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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 3, MARCH 2014 1275

Analysis of Unified Output MPPT Controlin Subpanel PV Converter System

Feng Wang, Student Member, IEEE, Xinke Wu, Member, IEEE, Fred C. Lee, Fellow, IEEE,Zijian Wang, Member, IEEE, Pengju Kong, Member, IEEE, and Fang Zhuo, Member, IEEE

Abstract—Photovoltaic (PV) systems frequently suffer dispro-portionate impacts on energy production due to mismatch cases.To remedy this, academia proposed a distributed max power pointtracking (MPPT) solution and has been implemented commer-cially. Taking the trend of the “distributed MPPT” concept a stepfurther, this paper discusses and analyzes an MPPT converter thatconnects to each PV cell string, called a subpanel MPPT converter(SPMC), to better address the real-world mismatch issues. TheSPMC system with a unified output MPPT control structure is alsoproposed in order to reduce the cost and simplify the distributedMPPT system. The proposal saves A/D units, current sensors, andMPPT controllers on the premise of guaranteeing that the SPMCis working on its optimal maximum power point regardless of themismatch case. This is favorable for the further integration andmakes the whole SPMC system less expensive and easier to realize.Finally, the effectiveness of the proposal is confirmed experimen-tally.

Index Terms—Photovoltaic (PV) system, subpanel MPPT(SPMC) converter, unified output control.

I. INTRODUCTION

A S global demand for energy continuously increases, sohas the need for renewable energy sources (RESs) that

minimize impact on the environment. It has given rise to thedevelopment of electronic power distribution systems (EPDS),such as nanogrid–microgrid–···–grid structure, utilizing multi-ple RES as supplementary energy source to utility grid. DCnanogrid, one kind of EPDS at low power level (10–100 kW), isaddressed as a promising EPDS comparing to ac nanogrid fromfollowing aspects: higher overall system efficiency, starting with

Manuscript received November 25, 2012; revised January 28, 2013 andMarch 27, 2013; accepted April 22, 2013. Date of current version September18, 2013. This work was supported in part by the CPES Industry PartnershipProgram, and in part by National Natural Science Foundation of China (No.51177130 and No. 51007081), and in part by Delta Science and TechnologyEducational Development Program (No. DREK2011002). Recommended forpublication by Associate Editor C. N. M. Ho.

F. Wang and F. Zhuo are with the State Key Laboratory of Electrical Insulationand Power Equipment, School of Electrical Engineering, Xi’an Jiaotong Uni-versity, Xi’an, Shaanxi 710049, China (e-mail: [email protected];[email protected]).

X. Wu is with the Department of Electrical Engineering, Zhejiang University,Hangzhou, Zhejiang 310058, China (e-mail: [email protected]).

F. C. Lee is with the Center for Power Electronics Systems, Virginia Tech,Blacksburg, VA 24061 USA (e-mail: [email protected]).

Z. Wang is with the Linear Technology Corporation, Milpitas, CA 95035USA (e-mail: [email protected]).

P. Kong is with the iWatt, Inc., Santa Clara, CA 95008 USA (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2013.2262102

Fig. 1. DC nanogrid structure.

fewer power converters, easier interface of RESs to a dc system,no frequency stability and reactive power issues, no skin effect,and ac losses. What is more, the consumer electronics, elec-tronic ballasts, LED lighting, and variable speed motor drivescan be more conveniently powered by dc. As shown in Fig. 1,all RES and appliances are integrated to dc bus by using bidi-rectional power electronic converters as energy control centerstaking charge of interfacing dc bus with utility ac grid [1]–[3].

Solar energy, therefore, is no doubt a suitable RES for sucharchitecture because of intrinsic dc output characteristics. How-ever, because of shadows, dirtiness, manufacturing tolerances,thermal gradients, aging, different module orientations and tilts,etc. [4], the ideal irradiance is practically impossible and themismatch cases always impact the performance of the PV sys-tems. For the centralized or string level MPPT PV systems, theconsequences of the aforementioned mismatch cases are degra-dations in total power harvest, multiple maxima power pointsissues on the power-voltage curve and MPPT algorithms canfail [5]–[7]. Moreover, even when the global maximum powerpoint of the shaded PV system is reached with some advancedalgorithms [8]–[14], because the shaded part of the PV systemwould limit the output current of the nonshaded part [15], sucha power is still lower than the sum of the available maximumpowers of the mismatch parts.

In reaction to these problems, a distributed MPPT solution,each PV panel connect with a dedicated MPPT converter, hasbeen proposed from academia and implemented commercially.The panel level MPPT converter is commonly referred to as “PVoptimizer” or “module integrated converters (MIC),” and it is

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1276 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 3, MARCH 2014

concerned essentially with the current PV system [4], [16]–[25].In [18], Walker and Sernia examined four nonisolated topologiesas possible cascadable converters for the PV optimizers. Theadvantages and drawbacks of such topologies are examined indetail. In [17] and [26], the authors proposed an improved multi-mode four switch Buck/Boost PV optimizer to increase energycapture in a PV optimizer string. The panel level distributedMPPT solution can, at best, eliminate the mismatch power lossamong PV panels. However, in a real-world mismatch case,a shaded PV panel cannot be just exactly obstructed, so theperformance of PV optimizer-based solar system is still lessthan satisfactory in such cases. Of similar concern are the smallscaled mismatch cases, such as dust, bird droppings, or damagedPV cells which can result in a disproportionate power loss inPV systems. Such cases happen more frequently but are usuallygiven less attention. Taking the trend of the “distributed MPPT”concept a step further, this paper focuses on a distributed MPPTstructure that connects each PV cell string with a dedicatedMPPT converter, called a subpanel MPPT converter (SPMC)module, to address the real-world mismatch issues and givenbetter performance in power recovery comparing with currentPV optimizers.

This paper is organized as follows: in the next part, the dis-tributed MPPT concept is introduced, which can be appliedto improve the performance of the PV system in real mismatchcases. The performance comparison of the current PV optimizerand the proposed SPMC system is given in Section III. Based onthe SPMC concept, a novel unified output MPPT control strat-egy is proposed accordingly in order to optimize and simplifythe distributed MPPT control solution as shown in part IV. Inthe fifth part, the reliable issue of the SPMC is discussed andin Section VI, simulation and test results are presented to verifythat the SPMC PV system can achieve a more effective powerharvest performance with the proposed control strategy. Finally,the paper ends with some concluding remarks and future work.

II. ANALYSIS OF DISTRIBUTED MPPT CONVERTER

Fig. 2(a) shows a standard PV panel consisting of PV cellstrings connected in series, divided in three parts by correspond-ing bypass diode. Bypass diodes prevent the appearance of hotspots and protect the PV module from potentially destructiveeffects. The PV module is connecting with a MPPT converterwhich always operates the PV module at its maximum powerpoint. So the MPPT converter together with the PV moduleis operating as a constant power source, the power of whichis determined by the peak power of the PV module, at a rel-atively wide voltage/current range at the output side, makingit possible to cascades with other converters in series or paral-lel. In other words, the distributed MPPT converter changes theMPP of the PV panel from a single voltage/current point into awide voltage/current range, shown as the green solid curve ofFig. 2(b). In a traditional PV system with centralized MPPT ar-chitecture, any disturbance can shift the maximum power pointof the module, and results in a significant power decrease un-less the module’s output voltage is adjusted. However, withdistributed MPPT structure, the peak power of the PV module

Fig. 2. Concept of distributed MPPT converter. (a) PV unit and distributedMPPT converter. (b) Output curve of PV unit and optimizer.

can be achieved over a very wide range of voltages, so evenwhen disturbances occur an adjustment to the output voltage ofthe distributed MPPT system, it still can maintain peak power.Distributed MPPT converter is usually implemented with a dc/dcpower converter. Three possible converter topologies are takeninto consideration in this paper because of their simplicity, highefficiency, and the capability of cascade operation as shown inFig. 3 [23], [24], [27]. The blue I–V and P–V curves indicate theoutput characteristic curves of an original PV panel, and theyare identical in each graph. The point M stands for the MPP ofthe original PV unit and the N1 and N2 indicate the initial pointand ending point of the MPP region, respectively, at the outputside of the distributed MPPT converter. The merit and demeritof the three topologies are given as follows: the Boost converteris only suitable for parallel connection, the output current ofBoost-type MIMC is inherently limited by the characteristic oforiginal PV panel. For the Buck converter, series connectionis a better choice and the inherent voltage limit characteristicis achieved and the Buck/Boost converter enjoys most of thebenefits of both Buck and Boost at the expenses of higher costand more complex control solution.

One important thing to note here is that the second stagecentral MPPT converter is still required in the distributed MPPTconverter-based PV system. However, the enlarged MPP regionmakes the MPPT of the second converter much easier, faster,more economical, and efficient when facing the mismatch [28].

III. STRUCTURE OF SUBPANEL MPPT CONVERTER

In most mismatch conditions, such as module-to-module dif-ference, different module orientations, and tilts, etc., about 10%–30% of annual performance loss or more can be recovered byusing the PV optimizers or PV MICs [28]–[31]. However, fre-quently, partial PV panel cannot work as expected which resultfrom dust and spot dirtiness such as leaves or bird droppingsor damage of PV cells, etc., the PV optimizer’s performanceis less than satisfactory in such cases. Since the panel is com-posed of several PV cell strings, taking the trend of “distributed

WANG et al.: ANALYSIS OF UNIFIED OUTPUT MPPT CONTROL IN SUBPANEL PV CONVERTER SYSTEM 1277

Fig. 3. Output characteristic curve of three topologies. (a) Boost converter. (b) Buck converter. (c) Buck/Boost converter.

MPPT” concept a step further, papers [31]–[37] propose to di-vide the standard PV module into several parts and implementdistributed MPPT solution into subpanel level. This part dis-cusses a SPMC system with three PV cell-string level dc/dcconverter that executes MPPT separately for sections of an in-dividual PV module which provides a better solution in orderto address the real-world mismatch impact. For the SPMC sys-tem, the output terminals of all the MPPT converters can beconnected either in parallel or in series. For the parallel con-nection, the control is relatively simple, but the high-voltagegain will increase the cost and reduce the efficiency. And forseries connection, lower rating devices and lower voltage gaincan be the promising candidate for a low cost and high effi-ciency distributed solar system [26]. Because of simple, highefficiency, and suitability for series connection as aforemen-tioned, the Buck-type converter is chosen as implementation ofthe SPMC. By employing low-voltage synchronous buck con-verters connected across each PV cell string, a high-frequency,high-efficiency SPMC power stage can be achieved as shownin Fig. 4. From the input side of each Buck converters, the con-verters are parallelly connected with each PV cell strings. Fromthe output side of the MPPT converters, they are connected inseries connection. One point should be noted that in this SPMCsystem, the bypass diodes inside the junction box of a standardPV module should be retained in case of the malfunction of theMPPT converters. For the convenience of theoretical expressionof the SPMC, the diodes are not shown here and the detailedinformation about the reliable issues is given in the fifth part.

The proposed SPMC provides the following benefits [29],[30]:

Fig. 4. SPMC diagram. (a) Distributed MPPT SPMC concept. (b) Implemen-tation of SPMC with Buck converter.

1) In such structure, the series rather than parallel connectionof MPPT converter allows the input–output voltage ratioto be close to unity in ideal irradiance case, which leadsto the highest switch utilization and is at a performanceversus cost disadvantage.


Fig. 5. Output I-V and P -V curve of SPMC system. (a) Original PV cell strings. (b) Each MPPT converter. (c) SPMC.

Fig. 6. Output I-V and P -V curve comparison. (a) Original PV panel. (b) PV optimizer. (c) SPMC.

2) Compared to a higher voltage level device used in theMICs, the lower voltage level device used in the SPMCapplication has better performance in efficiency.

3) Further distributed MPPT solution allows better perfor-mance in real-world mismatch cases comparing with PVoptimizers, and for series Buck MPPT converters, all thePV cell strings can guarantee always working on its indi-vidual MPP regardless of a mismatch case.

The output I–V and P–V curves of the three PV cell groups areshown in Fig. 5(a): blue curve and red curve indicate nonshadedand shaded PV cell string separately. In Fig. 5(b), the solid linesstand for typical output curves of a Buck MPPT converters innonshading (blue curve) and shading cases (red curve). Addingthem up, the output I–V and P − V curves of the SPMC systemof a PV panel are shown as black line in Fig. 5(c).

As we can see, if a few PV cells inside a PV panel are inshading case, the output characteristic of the shaded PV panel

suffers multipeak issues and power loss as shown in Fig. 6(a). Insuch conditions, the PV optimizer can only track the maximumpower point of the multipeak curve of the shaded PV paneleven adopting some advanced MPPT algorithms as shown inFig. 6(b), but still lose the power of the shaded PV cell string[24].

However, the SPMC introduces an autonomous MPPT con-verter for each PV cell string in a standard PV panel. So thecapability of performing the independent MPPT function oneach PV cell string basis is hereby achieved and it regulatesthe duty cycle of the power stage separately in order to de-couple a PV cell string from the others inside a PV panel.So a PV panel is divided into three independent parts andthe mismatch case in one cell string cannot affect the others,and the power loss resulting from mismatch among PV cellstrings, about 22% in this case, is thereby recovered as shown inFig. 6(c).


Fig. 7. Unified MPPT control of SPMC diagram.

In this part, the SPMC concept is proposed and the workingprinciple is introduced as well. However, although mismatchloss can be recovered through the SPMC with independentMPPT control, the implementation cost of the SPMC system ishigher due to the increase in component count. A set of MPPTcontrol IC, current sensor, voltage sensor, and correspondingA/D converters are needed for every PV cell string. In orderto address the above issues, an optimal control method for theSPMC solution is proposed in next section.

IV. UNIFIED OUTPUT MPPT CONTROL IN SPMC SYSTEM

In order to reduce the cost and simplify the independentMPPT control in SPMC structure, a unified output voltage con-trol with single MPPT detection strategy is proposed in thispart [38], [39], as shown in Fig. 7. In this structure: 1) a singleMPPT unit is sensing the output power of the SPMC systemwith only one pair of voltage and current sensors; 2) three BuckMPPT converters share a common Vref coming from the sin-gle MPPT unit; and 3) each Buck MPPT converter owns anindependent control loop.

Therefore, the output voltage signal of the MPPT control unitis the common MPPT voltage reference for all the converters ina SPMC module, during the MPPT period. The PWM controllerof each Buck converter in the SPMC system compares the sensedoutput voltage of each PV cell string and the common MPPTvoltage reference to control their respective switch. When thecommon voltage reference is perturbed by the unified outputMPPT controller, the input voltage of each Buck converter is

regulated by an independent closed PWM control loop. Hence,the input voltage perturbation can be achieved.

Because of their series connection, the Buck converters sharea same output current. Therefore, the output voltage of eachBuck converter will vary according to the extracted maximumpower from its individual PV cell strings and proportionate tothe maximum power. So the total output voltage of the SPMCis the sum of the output voltage of each MPPT converters

Vout =3∑

n=1

Vo n . (1)

Although the PV cell string MPP voltage may change withirradiance case or temperature, it is assumed that such changescan be considered relatively small [32]. For the same Vref signalis given to three independent control loops, so the output voltageof each PV cell string in steady state should be the same andequal to Vref

Vpv1 = Vpv2 = Vpv3 = Vref . (2)

And the duty cycle of each MPPT converter in steady statecan also derived

Vo1

Vpv1= D1 ,

Vo2

Vpv2= D2 ,

Vo3

Vpv3= D3 . (3)

If no mismatch happens, the SPMC should be working withhigh conversion efficiency and all the maximum power pointsof the three PV cell strings are exactly the same. Therefore, theoperating condition of each Buck converter in SPMC systemis same as well. If mismatch case happens with part of a PVmodule, the power coming from the shaded PV cell string isdecreased and the duty cycle of the corresponding MPPT con-verter is also decreased accordingly in order to save the powerof shaded PV cell string and adjust the common output currentlimitation. At this point, the SPMC system is working as a con-stant power source with different output voltage and current. Sowe can say that the conversion ratio and duty cycle for eachconverter can vary over wide range

D2 < D1 = D3 . (4)

Fig. 8(a) indicates the output I-V and P -V curves of shaded(red curve) and nonshaded (blue curve) PV cell strings, respec-tively. Because the voltage reference of the MPP is given by asingle MPPT unit, so the constant power curve of the output ofeach SPMC should start at a same voltage value and ending atcurrent limit of each SPMC as blue and red solid curve shownin Fig. 8(b). The final voltage reference from the MPPT unitis neither the MPP of shaded cell string nor the MPP of thenonshaded PV cell strings, it only stands for a tradeoff statepoint where the output power of three parallel PV cell stringscan reach the maximum in a same voltage value as shown in theenlarged view of the Fig. 8(b), adding the output curve up andthe characteristic curve of the whole SPMC system is shown asthe black curve in Fig. 8(c).

Final comparison is made among aforementioned structuresin Fig. 9. It shows the simulation comparison of the output P -Vcurves among the current PV optimizer, the distributed MPPT


Fig. 8. Output I-V and P -V curve of SPMC with proposed control solution. (a) Original PV cell strings. (b) Each MPPT converter. (c) SPMC.

Fig. 9. Comparison of different structure. (a) PV Optimizer. (b) SPMC with distributed MPPT. (c) SPMC with unified MPPT.

SPMC solution, and the proposed unified output MPPT SPMCsolution.

It is obvious that the proposed SPMC system with optimalsingle MPPT control has several advantages, both from the the-oretical and the practical point of view. First, the architecture ismore suitable for power recovery compared with PV optimizerin real mismatch cases. Second, the power rating of the de-vice can be reduced to lower level, which is good for efficiencyimprovement. And the proposed optimal control approach canrecover more than 90% power loss caused by mismatch casewith less circuit components and lower cost comparing with thesubpanel level distributed MPPT solution.

V. RELIABLE ISSUES OF THE SPMC SYSTEM

The junction box, presented in each standard PV panel, pro-vides the key bypass functionality (preventing hot-spot phenom-ena caused by reverse biasing due to defective cells or shading

in traditional PV module). Generally, the bypass diodes insidethe junction box are antiparallel and one-to-one connected tothe subpanel PV cell strings as shown in Fig. 10(a).

Regarding to the system reliability issues, the bypass diodesinside the junction box of the original PV module should beretained and antiparallel with the SPMC converter as shownin Fig. 10(b). Because the output side of a SPMC module-based PV system is connected with dc nanogrid, to simplifythe analysis for the reliable issues, we assume that a SPMCmodule is connecting with a constant voltage source Vout .Moreover, we need to make statements before the reliableanalysis:

1) the output voltage of each Buck converter inside a SPMCmodule is Vo1 , Vo2 , and Vo3 ;

2) the MPP voltage of each PV cell string is VMPPT1 ,VMPPT2 , VMPPT3 ;

3) the open circuit voltage of each PV cell string is VOC1 ,VOC2 , VOC3 .


Fig. 10. Structures of standard PV panel and SPMC system. (a) Standard PVPanel. (b) SPMC Module.

If one of the converters in SPMC, converter #1 for example,is failed, the analysis can be divided into following three cases:

A. Vout > VOC2+VOC3

In this case, the bypass diode of converter #1 will never con-duct because the maximum output voltage of the Buck converteris the open circuit voltage of the PV cell string, so the MPPTunit loses control in such case.

B. VMPPT2+VMPPT3 < Vout < Vo2+Vo3

In this case, the sum of the output voltages of the remainednormal converters #2 and #3 is slightly larger than the Vout ,so the failed converter is hereby bypassed by the correspond-ing diode and the Vout also clamps the output voltages of theconverters #2 and #3. As the purpose of the MPPT is keepingthe operating point of the PV cell string always stay on MPPthrough the control loop, and the higher output voltage requiresthe converters have boost function. So the Buck converters areworking at go-through mode at this time. In this case, the MPPTunit loses its control and the remaining two PV cell strings canbe seen as connected with the voltage source directly.

C. Vout < VMPPT2+VMPPT3

In this case, the input voltages of converters can be controlledat MPP through the MPPT control loop and the output voltageof the remaining two converters #2 and #3 are working towardanother steady-state point if converter #1 is failed. The two

Fig. 11. Experimental prototype.

converters are connecting with the dc bus through the bypassdiode of failed converter #1.

It needs to note that the consideration of the architecturedesign of the SPMC-based PV system should be paid moreattention. Especially, the number of the SPMC modules in astring should be large enough in case the bypass diode of thefailed converter blocks the power flow path. It also affectedby external factors such as the dc bus voltage level, the MPPparameters of the PV panel, and the irradiance case, etc.

VI. EXPERIMENTAL RESULTS

To verify the SPMC concept and proposed unified MPPTcontrol strategy, an experimental prototype is constructedas depicted in Fig. 11. The hardware setup consists of thefollowing parts.

A. Solar Simulators [40]

For the sources, three E4361 Agilent solar simulators are usedto simulate three PV cell strings inside a standard PV panel. Thesolar simulator is capable of quickly simulating the output char-acteristic curve of PV panels under different irradiance casesby setting the following parameters: open circuit voltage VOC ;MPP voltage VMPPT ; short circuit current ISC ; MPP currentIMPPT .

B. SPMC Power Stage

The power stage of the SPMC system is made up of threesynchronous Buck converters with a series connection on theoutput side as shown in Fig. 7. The input of each Buck con-verter has a one-to-one connection with all three E4361 solarsimulators, so the power rating of each buck stage is designed tomeet the power rating of one-third of a PV module. The outputstage of the SPMC was designed with the 9-A current limit andit is connected with an electrical load.

C. Control Board and Electrical Load

An ALTERA Cyclone III FPGA [41] development board withcorresponding AD9254 and DAC5672IPFBR converters is usedfor the single output MPPT realization, which can be taken placeby a MPPT IC if mass production is needed, and all the other con-trol components are all analog devices due to cost consideration.The static I-V and P -V curves of the proposed SPMC systemcan be derived through adjusting the output current through an


Fig. 12. Control signal for each MPPT converter under different cases.(a) Case A. (b) Case B. (c) Case C. (d) Case D.

TABLE ITEST PARAMETERS

electrical load to make the operating point of the SPMC boardscan from zero to current limit.

Fig. 12 indicates the control signal for each MPPT Buckconverter in different irradiance cases. In Fig. 12(a), all threePV cell strings are in unify ideal irradiance as condition I inTable I, so the maximum power of the three PV cell stringsare exactly the same, and the common maximum power voltagereference signal given by the single output MPPT unit can makeall three PV cell strings working on their maximum power point.Because the converters are working with same input and outputcurrents, so the duty cycles of each Buck converter are same aswell. In Fig. 12(b), cell string II is under shading condition IIIand the other two cell strings are under condition I, because themaximum power of the shaded PV cell string II is lower than theothers, so does the output voltage of the shaded Buck converter.Because the input voltage is set to be a same value, therefore itsduty cycle is smaller than the other two duty cycles. In Fig. 12(c),two of the cell strings are under shading condition III and theother one is under shading condition I, then the power comingfrom the shaded PV cell strings are decreased and the dutycycle is also decreased accordingly, similar as previous case. Ifall three PV cell strings are in different shading conditions I,II, III, respectively, and the duty cycles are also different as inFig. 12(d).

For the purposes of calculating the energy output of SPMCsystem, the dc model of the behavior is sufficient since thetime-scales of the transient behaviors in the power electronicconverters are short. The output characteristic I-V and P -Vcurves of the SPMC in different shading cases as mentionedbefore are shown in Fig. 13 which has a one-to-one relationship

Fig. 13. Tested output P–V curve of PV panel and SPMC. (a) Case A.(b) Case B. (c) Case C. (d) Case D.

with the cases in Fig. 12 because the current limit for the SPMCis 9 A. When the current reaches the current limit, the converteris shut down, so there is no current limit curve shown in the I-Vcurves.

From the earlier figures it’s clear that the SPMC system withunified output MPPT control has wider maximum power regionand higher output power compared with current PV optimizersolution in real-world mismatch case. The proposal also pro-vides comparable power recover ability regardless of shadingcase with less components, lower cost, and much simpler controlmethod comparing with subpanel distributed MPPT structure.

VII. CONCLUSION

For the purpose of improving the performance of PV systemin dc nanogrid under common mismatch conditions, this paperexplores the benefits of distributed MPPT solution through theuse of SPMC structure, which can be seen as the reduced versionof the current PV optimizer, connecting each PV cell string witha Buck converter. The approach offers many advantages includ-ing better power harvest ability, independent control loop, etc.In order to reduce the cost and simplify the SPMC structure,a unified input voltage control with single output MPPT de-tection strategy is proposed accordingly. The PV system basedon the proposed SPMC unit can recover nearly all of powerlosses caused by real-world mismatch case. Comparing the


distributed MPPT control structure with the SPMC PV system,this simplified control approach offers a number of additionalpractical implementation advantages such as: saves the numberof A/D units, current sensors, and MPPT controllers units on thepremise of guaranteeing maximum power statue regardless ofthe mismatch case. The simulation and experimental results ver-ify that the proposed SPMC with unified output MPPT controlsolution exhibits good performance under inhomogeneous andhomogeneous irradiations with an enhancement rate of about20% in power harvest.

Future work will envisage a deeper study of the distributedMPPT converter-based PV systems in order to better justify theapproach from a theoretical viewpoint. Possible extensions toother types of SPMC topologies are also of interest.

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Feng Wang (S’08) received the B.S. and M.S. de-grees in electrical engineering from Xi’an JiaotongUniversity, Xi’an, China, in 2005 and 2009, respec-tively, where he is currently working toward the Ph.D.degree in electrical engineering. From 2010 to 2012,he was an exchange Ph.D. student in the Center forPower Electronics Systems, Virginia Polytechnic In-stitute and State University, Blacksburg, USA.

He is currently with the State Key Laboratory ofElectrical Insulation and Power Equipment, Schoolof Electrical Engineering, Xian Jiaotong University

and also with the Center for Power Electronics Systems, Virginia PolytechnicInstitute and State University. His research interests include dc/dc conversion,digital control of switched converters, especially in distributed renewable en-ergy generation fields.

Xinke Wu (AM’09–M’10) received the B.S. andM.S. degrees in electrical engineering from theHarbin Institute of Technology, Harbin, China, in2000 and 2002, respectively, and the Ph.D. degreein electrical engineering from Zhejiang University,Hangzhou, China, in 2006.

From 2007 to 2009, he was a Postdoctoral Fel-low in the National Engineering Research Center forApplied Power Electronics, Zhejiang University, andfrom 2009 to 2010 he was an Assistant Research Fel-low. From 2011 to 2012, he was a Visiting Scholar

in the Center of Power Electronics System, Virginia Tech. Since 2011, he hasbeen an Associate Professor of electrical engineering with Zhejiang Univer-sity. His research interests include high efficiency LED driving technology, softswitching and high efficiency power conversion, and power electronics systemintegration.

Dr. Wu was awarded as Distinguished Young Scholar of Zhejiang Universityin 2012.

Fred C. Lee (S’72–M’74–SM’87–F’90) received theB.S. degree in electrical engineering from NationalCheng Kung University, Tainan, Taiwan, in 1968, andthe M.S. and Ph.D. degrees in electrical engineeringfrom Duke University, Durham, NC, USA, in 1972and 1974, respectively.

He is currently an University Distinguished Pro-fessor with Virginia Polytechnic Institute and StateUniversity (Virginia Tech), Blacksburg, USA. He di-rects the Center for Power Electronics Systems, aNational Science Foundation Engineering Research

Center. He is the holder of 35 U.S. patents and has published more than 200 jour-nal articles and more than 500 technical papers in conference proceedings. Hisresearch interests include high-frequency power conversion, distributed powersystems, electronic packaging, and modeling and control.

Zijian Wang (M’08) received the B.S. degreein electrical engineering from Zhejiang University,Hangzhou, China, in 2006. He received the M.S.degree in electrical engineering from Virginia Tech,Blacksburg, VA, USA, in 2010. Since 2007, he hasbeen working toward the Ph.D. degree in the Centerfor Power Electronics Systems, Virginia PolytechnicInstitute and State University.

From October 2011 to February 2013, he workedas the Applications Engineer in Monolithic PowerSystems, Inc. Since March 2013, he has been work-

ing as the Applications Engineer in Linear Technology Corporation, CA, USA.His research interests include power factor correction converters, electromag-netic interference modeling, and design optimization.

Pengju Kong (M’08) received the B.S. and Ph.D.degrees in electrical engineering from TsinghuaUniversity, Beijing, China, in 2003 and 2009,respectively.

Between 2005 and 2009, he was a Visiting Scholarat Center for Power Electronics Systems (CPES),Virginia Polytechnic Institute and State University,Blacksburg, VA, USA. He continued his research inCPES as a Postdoctoral Associate after receiving thePh.D. degree. He joined iWatt., Inc., Campbell, CA,USA, as a System and Application Engineer. His

research interests include EMI modeling and reduction techniques in powerelectronics systems, power factor correction techniques, high-frequency dc/dcconverter, photovoltaic converter, and modeling and control of converters.

Fang Zhuo (M’00) received the B.S., M.S., and Ph.D.degrees in electrical engineering from Xi’an JiaotongUniversity, Xi’an, China, in 1984, 1989, and 2001,respectively.

In 1984, he was a Lecturer at Xi’an Jiaotong Uni-versity, an Associate Professor in 1996, and a FullProfessor in power electronics and drives in 2004. In2004, he worked as a Visiting Scholar in NanyangTechnological University, Singapore. He was aSupervisor of Ph.D. student in 2006. He has pub-lished 160 articles, more than 30 papers were indexed

by SCI, EI, and ISTP, and he is also the coauthor of two handbooks, and holdsfour patents. He is the Power Quality Professional Chairman of Power SupplySociety of China. His research interests include motor driver control, powerquality improvement, grid-connected renewable energy system, and microgrid.

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