DC/DC Converter

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Design and Optimization of
an ESU for hybrid light
vehicles with the use of
Supercapacitors
Students:
Aniello Valentino
Francesco Villella
Supervisor:
Stefano Carabelli
Marcello Chiaberge
Index







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conclusions
2
Index







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conclusions
3
Introduction

Context

Motivations

Objectives
4
IntroductionContext
Contex
ESU with add-on
Supercapacitors
The Supercapacitors are
an addition to batteries
they can be inserted or
excluded depending on
the needs.
TTW Three Tilting Wheels
5
IntroductionMotivations
Motivations



Use supercapacitors in parallel with the battery to improve
acceleration and energy recovery during braking
Designed for peak power requirements to increase the
efficency and the life cycle of the ESU system
Feasibility study of an ESU
Why Supercapacitors?
The purpose is to allow higher accelerations and
deceleration of the vehicle with minimal loss of energy,
and conservation of the main battery pack.
6
IntroductionObjectives
Objectives
 Analysis and design of supercapacitor pack
 Design procedure definition for supercaps
 Analysis and design of supercap equalization net
 Analysis and design of DC/DC converter
 Definition of a dynamic model for supercap and
DC/DC converter with several degrees of
approximation
7







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conclusions
8
Supercapacitor
Battery vs Supercap
Type
Energy/
weight
[Wh/kg
]
Power/
Size
[W/kg]
Nom,
Cell
[V]
Cycles
Durabilit
y
[#cicli]
Charge
time
[h]
Lead
(Pb)
20÷30
1÷300
2
200÷300
8÷16
Ni-Cd
30÷55
10÷900
1.25
1500
1
Ni-MH
50÷80
20÷
1000
1.25
30÷
500
2÷4
Li-ion
110÷
160
1800
3.7
500÷
1000
2÷4
Li-ion
VHP Saft
74
6900
3.6
500000
20m
Nanosafe
90
4000
13.8
15000
<10m
Supercap
3.9÷5.7
470÷
13800
2.5÷
2.7
1000000
0÷30s
The non conventional
batteries have High Power
density but the charging time
is high for this application.
9
Supercapacitor
Supercapacitor





ADVANTAGES
High Capacitance and
ultra low ESR
High Density of Power
Fast charging /
discharging
High Available Current
High number of life
cycles



DRAWBACKS
Low voltage for each
cell
High Weight and
Volume
Very expensive
10
Supercapacitor
Equalization net
In power applications, supercapacitors are used in stacks where
many cells are connected in series or in parallel to obtain acceptable
voltages and energy.
The disparities among the cell's parameters won't exhibit the
same charging dynamic and, at end of charge transient, some
cells may present over-voltage while some others are
insufficiently charged.
 The tolerance of the supercaps is 20%, but presumably if you
buy supercaps from the same stock the tolerance reduces
itself.
11
 This involves the introduction of a control.

Supercapacitor
Possible Solution
DC/DC active solution
Switched Resistor
ADVANTAGES :
 High efficency
DRAWBACKS :
 Several DC/DC converter
 The implementation of
the hardware and its
control is very costly.
ADVANTAGES :
 Simplest Solution
DRAWBACK :
 Power loss
Integration Kit
ADVANTAGES :
 High efficency
 User friendly
DRAWBACK :
 Expensive 40$
12
Index







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conclusions
13
ESU with Supercapacitor
ESU with Supercapacitors
ACTUAL SYSTEM
ADVANTAGES
 Simple realization
DRAWBACKS
 Great stress for battery
 No longer battery life with high
absorbed currents
 Long charging time
 Few charge-discharge cycles
PROPOSED SOLUTION
ADVANTAGES
 In case of failure is always
guaranteed connection between the
battery and the inverter.
 During braking, the controller decides
which energy source recharge.
 This power system allows acceleration
and deceleration of the vehicle with
minimal loss of energy and minimizes the
stress of the batteries.
DRAWBACK
 We need to design a Bidirectional
DC/DC converter.
14
ESU with Supercapacitor
Specifications
 Ptraction = 22kW
Specifications
 Phase of Traction = 5 s
 Phase of Braking = 10 s
Matlab Algorithm
 Supercapacitor Add-on
 ESU must be fault tolerant
 Weight of ESU : less possible
Other important elements
Results SC bank
 Vbattery = 200V
 Restriction of DC/DC converter
15
ESU with Supercapacitor
Sizing supercapacitor bank
To respect the energy constraints, the
physical limits of supercapacitors and the
restrictions imposed by the DC/DC converter
must be considered.
In our analysis the following issues have been
taken into account:
 Supercapacitor working voltage
 Restriction of the DC/DC converter
16
ESU with Supercapacitor
Supercapacitor working voltage
The working voltage of the supercaps must be lower
than nominal voltage in order to lengthen life
expectation.
The aging processes of
supercapacitors are
mostly driven by
temperature and cell
voltage, which have an
influence on the calendar
life of the devices.
2,6V (96% of continuous voltage rating) was
chosen because it is a voltage that ensures a
sufficent life expectancy.
17
ESU with Supercapacitor
Supercapacitor working voltage
1,3V (50% of continuous voltage rating) was chosen because it is
a voltage that ensures a sufficient input voltage to the DC/DC
converter and keeps the ratio max-input / min-input near 2.
The discharge voltage ratio d (in %) of the supercapacitors bank is
defined as:
The DOD “Depth of Discharge” (in %) is then equal to:
Then the Energy of Supercapacitor bank is given by the
following equation:
This equation shows that, for a 50% DOD, the useful energy
represents 75% of the maximun energy. Is inefficent to
discharge the bank below 50% of its max voltage.
18
ESU with Supercapacitor
Restriction of the DC/DC converter
The converter imposes constraints on the
ratio between maximum input voltage and
minimum input voltage, also
between
output voltage and input voltage.
The restrictions refer to a non-isolated
converter.
19
Procedure
Add SC
Initial
condition
N=1
NO
# SC in
series=N
VinMAX
Vinmin
processing
Does the
number of SC
in series
respect the
costraint 5:1 ?
YES
Constraints
DC/DC
Choose a
model
Calculate
the energy
of a
module
specifications
Needed
Energy
datasheet
Repeat this procedure for all models and for
1<N<100 and Research the SC bank with
minimum weight.
Needed
Energy
>
E_module ?
Add Module
NO
YES
save data
20
ESU with Supercapacitor
Compromise Weight-Energy
Result:
The best compromise between weight and energy considering all the
constraints on supercap and DC / DC converter has been found
through a Matlab algorithm.









# cell in series (module): 35
# module: 1
# total of supercap : 35
Input voltage range:
45,5÷91 V
Weight : 11,19 kg
Energy: 133087J
36,96Wh
Power:26.62kW for 5s
Volume : 9000 cm3
Model:BCAP 1500
21
Index







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conclusions
22
DC/DC Converter
DC/DC Bidirectional Converter
It is necessary because the supercapacitors voltage (91V) is
different in comparison to the DC BUS voltage (200V).
Principle of Operation
Constraints
Constraint of application
Weight application : less possible
Step-down
Phase
Step-up
Phase
Vin[V]
200
45.5-91
Vout[V]
91
200
Iout[A]
100
110
Pout[kW]
9.1
22
Max time
phase [s]
10
5
23
DC/DC Converter
Comparison isolated-non isolated
Two main categories of bidirectional DC/DC converters can
be envisaged for this task:
Isolated converters
 Full Bridge
 Tapped Boost
Non isolated converters
 Buck+Boost
 Multiphase
Buck+Boost
Multiphase
Full Bridge
Tapped
Boost
Inductor
Very heavy
N but light
heavy
heavy
Trasformator
none
none
yes
Yes L couple
Diff.Control
Middle(2sw)
Hard(Nsw)
Hard(8sw)
Middle(2sw)
Efficiency
high
high
low
middle
24
DC/DC Converter
Non isolated converters
A variant of the Buck+Boost solution is the Multiphase Converter.
ADVANTAGES:
 Simplest topology of
the DC/DC converter
DRAWBACKS:
 Excessive weight
 Complicated Inductor
costruction
ADVANTAGES:
 The key principle of
these converters is the
output current sharing
among several parallel
channels.
DRAWBACK:
 Interleaved strategy is
very difficult.
25
Index







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conclusions
26
Modeling
Modeling
The modeling is needed to allow you to enter the ESU
designed in the system.
Virtual Prototype : Longitudinal dynamics model of the vehicle
Plant
ACU
ICE
ESU Modeling
Electric motor
Power Module
Battery
ECU System
DC/DC Converter
Supercap
Host
27
Modeling
Supercap
Laboratory Test
Simulink Model
Analysis of results
28
Modeling
DC/DC Converter + Supercap
FIRST APPROXIMATION
Assumptions:
SECOND APPROXIMATION
Assumptions:
 Linearity
 No Linearity,
 No losses (DC/DC)
 Losses (DC/DC)
Equations :
 Buck eq.
 Boost eq.
Equations :
 State Equations(L,C)
29
Modeling
Models Comparison
Comparison Parameters
Assumptions for the buck phase (braking):
 Static Simulations (fixed duty cycle)
 Iniatial SC Voltage:60V
 D = duty cycle = 40% T = Period = 20 us
 Simulation time : 40s
 In the first approx are
visible only the mean values.
 Very fast time simulation.
 Simulation Time(40s) :
0,001s
 In the second approx are
visible the instantaneous
values and you can see the
voltage/current ripple.
 Very long time simulation30
 Simulation Time(40s) : 30'
Modeling
First vs Second approximation
First Approx
Second Approx
Speed
Simulation
Very fast
Very slow
Accurancy
low
high
 If you need a fast simulation, and you do not want to see
the transient then you can use the first approximation
model.
 If you want to see the current and voltage ripples you
can use the second approximation model, this model is the
most similar to the electric model.
 For a more accurate comparison should have circuital
simulations.
31
Modeling
Dynamic Simulation
Real operating assumptions :
Dynamic Simulations (First approximation model with control)
Iniatial SC Voltage : 0V
D = duty cycle = variable
T = Period = 20 us
Simulation time : 100s
C/!D = Charge/!Discharge = '1', after 40 s '0'
In the figure you can
see the possible real
behaviour of the first
approximation model.
 Are visible the correct
functioning of the
system.
 Very fast time
simulation.
 Simulation Time(100s):
0,001s
32
Index







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conlusions
33
Results
Results
The choices made concerning the
following four points:
Topology
 DC/DC Converter
 SC Bank
 Modeling

34
ResultsTopology
Topology
DC/DC converter with high voltage battery pack
 In this solution we need to
design only one bidirectional
DC/DC converter.
 Inserting an electronic switch
in the converter it is possible
to guarantee the safeness of
the application.
 The number of the supercap
bank is not extreme.
 The supercap bank is an
add-on of the existing
system.
35
ResultsDC/DC Converter
Bidirectional DC/DC Converter
 The components are
commercially available
more easily
 It is a direct converter
then avoids losses related
to the transformer
Multiphase
36
ResultsSC Bank
Results Supercapacitor bank
Number of Scap = 35
Type of Scap = BCAP1500
Resulting Capacitance = 42,85F
Resulting ESR = 16,45mΩ
Energy storage=133087 J 36,97Wh
Volume = 9000cm3
Cost Scaps = 2750 dollars
Weight Scaps = 11,19 Kg
Estimated weight DC/DC converter 22 Kg
Max weight battery = 20 Kg
Estimated weight ESU 53Kg
Estimated operating temperature -25 ÷ 70 °C
37
ResultsModeling
ESU Model


The first approximation model is a simple solution and
has a short time simulation, in the future it will be placed
in the Virtual Prototype.
It will be used to evaluate the performance of the vehicle
with and without the use of the supercapacitors.
38
Index







Introduction
Supercapacitor
ESU with Supercapacitors
DC/DC Converter
Modeling
Results
Conclusions
39
Conclusions


Very high cost (Supercap + DC/DC Conv.)
High weight and volume (Supercap +
DC/DC Conv.)
In conclusion, for the requested application, the resulting data are
excessive in terms of weight and volume occupied.
However, to confirm these conclusions, it would be interesting
being able to perform tests, using the simulator. They will produce
curves that may highlight the performance gap with and without
the ESU.
40
Conclusions
The supercaps are
suitable to be used either
in buses, trains, trolley
buses...
...or in high performance
vehicles, such as sport
cars
and
competition
motorcycles.
41
Thanks
Aniello Valentino – Francesco Villella
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