elec 6411 - project final final

7/26/2019 ELEC 6411 - Project Final Final

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ELEC-6411 Final Project Report

Bi-directional Cascaded Buck-Boost ConverterDesign and simulation

Submitted to:

Dr Luiz A. C. Lopes

Submitted by:

Andrew Jensson !"""##$%

&a'endra ()ike *+$+#,%#

Date o Submission: December *% *"%

(erm: /all *"%

ABS(&AC(


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Power electronic converters are efficient in supplying power at a regulated voltage or regulated current.

Buck converters can supply power at a voltage level lower than the source while boost converters can

supply power at higher voltage level than the supply. Besides these converters, there are converters

which can source or sink the power depending on the requirement. In general, these are bi-directional

converters either with current reversal capability or voltage reversal capability.

In this project, a cascade type bi-directional buck-boost converter has been analyed and designed for

supplying power to charge an ultracapacitor or use the power stored in the ultracapacitor. !he

converter is capable of changing the direction of the current and incrementing the voltage level either

lower or higher compared to the source voltage. "nalytical design of each component#s parameters

including inductor, $%&'(! rating, and harmonic filter for safe operation of the selected )*+ '

ultracapacitor from $awell. !he converter was simulated in P&I$ for four of its operating conditions,

vi. buck charging of ultracapacitor, boost charging of the ultracapacitor, buck discharge of

ultracapacitor and boost discharge of ultracapacitor. !he harmonics from the simulation for each case

was analyed and a second order filter was designed for the supply and load accordingly. !he

resulting current ripple waveforms were found to have less harmonic magnitude as calculated.


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(ABL0 1/ C12(02(S

Abstract i

(able o contents ii

List o /i3ures iii

List o (ables iv

C)apter % %

1. Introduction 1

2. cope 2

C)apter * ,

1. Con!erter components "

2. Con!erter #nal$sis %

". Con!erter &peration %

C)apter , 4

1. peci'cation o( t)e ultracapacitor *

2. election o( t)e s+itc) *

". +itc)ing (re,uenc$ *

4. Inductor design

%. atter$

C)apter ! %"

1. PI/ simulation 102. uc - C)arge mode 11

". oost - C)arge /ode 12

4. uc Disc)arge /ode 1"

%. oost - Disc)arge /ode 14

C)apter %

1. 3armonic anal$sis 1%

2. umerical Fourier anal$sis 1%

". Filter design 16

4. Filter implementation 15

%. imulation 15

C)apter $ *"

1. Results 20

2. Conclusions 21

Appendi5


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Datas)eets

&eerences

L6S( 1/ /6Figure 1. 1

Conceptual Bi-directional Cascaded Buck-Boost Converter topology______________1YFigure 2. 1 MOSFE sc!e"atic diagra"____________________________________________________#

Figure 2. 2 $nductor sc!e"atic diagra"_____________________________________________________%

Figure 2. # Sc!e"atic diagra" o& a 'attery_________________________________________________%

Figure 2. % Basic structure o& a capacitor____________________________________________________(

Figure 2. ( Sc!e"atic diagra" o& t!e converter_____________________________________________)

YFigure %. 1 Converter sc!e"atic in *S$M__________________________________________________1+

Figure %. 2 Buck - C!arge ,ave&or"s______________________________________________________11

Figure %. # Boost - C!arge ,ave&or"s_____________________________________________________12

Figure %. % Buck - isc!arge ,ave&or"s___________________________________________________1#

Figure %. ( Boost - isc!arge ,ave&or"s___________________________________________________1%

YFigure (. 1 $deal rectangular current ,ave&or"___________________________________________1(Figure (. 2 Current !ar"onic content ,it!out lter________________________________________1)

Figure (. # /ar"onic lter i"ple"entation in si"ulation___________________________________10

Figure (. % Current ,ave&or"s &or Buck - C!arge "ode ,it! lter__________________________1

Figure (. ( Current ,ave&or"s &or Boost - C!arge "ode ,it! lter_________________________1

Figure (. ) Current ,ave&or"s &or Buck - isc!arge "ode ,it! lter_______________________1

Figure (. 0 Current ,ave&or"s &or Boost - isc!arge "ode ,it! lter______________________1


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L6S( 1/ (ABLa'le 2. 1 Converter "odes o& operation____________________________________________________)

a'le 2. 2 3elation o& drive-"ode and s,itc!ing state______________________________________)Y

a'le #. 1 4ltracapacitor para"eters________________________________________________________

a'le #. 2 MOSFE para"eters______________________________________________________________

a7le ". " Inductor parameters

a'le ). 1 esign and si"ulated converter results_________________________________________2+

a'le ). 2 esign and si"ulated lter results______________________________________________2+

C7A8(0& %

1. Introduction

Power electronics converters have a wide range of applications. %ne of particular interest is the

regulated dc supply. In many applications, the output voltage may be required to be higher or

lower compare to the source. Buck-Boost converters can be used to achieve this requirement.

/owever, in some applications, the direction of power flow should also be reversible. 'or such

conditions, bi-directional converters are used. !here are many different topologies for the bi-

directional converters with each topology having certain advantages over others.

!he Bi-directional ascaded Buck-Boost onverter, which for simplicity will be referred to as

0converter1 from herein, is one topology of dc-dc converters that has unique features andfunctions that make it an attractive option for certain applications. 2hen variable power must

be sent and returned to a dc source 3hence bi-directional4, a dc-dc converter may be selected5

even providing a more stable operating dc source for critical loads, a Bi-directional ascaded

Buck-Boost onverter may be utilied. !he 0cascaded1 component stems from the series

connection of both types of converters. !he converter presented in this project has application

in many systems.

(lectric 6ehicle 7rive and 8egenerative Brakingi

Back-up Power &upplyii

Photo-voltaic &ystems

(nergy 8ecovery &ystems

Back-to-back 2ind Power &ystems


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'or the purpose of this project, the converter will be modeled in an electric vehicle inspired

application, with the load of the converter being an ultracapacitor. 7epending on the status of

the ultracapacitor, it will be charged or discharged using a Buck or Boost configuration. !he

topology for this converter is shown in figure ).) iii.

Figure 1. 1Conceptual Bi-directional Cascaded Buck-Boost Converter topology

2. Scope

!his project is limited to the design of the inductor between the two legs, selection of the

switching devices and the rating of the switches based on the load. "dditionally, a second order

filter is designed and component are selected for the battery side and the ultracapacitor side.

7esign of the controller is out of the scope of this course project, so it is not discussed in this

report. !he designed converter is simulated in P&I$ software for validation.


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C7A8(0& *

1. Converter components

Before designing a functional converter, one must understand the fundamentals of operation of

the circuit, as well as fundamental components and concepts individually.

i. $%&'(!

Based on the characteristics of different available power electronic switches, the $%&'(!

was selected5 the criteria which will be discussed in a later chapter of this report. 'or the

purpose of design and analysis of the converter, the $%&'(! operates as an idealied

switch. !his means that the $%&'(! will not have a voltage drop across it when closed and

no current will leak through it when opened. !he idealied $%&'(! will also have an

instantaneous turn-on and turn-off time and is capable of operating under all voltage and

current conditions present in the converter. !o operate the $%&'(!, a gate signal will be

applied to the 091 node when a closed switch is desired5 under all other conditions, the gate

signal will be grounded with : v.

Figure 2. 1 MOSFE sc!e"atic diagra"iv

ii. Inductor

!he inductor plays a vital role in the converter circuit5 namely to act as the energy storage

medium while the $%&'(!s are switching to provide the desired output voltage. !he basic

inductor is a simple component in that it has a magnetic or air core with a wire coiled

around. !here are many other types of inductors, but the scope of this project will not

eplore these configurations, rather the inductance is the only parameter considered. 2hen

current passes through the coil of wire, the inductor presents 0inertia1, or resists the changeof current, depending upon the current applied and the inductance, measured in henries.


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Figure 2. 2 $nductor sc!e"atic diagra"v

iii. Battery

Batteries are the portable source of power which can store and generate electrical power as a

result of a chemical reaction. 'or electrical vehicle and other applications where power is

bi-directional, rechargeable batteries are used. !hese batteries can go through many charge

and discharge cycles, however there is some limit on the continuous current and the current

should have low ripple to maintain epected life of the battery.

Figure 2. # Sc!e"atic diagra" o& a 'attery

iv. apacitor

!he capacitor is another energy storing element in the converter to aid in reducing the

voltage ripple and is often applied on the supply and output of a converter. 'or the design of

the converter within the scope of this project, the capacitor will be considered ideal,

meaning that it contains no parasitic resistance. " capacitor acts as an energy storage device

and will oppose the change in voltage by drawing in the ripple currents, so the current that is

supplied to the capacitor is dependent on the rate of change of the voltage applied, and the

capacitance, measured in farads.


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Figure 2. % Basic structure o& a capacitorvi

v. ;ltracapacitor

"n ultracapacitor is an energy storage device that is utilied in applications where

frequent bursts of power is required for short duration. !he energy storage density for

ultracapacitor is very high compared to normal capacitors. ;ltracapacitors typically

store ): to ):: times more energy per unit volume or massthan electrolytic capacitors,

can accept and deliver chargemuch faster than batteries, and tolerate many more charge

and discharge cycles than rechargeable batteries.

2. Converter Analysis

i. &teady-state "nalysis

&teady-state analysis is an important tool used by designers and engineers to allow for

certain assumptions that simplify the design process. !he assumptions presume that all

analyses of the circuit will be done after any transient or sub-transient responses of the

components have been cleared. !his is done to allow designers to calculate parameters and

components while utiliing simplified equations and processes.

ii. ontinuous onduction $ode

"lso known as $, this describes the operation principle of the converter when the

inductor is not allowed to fully discharge its stored energy. 'orcing the converter to operate

in $ will simplify the design procedure and analysis of the circuit. !his is done by siing

the inductor so that the current flowing through the inductor never reaches a ero-value andthis allows for a designer to use common analysis and design equations to create a converter

that operates as epected.

iii. 7iscontinuous onduction $ode


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a'le 2. 1 Converter "odes o& operation

Mode Current Source Current Sink Description

Buck - Car!e Battery ;ltracapacitor Vbattery>Vsupercap

Boost - Car!e Battery ;ltracapacitor Vbattery Vsupercap

Buck - Discar!e ;ltracapacitor Battery Vbattery Vsupercap

Boost - Discar!e ;ltracapacitor Battery Vbattery>Vsupercap

!he relation of the drive-mode and switching states are shown in table =.=.

a'le 2. 2 3elation o& drive-"ode and s,itc!ing state

Mode "1 "2 "3 "# Description

Buck - Car!e P2$ %'' %'' %'' harging ;ltracapacitor

Boost - Car!e %> %'' %'' P2$ harging ;ltracapacitor

Buck - Discar!e %'' %'' P2$ %'' harging Battery

Boost - Discar!e %'' P2$ %> %'' harging Battery

"s table =.= has identified, the modulation of specific configurations of the switches will

result in measurable and distinct changes in the output voltage. !his converter requires

relatively more comple control and a larger quantity of switching devices, but is able to

operate in a wide range of applications and maintain suitable performance under all current

and voltage conditions presented in this arrangement. !he critical modes of converter

operation are discussed below.


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i.Buck - harge

In this mode the ultracapacitor voltage is less than the battery voltage. &witch ?) is

switched in pwm mode to charge the ultracapacitor in current controlled mode. !he

switching is based on the maimum current through the ultracapacitor.

ii. Boost - harge

2hen the ultracapacitor voltage is greater than the battery voltage, switch ?) is kept on and

switch ?@ will be switched on pwm mode to get higher output voltage than the battery

voltage. /ere again the control is done based on the current through the inductor.

iii. Buck - 7ischarge

!o discharge the ultracapacitor when its voltage is greater than the battery voltage, the

converter is operated in current controlled mode operating the switch ?A in pwm mode.

iv. Boost - 7ischarge

2hen the ultracapacitor voltage is lower than the battery voltage, the converter is operated

in this mode to supply power to the battery. !his is done by keeping ?A on all the time and

switching ?= in pwm mode with current through inductor being controlled.


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C7A8(0& ,

1. Speci$ication o$ te ultracapacitor

!o specify the individual components that make the converter, a circuit consisting of a battery

and an ultracapacitor was selected. !his type of load and source is an analogy to an electric

vehicle application. In this application, the current flowing is bi-directional and the magnitude

must be limited to a value within the tolerable levels of the components. !he $awell

BMOD%1&' (%#) B** ultracapacitor with the parametersviigiven in table A.) was selected as

an appropriate load for this study.

a'le #. 1 4ltracapacitor para"eters

Capacitanc

e

+ated ,olta!e Ma Continuous Current eaka!e Current /S+

)*+ ' @ 6 CC " +.= m" *.A mD

'rom the leakage current, the leakage resistance is calculated to be E,=A) D.

2. Selection o$ te s0itc

'rom the rating of the ultracapacitor and the level of the operating voltage, the switching device

should be selected. !he maimum voltage across each switch is @ 6 and the average current

through each of the switch is maimum continuous current through the ultracapacitor. Because

of the low voltage, high current, high switching frequency capability, a $%&'(! is selected as

the switching device. !he $%&'(! must be capable of handling the application demands

without damage5 therefore the &!$icroelectronics S1)'1%43-2 $%&'(!viiiwith )::F

safety margin was selected. !he parameters of the $%&'(! are listed in table A.=.

a'le #. 2 MOSFE para"eters

,olta!e5VDS ):: 6

Continuous current5ID ): "

ermal resistance 6unction-case5Rthjcase :.@ GH2

urn on delay time5td ,o n =+.* ns

+ise time5 tr EC.) ns

urn-o$$ delay5td,off EE.E ns

4all time5tf *.E ns

+esistance o$ drain-source5 +DS7on8ma @.+ mD


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3. S0itcin! $re9uency

!he $%&'(! must be able to dissipate the heat generated due to conduction and switching

losses. !he switching frequency,fS of the converter, is calculate by balancing the rate of

heat generation and the rate of heat loss.

!he conduction loss is given by Pcond 8ds3on4JI7=JdutyKcycle L3=.)4

'or the worst case, dutyKcycle is considered unity.

!he switching loss is given by Psw MJ67&JI7&J3td3on4N trNtd3off4Ntf4Jfsw L3=.=4

!he maimum allowable power loss can be calculated using equation 3=.A4

Pallowable T

R thjcase L3=.A4

!o limit the junction temperature to )::o, the maimum allowable power loss is calculated to

be )+*.=+ watts. "s per the datasheet, the drain to source resistance varies with temperature, at

)::o, it is given by equation 3=.@4.

87&3on4).+J87&3normal4L3=.@4

onsidering the non-perfect junction between the $%&'(! and the heat sink the switching

frequency is selected to be )H+ thof the switching frequency given by the calculations using

equations 3=.)4 to 3=.@4 which is +: k/.

#. Inductor desi!n

%nce the switching frequency is fied, the sie of the coupling inductor, , can be properly

designed. In order to maintain the continuous current mode operation of the converter, the

inductor can be sied according to the following equationiO

L= TSVbat

8ILB ,max

;sing a fair assumption of maintaining continuous conduction mode at +F of the safe

continuous current value, which is set at :F of the maimum continuous allowable current,the following values are obtained and are listed in table A.A.

a'le #. # $nductor para"eters

S0itcin! ime5

TS

Battery

,olta!e5

)%: Ma Continuous

Current5 Sa$e Operatin! (oint

Minimum Inductor

Current5ILB ,max

Induct

ance5


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Vbat

1

50kH=20!s 24V 8077"=61.6" 561.6"=3.08"

2% ;

'. Battery

'or this project, an ideal battery in the P&I$ software component library is selected. " small

resistance of + mD is added in series with the battery to represent the internal resistance of the

battery. %ther parasitic and intrinsic factors of the battery are not considered in this project.


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C7A8(0& !

1. (SIM simulation

" circuit shown in figure @.) is made in P&I$ for simulation realiation.

Figure %. 1 Converter sc!e"atic in *S$M


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2. Buck - Car!e mode

Figure %. 2 Buck - C!arge ,ave&or"s

In this mode ?) is operated in pwm mode with ?=, ?A and ?@ turned off. 'or simulation

purpose, the battery voltage is fied at =@ 6 and ultracapacitor voltage is set to =: 6. !he

inductor current is forced to maintain :F of rated ultracapacitor current i.e. *= ". !he

modulating triangular waveform, gate signal to transistor ?), battery voltage, ultracapacitor

voltage, capacitor current, inductor current and the battery current are plotted


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3. Boost - Car!e ModeFigure %. # Boost - C!arge ,ave&or"s

In this mode ?) is on all the time with ?@ operated in pwm mode while ?= and ?A are turned

off. 'or simulation purpose, the battery voltage is fied at =@ 6 and the ultracapacitor voltage is

set to @: 6. !he inductor current is forced to maintain an average value of *= ". !he waveforms

as in buck charge mode are plotted in figure @.@ and figure @.+. It shows that ultracapacitor

current has a pulsed waveform while inductor current and battery current has an average value

of current close to *+ ".


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#.

#.

#.

#.

#.#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.

#.#.

#.

#.

Buck < Discar!e Mode

Figure %. % Buck - isc!arge ,ave&or"s

!he ultracapacitor is discharging into the battery with current being maintained within a ripple

of approimately A " near an average value of *= ". !he voltage of the ultracapacitor in this

simulation is @: 6, and the battery voltage is =@ 65 this gives a duty cycle for the switching$%&'(! 3?A4 of :.*, which can be seen in the gate voltage waveform. !he current in the

ultracapacitor is pulsed waveform with a peak to peak ripple of around *+ ".


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'. Boost - Discar!e Mode

Figure %. ( Boost - isc!arge ,ave&or"s

!he voltage of the ultracapacitor is measured to be =: 6 and is slowly decreasing as it discharges into

the battery5 this eplains the negative values for current in figure @.+. !he duty cycle of the switching

$%&'(!, ?= is calculated as :.=, which can be seen in the gate voltage waveform. !he inductor

current is held within the A " ripple around C= ", while the battery current is a pulsed waveform with a

peak to peak ripple of nearly C+ ".


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C7A8(0&

1. armonic analysis

'rom the waveforms shown in figure @.= to @.+, the battery current is pulsed in buck charge

mode and boost discharge mode while the ultracapacitor current is pulsed in boost charge mode

and buck discharge mode. !his is due to the nature of the circuit and the switching operation

required to obtain the buck and boost modes. !o determine the harmonic content present in the

pulsed waveforms, the pulsed waveform is assumed to have an ideal rectangular shape as

shown in figure +.).

Figure (. 1 $deal rectangular current ,ave&or"

2ith the idealied rectangular waveform shown in figure +.), the 'ourier series analysis is

simplified and using equation +.), the amplitude of the harmonic currents is calculated.

ah=2"

h# (sinhd#)

L 3+.)4

2hen d is :.+, the amplitude of harmonics is largest. !he fundamental component,a1 , with a

frequency of +: k/, which is the same as the switching frequency,fS , has a peak to peak

amplitude of4"

# .

!his large amplitude is undesirable for the life of the battery and ultracapacitor, and a filter is

needed to limit this value.


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fres= fh

10

$a%n(dB)40dB /Dec L 3+.+4

;sing equations +.= to +.+, the resonant frequency of the filter,

fres

, is calculated to be E.E

k/. !he angular frequency becomes )res=2 # fres=62.25krad /sec .

!he resonant angular frequency of the second order harmonic filter can be written as equation

+.*.

)res= 1

L* L 3+.*4

!o limit the voltage ripple of the filter capacitor to +F of the battery voltage, the sie ofcapacitance for the worst-case of duty cycle equal to ), is calculated to be )::: ' using

equation +.C.

V=+

* =

I"V$t

* L 3+.C4

>ow the filter inductance is determined from equation +.* to be =*: n/.

#. 4ilter implementation

!he converter with the battery and load second order filters implemented is shown in figure +.A.

Figure (. # /ar"onic lter i"ple"entation in si"ulation

'. Simulation

'or the same cases in chapter @ sections = to +, P&I$ simulation with filter results the

waveforms shown in figure +.@ to figure +.C. !he waveforms shows that the peak to peak ripple


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of the fundamental harmonics were reduced. /owever, harmonics at the resonant frequency of

the filter were introduced.


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Figure(. %

Current ,ave&or"s &or Buck - C!arge "ode,it!lter

Figure (. ( Current ,ave&or"s &or Boost - C!arge "ode ,it! lter


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Figure (. ) Current ,ave&or"s &or Buck - isc!arge "ode ,it! lter

Figure (. 0 Current ,ave&or"s &or Boost - isc!arge "ode,it! lter

omparing the results in figure @.= to figure @.+ with figure +.@ to figure

+.C respectively, the current waveforms have lost their pulsed rectangular shape and are now much

closer to an ideal dc voltage. !here is still a ripple due to the siing of the filter, which was necessary to

maintain designed operation characteristics after implementation, while removing as much of the

harmonic current content as possible. !here is a possible resonance effect shown in the waveform

shape, but this impact on the components is much less than the large pulsed current waveform that was

present prior to filter implementation.

C7A8(0& $

1. +esults

!he average inductor current was designed to sustain :F of the maimum continuous current

rating of the ultracapacitor with a maimum ripple of +F at a switching frequency of +: k/.

!he second order filters were designed to limit the current ripple in the battery and

ultracapacitor to +F of the battery voltage and +F peak-to-peak of the current amplitude. "

summary of the results are listed in table *.) and table *.=.

a'le ). 1 esign and si"ulated converter results

Mode o$

Operation

Inductor Avera!e

Current5IL

Inductor +ipple

Current5IL Comments

Desi!ned Simulate Desi!ned Simulate


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d d

Buck - Car!e *).* " *= " A.: " =.= " &imulation as designed

Boost - Car!e *).* " *@ " A.: " C.: " IL higher than

designed

Buck - Discar!e *).* " *: " A.: " E.: " ILhigher than

designed

Boost - Discar!e *).* " C= " A.: " @.: "I,design

IL


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2. Conclusions

!he Bi-directional ascaded Buck-Boost onverter topology was analyed and designed for a

specific application supplying power to charge the ultracapacitor or use the power in the

ultracapacitor. !he sie of the switches, inductor and filter were determined using analytical

equations. !he converter was simulated in P&I$ to verify its operation. !he harmonics in

current was analyed using both 'ourier series method and numerical 'ourier transform in

software. 7esigning a second order filter, the dominant harmonic current was limited to

with +F of the peak amplitude which was verified in simulation.


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A8802D69


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DA(AS700(S

&0/0&02C0S


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i tud$ o( i-Directional uc-oost Con!erter opologies (or #pplication in Electrical

8e)icle /otor Dri!es F. Caricc)i9 F. Crescim7ini9 F. :iulii Capponi9 L. olero.

ii /odelling and Control (or a idirectional uc-oost Cascade In!erter 3onglin ;)ou9

)uai ??+ps.pren)all.com?c)et@pa$nter@introduct@6?6?1664?4261**.c+?indeA.)tml

! )ttps>??en.+iipedia.org?+ii?InductorB?media?File>InductorignalFilter1.png

!i )ttp>??+++.electronics-tutorials.+s?capacitor?cap@1.)tml

!ii )ttps>??+++.maA+ell.com?images?documents?),@4*!@ds1016201".pd(

!iii )ttp>??+++.st.com?st-+e7-

ui?static?acti!e?en?resource?tec)nical?document?datas)eet?D/001"""20.pd(

iAiAPo+er Electronics> Con!erters9 #pplications9 and Design - ed /o)an9 ore /.

ndeland9 illiam P. Ro77ins

A eamless Controlled Parallel i-directional DC-DC Con!erter (or Energ$ torage $stem

- aa$ui &uc)i9 #i)io anouda9 ao$a aa)as)i and /inoru /otei

elec 6411 - project final final

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