Sub-Module Differential Power Processing for

Photovoltaic Applications

Shibin Qin, Robert C.N. Pilawa-Podgurski

UNIV ERSI TY OF ILLINOIS AT URBANA-CHAMPAIGN

URBANA, ILLINOIS 61801

EMAIL: sqin3@illinois.edu

Abstract-In this paper, a sub-module differential power pro­

cessing (DPP) architecture for solar photovoltaic (PV) applica­

tions is presented, along with a maximum power point tracking

(MPPT) control scheme that requires minimum communication

and no local current sensing. The efficiency, size and cost benefit

of this architecture are analyzed, and the control challenges asso­

ciated with this architecture are addressed. A hardware prototype

is implemented and tested in a controllable indoor test setup that

enables replication of real-world partial shading conditions. We

experimentally verified a substantial increase in total system en­

ergy capture compared to module-level MPPT solutions, through

the use of the proposed DPP architecture and control strategy.

I. INTRODUCTION

Solar photovoltaic (PV) energy, a promising renewable en­

ergy source, is undergoing a rapid growth world-wide thanks to

reduced manufacturing cost and increased production capacity.

However, PV still remains at a cost disadvantage compared

with conventional energy sources, such as natural gas and

coal. In order to reach grid parity it is therefore important

to - in addition to reduce manufacturing cost - ensure that

solar installations extract the maximum possible energy from

the system. For this reason, much research has focused on

the development of power electronics solutions that have high

efficiency, increased power yield and reduced system cost. In

most PV installations, solar modules are often connected in

series to form a large PV array for increased output voltage.

However, due to partial shading of individual modules by

external objects, manufacturing variability, and non-uniform

ageing characteristics, the output power of each module may

differ, and series connected modules will then suffer from cur­

rent mismatch. Since the current of the entire string is limited

by the weakest module, even relatively minor non-uniformity

in the power production of individual modules can greatly im­

pair the overall power yield. To address this problem, several

types of power electronics solutions have been proposed to

perform maximum power point tracking (MPPT) of individual

PV modules [1]-[6], and even at the sub-module level [7]­

[9]. Many of these proposed solutions remain unattractive,

however, primarily due to issues of high cost and relatively low

power conversion efficiency. By processing only the mismatch

power, substantial improvements in system efficiency and costs

can be realized [1], [10], [11]. The concept, denoted differ­

ential power processing (DPP), employs converters that only

need to process the mismatch power of the PV modules, which

is typically just a small portion of the total power output. This

leads to reduced size and cost of the local converter, which is

a significant benefit compared to conventional DC optimizers

that have to process the whole output power. With the same

converter efficiency, the power loss of a DPP system can thus

be much smaller than that of a DC optimizer system. Although

DPP systems show great promise to reduce the cost and power

losses of PV systems, their potential has not yet been fully

explored, in particular with regards to implementations that

achieve low-cost and low-complexity control. In this paper,

we present a sub-module-to-sub-module DPP converter system

that achieves high efficiency, small size, maximum power yield

and reduced cost. Additionally, we propose and experimentally

978-1-4673-4355-8/13/$3l.00 ©2013 IEEE 101

evaluate a control method that can realize MPP operation

of each sub-module without any local current sensing, while

achieving improved performance compared to prior solutions.

II. BACKGROUND

There have been several research endeavors to apply the dif­

ferential power processing technique to the power processing

stage of solar photovoltaic system, and a number of architec­

tures have been proposed to implement this technique. Refer­

ence [11] provides a high-level analysis of the relative merits

of different circuit topologies. Most architectures consist of

several DPP converters and one central converter to perform

MPPT. In the generation control circuit architecture proposed

in [1], [10], a multistage chopper circuit is used to process the

differential power, while the global power output is controlled

by a central converter. One of the advantages of this approach

is that it does not require any local current measurement, which

would introduce extra loss and cost to the system. However,

this approach requires extra communication wires between the

central converter and each differential power converter, and

the control algorithm becomes more complex and slower as

the number of modules in the system increases, making the

solution difficult to implement for a large system. In addition,

each switch in the multistage chopper circuit has to be rated

for the full output voltage, so slow, high-voltage switches must

be used, which may impair the conversion efficiency and limit

further miniaturization of the system.

Recent techniques from battery-equalization circuits have

been adapted for use in solar PV, in a method commonly

known as voltage equalization or "virtual parallel" operation,

in which power converters maintain all modules at the same

voltage [8], [9]. The work presented in [8] make use of res­

onant switched-capacitors to achieve the voltage equalization,

while that of [9] employs fJyback converters. Voltage equal­

ization architectures overcome some of the disadvantages of

the generation control circuit proposed in [1], since they run

open-loop without any local control. However, this approach

does not actually perform any individual maximum power

point tracking of the modules, but leverages the fact that the

maximum power point is less sensitive to mismatch in module

Module

I Istring

I '-...../

Sub-A Module

1 + DPP I

Converter B I Central '-...../ Vstring

Converter C Sub-

Module 2 -I DPP

I Converter '-......../

Sub-Module

3

I I

Fig. 1. Schematic drawing of the differential power processing architecture.

voltages than module currents to achieve near MPP operation.

This approach will thus only be effective when the modules

or sub-modules connected in the string have very similar I-V

characteristics (e.g small amounts of shading, uniform tem­

peratures, and carefully matched modules), such that the MPP

voltages are nearly identical.

The objective of this work is to develop a DPP architecture

together with suitable control that addresses the shortcomings

of the previous solutions and enables an efficient, scalable, and

low-cost system implementation. More specifically, we seek a

system where each differential converter employs low-voltage

transistors, minimum communication, as well as no local cur­

rent sensing, while still performing true MPPT functionality,

rather than voltage equalization.

III. PROPOSED ARCHITECTURE

A schematic overview of our proposed DPP architecture is

shown in Fig. 1. In this discussion we present a system consist­

ing of three sub-modules only (a typical PV module has three

sub-modules), but the system is scalable to a large number of

modules and converters. In this architecture, each converter

only needs to be rated at twice the sub-module open circuit

voltage, which enables the use of low-voltage transistors that

can efficiently operate at a high switching frequency, leading to

a small converter size. Each DPP converter is implemented as

a synchronous buck-boost circuit, as shown in the schematic

102

TABLE I COMPONENT LISTING

Device Model Value Manufacturer

Integrated Power Stage FDMF6704V Fairchild L SER1360-103KL 10 J.tH Coilcraft RI, R3 0402 100 H1 Panasonic R2, R4 0402 10 kSl Panasonic Cfl, Cf2 0402 I J.tF Murata Csum 1206, X5R, 25V 4 x 10 J.tF Murata Cmid 1206, X5R, 25V 5 x 10 J.tF Murata Microcontroller ATtiny861 Atmel

TABLE II CONVERTER SPECIFICATIONS

Sub-module Voltage Range

Converter Power Rating

Switching Frequency

Converter Peak Efficiency

Converter Weight

Converter Volume

3-13.5 V

60 W

250 kHz

97%

4,55 grams

1,91 cm3

drawing of Fig. 2. Each converter is also equipped with an

isolated 12C communication interface to acquire diagnostic

data. Table I provides a listing of the main components used

in the design.

Shown in Fig. 3a is an annotated photograph of a converter

prototype, together with the 12C communication hardware.

Note that the inductor is placed on the back-side of the 2-

layer PCB. and that a large portion of the PCB is consumed by

connectors and diagnostic communication hardware. Only the

core part of the converter will be used in practice, as shown in

the illustration of Fig. 3b, where two DPP converters have been

placed in a standard PV module junction box. Finally, Table

II provides an overview of the specifications of the converter.

IV. CONTROL

In the system of Fig. 1, the overall maximum power output

(global MPP) can be achieved only when each sub-module

operates at its own local MPP. Therefore, a control method

must be designed to allow each DPP converter to track the

local MPP of its corresponding sub-modules, while simulta­

neously allowing all other converters to do the same. For a

system consisting of a string of 3 sub-modules, as shown in

Fig 1, two differential converters and one central converter is

required. In general, (N-I) differential converters along with

one central converter are required for a system comprising N

sub-modules. If the current and voltage of each sub-module

A DPP Converter

Integrated Power Stage

R,

R,

C Fig. 2. Schematic drawing of the DPP converter, with terminal labels corresponding to those of Fig. 1.

(a) (b)

Fig. 3. Photograph of the DPP converter experimental prototype (a) and DPP converters placed in a standard junction box (b).

are given (in this case, we want each sub-module to operate at

MPP current and MPP voltage), the entire circuit - including

current, voltage and duty ratio of each DPP converter as well as

the system output power, current and voltage - can be uniquely

solved. This global MPP operating point corresponds to one

unique set of duty ratios (D1-mpp, D2-mpp). The task of the

overall control scheme is thus to find the unique combination

of each DPP converter's duty ratio as well as the overall string

current and voltage that correspond to the global MPP.

To accomplish the control task, and to avoid any current

measurement in the DPP converters, we propose an MPPT

control scheme as follows: In order to maximize the total

output power of the system, the central controller strives to

maximize Pstring = Vstring x 1string. The string current is

shared by all sub-modules, so if the central converter regulates

this current to a constant value temporarily, maximum output

power at this operating current can be achieved by maximizing

the sum of the sub-module voltages (the string voltage). For

any constant string current and given insolation of each sub­

module, the string voltage is only a function of duty ratios

103

Fig. 4. A 3-dimensional plot of string voltage (VatTing) versus DPP converter duty ratios (Dl, D2) for a 3-sub-module 2-converter system.

of all DPP converters, so the overall control algorithm can be

described as

(1)

For any given string current, there exists a unique combination

of duty ratios of all DPP converters in the string that allows for

the maximum string voltage. For a 3-sub-module 2-converter

system, this can be seen from a three dimensional surface plot

of string voltage Vstring versus DPP converter duty ratios,

D1, D2 at a fixed string current, Istring. One such example is

shown in Fig. 4. It can be seen that the string voltage surface

only has one peak, which corresponds to a unique combination

of Dl and D2, and no local maxima exists on the surface. It

can be shown that there is only one single maximum string

voltage for a given string current.

A challenge associated with this algorithm is the acquisition

of global string voltage information for each local DPP con­

verter, as each converter can only measure the voltage across

the two sub-modules that are connected to it, which is only

a portion of the string voltage. As shown in [7], in cascaded

sub-module MPPT converters, it is possible to achieve string

voltage maximization by maximizing the output of each series­

connected converter. However, this simple approach does not

work for the DPP architecture owing to the differences in cir­

cuit topologies between the two architectures. As is observed

in Fig. 1, each differential converter transfers energy between

two sub-modules, so any attempt to maximize the voltage of

only a single sub-module will result in minimizing the voltage

of the other (such a control scheme would drive the duty ratio

to zero or one). It is therefore expected that some form of

communication will be required for each DPP converter to

acquire the string voltage information.

Based on the information acquired from communication, our

proposed local DPP control scheme lets each converter strive

to maximize the string voltage by adjusting its own duty ratio.

For the ith DPP converter, the control scheme is given as:

arg max Vstring(Di) DiE(O,l)

(2)

where the duty cycle of the ith converter, Di, is the control

handle. Each converter performing Eq. 2 will lead to the com­

bination of duty ratio as in Eq. 1. The global output power is

then given by:

Pstring = Vstring x Istring (3)

which, provided that Eq. 1 is true, represents a maximum in

the global output power space.

We have implemented the local control objective of Eq. 2

using a perturb and observe (P&O) MPPT algorithm. In this

way, by adjusting its duty ratio (in a "fast" loop), each DPP

converter maximizes the string voltage. The central converter

performs conventional P&O algorithm (in a "slow" loop) and

gradually adjusts the string current to the optimal string cur­

rent. In this control algorithm, it is important that the DPP

MPPT algorithm is significantly faster than that of the central

converter, so that the DPP converters converge to the new

optimum operating point for each string current iteration.

It should be noted that although this control method requires

extra communication hardware, it may not significantly impact

the cost of this system. In some cases, the communication

hardware may already exist in the system for other purposes.

Even if the communication hardware is not present in the sys­

tem, its implementation can bring benefits besides MPPT, such

as performance monitoring of each individual sub-module.

lO4

In this setup, 1ext replaces the photo-diode current (Iph) that

would be present if the modules were to be placed in sunlight.

The advantage of this testing method is that we can perform

repeated experimental measurements, which is very difficult

to obtain in an outdoor environment, where conditions change

constantly. Shown in Fig. 6 is measured power versus voltage

for a sub-module operated in the manner shown in Fig. 5, for

a number of different current settings. Note that the shape of

the curve agrees well with what would be observed if the sub­

Fig. 5. Schematic drawing of the laboratory test setup. Each sub-module module is illuminated by sunlight, but can now be achieved is connected in parallel to an external power supply in current-limited mode that can model sub-module mismatch. in repeatable and controlled experimental measurements.

With the test setup described above, we conducted the fol­

lowing experiment on a 3 sub-module system (as shown in

Figures 1 and 5): An electronic load (HP 6060B) was used to

act as a central converter, and 5 multi-meters (Agilent 3441OA)

were used to measure the string voltage and current, and volt­

age of each sub-module. A computer was used to control the

electronic load and multi-meters through a GPIB interface and

record all the data. The communication was implemented over

an 12C link between the computer and each DPP converter.

A photograph of the entire test setup is shown in Fig. 7.

6 8 The short circuit currents of the 3 sub-modules were set to Voltage [VI

Fig. 6. Power versus voltage characteristics of controlled laboratory PY h, 12, and h (within 0 A to 3 A range) to simulate a partial sub-module setup.

Therefore, the cost-effectiveness of the system is not believed

to be significantly impaired.

In addition, the communication required in this control al­

gorithm is only for DPP converters to acquire string voltage

information, and no central control unit is necessary. Each

DPP converter can adjust its own behavior based on the string

voltage change, so distributed control can be implemented,

which yields extra benefit in terms of robustness and execution

speed.

V. EXPERIMENTAL VERIFICATION

Shown in Fig. 5 is a schematic drawing of our controlled

laboratory test setup. It comprises three PV sub-modules that

are connected in series to form a module. Each sub-module

is connected in parallel to a power supply (HP6634A) that is

operating in current-limit mode (Iext of Fig. 5). By adjusting

the values of 1ext, different levels of mismatch can be tested.

shading condition. The electronic load was at first connected to

the entire sub-module string without DPP converters, and then

to the entire sub-module string with DPP converters, respec­

tively. In each case the electronic load was controlled by the

computer to perform a current sweep from 0 A to 3 A with 60

steps. Two examples of the results under a moderate mismatch

condition and a severe mismatch condition are shown in Fig. 8

as plots of output power versus string current. The maximum

output power improvement in each scenario is also shown.

Another advantage of the DPP method observed from these

plots is that the local maximum caused by a conducting bypass

diode in the conventional operation is eliminated, since the

mismatch power is transferred around the weak sub-module,

without causing any bypass diodes to conduct.

To get a complete output power improvement characteri­

zation of the system, the same experiment described above

was repeated under a variety of mismatch conditions. In these

105

Fig. 7. Photograph of the laboratory test setup.

80ir===������· ��=3A�'�I�""��=2�. 1�A�' �I���1�.�5A��;;=;�;l

Sub-module 1 Sub-module 2 Sub-module 3 Without DPP

70

60

3.0 Current [A]

(a) Moderate mismatch.

(b) Severe mismatch.

Fig. 8. Plots of power versus current with and without DPP MPPT, for mismatched sub-modules.

experiments, the short circuit current of sub-module 1 was

always set to 3 A, which corresponds to 100% insolation. Sub­

module 2 was set to insolation level between 0 and 100%, and

the insolation of sub-module 3 was swept from 0 to 100%. The

resulting increase in maximum power capture with the DPP

� 0. 0. " :5 .� � " E " e Q.

.s

.. .... 60% - 100%

50 60 70 Insolation Level of the 3rd Sub-module [%]

100

Fig. 9. Inprovement with DPP under a variety of insolation conditions.

converters compared to operating without DPP converters was

measured and calculated, as shown in Fig. 9. As can be seen, a

significant increase in maximum power output can be achieved

with DPP technology, especially under moderate to severe

mismatch conditions. In these scenarios, the string current is

severely limited by the weakest sub-module and large portion

the the energy potential of the strongest sub-module can not

be utilized for the reference system without DPP converters.

It can be also seen that when the mismatch of the three sub­

modules is small, each sub-module is operating very close to

MPP even without DPP converters, therefore the increase of

maximum power output with DPP is limited. In such small

or no mismatch scenario, PV arrays typically operate near

the theoretical maximum output power, and subsequently each

DPP converter only needs to processe a small mismatch cur­

rent. The bulk power delivery occurs directly through the string

to the central converter without any intermediate conversion,

so the overall power loss of a DPP system can be very small.

In particular, during light-load operation, the DPP converters

are processing very little power (owing to small sub-module

mismatch), and their negative impact on overall system effi­

ciency is quite limited, since the vast majority of the power is

not processed by the DPP converters in this case.

VI. EFFICIENCY CONSIDERATIONS

In this work, the DPP converters are implemented as syn­

chronous buck-boost circuits operating in fixed frequency pulse

width modulation (PWM) mode. However, synchronous buck­

boost converters typically operate in continuous conduction

106

mode (CCM). In light load condition, when the inductor cur­

rent starts to flow reversely, the low side MOSFET will remain

on and allow current to flow back to the input. As a result,

energy will be transferred back and forth between converter

input and output, while only a small amount of net energy

is delivered to the output. Large loss will be incurred during

the unnecessary back-and-forth transfer of energy. At the same

time, as the converter operates at a fixed switching frequency,

the switching loss of the converter is nearly constant over

the entire output current range and does not scale down with

decreasing load [12]. All these factors lead to an efficiency

penalty of the CCM synchronous buck-boost circuit in light

load operation.

Many solutions for light load efficiency optimization have

been proposed, including pulse frequency modulation (PFM)

[13], burst mode operation [14] [15], etc. Burst mode operation

is adopted in this work to improve the light load efficiency of

DPP converters. The integrated power stage we employed in

this circuit allows low side switch selection between MOSFET

and diode by an external signal, so in burst mode, the load side

switch is implemented with a diode to prevent any negative

current flow in light load condition. The power conversion will

be activated to charge up the output capacitor when the output

voltage drops below a lower limit, and deactivated once the

output voltage reaches an upper limit. The efficiency of the

converter operating in CCM and in burst mode over the light

load range was measured and plotted in Fig. 10. To provide

a straightforward illustration of the converter efficiency in the

different operation modes, for all the measurements in Fig. 10,

the power consumption of the micro-controller is excluded by

powering it from a separate source. Also shown in Fig. lO is

the efficiency of DPP converter in dual mode, in which the

converter switches between CCM and burst mode depending

on the load current. The dual mode is implemented as follows:

When the converter operates in burst mode, as the output

current increases, the output capacitor discharges faster and

thus the ratio of converter in active state to converter in idle

state increases. Once this ratio is higher than a certain value,

which corresponds to the critical current (about 0.9 A in our

case), the converter will switch its operation to CCM. When

100

95

90

� 85

� � ·u tt

Output Current [AJ

Fixed Frequency CCM Burst Mode Operation Dual Mode Operation

1.5 2.0

Fig. 10. Efficiency of DPP converter excluding micro-controller power consumption.

the converter operates in CCM, the operation control function

will periodically switch to burst mode. If the converter is in

moderate to heavy load condition, it will immediately switch

back to CCM by the mechanism described above. Otherwise,

it will stay in burst mode until the load increases. In this way,

dual mode operation is achieved for high efficiency over the

full load range, as shown in Fig. 10.

When the micro-controller is powered directly by the in­

put voltage (through the integrated linear regulator on the

FDMF6704V), the static micro-controller power consumptions

can have a significant impact on the overall efficiency of the

system, especially in light load condition. As a result, special

care has been taken to reduce the power consumption of the

micro-controller (ATtiny 861) in our system. The analog to

digital conversion (ADC) takes up most of the micro-controller

operating time in light load operation. To conserve power in

light load operation, the micro-controller enters sleep mode,

in which most of the clocks are shut down, immediately after

an ADC sample has begun. The micro-controller is woken up

from sleep mode by the completion of ADC and performs

the necessary calculations, after which it again enters sleep

mode after beginning another ADC sample. The efficiency of

the entire converter system in dual mode operation, including

micro-power consumption before and after micro-controller

power optimization, is shown in Fig. 11

The application of existing light load operation techniques

to DPP faces some unique challenges. Most of the existing

107

65 60

50.0 0.8 1.0 1.2 1.4 1.6 Output Current [AI

Fig. 11. Efficiency of DPP converter in dual mode including micro-controller power consumption.

light load solutions aim at regulating the output voltage to a

fixed value, while in our proposed DPP structure, the convert­

ers should instead enforce a variable ratio between input and

output voltage. This can be addressed by constantly sampling

the input voltage, and the product of sampled input voltage

and the voltage ratio decided by the P&O MPPT algorithm can

be used as the output voltage reference. Another challenge is

that since the distribution of mismatch on a sub-module string

is random, the direction towards which each DPP converter

needs to transfer energy can not be decided beforehand, and

may alternate. Thus, the DPP converter needs to be able to

operate bidirectionally. Although a synchronous buck-boost

converter running in PWM CCM readily lends itself to bidirec­

tional operation, most of the light load operation techniques

require a clearly defined operation direction to regulate the

output voltage. This problem can be addressed by periodically

forcing the operation mode to work in CCM, and measuring

the average voltage across the inductor. As the inductor has a

parasitic resistance, its average voltage will not be zero and

the polarity of the average inductor voltage can be used to

decide the operation direction.

VII. CONCLUSI ONS

We have presented a sub-module differential power process­

ing MPPT architecture for solar PV applications, which en­

ables more energy to be extracted from the system. By employ­

ing low-voltage synchronous buck-boost converters connected

across each pair of sub-modules of the panel, a high frequency,

high efficiency power stage can be used. We have implemented

a hardware prototype for use in sub-module tracking of a PV

module, and presented a control method that achieves global

and local MPPT operation without any current sensing and

with improved performance compared to the state-of-the-art.

We have experimentally measured substantial improvement in

overall energy capture compared to per-module MPPT imple­

mentations.

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