FACULDADE DE E NGENHARIA DA UNIVERSIDADE DO P ORTO DC/DC Converter with Transparent Electronics for application on Photovoltaic Panels Romano Jorge de Sousa Torres P REPARAÇÃO DA DISSERTAÇÃO P REPARAÇÃO DA DISSERTAÇÃO Orientador: Vitor Grade Tavares (PhD) Co-orientador: Pedro Miguel Cândido Barquinha (PhD) Co-orientador: Ganga Mamba Bahubalindruni February 13, 2013
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FACULDADE DE ENGENHARIA DA UNIVERSIDADE DO PORTO
DC/DC Converter with TransparentElectronics for application on
Photovoltaic Panels
Romano Jorge de Sousa Torres
PREPARAÇÃO DA DISSERTAÇÃO
PREPARAÇÃO DA DISSERTAÇÃO
Orientador: Vitor Grade Tavares (PhD)
Co-orientador: Pedro Miguel Cândido Barquinha (PhD)
DC/DC Converter with Transparent Electronics forapplication on Photovoltaic Panels
Romano Jorge de Sousa Torres
PREPARAÇÃO DA DISSERTAÇÃO
February 13, 2013
Sumário
A eletrónica transparente é uma tecnologia emergente que pode proporcionar sistemas de baixocusto dada a possibilidade de fabricação de dispositivos a baixa temperatura. A sua aplicação podeser útil em vários domínios como indumentária eletrónica e sensores de monitorização de saúde.Com a implementação de conversores DC/DC em eletrónica transparente, as aplicações poderiamextender-se para dispositivos com fonte de bateria ou painéis fotovoltáicos.
A tecnologia transparente é baseada em transístores de filme fino (TFT) com semicondutores a-IGZO, material que emergiu nos últimos anos. Os transístores podem ser utilizados para a criaçãodo Conversor DC/DC desejado, o qual pode ser depositado do vidro dos painéis, reduzindo oscustos de montagem e conceção de todo o sistema.
Esta dissertação tem como principal objetivo o projeto e desenvolvimento para construçãode uma topologia de conversor DC/DC com eletrónica transparente para aplicação em painéisfotovoltáicos. Inclui o estudo de elementos passivos, como bobinas e condensadores, e a suapossibilidade de utilização.
O trabalho tem a colaboração do grupo CENIMAT da Universidade Nova de Lisboa. Nesselocal será construído o conversor DC/DC e os circuitos desenvolvidos durante esta dissertação.
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ii
Abstract
Transparent electronics is a forthcoming technology, which allows for low-cost systems with de-vices fabricated at low temperatures. Its applications may be useful in various domains, suchas wearable electronics and sensors for health monitoring. Designing DC/DC converters withtransparent technology, the possible applications could extend to battery supplied devices or pho-tovoltaic panels.
Transparent technology, in this work, is based on thin-film transistors (TFT) with a-IGZOsemiconductors, material that has emerged in recent years. Their effectiveness for designingDC/DC converters will be assessed. Such converter could eventually be deposited on the glasscasing of photovoltaic panels, reducing installation and designing costs of the whole system.
This dissertation has in its main objective the development, design and fabrication of a DC/DCconverter topology with transparent electronics, for application in photovoltaic panels. It includesthe study of some passive elements, such as inductors and capacitors, and their effectiveness for apossible use.
The work is developed in collaboration with the CENIMAT group at UNL, where the devel-oped circuits and DC/DC converter will be fabricated.
3.1 Simulated results of negative DC/DC converters with load current . . . . . . . . 24
ix
x LIST OF TABLES
Abreviaturas e Símbolos
a-IGZO amorphous Indium Gallium Zinc OxideDC Direct currentFEUP Faculdade de Engenharia da Universidade do PortoITO Indium Tin OxideMOSFET Metal Oxide Semiconductor Field Effect TransistorMPDV Multiphase voltage doublerTFT Thin-Film TransistorTPVD Two-phase voltage doublerUNL Universidade Nova de LisboaUV Ultra-violet
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Chapter 1
Introduction
The main focus of this thesis is the study and implementation of DC/DC converters with transpar-
ent electronics. Transparent electronics is a forthcoming technology that promises cost effective
solutions and opens a new set possibilities, such as electronics on glass. The discover of fabri-
cation processes at low temperature, allows the development of TFT on flexible substrates at an
evermore low cost. The transparent DC/DC converters can be used on photovoltaic panels by
deposition on glass.
1.1 Motivation
Today, there are many studies concerning climate changes that result from the burning of non-
renewable energy sources, causing emissions of carbon dioxide to atmosphere and developing
greenhouse effect. Due to an increasing world population, the energy demand rises, depleting
petroleum and other non-renewable energy sources. The research on alternative and renewable
energy sources is then of outmost importance. Photovoltaic energy is one possibility.
Photovoltaic panels often use DC/DC Converters. They are mainly electronic circuits that
change the voltage value from input to output, working with direct currents (dc). Their application
extend to battery supplied devices, like mobile phones, laptops, typically to increase the voltage
provided. Converters are used also on photovoltaic panels. In this case, the converter produces a
regulated output voltage from energy temporarily saved, releasing it on a different voltage value,
with increasing efficiency.
Now-a-days, the inclusion of photovoltaic panels on daily life is possible, by either using the
roofs of buildings or large fields, but the creation of transparent devices can be considered as a
possibility of embodying the electronics with panels. Such approach could effectively down-size
the overall cost of the electronics (low-temperature fabrication) and of assembly (deposition of
electronic system on the glass casing). This is the main motivation behind the present proposal for
dissertation.
1
2 Introduction
1.2 Generic Proposed Solution
The first proposal described in this report, for a possible DC/DC converter topology to be imple-
mented with transparent electronics, uses switched-capacitor concepts to circumvent the need for
inductive elements. However, there are other issues that need to be taking into account. This report
presents in chapter 4 the main difference between a positive and negative DC/DC converter of this
type, describing the ideal behaviour expected from each topology.
1.3 Structure of the Document
Besides the introduction, this report has 4 more chapters.
Chapter 2 discusses the fundamentals and theory behind the construction of DC/DC converters
with transparent electronics.
Chapter 3 presents the state of the art, where various studies, closely related to the goal of this
dissertation, are exposed.
Chapter 4 presents the specifications and an initial approach to the desired converter.
Chapter 5 presents the conclusions and the work plan for the thesis.
Chapter 2
Background
2.1 DC/DC Converters
M. Rashid [1] describes DC/DC Converters as electronic circuits that receive an input voltage,
increase or decrease its level, resulting in a different voltage level at the output. They are important
for electronic voltage-supply systems.
Linear power regulators are systems that impose a steady and constant voltage at the output,
with a changing (and different) voltage level at input. Linear regulators are limited to an output
voltage level smaller than the input, but their power efficiency is in general reasonably small.
Switched-mode regulators are another way to get an output voltage level different from input.
They use on and off states to achieve good efficiency.
2.1.1 Step-Down Buck Converter
This circuit allows a smaller voltage level at output than the input. It uses a switch, a diode, an
inductor, a capacitor and an output resistor (the load). The circuit is shown in figure 2.1.
Figure 2.1: Step-down buck DC/DC converter [1]
When the switch is on, the current increases linearly in the inductor that it is divided to the
capacitor and the resistor. When the switch is off, the energy stored in the inductor is used to
3
4 Background
feed the output, and the current flows throw the diode which decreases linearly and smoothly. The
frequency of switching allows an adequate constant voltage level at the output.
The average output voltage level can be determined by the duty cycle "D" — the ratio between
time when the switch is on and the period. The following equation [1] shows the relation between
the output and input voltages for this Step-Down Buck Converter, calculated by Faraday’s Law:
Vout
Vin= D (2.1)
The output voltage is always smaller than the input, given the fact that the duty cycle "D" value
is smaller than 1.
2.1.2 Step-Up Boost Converter
This circuit allows an output voltage level greater than the input. It uses the same devices as the
Buck converter, but the layout changes the characteristics. The circuit can be seen in figure 2.2.
Figure 2.2: Step-up boost DC/DC converter [1]
When the switch is on, the current in the inductor increases, and energy is stored. The diode
is off, and the capacitor feeds the load. When the switch goes to off, the energy in the inductor
is released to the capacitor and resistor. The average voltage ratio is also determined by the duty
cycle, resulting in higher output voltage level:
Vout
Vin=
11−D
(2.2)
2.1.3 Buck-Boost Converter
It was created to allow an output voltage level greater or smaller than the input, depending of duty
cycle. The circuit can be seen in figure 2.3.
2.1 DC/DC Converters 5
Figure 2.3: Buck-boost DC/DC converter [1]
When the switch is on, the diode is off and the current in the inductor increases. When the
switch is off, energy in the inductor is released through the capacitor and resistor. The average
voltage ratio is:
Vout
Vin=− D
1−D(2.3)
2.1.4 Cuk converter
The basic function of this circuit is the same as Buck-boost, but it has the advantage of having
filtered currents at the input and the output, while the previous ones do not. The improved current
characteristics comes at the cost of more reactive components added. The layout can be seen on
figure 2.4.
Figure 2.4: Cuk DC/DC converter [1]
Here the energy storage and transfer is performed by a capacitor. When the switch is on, the
capacitor is discharged by the inductor on the right and the diode is off. When the switch is off,
the left inductor charges the capacitor, and the diode is on. In both switch states, current flows
through both inductors, providing filtering and improving the characteristic of continuous current.
The average voltage ratio is the same as Buck-boost converter.
6 Background
2.2 Transparent electronic devices
DC/DC converters use various components such as resistors, capacitors, diodes and inductors.
Regarding the goal of creation such device with transparent electronics, a study about transparency
is very important.
To create transparent devices, different materials should be combined to perform the functions
of non-transparent devices. The efficiency of new devices created should be, as possible, equal
or better to the existing ones. However, the characteristics of transparent electronic materials
comparing with the others, like smaller conductivity, decrease their efficiency.
There have been many studies to create high performance devices. They focus on the layout
of materials, and the results are sometimes very different from what could be expected with non-
transparency.
2.2.1 Resistors
According to D. Keszler et al. [2], a good performance resistor should fulfil the characteristic
defined by Ohm’s law; it should have a linear behaviour according to this law. Parasitic capacitance
is undesirable, therefore those should be fabricated on insulating substrates.
Taking advantage of the smaller conductivity of transparent materials, resistors are created
with a long path between two contacts. The layout can be seen in figures 2.5 and 2.6.
Figure 2.5: Transparent resistor from a planar view [2]
Figure 2.6: Transparent resistor from cross-sectional view [2]
The passivation layer is created to protect the resistor physically and chemically. Hence, the
conductance of resistor establishes. The resistance value is proportional to the resistivity of mate-
rial and the path length. It is inversely proportional to the width.
2.2.2 Capacitors
Capacitors should have a linear current-voltage derivative characteristic, according to D. Keszler
et al. [2]. They are created to store energy in an electric field form. A transparent capacitor can
2.2 Transparent electronic devices 7
be achieved with a transparent electric insulator between two contacts. The layout can be seen in
figures 2.7 and 2.8.
Figure 2.7: Transparent capacitor from plan view [2]
Figure 2.8: Transparent capacitor from cross-sectional view [2]
The capacitance is proportional to the insulator dielectric constant and the plates area. It is
inversely proportional to the insulator thickness.
2.2.3 Inductors
Inductors store energy in a magnetic field, but according to D. Keszler et al. [2], due to poor
conductance and consequently high parasitic resistance of transparent materials, it is difficult to
create these devices with linear voltage-current derivative as expected.
In fact, the resistance related to the transparent material is so high, that the large number of
turns needed for inductor increases the length of material and consequently the parasitic resistance
[2]. With high parasitic resistance, the quality factor of inductor will be low. Therefore, other ways
to create DC/DC converters need to be studied. One possible solution can be voltage multipliers,
referred in next section.
2.2.4 Thin-film transistors (TFT)
The basic function of these devices is to control the current between drain and source with an input
voltage at the gate (to the source). Due to their characteristics, transistors can be used as a switch
or even as a diode. The ability to set the device in multiple operation modes makes them the core
of electronics. Different structures of the device are possible and various technological parameters
can change their characteristics. There are four different structures of TFTs studied, the staggered
8 Background
bottom-gate, staggered top-gate, co-planar bottom-gate and co-planar top-gate structures. The
different layouts can be observed in figures 2.9 and 2.10.
Table 3.1: Simulated results of negative DC/DC converters with load current
efficiency. The results for both proposals suggests a good efficiency and constant output voltage,
which allows an increasing knowledge on how to reduce undesired behaviours, such as parasitic
capacitance and resistance.
24 Bibliographic Review
3.2.3 DC/DC Converters in Organic Thin-Film Transistor Technology
H. Marien et al. [7] propose a DC/DC converter using organic thin-film transistors with a mod-
ified Dickson’s charge pump. Organic technology only uses p-type transistors in contrast to
transparent TFTs. The authors report some advantages of this technology, as the production at
low-temperatures, reduced complexity and cost of the process. However, low mobility, transis-
tor parameters variations and influence of oxygen and water, like transparent technology, are real
limitations. The schematic is shown in figure 3.6.
Figure 3.6: DC/DC converter design in organic technology [7]
The authors [7] describe the ideal output voltage with the following formula:
Vout =Vin +N(Vdd−Iout
fCD) (3.1)
N is the number of stages of charge pump, Iout is the output current, f is the clock frequency and
CD the capacitor value for each stage. Internal leakage current is estimated with and without 10µA
load current by:
Iint = 10µA4Vdd−Vout(Iout = 0µA)
Vout(Iout = 0µA)−Vout(I = 10µA)(3.2)
and power efficiency follows the equation:
η power =10µA
Iint +10µA(3.3)
Simulation shows an output voltage level around 75 V with a voltage source of 25 V and no
load current. With 10µA load current, output voltage is more than 10 V lower. It is very unstable,
with a visible difference. Efficiency achieved is around 48%, which is relatively small. Authors
[7] explain it with leakage in capacitors. This proposal use a design based on Dickson’s charge
pump. It uses a large number of capacitors and transistors, increasing the problems referred in
chapter 2 with large use of these devices. Dickson’s charge pump was compared with other
topologies in [6] and [4], with worse results for the first one.
Chapter 4
Specifications
This chapter presents an initial proposal for DC/DC converter using transparent electronics, based
on switched-capacitors. Due to problems referred in chapter 2 with inductors, this suggestion only
uses the capacitors and TFTs. First section presents a boost converter. The second one presents a
buck converter.
4.1 Proposal for Boost DC/DC Converter
Boost DC/DC converter using switched capacitors have two different phases. The first one has
the flying capacitor connected to voltage source. The second has this capacitor connected to input
voltage and output. The design is shown on figure 4.1.
Figure 4.1: Proposal for Boost DC/DC Converter design
When CLK = 1, the flying capacitor C1 stores energy from voltage source. This capacitor is
discharged when CLK = 0, and the load capacitor receives energy from voltage source and C1.
The output voltage expected is 2VDD−Vth, due to the voltage fall in N2. The right side of converter
25
26 Specifications
has the same behaviour as the left one with exchanged clock signals. This makes a more constant
output voltage level.
4.2 Proposal for Buck DC/DC Converter
Buck DC/DC converter uses the same fundamentals of boost converter, but it needs the flying
capacitor always connected to output instead of input. The circuit design is shown in figure 4.2.
Figure 4.2: Proposal for Buck DC/DC Converter design
When CLK = 1, VDD charges C1 and CL, but when CLK = 0, only C1 charges the load capacitor.
This creates a lower voltage level at the output. Ideally, this voltage level is VDD/2, but the load
resistance and parasitic capacitance will decrease it as referred in chapter 2.
Chapter 5
Conclusions and Future Work
This report contains the background necessary for the development of a DC/DC Converter with
transparent electronics and the bibliographic review with the state of the art on transparent con-
verters made so far. It was presented different device masks, the characteristics of TFT technology
and a study on voltage multipliers. During the remaining of the Master thesis, a physical imple-
mentation of the transparent DC/DC converter will be developed. The following tasks are to be
accomplished in order to complete the Master Thesis:
• Structural design of passive elements masks using CAD tools
• Proposal of a DC/DC Converter topology (possibly boost) with TFT’s and respective elec-
trical simulation
• Structural design of the masks for the converter and sending for fabrication
• Start writing the report
• Manufacturing and testing of developed circuits in CENIMAT-UNL
• Finishing writing the report
The work plan for second semester is described on Gantt diagram on figure 5.1.
Figure 5.1: Gantt diagram with work plan
27
28 Conclusions and Future Work
Bibliography
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[3] M. Wens and M. Steyaert. Design and Implementation of Fully-Integrated Inductive DC-DCConverters in Standard CMOS. Springer, 2011. ISBN 978-94-007-1435.
[4] J. Starzyk. A dc-dc charge pump design based on voltage doublers. IEEE Transactions onCircuits and Systems-I: Fundamental Theory and Applications, 48:350–358, 2001.
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