Asymmetric Interleaved Multiphase Dc Dc … new.pdfPriyanka Reddy.G, et,al., International Journal...

Priyanka Reddy.G, et,al., International Journal of Technology and Engineering Science [IJTES]TM

Volume 4[1], pp: 7048-7055, 2016

ISSN: 2320-8007 7048

Asymmetric Interleaved Multiphase Dc–Dc Converters

with Unbalanced Nonlinear Loads for Ripple

Minimization

G.Priyanka Reddy1, Dr.D.BalaGangi Reddy2, Department of Electrical & Electronics Engineering,

PG Student1, 2Vidya Jyothi Institute of Technology, [email protected]

Abstract: Non-linear loads, especially power

electronic loads, create harmonic currents and

voltages in the power systems. If the mains voltage

is undistorted, but nonlinear loads are connected to

the electrical grid, the current harmonics produced

will cause voltage distortions in the line

impedances, and the voltage at the load terminals

will also be distorted. Proposed control of harmonic

elimination that allows for ripple minimization

under asymmetric conditions can be extended for

unbalanced nonlinear loads the indiscriminate use

of non-linear loads has given rise to investigation

into new compensation equipment based on power

electronics. The aim of this equipment is the

elimination of harmonics in the system thereby

effectively minimize the current ripples under

unbalanced nonlinear loads

Keywords: DC-DC Converter, Ripple

minimization, nonlinear loads, Harmonic

Elimination

Introduction:

Current ripple cancellation is an important feature

of multiphase switching converters, as it enables

each individual converter of the system to operate

at a higher ripple than the overall load-current

ripple through interleaving of the phases. This yield

significantly lower value for the inductor and

capacitors of each converter, and it can lead to

substantial reductions in converter size and cost,

while increasing the efficiency. Symmetric

multiphase dc-dc converters are widely used in

power electronics, as they enable the processing of

high power through splitting the overall load-

current into multiple phases. Distributing the

processed power symmetrically between the phases

and performing ripple minimization through

interleaving is well understood. However, in recent

applications such as maximum power point (MPP)

tracking for solar photovoltaic (PV), converters are

forced to operate under asymmetric conditions, due

to differences in the sources or loads of each

converter. This work presents a control technique,

based on harmonic elimination that allows for

ripple minimization under asymmetric

conditions.In standard symmetric multiphase

converters(see Fig. 1), where the output current is

the sum of allphase currents, it is possible to

accomplish a load-current rippleminimization by

phase-shifting the switching functions of eachphase

by an angle determined by

Fig. 1. Standard multiphase-interleaved converter

topology with common inputand output voltages.

Existing system:

Recently, asymmetric phase-shifting has been used

to account for imbalances in the converter phases

due to component tolerances, and in the context of

EMI noise shaping, where certain higher order

harmonics can be reduced, which therefore reduces

the filter size as dictated by EMI regulations.

However, little improvement can be achieved due

to practical limitations such as measurement errors

and signal delays in the complex control circuitry.

Usually, component tolerances are small, which

means that the deviations from symmetrical

operation are limited. Consequently, the additional

cost introduced by the more sophisticated control

might not be justified.


Volume 4[1], pp: 7048-7055, 2016

ISSN: 2320-8007 7049

Proposed system:

Interleaving of the different converters and

applying a symmetric phase-shift will yield some

benefits in the architecture. However, methods that

go beyond this technique are required to minimize

the output current ripple under asymmetric

operating conditions, which will be explored in this

work. The presented results are universally

applicable for different dc-dc converter topologies,

such as buck-type (buck, buck-boost, flyback) and

boost-type (boost, boost-buck, SEPIC) converters.

In the system, all the outputs of the dc-dc

converters are connected in series - supplying one

common load. However, since the operating point

of each sub-module may differ due to shading,

manufacturing tolerances, cell damage, and aging;

the duty cycles of the individual converters are

oftentimes different. These operating conditions are

different from what is usually referred to as multi-

phase interleaved converters as described above.

The average output current is identical for all three

converters, but the output voltages are different due

to the series connection.

Fig. 2. Multiphase dc–dc buck converter in a solar

PV application.

Recently, asymmetric phase-shifting has been used

to accountfor imbalances in the converter phases

due to component tolerances[12]–[14], and in the

context of EMI noise shaping, wherecertain higher

order harmonics can be reduced, which

thereforereduces the filter size as dictated by EMI

regulations [15]–[17].This method is also employed

in multilevel-cascaded H-bridges with non equal

dc-link voltages to reduce sideband harmonics[18],

[19]. However, as demonstrated in [12], [13], little

improvementcan be achieved due to practical

limitations suchas measurement errors and signal

delays in the complex controlcircuitry. Usually,

component tolerances are small, whichmeans that

the deviations from symmetrical operation are

limited.Consequently, the additional cost

introduced by the moresophisticated control might

not be justified. Contrarily, phaseshiftingfor non-

uniform duty ratios and different input sourcesfor

each phase is a more relevant field of application

for thesetechniques that is not well understood, and

has not been exploredin the literature. Fig. 2 shows

a schematic drawing ofan emerging application

where nonuniform duty ratios occura single

photovoltaic (PV) panel is usually divided into

threesubmodules, which are connected to separate

inputs of the converters.Each power converter,

performs MPP tracking for onesubmodule. The

MPP of a PV cell varies with irradiation

andtemperature. It may also change throughout the

lifetime of acell due to aging. Fig. 3 shows the

variation of the MPP andthe change in the

respective voltage VMPP for different levelsof

irradiation in the case of a single PV cell. As has

beenshown, operating individual panels [11], or

even submodules[20]–[23] at their individual

MPPs, can yield a significant improvementin

energy capture in PV applications. In

addition,possible implementations and the

corresponding control of distributedmaximum

power point tracking (DMPPT) are also presentedin

[24]–[28]. The topology shown in Fig. 2 is used as

anexample in this paper. MPP tracking is

performed locally on asubmodule level through a

dc–dc converter that is connected toa string of solar

cells at its input. The global maximum powerpoint

tracking is done by the inverter and can be

performed ondifferent levels, e.g., for all panels, a

single string, or a singlepanel. In the system of Fig.

3, all the outputs of the dc–dc convertersare

connected in series—supplying one common

load.However, since the operating point of each

submodule may differdue to shading,

manufacturing tolerances, cell damage, andaging;

the duty cycles of the individual converters are

oftentimesdifferent. These operating conditions are

different fromwhat is usually referred to as

multiphase-interleaved convertersas described

previously. The average output current is

identicalfor all three converters, but the output

voltages are different dueto the series connection.

In this application, it is desirable toemploy

interleaving of the module (or submodule)

convertersto reduce the overall current ripple, and

enable the use of small,low-cost inductors in each

converter. This is still possible, becauseof the

common output current. Furthermore, the

DMPPTconverters do not require large electrolytic

capacitors, as they donot need to buffer the line-


Volume 4[1], pp: 7048-7055, 2016

ISSN: 2320-8007 7050

frequency power ripple. This ripple isbuffered by

the inverter, which is connected to the output of the

DC–DC converters. The dc input ripple is typically

buffered byPV interfacing circuits in modern grid-

connected PV inverters 29].Interleaving of the

different converters and applying a

symmetricphase-shift will yield some benefits in

the architectureof Fig. 2. However, methods that go

beyond this technique arerequired to minimize the

output current ripple under asymmetricoperating

conditions, which will be explored in this study.

The presented results are universally applicable for

different dc–dcconverter topologies, such as buck-

type (buck, buck-boost, flyback)and boost-type

(boost, boost-buck, SEPIC) converters.This study

represents an expansion of our earlier

conferencepaper [30], and includes a more detailed

description of the proposedcontrol method, as well

as additional discussions andmore extensive

experimental results.

Fig. 3. DMPPT topology used as an example in this

study.

2. Interleaved Buck Converters To solve the problems of large current ripples

associated with conventional buck converters,

interleaved buck converter are used by connecting

two or more buck converters in parallel.

Interleaving technique not only reduces output

current ripples but also increase the power ratings

[8, 9]. Figure 4 shows the circuit diagram of a two

phase interleaved buck converter. This converter

uses two same inductors in two parallel phases.

There are four states of operation in one switching

period and during state-I and state-III, when current

in one phase increases the current in the other

phase decreases and there is a ripple cancellation

effect which results in small output current ripples.

The current is shared among the two phase which

decrease the conduction losses and improve

efficiency and can be used in high power ratings.

However, this topology does not lowers the voltage

conversion ratio and still suffer from small duty

ratio operation for high step down voltage

conversion. The voltage gain of a simple buck and

interleaved buck converter is same.

Fig.4. Two Phase Interleaved Buck Converter

Interleaved buck converters with extended duty

cycle have been proposed for high step-down

voltage conversion [10-12]. The concept can be

generalized to more than two phase. This topology

can perform a high step down voltage conversion

along with ripples cancellation feature of simple

interleaved buck converter.

Multilevel buck converters Multi-level (2-level, 3-level, and 5-level) buck

converters are proposed for decreasing the current

ripples and voltage stress of the switches [50-52].

Flying capacitors along with additional switches

are used to get the same characteristic of

interleaving buck converter in minimizing

switching ripples and an additional advantage of

reducing the switch stress.

3.Mathematical Description of the Problem

The previous section provided numerous examples

in the literature[1]–[11] that describe standard

interleaved multiphaseconverters in different

applications. Additionally, asymmetricphase-

shifting techniques have recently been proposed to

compensatefor component tolerances [12]–[14],

and a number ofdifferent applications for these

techniques have been proposedin [15]–[17]. This

paper extends the aforementioned studies tothe

general case of different operating points for the

converter ofeach phase inmultiphase circuits.

Thiswill facilitate rippleminimizationin DMPPT

applications such as described in [20]–[28].A

keycontribution of our proposed analysis technique

is that it isbased on a more universally applicable

frequency-domain descriptionof the converter

current waveforms, as opposed to atime-domain

description as outlined in [34], [35]. [36]–[38]

describethe mathematical fundamentals of the

presented analysis.


Volume 4[1], pp: 7048-7055, 2016

ISSN: 2320-8007 7051

Fig.5. Characteristics of Two Phase Interleaved

Buck Converter

Current ripple cancellation for asymmetric

multiphase interleaved dc-dc switching converters

A. Two Phases (N = 2)

For the case of two converter phases (N = 2), a180◦

shiftof the operation between phases yields the

optimal cancelationeffect. However, a complete

cancelation of a certain harmonic ripple component

is not possible under asymmetric operating

conditions. The conditions for improved ripple

cancelation in asymmetricmultiphase circuits are

derived for the case of general N here.

B. Derivations for General N

In multiphase dc–dc converters N current ripple

waveforms can be observed where the shape varies

depending on the converter type and operation

mode (see [31] for an overview). The framework

presented here is universally applicable and can be

used for various converter types and modes of

operation.

Fig.6. Two Converter Phases (N = 2), a180◦ shift of

the operation

As one illustrative example, consider the buck

converter. Fig. 5 shows the inductor current ripple

waveform for buck-type converter topologies

(buck, buck-boost, flyback) in continuous

conduction mode (CCM). This waveform is used as

an example in the derivations here. However, it

should be noted that the general method presented

here is applicable to other waveforms as well. In

the considered PV application, each submodule

may operateat a slightly different voltage and

current, owing to a mismatch in the I–V

relationship between submodules. This mismatch

can occur due to manufacturing variability, partial

shading, aging, and dirt accumulation. The

different input sources cause different magnitudes

of the average output voltages Vout,n in the output

The overall current ripple can be obtained by

summing up all N ripple components, In standard

symmetric multiphase converters where the output

current is the sum of all phase currents, it is

possible to accomplish a load-current ripple

minimization by phase-shifting the switching

functions of each phase by an angle determined by

which are denoted by˜in The magnitude of˜isum(t)

can be minimized by adequately phase-shifting the

ripple components of the individual phases.

Performance Evaluation by Simulation

To evaluate the performance of the proposed ripple

cancelationmethod, simulations in MATLAB were

carried out. Additionalsimulation results using the

circuit simulator LT spice have been presented in

[30] and [32]. The three-phase buck

convertertopology shown in Fig. 3 is used as an

example. The simulationsrelate to the summed


Volume 4[1], pp: 7048-7055, 2016

ISSN: 2320-8007 7052

inductor currents.We explore the improvementof

our proposed method compared to the

conventionalphase-shifting technique (φ01 = 0◦,

φ02 = 120◦, φ03 = 240◦)over a wide range of

operating conditions. For each set of

operatingconditions, the converter operation was

simulated withthe conventional, and subsequently

with the proposed, phaseshiftingmethod applied.

We considered again the application of submodule

MPP tracking.In this case, it is possible to reduce

the variable space tothree dimensions by assuming

that the input voltages are approximatelyidentical

for all converters. This is valid for slightvariations

between submodules, as the MPP voltages do

notchange substantially [11] Moreover, the current

ripple Δin, as given in (17), is only linearly

dependent on theinput voltages Vin,n, while it

shows a quadratic dependence on Dn. This means

that in case of a change in input voltage (e.g.,due to

an irradiation change of the submodules), the

magnitudeof Δin is scaled by a linear factor,

yielding similar results.Consequently, the

magnitudes of Ankare approximately

onlydependent on the duty cycle Dnof each phase,

as the currentripple is now also only dependent on

this variable, if Vin,nisfixed. For the simulations,

the input voltages havebeen chosen to be Vin,

1 = Vin, 2 = Vin,3 = 12 V. The operating duty ratio

Dnof each converter has been varied between0.1

and 0.9 with a step size of 0.1. The calculations

were carriedout with the goal of minimizing the

fundamental ripplecomponent (k = 1). The result is

a 9 × 9 × 9 matrix holdingthe decrease of the

summed inductor current ripple in

percent.However, not all values within the matrix

are independent, ascombinations of converters

running at certain duty ratios arecalculated multiple

times. It shows the results of the aforementioned

simulations, where the averageimprovement was

calculated over D3 = 0.1 to 0.9, whileD1 and D2

have been varied. By averaging over D3, the

aforementioned9 × 9 × 9 matrix is reduced to the

displayed 9 × 9table. Consequently, the entries do

not represent a single operatingcondition for D3 but

rather an average. The resulting matrixis

bisymmetric, due to the previously mentioned

dependence ofthe results. The main diagonals have

been highlighted in red and blue, respectively.

However, this is only requiredif it is intended to use

more than the three phases to improveripple

cancelation. In the PV application discussed here,

there is additional complexity of synchronization

between PV modules(e.g., added wiring) if an

asymmetric phase-shift beyonda single PV module

is desired. In a typical scenario, we envisionthat

ripple cancelation between three submodules

alone(i.e., at the PV module level) provides the best

trade-off betweenimplementation complexity and

overall component size/cost reduction.

Fig. 7. Shows the calculated phase currents for

three converters at the operating conditions


Volume 4[1], pp: 7048-7055, 2016

ISSN: 2320-8007 7053

Fig.8. Shows the calculated phase currents THD at

the operating conditions

Fig.9.the waveform shown in performing the

calculations in the frequency domain

Fig.10. The waveform shown in performing the

calculations in the frequency domain

Fig.11. In terms of the phase-shift angle φ0n in

Fourier series.

Fig.12.Harmonic Frequency Components of the

Ripple

Fig.13.The phase shift is left as the only degree of

freedom to influence the current ripple

Fig.14.The phase shift is left as the only degree of

freedom to influence the current ripple

Fig.15.Multiples of the switching frequency

The input voltages, input currents, output voltages,

output currents, and duty ratios of all converters are

determined by the operating conditions.

Consequently, the phase shift is left as the only

degree of freedom to influence the current ripple.

To describe the waveform shown in performing the

calculations in the frequency domain makes it

possible to directly influence certain harmonic

frequency components of the ripple, which are

occurring at multiples of the switching frequency.

This makes the proposed solution more universally

applicable than a time-domain analysis such as

outlined in [34], [35]. In practice, the goal is

typically to minimize the lowest harmonics of the


Volume 4[1], pp: 7048-7055, 2016

ISSN: 2320-8007 7054

ripple, as they are usually dominant and dictate the

output filter requirements.

Conclusions

A brief survey of high step-down dc-dc converters

is carried out in this study. The limitations of

conventional dc-dc converters used for high step-

down dc-dc voltage conversion are discussed.

Various non-isolated dc-dc step-down converter

topologies solving these limitations are reviewed.

The circuit diagram of each technique/topology is

given and main points are discussed. Interleaving

technique is good for reducing ripples: quadratic,

tapped & switched capacitor can avoid narrow duty

cycle for high step down conversion, coupled

inductors can minimize ripple as well as extend

duty cycle & multilevel is good for reducing switch

stress. A comprehensive review of the published

research work on non-isolated dc-dc step down

converters is given and around twenty two different

techniques used for such type of conversion are

briefly explained. A comparison of these topologies

is carried out in relation to step down conversion

ratio/voltage gain, switch stresses, output current

ripple and number of components used. Two best

topologies are identified and there simulations

results are given. This paper well provides a clear

background to the students and researchers who

want to do further research on step down dc-dc

converters.

• Control strategy that enables improved ripple

cancellationfor multiphase converters with

asymmetric operatingconditions has been

presented.

• Performance has been verified by simulations

andexperimental results.

• Calculation of uneven phase angles based on

thefrequency domain representation of the current

waveformsgoes beyond the previous works in the

field.

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Author Biography

Dr.D.Bala Gangi Reddy received his

B.Tech, M.Tech and Ph.D degrees

from the College of Engineering,

Jawaharlal Nehru Technological

University, Hyderabad in 1999, 2005

and 2014 respectively. Currently he is working as

Professor in the Department of Electrical and

Electronics Engineering at Vidya Jyothi Institute

of Technology, Aziz Nagar Gate, C.B. Post,

Hyderabad-75; He published numerous papers on

Power systems and FACTS. His current research

interests include Control of Electrical Drives,

Electrical Distribution System and Power Quality

& FACTS.

G.Priyaka Reddy is pursuing her

MTech in Vidya Jyothi Institute of

Technology, She received her

BTech from JNTUH Hyderabad. Her

current research interests include

Control of Electrical Drives, Electrical

Distribution System and Power Quality

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