This document analyzes a unified output MPPT control strategy for a subpanel PV converter system (SPMC) to address real-world mismatch issues in photovoltaic systems. The SPMC system connects a dedicated MPPT converter to each PV cell string in a panel. This allows each string to operate at its individual maximum power point regardless of mismatch conditions. However, implementing independent MPPT control for each string increases costs. Therefore, the document proposes a unified output MPPT control structure that reduces costs by saving on analog-to-digital units, current sensors, and MPPT controllers while still allowing each SPMC to operate at its optimal maximum power point. Simulation and experimental results confirm the effectiveness of this unified output control approach.

1. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 3, MARCH 2014 1275
Analysis of Unified Output MPPT Control
in 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 point
tracking (MPPT) solution and has been implemented commer-
cially. Taking the trend of the “distributed MPPT” concept a step
further, this paper discusses and analyzes an MPPT converter that
connects to each PV cell string, called a subpanel MPPT converter
(SPMC), to better address the real-world mismatch issues. The
SPMC system with a unified output MPPT control structure is also
proposed in order to reduce the cost and simplify the distributed
MPPT system. The proposal saves A/D units, current sensors, and
MPPT controllers on the premise of guaranteeing that the SPMC
is working on its optimal maximum power point regardless of the
mismatch case. This is favorable for the further integration and
makes 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
AS global demand for energy continuously increases, so
has the need for renewable energy sources (RESs) that
minimize impact on the environment. It has given rise to the
development of electronic power distribution systems (EPDS),
such as nanogrid–microgrid–···–grid structure, utilizing multi-
ple RES as supplementary energy source to utility grid. DC
nanogrid, one kind of EPDS at low power level (10–100 kW), is
addressed as a promising EPDS comparing to ac nanogrid from
following aspects: higher overall system efficiency, starting with
Manuscript received November 25, 2012; revised January 28, 2013 and
March 27, 2013; accepted April 22, 2013. Date of current version September
18, 2013. This work was supported in part by the CPES Industry Partnership
Program, and in part by National Natural Science Foundation of China (No.
51177130 and No. 51007081), and in part by Delta Science and Technology
Educational Development Program (No. DREK2011002). Recommended for
publication by Associate Editor C. N. M. Ho.
F. Wang and F. Zhuo are with the State Key Laboratory of Electrical Insulation
and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong Uni-
versity, Xi’an, Shaanxi 710049, China (e-mail: fengwang83413@gmail.com;
zffz@mail.xjtu.edu.cn).
X. Wu is with the Department of Electrical Engineering, Zhejiang University,
Hangzhou, Zhejiang 310058, China (e-mail: wuxinke@zju.edu.cn).
F. C. Lee is with the Center for Power Electronics Systems, Virginia Tech,
Blacksburg, VA 24061 USA (e-mail: fclee@vt.edu).
Z. Wang is with the Linear Technology Corporation, Milpitas, CA 95035
USA (e-mail: wzj.zju@gmail.com).
P. Kong is with the iWatt, Inc., Santa Clara, CA 95008 USA (e-mail:
pengjukong@gmail.com).
Color versions of one or more of the figures in this paper are available online
at 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 drives
can 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 centers
taking charge of interfacing dc bus with utility ac grid [1]–[3].
Solar energy, therefore, is no doubt a suitable RES for such
architecture 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 the
mismatch cases always impact the performance of the PV sys-
tems. For the centralized or string level MPPT PV systems, the
consequences of the aforementioned mismatch cases are degra-
dations in total power harvest, multiple maxima power points
issues on the power-voltage curve and MPPT algorithms can
fail [5]–[7]. Moreover, even when the global maximum power
point of the shaded PV system is reached with some advanced
algorithms [8]–[14], because the shaded part of the PV system
would limit the output current of the nonshaded part [15], such
a power is still lower than the sum of the available maximum
powers of the mismatch parts.
In reaction to these problems, a distributed MPPT solution,
each PV panel connect with a dedicated MPPT converter, has
been proposed from academia and implemented commercially.
The panel level MPPT converter is commonly referred to as “PV
optimizer” or “module integrated converters (MIC),” and it is
0885-8993 © 2013 IEEE

2. 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 topologies
as possible cascadable converters for the PV optimizers. The
advantages and drawbacks of such topologies are examined in
detail. In [17] and [26], the authors proposed an improved multi-
mode four switch Buck/Boost PV optimizer to increase energy
capture in a PV optimizer string. The panel level distributed
MPPT solution can, at best, eliminate the mismatch power loss
among PV panels. However, in a real-world mismatch case,
a shaded PV panel cannot be just exactly obstructed, so the
performance of PV optimizer-based solar system is still less
than satisfactory in such cases. Of similar concern are the small
scaled mismatch cases, such as dust, bird droppings, or damaged
PV cells which can result in a disproportionate power loss in
PV systems. Such cases happen more frequently but are usually
given less attention. Taking the trend of the “distributed MPPT”
concept a step further, this paper focuses on a distributed MPPT
structure that connects each PV cell string with a dedicated
MPPT converter, called a subpanel MPPT converter (SPMC)
module, to address the real-world mismatch issues and given
better performance in power recovery comparing with current
PV optimizers.
This paper is organized as follows: in the next part, the dis-
tributed MPPT concept is introduced, which can be applied
to improve the performance of the PV system in real mismatch
cases. The performance comparison of the current PV optimizer
and the proposed SPMC system is given in Section III. Based on
the SPMC concept, a novel unified output MPPT control strat-
egy is proposed accordingly in order to optimize and simplify
the distributed MPPT control solution as shown in part IV. In
the fifth part, the reliable issue of the SPMC is discussed and
in Section VI, simulation and test results are presented to verify
that the SPMC PV system can achieve a more effective power
harvest 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 cell
strings connected in series, divided in three parts by correspond-
ing bypass diode. Bypass diodes prevent the appearance of hot
spots and protect the PV module from potentially destructive
effects. The PV module is connecting with a MPPT converter
which always operates the PV module at its maximum power
point. So the MPPT converter together with the PV module
is operating as a constant power source, the power of which
is determined by the peak power of the PV module, at a rel-
atively wide voltage/current range at the output side, making
it possible to cascades with other converters in series or paral-
lel. In other words, the distributed MPPT converter changes the
MPP of the PV panel from a single voltage/current point into a
wide voltage/current range, shown as the green solid curve of
Fig. 2(b). In a traditional PV system with centralized MPPT ar-
chitecture, any disturbance can shift the maximum power point
of the module, and results in a significant power decrease un-
less the module’s output voltage is adjusted. However, with
distributed MPPT structure, the peak power of the PV module
Fig. 2. Concept of distributed MPPT converter. (a) PV unit and distributed
MPPT converter. (b) Output curve of PV unit and optimizer.
can be achieved over a very wide range of voltages, so even
when disturbances occur an adjustment to the output voltage of
the distributed MPPT system, it still can maintain peak power.
Distributed MPPT converter is usually implemented with a dc/dc
power converter. Three possible converter topologies are taken
into consideration in this paper because of their simplicity, high
efficiency, and the capability of cascade operation as shown in
Fig. 3 [23], [24], [27]. The blue I–V and P–V curves indicate the
output characteristic curves of an original PV panel, and they
are identical in each graph. The point M stands for the MPP of
the original PV unit and the N1 and N2 indicate the initial point
and ending point of the MPP region, respectively, at the output
side of the distributed MPPT converter. The merit and demerit
of the three topologies are given as follows: the Boost converter
is only suitable for parallel connection, the output current of
Boost-type MIMC is inherently limited by the characteristic of
original PV panel. For the Buck converter, series connection
is a better choice and the inherent voltage limit characteristic
is achieved and the Buck/Boost converter enjoys most of the
benefits of both Buck and Boost at the expenses of higher cost
and more complex control solution.
One important thing to note here is that the second stage
central MPPT converter is still required in the distributed MPPT
converter-based PV system. However, the enlarged MPP region
makes 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 moduleorientations, andtilts, etc., about 10%–
30% of annual performance loss or more can be recovered by
using the PV optimizers or PV MICs [28]–[31]. However, fre-
quently, partial PV panel cannot work as expected which result
from dust and spot dirtiness such as leaves or bird droppings
or damage of PV cells, etc., the PV optimizer’s performance
is less than satisfactory in such cases. Since the panel is com-
posed of several PV cell strings, taking the trend of “distributed

3. 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 implement
distributed MPPT solution into subpanel level. This part dis-
cusses a SPMC system with three PV cell-string level dc/dc
converter that executes MPPT separately for sections of an in-
dividual PV module which provides a better solution in order
to address the real-world mismatch impact. For the SPMC sys-
tem, the output terminals of all the MPPT converters can be
connected either in parallel or in series. For the parallel con-
nection, the control is relatively simple, but the high-voltage
gain will increase the cost and reduce the efficiency. And for
series connection, lower rating devices and lower voltage gain
can be the promising candidate for a low cost and high effi-
ciency distributed solar system [26]. Because of simple, high
efficiency, and suitability for series connection as aforemen-
tioned, the Buck-type converter is chosen as implementation of
the 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 shown
in Fig. 4. From the input side of each Buck converters, the con-
verters are parallelly connected with each PV cell strings. From
the output side of the MPPT converters, they are connected in
series connection. One point should be noted that in this SPMC
system, the bypass diodes inside the junction box of a standard
PV module should be retained in case of the malfunction of the
MPPT converters. For the convenience of theoretical expression
of the SPMC, the diodes are not shown here and the detailed
information 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 connection
of MPPT converter allows the input–output voltage ratio
to be close to unity in ideal irradiance case, which leads
to the highest switch utilization and is at a performance
versus cost disadvantage.

4. 1278 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 3, MARCH 2014
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 the
MICs, the lower voltage level device used in the SPMC
application has better performance in efficiency.
3) Further distributed MPPT solution allows better perfor-
mance in real-world mismatch cases comparing with PV
optimizers, and for series Buck MPPT converters, all the
PV 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 are
shown in Fig. 5(a): blue curve and red curve indicate nonshaded
and shaded PV cell string separately. In Fig. 5(b), the solid lines
stand for typical output curves of a Buck MPPT converters in
nonshading (blue curve) and shading cases (red curve). Adding
them up, the output I–V and P − V curves of the SPMC system
of 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 in
shading case, the output characteristic of the shaded PV panel
suffers multipeak issues and power loss as shown in Fig. 6(a). In
such conditions, the PV optimizer can only track the maximum
power point of the multipeak curve of the shaded PV panel
even adopting some advanced MPPT algorithms as shown in
Fig. 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 the
capability of performing the independent MPPT function on
each PV cell string basis is hereby achieved and it regulates
the 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 and
the mismatch case in one cell string cannot affect the others,
and the power loss resulting from mismatch among PV cell
strings, about 22% in this case, is thereby recovered as shown in
Fig. 6(c).

5. WANG et al.: ANALYSIS OF UNIFIED OUTPUT MPPT CONTROL IN SUBPANEL PV CONVERTER SYSTEM 1279
Fig. 7. Unified MPPT control of SPMC diagram.
In this part, the SPMC concept is proposed and the working
principle is introduced as well. However, although mismatch
loss can be recovered through the SPMC with independent
MPPT control, the implementation cost of the SPMC system is
higher due to the increase in component count. A set of MPPT
control IC, current sensor, voltage sensor, and corresponding
A/D converters are needed for every PV cell string. In order
to address the above issues, an optimal control method for the
SPMC solution is proposed in next section.
IV. UNIFIED OUTPUT MPPT CONTROL IN SPMC SYSTEM
In order to reduce the cost and simplify the independent
MPPT control in SPMC structure, a unified output voltage con-
trol with single MPPT detection strategy is proposed in this
part [38], [39], as shown in Fig. 7. In this structure: 1) a single
MPPT unit is sensing the output power of the SPMC system
with only one pair of voltage and current sensors; 2) three Buck
MPPT converters share a common Vref coming from the sin-
gle MPPT unit; and 3) each Buck MPPT converter owns an
independent control loop.
Therefore, the output voltage signal of the MPPT control unit
is the common MPPT voltage reference for all the converters in
a SPMC module, during the MPPT period. The PWM controller
of each Buck converter in the SPMC system compares the sensed
output voltage of each PV cell string and the common MPPT
voltage reference to control their respective switch. When the
common voltage reference is perturbed by the unified output
MPPT 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 share
a same output current. Therefore, the output voltage of each
Buck converter will vary according to the extracted maximum
power from its individual PV cell strings and proportionate to
the maximum power. So the total output voltage of the SPMC
is 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 with
irradiance case or temperature, it is assumed that such changes
can be considered relatively small [32]. For the same Vref signal
is given to three independent control loops, so the output voltage
of each PV cell string in steady state should be the same and
equal to Vref
Vpv1 = Vpv2 = Vpv3 = Vref . (2)
And the duty cycle of each MPPT converter in steady state
can also derived
Vo1
Vpv1
= D1,
Vo2
Vpv2
= D2,
Vo3
Vpv3
= D3. (3)
If no mismatch happens, the SPMC should be working with
high conversion efficiency and all the maximum power points
of the three PV cell strings are exactly the same. Therefore, the
operating condition of each Buck converter in SPMC system
is same as well. If mismatch case happens with part of a PV
module, the power coming from the shaded PV cell string is
decreased and the duty cycle of the corresponding MPPT con-
verter is also decreased accordingly in order to save the power
of shaded PV cell string and adjust the common output current
limitation. At this point, the SPMC system is working as a con-
stant power source with different output voltage and current. So
we can say that the conversion ratio and duty cycle for each
converter 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 a
single MPPT unit, so the constant power curve of the output of
each SPMC should start at a same voltage value and ending at
current limit of each SPMC as blue and red solid curve shown
in Fig. 8(b). The final voltage reference from the MPPT unit
is neither the MPP of shaded cell string nor the MPP of the
nonshaded PV cell strings, it only stands for a tradeoff state
point where the output power of three parallel PV cell strings
can reach the maximum in a same voltage value as shown in the
enlarged view of the Fig. 8(b), adding the output curve up and
the characteristic curve of the whole SPMC system is shown as
the black curve in Fig. 8(c).
Final comparison is made among aforementioned structures
in Fig. 9. It shows the simulation comparison of the output P-V
curves among the current PV optimizer, the distributed MPPT

6. 1280 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 3, MARCH 2014
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 SPMC
solution.
It is obvious that the proposed SPMC system with optimal
single MPPT control has several advantages, both from the the-
oretical and the practical point of view. First, the architecture is
more suitable for power recovery compared with PV optimizer
in real mismatch cases. Second, the power rating of the de-
vice can be reduced to lower level, which is good for efficiency
improvement. And the proposed optimal control approach can
recover more than 90% power loss caused by mismatch case
with less circuit components and lower cost comparing with the
subpanel 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 inside
the junction box are antiparallel and one-to-one connected to
the subpanel PV cell strings as shown in Fig. 10(a).
Regarding to the system reliability issues, the bypass diodes
inside the junction box of the original PV module should be
retained and antiparallel with the SPMC converter as shown
in Fig. 10(b). Because the output side of a SPMC module-
based PV system is connected with dc nanogrid, to simplify
the analysis for the reliable issues, we assume that a SPMC
module is connecting with a constant voltage source Vout.
Moreover, we need to make statements before the reliable
analysis:
1) the output voltage of each Buck converter inside a SPMC
module 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.

7. WANG et al.: ANALYSIS OF UNIFIED OUTPUT MPPT CONTROL IN SUBPANEL PV CONVERTER SYSTEM 1281
Fig. 10. Structures of standard PV panel and SPMC system. (a) Standard PV
Panel. (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 converter
is the open circuit voltage of the PV cell string, so the MPPT
unit loses control in such case.
B. VMPPT2+VMPPT3 < Vout < Vo2+Vo3
In this case, the sum of the output voltages of the remained
normal 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 the
converters #2 and #3. As the purpose of the MPPT is keeping
the operating point of the PV cell string always stay on MPP
through the control loop, and the higher output voltage requires
the converters have boost function. So the Buck converters are
working at go-through mode at this time. In this case, the MPPT
unit loses its control and the remaining two PV cell strings can
be seen as connected with the voltage source directly.
C. Vout < VMPPT2+VMPPT3
In this case, the input voltages of converters can be controlled
at MPP through the MPPT control loop and the output voltage
of the remaining two converters #2 and #3 are working toward
another steady-state point if converter #1 is failed. The two
Fig. 11. Experimental prototype.
converters are connecting with the dc bus through the bypass
diode of failed converter #1.
It needs to note that the consideration of the architecture
design of the SPMC-based PV system should be paid more
attention. Especially, the number of the SPMC modules in a
string should be large enough in case the bypass diode of the
failed converter blocks the power flow path. It also affected
by external factors such as the dc bus voltage level, the MPP
parameters of the PV panel, and the irradiance case, etc.
VI. EXPERIMENTAL RESULTS
To verify the SPMC concept and proposed unified MPPT
control strategy, an experimental prototype is constructed
as depicted in Fig. 11. The hardware setup consists of the
following parts.
A. Solar Simulators [40]
For the sources, three E4361 Agilent solar simulators are used
to simulate three PV cell strings inside a standard PV panel. The
solar simulator is capable of quickly simulating the output char-
acteristic curve of PV panels under different irradiance cases
by setting the following parameters: open circuit voltage VOC ;
MPP voltage VMPPT ; short circuit current ISC ; MPP current
IMPPT .
B. SPMC Power Stage
The power stage of the SPMC system is made up of three
synchronous Buck converters with a series connection on the
output side as shown in Fig. 7. The input of each Buck con-
verter has a one-to-one connection with all three E4361 solar
simulators, so the power rating of each buck stage is designed to
meet the power rating of one-third of a PV module. The output
stage of the SPMC was designed with the 9-A current limit and
it is connected with an electrical load.
C. Control Board and Electrical Load
An ALTERA Cyclone III FPGA [41] development board with
corresponding AD9254 and DAC5672IPFBR converters is used
for the single output MPPT realization, which can be taken place
byaMPPTICif mass productionis needed, andall theother con-
trol components are all analog devices due to cost consideration.
The static I-V and P-V curves of the proposed SPMC system
can be derived through adjusting the output current through an

8. 1282 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 3, MARCH 2014
Fig. 12. Control signal for each MPPT converter under different cases.
(a) Case A. (b) Case B. (c) Case C. (d) Case D.
TABLE I
TEST PARAMETERS
electrical load to make the operating point of the SPMC board
scan from zero to current limit.
Fig. 12 indicates the control signal for each MPPT Buck
converter in different irradiance cases. In Fig. 12(a), all three
PV cell strings are in unify ideal irradiance as condition I in
Table I, so the maximum power of the three PV cell strings
are exactly the same, and the common maximum power voltage
reference signal given by the single output MPPT unit can make
all three PV cell strings working on their maximum power point.
Because the converters are working with same input and output
currents, so the duty cycles of each Buck converter are same as
well. In Fig. 12(b), cell string II is under shading condition III
and the other two cell strings are under condition I, because the
maximum power of the shaded PV cell string II is lower than the
others, so does the output voltage of the shaded Buck converter.
Because the input voltage is set to be a same value, therefore its
duty cycle is smaller than the other two duty cycles. In Fig. 12(c),
two of the cell strings are under shading condition III and the
other one is under shading condition I, then the power coming
from the shaded PV cell strings are decreased and the duty
cycle is also decreased accordingly, similar as previous case. If
all three PV cell strings are in different shading conditions I,
II, III, respectively, and the duty cycles are also different as in
Fig. 12(d).
For the purposes of calculating the energy output of SPMC
system, the dc model of the behavior is sufficient since the
time-scales of the transient behaviors in the power electronic
converters are short. The output characteristic I-V and P-V
curves of the SPMC in different shading cases as mentioned
before 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 SPMC
is 9 A. When the current reaches the current limit, the converter
is shut down, so there is no current limit curve shown in the I-V
curves.
From the earlier figures it’s clear that the SPMC system with
unified output MPPT control has wider maximum power region
and higher output power compared with current PV optimizer
solution in real-world mismatch case. The proposal also pro-
vides comparable power recover ability regardless of shading
case with less components, lower cost, and much simpler control
method comparing with subpanel distributed MPPT structure.
VII. CONCLUSION
For the purpose of improving the performance of PV system
in dc nanogrid under common mismatch conditions, this paper
explores the benefits of distributed MPPT solution through the
use of SPMC structure, which can be seen as the reduced version
of the current PV optimizer, connecting each PV cell string with
a 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 based
on the proposed SPMC unit can recover nearly all of power
losses caused by real-world mismatch case. Comparing the

9. WANG et al.: ANALYSIS OF UNIFIED OUTPUT MPPT CONTROL IN SUBPANEL PV CONVERTER SYSTEM 1283
distributed MPPT control structure with the SPMC PV system,
this simplified control approach offers a number of additional
practical implementation advantages such as: saves the number
of A/D units, current sensors, and MPPT controllers units on the
premise of guaranteeing maximum power statue regardless of
the mismatch case. The simulation and experimental results ver-
ify that the proposed SPMC with unified output MPPT control
solution exhibits good performance under inhomogeneous and
homogeneous irradiations with an enhancement rate of about
20% in power harvest.
Future work will envisage a deeper study of the distributed
MPPT converter-based PV systems in order to better justify the
approach from a theoretical viewpoint. Possible extensions to
other 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 Jiaotong
University, 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 for
Power Electronics Systems, Virginia Polytechnic In-
stitute and State University, Blacksburg, USA.
He is currently with the State Key Laboratory of
Electrical Insulation and Power Equipment, School
of Electrical Engineering, Xian Jiaotong University
and also with the Center for Power Electronics Systems, Virginia Polytechnic
Institute 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. and
M.S. degrees in electrical engineering from the
Harbin Institute of Technology, Harbin, China, in
2000 and 2002, respectively, and the Ph.D. degree
in 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 for
Applied Power Electronics, Zhejiang University, and
from 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 has
been an Associate Professor of electrical engineering with Zhejiang Univer-
sity. His research interests include high efficiency LED driving technology, soft
switching and high efficiency power conversion, and power electronics system
integration.
Dr. Wu was awarded as Distinguished Young Scholar of Zhejiang University
in 2012.
Fred C. Lee (S’72–M’74–SM’87–F’90) received the
B.S. degree in electrical engineering from National
Cheng Kung University, Tainan, Taiwan, in 1968, and
the M.S. and Ph.D. degrees in electrical engineering
from Duke University, Durham, NC, USA, in 1972
and 1974, respectively.
He is currently an University Distinguished Pro-
fessor with Virginia Polytechnic Institute and State
University (Virginia Tech), Blacksburg, USA. He di-
rects the Center for Power Electronics Systems, a
National 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. His
research interests include high-frequency power conversion, distributed power
systems, electronic packaging, and modeling and control.
Zijian Wang (M’08) received the B.S. degree
in 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 has
been working toward the Ph.D. degree in the Center
for Power Electronics Systems, Virginia Polytechnic
Institute and State University.
From October 2011 to February 2013, he worked
as the Applications Engineer in Monolithic Power
Systems, 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 Tsinghua
University, Beijing, China, in 2003 and 2009,
respectively.
Between 2005 and 2009, he was a Visiting Scholar
at Center for Power Electronics Systems (CPES),
Virginia Polytechnic Institute and State University,
Blacksburg, VA, USA. He continued his research in
CPES as a Postdoctoral Associate after receiving the
Ph.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 power
electronics systems, power factor correction techniques, high-frequency dc/dc
converter, 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 Jiaotong
University, 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 Full
Professor in power electronics and drives in 2004. In
2004, he worked as a Visiting Scholar in Nanyang
Technological University, Singapore. He was a
Supervisor 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 holds
four patents. He is the Power Quality Professional Chairman of Power Supply
Society of China. His research interests include motor driver control, power
quality improvement, grid-connected renewable energy system, and microgrid.