/DC converter.With the DC/DC converter working in current mode and controlled by the UCC3802 controller,one can get some new features based on the following observations:The powering circuit of UCC
11、3802 was greatly simplified but the time delay due to the capacitor charging that make VCC reach its turn-on threshold could not be eliminated.And its large hysteresis optimized for off-line power operation also add difficulties to define the turn-on and turn-off moment that must be considered when
12、modeling this controllers behavior;When UCC3802 leaves its normal operation area for any reasons,e.g.when the input voltage drops to a value below 250V,the controller holds the output low,which makes the MOSFET switch turn off,and the DC/DC converter becomes equivalent to open-circuit to the PFC rec
13、tifier,rather than a constant resistor;In internal circuits of UCC3802,the 1.0 volt cycle by cycle current limit threshold and the soft-starting features effectively limit the input current of DC/DC converter and eliminate the potential for excessive current overshoot while the system is starting up
14、.Actually,when starting up or experiencing a large-signal transient process,both input current of the forward DC/DC converter and filter inductor current in the secondary side have envelope or waveform with the almost same shape as shown in Fig.2,although they have different meaning.Wire current:-:V
15、IN#P/A02468101214time/mSecs2mSecs/div024681012141618Inductor_Current/A024681012141618202224Inductor_CurrentWire current:-:VIN#P Figure 2:Input current of a forward DC/DC Converter and its filter inductor current in secondary side share a waveform(envelope)of almost same shape.http:/-3-3 Improved lar
16、ge-signal model and its implementation in SIMPLIS environment According to above analysis,some additional features should be taken into consideration when applying the large-signal model to the investigated converter.This paper contributed the following improvements to the larger-signal model and im
17、plemented this improved model in SIMPLIS environment as shown in Fig.3.Firstly,a newly established time delay and hysteresis module was added into the model for dealing with potential time delay and hysteresis characteristics.In Fig.3,two hysteresis comparators serve the input voltage and VCC of the
18、 controller respectively.And another resistor and capacitor were used to generate the delay time before VCC reaches its high threshold value;Then,being new members in the current limit and waveform definition group,a timer and a bypassing switch are always ready to shape the current waveform accordi
19、ng to Fig.2 at the beginning of each large-signal transient process,working together with the soft-start circuit;Finally,the definition of piece-wise linear(PWL)has to be modified according to the new input-to-output characteristics.For model of the investigated converter,it could be left as a linea
20、r resistor when input voltage is below its minimum operation value(250V)because the circuit has ensured that operation point would never enter this area,but as an engineering rule,this step is necessary.S3U32V_ZENERIDEAL_DIODE1PWLE21E11G11ZENERC12U6V1C13C14U4R2V3C15U2IDEAL_DIODE21 R19F21G2200mS1C16U
21、5R20 Figure 3:SIMPLIS-based improved model.4 Comparison and analysis of simulation results Simulation results of the system while using switching model for both PFC rectifier and DC/DC converter are shown in Fig.4,and those in circumstance while using switching model for PFC rectifier but the large-
22、signal model for DC/DC converter are shown in Fig.5.The simulation objective is to get key waveforms of the system during a time range from 0 ms to 300 ms,in which both startup process and another transient process reasoned by voltage sag between 80 ms and 200 ms are included.http:/-4-The waveforms
23、of AC current and PFC rectifier output voltage in the two different circumstances as shown in Fig.4(a)and Fig.5(a)are so similar that even authors were surprised when they were first time displayed.Fig.4(b)shows the load voltage waveform that is easy to be imagined and also the filter inductor curre
24、nt with the same shape shown in Fig.2.Fig.5(b)is a purely virtual waveform,which can be obtained only by the large-signal model.What it revealed is the average current fed into the input terminal of the DC/DC converter by PFC rectifier and one can clearly see the constant power characteristics of th
25、e downstream DC/DC converter when its operated in normal status.The only big difference lays that it took 4 minute and 4.35 second of CPU time to get the simulation result by using the large-signal model,which is much less than 35 minute and 17.07 second of CPU time,taken by its counterpart using sw
26、itching model.time/mSecs50mSecs/div050100150200250300VY2-300-200-1000100200300400I_AC/AY1020406080I_AC(Y1)VIN_AC_side(Y2)VOUT_PFC(Y2)time/mSecs50mSecs/div050100150200250300V_load/VY2051015202530354045Inductor_Current/AY105101520Inductor_Current(Y1)V_load(Y2)Figure 4:Switching model simulation result
27、 for startup and transient process due to voltage sag during 80 ms to 200 ms.5 Conclusion This paper applied the whole modeling process of the large-signal model to an UCC3802-controlled forward DC/DC converter,which was fed by an UC3854-controlled PFC rectifier,and also contributed some improvement
28、s to the definition of this model.Simulation results showed that the improved large-signal model was accurate enough in treating the large-signal transient process and it was also more efficient in computer simulation.http:/-5-time/mSecs50mSecs/div050100150200250300VY2-300-200-1000100200300400I_AC/A
29、Y1020406080I_AC(Y1)VIN_AC_side(Y2)VOUT_PFC(Y2)(a)AC voltage(only for time reference),AC current and output voltage of PFC rectifier.time/mSecs50mSecs/div050100150200250I_in_Model/A00.20.40.60.811.21.41.61.8I_in_Model(b)Input current to the large-signal model emulating the forward DC/DC converter.Out
30、put voltage ofPFC rectifierAC currentAC voltage(onlyfor time reference)Figure 5:Simulation result for startup and transient process due to voltage sag during 80 ms to 200 ms,while emulating the DC/DC converter by large-signal model.Acknowledgments The authors thank Transim Technology Corporation for
31、 SIMPLIS software supports.References 1 Jinjun Liu,T.G.Wilson,Qun Zhao,Wei Dong and F.C.Lee,“Large-signal model of a downstream DC/DC converter for analysis and design of front-end PFC rectifier using computer simulation,”IEEE APEC 2003,pp.1002-1007,2003.2 Lu Chen,Jinjun Liu,Pu Zhang and Zhaoan Wang
32、,“A model of DC/DC converters in multi-module inter-connected power electronic systems and its implementation in SABER environment,”(in Chinese)The 9th PES/CES Annual meeting,Qinhuangdao,China,2004.3 Lu Chen,“Modeling and Simulation for DC/DC Converters in Multi-Module Power Electronic System,”(in Chinese)Master degree thesis in Xian Jiaotong University,Xian,China,2005.Authors Brief Introduction：Runxin Wang,a Ph.D.a student in school of electrical engineering,Xian Jiaotong University.Jinjun Liu,a professor in school of electrical engineering,Xian Jiaotong University.