UNIVERSITY OF NAIROBI
FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING
100 W LOW COST INVERTER FOR SOLAR ENERGY CONVERSION
PROJECT INDEX: PRJ 027
BY
KETTER RONALD KIPKOECH
F17/2140/2004
SUPERVISOR: DR. MWANGI MBUTHIA
EXAMINER: MR. OGABA
Project report submitted in partial fulfillment of the
requirement for the award of the degree
of
Bachelor of Science in ELECTRICAL AND ELECTRONIC ENGINEERING of the
University of Nairobi
Submitted On: 20th May, 2009.
i ABSTRACT This project report outlines the research, design and implementation of a low power direct
current (DC) to alternating current (AC) inverter with particular focus on developing a low cost
transformer-less voltage source conversion that has a higher efficiency as well as compact in
size. The final design consisted of a two-stage converter consisting of a DC-DC Step Up stage
and a DC-AC Inverter stage. The DC-DC Step up converter was based on switched capacitor
techniques and steps 12Vdc to 240Vdc. The inverter stage was based on a full-bridge
configuration that generates a 240Vac output from 240Vdc.
To achieve this improvement in inverter efficiency and a reduction in cost, the power
transformer and magnetic components such as inductors have been eliminated. In addition,
inverter voltage control techniques such as pulse width modulation (PWM) and switching of
MOSFETs have been optimized through digital control using ATtiny26L microcontroller unit.
The concern for environmental degradation from fossils fuel as well as well as the depletion of
these resources has led to major investment in the research and development of the renewable
energy sources. Solar energy has emerged as one of the major alternative source. The
disadvantages of using solar energy are its higher capital cost in comparison to that of
conventional sources of energy, and its conversion efficiency, which in commercially available
Photovoltaic (PV) systems is less than 90%.Consequently, for utility connected PV generation to
become a viable alternative energy source, its efficiency needs to be improved, its cost reduced,
and the quality of power supplied by the inverters must meet standards. Therefore, this project
seeks to achieve this improvement as outlined in the report.
ii ACKNOWLEDGEMENT The achievements made in this project have been realized with assistance from a number of
people, and the author wishes to express his gratitude to them for their contributions.
The author also wishes to sincerely thank his project supervisor, Dr. Mwangi Mbuthia for his
guidance and encouragement during the course of this project.
A special thanks to fellow students and friends- Gideon, Raphael, Edwin and Boniface for their
assistance when it was required and moral support over the years.
Finally, the author would also like to thank his parents for their love, encouragement and support
throughout this long journey.
iii List of Figures Figure 1.0 System Schematic ..........................................................................................3
Figure 2.1 Block diagram of Solar PV grid connected system........................................6
Figure 2.2 Basic DC-DC converter..................................................................................8
Figure 2.3 DC-DC converter voltage waveforms.............................................................9
Figure 2.4 Pulse width modulation concepts....................................................................9
Figure 2.5 Basic boost converters.....................................................................................10
Figure 2.6 Basic boost converter switch states.................................................................10
Figure 2.7 Positive output switched-capacitor converters................................................13
Figure 2.8 A simple voltage doubler……………………................................................15
Figure 2.9 A practical voltage doubler………….………................................................16
Figure 2.10 Four-level MMCCC with three modular blocks...........................................17
Figure 2.11 DC-AC Converter……………………………….........................................20
Figure 2.12 Topology of a single-phase, full-bridge inverter...........................................22
Figure 2.13 MOSFET schematic diagram and characteristics..........................................24
Figure 2.14 Switching waveforms at (a) turn-on and (b) turn-off....................................26
Figure 2.15 Totem-pole gate drive circuit.........................................................................27
Figure 2.16 Triangular carrier wave and PWM signal for 80% duty cycle.......................29
Figure 2.17 Spectrum of a PWM signal with 25% duty cycle ………………..................30
Figure 2.18 Uni-Polar PWM………………......................................................................32
Figure 3.2 Gate driver circuit …........................................................................................38
Figure 3.3 H-bridge inverter circuit....................................................................................38
Figure 4.1 Four Modules Simulation result of the Dc-Dc converter stage.........................42
Figure 4.2 Prototype Circuit………………………………………………........................44
iv Table of Contents Abstract .......................................................................................................................................... ii
Acknowledgement ........................................................................................................................ iii
List of Figures.............................................................................................................................. .iv
1.0 INTRODUCTION....................................................................................................................1
1.1 Background ............................................................................................................................1
1.2 Project Objectives………………………………..……………………….………………...2
1.3 Justification ……….…………………………………………………….……..…………...3
1.4 Outline of the report ……….……….…………………………………….………………...3
2.0 REVIEW OF LITERATURE ............................................................................................5
2.1 Solar Photovoltaic Energy System overview .........................................................................5
2.2 DC-DC Conversion ................................................................................................................7
2.2.1 Ideal Boost Converter ......................................................................................................9
2.2.1.1 Continuous Conduction mode ...............................................................................10
2.2.1.1 Discontinuous Conduction mode ..........................................................................11
2.2.2 Switched- Capacitor Converters ..................................................................................11
2.2.3 Charge Pump DC–DC Converter ................................................................................13
2.2.4 Capacitor-clamped multilevel dc-dc converter ............................................................16
2.3 DC-AC Conversion ..............................................................................................................18
2.3.1 Full-bridge Single phase inverters ................................................................................21
2.4 MOSFETs ............................................................................................................................24
2.4.1 MOSFETs Used As Switches ......................................................................................25
2.4.2 MOSFET Drivers .........................................................................................................27
2.4.3 Power Losses ................................................................................................................28
2.5 Pulse width modulation (PWM) ..........................................................................................29
2.5.1 Bi-polar PWM ..............................................................................................................31
2.5.2 Uni-polar PWM ............................................................................................................31
2.6 Digital Control ....................................................................................................................33
3.0 DESIGN AND IMPLEMENTATION ................................................................................35
3.1 DC-DC converter Design .....................................................................................................35
3.1.1 Capacitor Selection .....................................................................................................36
v 3.1.2 MOSFET Selection .....................................................................................................37
3.1.3 Gate Drive ...................................................................................................................37
3.2 DC-AC Inverter Design ......................................................................................................38
3.3 Control Circuit .....................................................................................................................39
4.0 ANALYSIS AND RESULTS ...............................................................................................41
4.1 Calculated Efficiency ..........................................................................................................41
4.2 Simulation Results ...............................................................................................................41
4.2.1 DC-DC converter stage ...............................................................................................41
4.3 Testing gating Signal ..........................................................................................................42
4.4 Prototype Results .................................................................................................................43
5.0 CONCLUSION AND FUTURE WORK ............................................................................45
5.1 Conclusion ...........................................................................................................................45
5.2 Future Work ........................................................................................................................45
APPENDIX ..................................................................................................................................46
A PWM ATtiny26L Program ....................................................................................................46
B Microcontroller Power Supply Circuit ..................................................................................49
C ATtiny26 Datasheet ..............................................................................................................50
D IRF740 Datasheet ..................................................................................................................55
vi CHAPTER 1 1.0 INTRODUCTION 1.1 Background An inverter is a device that takes a DC (Direct Current) input and produces a sinusoidal AC
(Alternating Current) output. An inverter needs to be designed to handle the requirements of an
energy hungry household yet remain efficient during periods of low demand. Inverters can be
designed in a number of topologies depending on the situation and its requirements. The
efficiency of the inverter is highly dependent on the switching device, topology and switching
frequency of the inverter.
Nowadays, the main energy supplier of the worldwide economy is fossil fuel. This however has
led to many problems such as global warming and air pollution. Therefore, with regard to the
worldwide trend of green energy, solar power technology has become one of the most promising
energy resources. The number of PV installations has had an exponential growth, mainly due to
general public, the governments and utility companies who support the idea of the green energy.
The solar cell transforms the light energy into electric energy. It represents a source with a good
energy density and a high theoretical efficiency. From an electric point of view, the solar cell is
considered as a voltage source. This source is nevertheless imperfect. Therefore it is necessary to
insert an inverter between the solar cell and the network in order to obtain the alternating electric
source.
PV systems are connected to domestic systems or the grid via inverters. Therefore criterions
have to be defined to choose the inverter, taking into account the difference of voltage values
between a typical solar cell 12V and the required voltage for home applications which is 240V
AC in Kenya. In most of the current systems, transformers are used in the inverter. In my project
1 however, one of the methods used to reduce power loss, cost and size of the inverter system was
to avoid the use of power transformers.
The amount of electrical energy produced by a PV system connected to a consumer does not
always coincide with the energy demand of that consumer. Therefore, to use all the energy
produced from such a system requires large energy storage capacity to be used in conjunction
with the PV generation. Alternatively, a more economical method would be to directly feed any
excess energy from the PV system to the grid network. The advantage of utility connected PV
generation over the stand-alone systems is that back-up generation and bulk storage are shared.
Grid-connected PV systems, when located at the point of use have potential benefits such as
reduction in:• Transmission power losses,
• Transmission line capacity requirement,
• Conventional generation capacity requirement,
• CO2 emissions and fuel costs.
The main disadvantages of using PV energy are its capital cost in comparison to that of the
conventional sources of energy and its conversion efficiency. High efficiency is invariably
required, since cooling of inefficient power converters is difficult and expensive. The ideal
power converter exhibits 100% efficiency; in practice, efficiencies of 70% to 95% are typically
obtained. Consequently, for utility connected PV generation to become a viable alternative
energy source not only must the cost of the PV panels and the inverter system be reduced but
also its efficiency needs to be improved.
1.2. Project Objectives The main aim of this project was to design and implement a single phase, transformer-less PV
inverter system suitable for both domestic applications, which would lead to higher inverter
efficiencies, improved output power quality and reduced cost.
2 The specific objectives of the project are summarised as: a. Design a low cost 12v DC to 240v AC, 100w inverter.
b. To construct and test the inverter based on the design specifications.
c. To obtain experimental results to validate the higher efficiencies of the proposed inverter
system can be achieved without compromising the quality of output power fed to the
domestic or grid system or its cost.
Therefore the project can be split into two major components- A DC-DC step up converter and a
DC-AC inverter. The overall system schematic can be seen in Figure 1.0.
12 VDC
12VDC to 240VDC
Switched Capacitor
Converter
240VDC to
240VAC Full
Bridge Inverter
240VAC
Digital
ControllerAtmel ATtiny26
Figure 1.0 System Schematic
The system consists of microcontroller circuit for generating sinusoidal pulse width modulation
(SPWM) pulses to control switches in both the DC-DC step up converter stage and the full
bridge circuit. SPWM signal generated by the microcontroller
1.3 Justification All over the world the importance of renewable energy sources is recognized by both the general
public and the power industries. The concern for environmental degradation from fossils fuel as
3 well as well as the depletion of these resources has led to a shift in focus into research on the use
of renewable energy sources such as solar power, wind, running water, biomass and geothermal.
In addition, there are still a significant number of remote locations that are not connected to a
supply grid. Often, the cost of connecting these locations to the grid far outweighs the initial
expenditure required to set up some form of renewable energy source on location.
This proposition is made more attractive with the introduction of high efficiency inverters. This
is due to the fact that an inefficient inverter leads to higher rating requirements on all preceding
devices - often expensive solar panels and battery banks. The cost of higher ratings on these
preceding devices far outweighs the cost of researching, designing and developing an efficient
high power inverter. Recent advances in microprocessor technology have led to ‘single-chip’
solutions for the control of inverters which makes complicated control systems cheap and
efficient.
1.4 Outline of the report Chapter 2 will begin with a literature review on all relevant recent articles written on the subject
of both power inverters and their control.
Chapter 3 will explain the implementation of both stages of the inverter and the control system,
detailing both hardware and software design.
Chapter 4 will give the results of testing on both stages of the inverter implementation of the
design
Chapter 5 will give an overall conclusion of the project and some suggestions for further
research.
4 CHAPTER TWO 2.0 REVIEW OF LITERATURE There have been a large number of articles written concerning power conversion in recent years.
This can be attributed in part to the increased use of renewable energy sources producing DC
voltage which has to be converted to AC for use in applications. In addition, the rise in
popularity of high voltage DC transmission systems - and their integration with existing AC
supply grids has contributed to increase in research and development in power conversion. There
is also a consistent demand for high efficiency inverter devices for lower power applications like residential houses, UPS and remote areas of the world. This chapter will discuss and contrast
recent literature concerning high power inverters and their control.
2.1 Solar Photovoltaic Energy System overview Solar photovoltaic (PV) systems use solar panels made of silicon to convert sunlight into
electricity. The electricity is direct current and can be used that way, converted to alternating
current or stored for later use. Sunlight is made of photons, small particles of energy. These
photons are absorbed by and pass through the material of a solar cell or solar photovoltaic panel.
The photons 'agitate' the electrons found in the material of the photovoltaic cell. As they begin to
move (or are dislodged), these are 'routed' into a current - the movement of electrons along a
path.
Wire conducts these electrons, either to batteries or to the regular electrical system of the house,
to be used by appliances and other household electrical items. In many solar energy systems, the
battery stores energy for later use. This is especially true when the sun is shining strongly. Grid
connection of photovoltaic systems is becoming popular as they can contribute electrical energy
to the utility system under energy buy back schemes to reduce the payback period. The excess
energy from the photovoltaic system is absorbed by the grid and the grid in turn feeds the house
5 when the electricity from the PV system is inadequate. The user of a photovoltaic system can get
benefits from the grid with appropriate load management within the system. The various
components in a solar photovoltaic energy system are shown in figure 2.1.These components
include: solar panel arrays, maximum power point tracking (MPPT), charge controllers, inverter
and meter for grid connected PV system.
Figure 2.1 Block diagram Solar PV grid connected system
The solar panel consists of photovoltaic cells which come in a variety of forms, the most
common structure is a semiconductor material into which a large-area diode, or p-n junction, has
been formed. An installation of photovoltaic modules or panels is known as a photovoltaic array.
Photovoltaic cells typically require protection from the environment. For cost and practicality
reasons a number of cells are connected electrically and packaged in a photovoltaic module,
while a collection of these modules that are mechanically fastened together, wired, and designed
to be a field-installable unit, sometimes with a glass covering and a frame and backing made of
metal, plastic or fiberglass, are known as a photovoltaic panel or simply solar panel.
The maximum power point tracking (MPPT) is feature that optimizes the solar photovoltaic
array's energy production. The power output of a module varies as a function of the voltage in a
way that power generation can be optimized by varying the system voltage to find the 'maximum
power point'. This can be part of the charge controllers and it makes use of set points- specific
voltages at which the charge controller changes its charge rate. Some allow the user to adjust
6 these set points himself but most modern MPPT use digital control techniques to adjust to the
desired set point.
A charge controller is used in combination with a battery, and it does just what its name implies:
it controls the charge your battery is receiving. The charge controller is placed between the solar
module and the battery. When the battery has received enough charge, the charge controller will
withhold further charging. This is also called "voltage regulating" within the solar industry.
Without a charge controller, your battery will over-charge. This can damage your battery and it
may be dangerous in other ways-it can even cause fires. In addition, some charge controllers also
have a feature called low-voltage disconnect (LVD). This feature prevents the battery from overdischarging. As with any other battery, draining causes damage. Almost all modern inverters,
even cheap pocket-sized ones, have this feature built in.
Solar energy systems use a lead-acid deep cycle battery. This type of battery is different from a
conventional car battery, as it is designed to be more tolerant of the kind of ongoing charging and
discharging you would expect when you have variable sunshine from one day to the next. Leadacid deep cycle batteries last longer but they also cost more than a conventional battery. The
major difference between lead acid batteries and other batteries is that they have solid lead
plates; in conventional car batteries, the plate is made of a sponge-like material. Depth of
discharge is a measure of how much energy has been taken from a battery. With the lead-acid
deep cycle battery used in a solar electric system, there is more tolerance for discharging. You
can discharge the battery of a solar energy system 50% to 80% with no damage to the battery
2.2 DC­DC Conversion The purpose of a DC-DC converter is to supply a regulated DC output voltage to a variable-load
resistance from a fluctuating DC input voltage. In many cases the DC input voltage is obtained
by rectifying a line voltage that is changing in magnitude. DC-DC converters are commonly used
in applications requiring regulated DC power, such as computers, medical instrumentation,
communication devices, television receivers, and battery chargers. DC-DC converters are also
used to provide a regulated variable DC voltage for DC motor speed control applications.
7 The output voltage in DC-DC converters is generally controlled using a switching concept, as
illustrated by the basic DC-DC converter shown in Figure 2.2. Early DC-DC converters were
known as choppers with silicon-controlled rectifiers (SCRs) used as the switching mechanisms.
Switc
RL
VIN
VOUT
Figure 2.2 basic DC-DC converters
Modern DC-DC converters classified as switch mode power supplies (SMPS) employ insulated
gate bipolar transistors (IGBTs) and metal oxide silicon field effect transistors (MOSFETs).
The switch mode power supply has several functions:
1. Step down an unregulated DC input voltage to produce a regulated DC output voltage
using a buck or step-down converter.
2. Step up an unregulated DC input voltage to produce a regulated DC output voltage
using a boost or step-up converter.
3. Step down and then step up an unregulated DC input voltage to produce a regulated
DC output voltage using a buck–boost converter.
4. Produce multiple DC outputs using a combination of SMPS topologies
The regulation of the average output voltage in a DC-DC converter is a function of the on-time,
ton of the switch, the pulse width, and the switching frequency as illustrated in Figure 2.3[1]
Pulse width modulation (PWM) is the most widely used method of controlling the output
voltage. The PWM concept is illustrated in Figure 2.4[1]. The output voltage control depends on
the duty ratio, D. The duty ratio is defined as
8 (2.1)
Based on the on-time ton of the switch and the switching period Ts. PWM switching involves
comparing the level of a control voltage,
to the level of a repetitive waveform as
illustrated in Fig. 2.3. The on-time of the switch is defined as the portion of the switching period
where the value of the repetitive waveform is less than the control voltage. The switching period
(switching frequency) remains constant while the control voltage level is adjusted to change the
on-time and therefore the duty ratio of the switch. The switching frequency is usually chosen
above 20 kHz so the noise is outside the audio range.
DC-DC converters operate in one of two modes depending on the characteristics of the output
current:
1. Continuous conduction
2. Discontinuous conduction
The continuous-conduction mode is defined by continuous output current (greater than zero)
over the entire switching period, whereas the discontinuous conduction mode is defined by
discontinuous output current (equal to zero) during any portion of the switching period.
Figure 2.3 DC-DC converter voltage waveforms
Figure 2.4 Pulse width modulation concepts
2.2.1 Ideal Boost Converter The circuit that models the basic operation of the boost converter is shown in Figure 2.5
9 Figure 2.5 Basic boost converters
The input voltage in series with the inductor acts as a current source. The energy stored in the
inductor builds up when the switch is closed. When the switch is opened, current continues to
flow through the inductor to the load. Since the source and the discharging inductor are both
providing energy with the switch open, the effect is to boost the voltage across the load. The load
consists of a resistor in parallel with a filter capacitor. The capacitor voltage is larger than the
input voltage. The capacitor is large to keep a constant output voltage and acts to reduce the
ripple in the output voltage.
2.2.1.1 Continuous Conduction mode The continuous-conduction mode of operation occurs when the current through the inductor in
the circuit of Figure 2.6 is continuous with the inductor current always greater than zero. The
operation of the circuit in steady state consists of two states. The first state with the switch closed
has current charging the inductor from the voltage source. The switch opens at the end of the ontime and the inductor discharges current to the load with the input voltage source still connected.
This results in an output voltage across the capacitor larger than the input voltage. The output
voltage remains constant if the RC time constant is significantly larger than the on-time of the
switch.
Figure 2.6 Basic boost converter switch states: (a) switch closed (b) switch open
10 The voltage ratio for a boost converter is derived based on the time-integral of the inductor
voltage equal to zero over one switching period. The voltage ratio is equivalent to the ratio of the
switching period to the off-time of the switch as shown by the equation 2.2
(2.2)
2.2.1.2 Discontinuous­Conduction Mode The discontinuous mode of operation occurs when the value of the load current is less than or
equal to zero at the end of a given switching period. Assuming a linear rise and fall of current
through the inductor, the boundary point between continuous- and discontinuous-current
conduction occurs when the average inductor current over one switching period is half the peak
value. The average inductor current at the boundary point is calculated using the equation 2.3
(2.3)
And the expression for
can be obtained as:
(2.4)
2.2.2 Switched­ Capacitor Converters Classic DC/DC converters consist of inductors and capacitors. They are large because of the
mixture of inductors and capacitors. The demand small efficient DC-DC converters has led to
development of several types of switched component converters and are made of either inductors
or capacitors, so-called switched-inductor and switched-capacitors. They can perform in two- or
four-quadrant operation with high output power range (thousands of Watts) and efficiency.
Switched-capacitor DC/DC, according to [3], converters consist of only capacitors. Because
there is no inductor in the circuit, their size is small. They have outstanding advantages such as
low power losses and low electromagnetic interference (EMI). Since its electromagnetic
radiation is low, switched-capacitor DC/DC converters are required in certain equipment. The
11 switched-capacitor can be integrated into an integrated-chip (IC). Hence, its size is largely
reduced, has very high power density and is more efficient. Much attention has been drawn to the
switched-capacitor converter since its development. Many papers have been published
discussing its characteristics and advantages. However, most of the converters in the literature
perform a single-quadrant operation. Some of them work in the push-pull status. In addition,
their control circuit and topologies are very complex, especially, for the large difference between
input and output voltages.
Switched-capacitor (SC) converters can perform in push-pull state with conduction duty cycle
k = 0.5. There exist several topologies in the design of positive output SC converters. Each
circuit has one main switch S and several slave switches as Si (i = 1, 2, 3 …n). The number n is
called stage number. The main switch S is on and slaves off during switch-on period kT, and S is
off and slaves on during switch-off period (1 – k)T. The load is resistive load R. Input voltage
and current are VIN and IIN, output voltage and current are VO and IO. The elementary circuit of
positive output SC converter and its equivalent circuits during switch-on and switch-off are
shown in Figure 2.7[3]. The voltage across capacitor C1 is charged to VIN during switch on.
The voltage across capacitor C2 is charged to VO = 2 VIN during switch off.
Therefore, the output voltage is
VO = 2 VIN
Considering the voltage drops across the diodes and switches, we combine all values in a figure
of ∆V. Therefore the real output voltage is
VO = 2 VIN – ∆V
(2.5)
12 Figure 2.7 Positive output switched-capacitor converters
The re-lift circuit is derived from the elementary circuit by adding parts set: one slave switch,
two switched-capacitors and three diodes. The higher order lift circuit is designed by just
multiple repeating of the parts in the re-lift circuit. The output voltage of the nth-order lift circuit
is
(2.6)
2.2.3 Charge Pump DC–DC Converter Charge pump is a popular dc-dc circuit which is used to obtain a voltage higher than the supply
voltage or a voltage with reverse polarity. In many applications such as the Power IC, continuous
time filters, EEPROMs, and switched-capacitor transformers, voltages higher than the power
13 supplies are frequently required. Increased voltage levels are obtained as a result of transferring
charges to a capacitive load, and do not involve amplifiers or regular transformers. It is well
suited for monolithic integration and those applications where high voltage and limited current
drivability are required.
Charge-pump voltage converters use ceramic or electrolytic capacitors to store and transfer
energy. Although capacitors are more common and much cheaper than the coils used in other
types of DC/DC converters, capacitors can't change their voltage level abruptly. A changing
capacitor voltage always follows the exponential function, which imposes limitations that
inductive voltage converters can avoid. On the other hand, inductive voltage converters are more
expensive. Capacitive voltage conversion is achieved by switching a capacitor periodically.
Passive diodes can perform this switching function in the simplest cases, provided an alternating
voltage is available. Otherwise, DC voltage levels require the use of active switches, which first
charge the capacitor by connecting it across a voltage source and then connect it to the output in
a way that produces a different voltage level.
Charge pumps usually operate at a high- frequency level in order to increase their output power
within a reasonable size of total capacitance used for charge transfer. This operating frequency
may be adjusted by compensating for changes in the power requirements and saving the energy
delivered to the charge pump. With the rapid development of portable electronic equipments,
low-power low-voltage techniques are desired for all kinds of digital and analog system designs,
so that charge pump switched capacitor (SC) dc-dc converter has been recently receiving
renewed interest.
Most topologies of charge pumps are based on three types: Dickson, cross-connecting, and
Makowski. A common disadvantage of Dickson and cross-connecting charge pump is that, they
may require a large number of devices and silicon area when a high voltage gain is needed, while
with the development in electronic technology, larger voltage gains are desired in various
applications. A basic cell of charge pump cell is as shown in Figure 2.8. It consists of a single
capacitor and three switches.
14 3
2
S1
AM
Vin
C1
1
4
S2
S3
0
Figure 2.8 A simple voltage doubler
During first clock phase Ф1, switches S1 and S3 are closed and the capacitor is charged to the
supply voltage, VIN. In the next clock phase Ф2, switch S2 is closed and the bottom plate of the
capacitor assumes a potential VIN, while the capacitor maintains its charge of VINC from the
previous phase. This means that during Ф2
(VOUT- VIN) ×C = VIN× C
(2.7)
VOUT = 2 VIN
(2.8)
Therefore,
Thus, in the absence of a DC load, an output voltage has been generated that is twice the supply
voltage. In order to accommodate a load at the output, the circuit would be modified by adding
an output capacitance as shown in Figure. 2.9
15 3
2
5
S1
AM
S4
C1
Vin
Cout
Load
0
0
4
S2
1
S3
0
Figure2.9 Practical Voltage Doubler
In this case, the ideal output voltage is given by
(2.9)
If a load RL is present, then a ripple voltage, VR, is generated at the output. The ripple voltage
can be reduced by making Co sufficiently large so that VR is negligible compared to Vout.
Voltage multiplication greater than twice the supply voltage can be achieved by cascading more
than one capacitor in series.
2.2.4 Capacitor­clamped multilevel dc-dc converter Multilevel dc-dc converter is becoming more popular for its high efficiency power conversion.
Some of them have bidirectional power handling capability, which is one of the key
requirements in future hybrid electric or fuel-cell-powered automobile power systems. There are
several types of multilevel dc-dc converters that can be broadly categorized as diode-clamped
multilevel dc-dc converters and capacitor-clamped multilevel converters. The capacitor-clamped
multilevel converters are the most implemented because of higher efficiency such as flying
capacitor and multilevel modular capacitor clamped dc-dc converter (MMCCC) [4], [5]. Unlike
16 other topologies such as the flying capacitor multilevel dc-dc converters, MMCCC, which is
based on Fibonacci switched-capacitor converter topology is completely modular and requires a
simpler switching scheme.
In addition, the new topology has many advantageous features such as high frequency operation
capability, low input/output current ripple, low ON-state voltage drop, and bidirectional power
flow management. Its modular structure allows for the design to achieve any conversion ratio
(CR). Each modular block has one capacitor and three transistors leading to three terminal
points. Figure 2.10 shows the simplified operational circuit of the MMCCC
SB6
Vin
SR1
SB1
Vou
SR4
C
C
C
SB1
SR3
SB5
SR
SB
SR
Figure 2.10 Four-level MMCCC with three modular blocks
The switching sequence in the new converter works in a simpler way than the conventional
converter. As there are only two subintervals, two switching states are present in the circuit.
17 Switches SR1 to SR5 in Figure 2.10 are switched on at the same time for duty cycle, D= 0.5 to
achieve state 1; while in the same way, switches SB1 to SB6 are operated simultaneously to
make the steady-state 2. The simpler switching scheme enables high-speed operation for the new
MMCCC circuit.
The other major advantage of the modular structure is the flexibility to change the conversion
ratio. For a five level conversion, four modules are connected in cascade configuration.
Thus, the number of modules is (N − 1) where N is the conversion ratio. In this way, any number
of modules can be connected in cascade and the corresponding conversion ratio can be achieved.
The MMCCC topology is a capacitor-clamped converter, and energy is transferred from one bus
to another by capacitors only. This topology uses the same capacitors in each module making the
circuit modular, although the capacitors in different modules will experience different voltage
stresses. To ensure equal voltage stress across the transistors, capacitors in different modules
have to withstand unequal voltage stress.
2.3 DC­AC Inversion Inversion is the conversion of dc power to ac power at a desired output voltage or current and
frequency and is shown in Figure 2.11. This conversion can be achieved either by transistors or
by silicon controlled rectifiers (SCRs). For low to medium power outputs, transistorised inverters
are suitable but for high power outputs, SCRs should be used.For low power, self oscillating
transistorized inverters are suitable but for high-power outputs, driven inverters are more
common. Moreover for multiphase Ac output, there is no alternative other than driven inverters.
The driven inverters have better frequency stability because a separate master oscillator is used
for the purpose. For the application in inverters transistors have an edge over SCRs regarding
switching speed, simplicity of the control circuits, higher efficiency and greater reliability. This
is mainly due to the fact that SCRs require extra circuits to turn off, moreover additional
complex logic circuits may be required to prevent false triggering and provide proper
commutating timing. SCRs can handle much higher load current than transistors and
consequently, for high outputs, SCRs become more desirable under high current conditions. The
terms voltage-fed and current-fed are used in connection with inverter circuits.
18 A voltage-fed inverter is one in which the dc input voltage is essentially constant and
independent of the load current drawn. The inverter specifies the load voltage while the drawn
current shape is dictated by the load.
A current-fed inverter (or current source inverter) is one in which the source, hence the load
current is predetermined and the load impedance determines the output voltage. The supply
current cannot change quickly. This is achieved by series dc supply inductance which prevents
sudden changes in current. The load current magnitude is controlled by varying the input dc
voltage to the large inductance; hence inverter response to load changes is slow. Being a current
source, the inverter can survive an output short circuit thereby offering fault ride-through
properties.
Inverter circuits can also be broadly classified into two classes, namely, amplifier-type sine-wave
inverters and saturated-switch type square-wave inverters.
In amplifier-type inverters, transistors are used as amplifiers operating in a non-saturated
condition. The efficiency of this type of inverters is generally low because of high power
dissipation in the transistors. There are other problems such as cross-over distortions. These
circuits are suitable for low power applications and where load power factor and load regulation
are not important and efficiency is not a criterion.
The saturated-switch type inverters have a high efficiency because transistors or SCRs are used
to operate like switches, that is, in either a fully saturated conducting mode or a cut-off blocking
mode. The losses in the semiconductor devices reduce considerably due to their mode of action
and consequently not only does the efficiency become high but much more power can be drawn
at the output than in the case of the amplifier-type circuits using the same rating of the
transistors.
Voltage control may be required to maintain a fixed output voltage when the dc input voltage
regulation is poor, or to control power to a load. The inverter and its output can be single-phase,
three-phase or multi-phase. Variable output frequency may be required for ac motor speed
control where, in conjunction with voltage or current control, constant motor flux can be
maintained.
Inverter output waveforms are usually rectilinear in nature and as such contain harmonics which
may lead to reduced load efficiency and performance. Load harmonic reduction can be achieved
by either filtering, selected harmonic reduction chopping or pulse-width modulation.
19 Figure 2.11 DC-AC Converter
Typically the inverters are “hard switched” voltage source producing pulse-width modulated
(PWM) signals with a sinusoidal fundamental. The PWM signal is then filtered to obtain a near
sinusoidal waveform. A very common application for single-phase inverters are so-called
“uninterruptable power supplies” (UPS) for computers and other critical loads. Here, the output
waveforms range from square wave to almost ideal sinusoids. UPS designs are classified as
either “off-line” or “online”. An off-line UPS will connect the load to the utility for most of the
time and quickly switch over to the inverter if the utility fails. An online UPS will always feed
the load from the inverter and switch the supply of the DC bus instead. Since the DC bus is
heavily buffered with capacitors, the load sees virtually no disturbance if the power fails.
In addition to the very common hard-switched inverters, active research is being conducted on
soft switching techniques. Hard-switched inverters use controllable power semiconductors to
connect an output terminal to a stable DC bus. On the other hand, soft switching inverters have
an oscillating intermediate circuit and attempt to open and close the power switches under zerovoltage and or zero-current conditions.
Modern inverters use insulated gate bipolar transistors (IGBTs) as the main power control
devices. Besides IGBTs, power MOSFETs are also used especially for lower voltages, power
ratings, and applications that require high efficiency and high switching frequency. In recent
years, IGBTs, MOSFETs, and their control and protection circuitry have made remarkable
progress. IGBTs are now available with voltage ratings of up to 3300 V and current ratings up to
1200 A. MOSFETs have achieved on-state resistances approaching a few milliohms. In addition
to the devices, manufacturers today offer customized control circuitry that provides for electrical
isolation, proper operation of the devices under normal operating conditions and protection from
20 a variety of fault conditions. In addition, the industry provides good support for specialized
passive devices such as capacitors and mechanical components such as low inductance bus-bar
assemblies to facilitate the design of reliable inverters. In addition to the aforementioned
inverters, a large number of special topologies are used.
Inverters fall in the class of power electronics circuits. One of the most important performance
considerations of power electronics circuits, like inverters, is their energy conversion efficiency.
The most important reason for demanding high efficiency is the problem of removing large
amounts of heat from the power devices. Of course, the judicious use of energy is also
paramount, especially if the inverter is fed from batteries such as in PV systems and electric cars.
For these reasons, inverters operate the power devices, which control the flow of energy, as
switches. In the ideal case of a switching event, there would be no power loss in the switch since
either the current in the switch is zero (switch open) or the voltage across the switch is zero
(switch closed) and the power loss is computed as the product of both. In reality, there are two
mechanisms that do create some losses, however; these are on-state losses and switching losses
[3]. On-state losses are due to the fact that the voltage across the switch in the on state is not
zero, but typically in the range of 1 to 2 V for IGBTs. For power MOSFETs, the on-state voltage
is often in the same range, but it can be substantially below 0.5 V due to the fact that these
devices have a purely resistive conduction channel and no fixed minimum saturation voltage like
bipolar junction devices (IGBTs). The switching losses are the second major loss mechanism and
are due to the fact that, during the turn-on and turn-off transition, current is flowing while
voltage is present across the device. In order to minimize the switching losses, the individual
transitions have to be rapid (tens to hundreds of nanoseconds) and the maximum switching
frequency needs to be carefully considered. In order to avoid audible noise being radiated
equipment such as motors, most modern inverters operate at switching frequencies substantially
above 10 kHz.
2.3.1 Full­bridge Single phase inverters
Figure2.12 [2] shows the basic topology of a full-bridge inverter with single-phase output. This
configuration is often called an H-bridge, due to the arrangement of the power switches and the
load. The inverter can deliver and accept both real and reactive power. The inverter has two legs,
left and right. Each leg consists of two power control devices (here IGBTs) connected in series.
The load is connected between the midpoints of the two phase legs. Each power control device
21 has a diode connected in anti-parallel to it. The diodes provide an alternate path for the load
current if the power switches are turned off. For example, if the lower IGBT in the left leg is
conducting and carrying current towards the negative DC bus, this current would “commutate”
into the diode across the upper IGBT of the left leg, if the lower IGBT is turned off. Control of
the circuit is accomplished by varying the turn on time of the upper and lower IGBT of each
inverter leg, with the provision of never turning on both at the same time, to avoid a short circuit
of the DC bus. In fact, modern drivers will not allow this to happen, even if the controller would
erroneously command both devices to be turned on. The controller will therefore alternate the
turn on commands for the upper and lower switch, i.e., turn the upper switch on and the lower
switch off, and vice versa. The driver circuit will typically add some additional blanking time
(typically 500 to 1000 ns) during the switch transitions to avoid any overlap in the conduction
intervals. The controller will hereby control the duty cycle of the conduction phase of the
switches. The average potential of the center-point of each leg will be given by the DC bus
voltage multiplied by the duty cycle of the upper switch, if the negative side of the DC bus is
used as a reference. If this duty cycle is modulated with a sinusoidal signal with a frequency that
is much smaller than the switching frequency, the short-term average of the center-point
potential will follow the modulation signal. “Short-term” in this context means a small fraction
of the period of the fundamental output frequency to be produced by the inverter.
FIGURE 2.12 Topology of a single-phase, full-bridge inverter
22 For the single phase full-bridge inverter, the modulations of the two legs are inverse of each
other. When S1 and S4 are turned on simultaneously, the input voltage Vs appears across the load.
If S2 and S3 are turned on at the same time, the voltage across the load is reversed and is -Vs. The
rms output voltage, according to [2], can be found from
Vo =
(2.10)
Equation 2.9 can be extended to express the instantaneous output voltage in Fourier series as
Vo =
(2.11)
and for n= 1 equation gives the rms value of the fundamental component as
V1
Vs
(2.12)
When the H-bridge is implemented using MOSFETs then use of P-Channel MOSFETs on the
high side and N-Channel MOSFETs on the low side is easier, but using all N-Channel
MOSFETs and a FET driver, lower “on” resistance can be obtained resulting in reduced power
loss. The use of all N-Channel MOSFETs requires a driver, since in order to turn on a high side
N-Channel MOSFET, there must be a voltage higher than the switching voltage (in the case of a
power inverter, 170V). This difficulty is often overcome by driver circuits capable of charging an
external capacitor to create additional potential.
This voltage can be filtered using an LC or RC low-pass filter. The voltage on the output of the
filter will closely resemble the shape and frequency of the modulation signal. This means that the
frequency, wave-shape, and amplitude of the inverter output voltage can all be controlled as long
as the switching frequency is at least 25 to 100 times higher than the fundamental output
frequency of the inverter. The actual generation of the PWM signals is mostly done using
microcontrollers and digital signal processors
23 2.4 MOSFETs
Metal Oxide Semiconductor Field effect Transistors (MOSFETs) are three terminal devices
consisting of a gate, drain and source. A power MOSFETs is a voltage-controlled device and
requires a very small input current. The switching speed is very high of the order of
nanoseconds. MOSFETs are commonly used in power electronics design as they have very good
current-carrying capability in their off-state. MOSFET is the most commonly used active device
in very large scale integrated (VLSI) circuits. Figure2.13 [3] shows the device schematic,
current- voltage characteristics, transfer characteristics and device symbol for a MOSFET.
FIGURE 2.13(a) Schematic diagram, (b) current-voltage characteristics, (c) transfer
characteristics, and (d) device symbol for an n-channel enhancement mode MOSFET.
24 2.4.1 MOSFETs Used As Switches Power MOSFETs are commonly used as switches in power electronic applications. It is always
desirable to have power switches perform as close as possible to the ideal case. For a
semiconductor device to operate as an ideal switch, it must possess the following features:
1. No limit on the amount of current (known as forward or reverse current) the device can
carry when in the conduction state (on-state).
2. No limit on the amount of device-voltage (known as forward- or reverse-blocking
voltage) when the device is in the non-conduction state (Off-state).
3. Zero on-state voltage drop when in the conduction state.
4. Infinite off-state resistance, that is, zero leakage current when in the non-conduction
state.
5. No limit on the operating speed of the device when a state is changed, that is, zero rise
and fall times.
For the ideal switch, during switching and conduction periods the power loss is zero, resulting in
100% efficiency; with no switching delays, an infinite operating frequency can be achieved. In
short, an ideal switch has infinite speed, unlimited power handling capabilities, and 100%
efficiency. It must be noted that it is not surprising to find semiconductor-switching devices that
for all practical purposes can almost perform as ideal switches for number of applications
For a MOSFET to be used as a switch it is important to keep the time that the gate voltage
spends between VTH and the fully ‘on’ voltage to a minimum. The reason for this is that power
loss occurs when there is a potential difference between the source and drain at the same time as
drain current flowing. However, there exist capacitive effects within the MOSFET substrate,
stopping VGS from changing instantaneously. These capacitive effects result in the switching
waveforms as seen in figure 2.14[2]. To reduce the switching losses and enable the use of
MOSFETs as switches, the stray capacitances need to be charged quickly which can be achieved
by high-current gate drive signals.
25 (a )
(b)
Figure 2.14 Switching waveforms at (a) turn-on and (b) turn-off
2.4.2 MOSFET Drivers To turn a power MOSFET on, the gate terminal must be set to a voltage at least 5 volts greater
than the source terminal. One feature of power MOSFET is that they have a large stray
capacitance between the gate and the other terminals. The effect of this is that when the pulse to
the gate terminal arrives, it must first charge this capacitance up before the gate voltage can
reach the 10 volts required. The gate terminal then effectively does take current. Therefore the
circuit that drives the gate terminal should be capable of supplying a reasonable current so the
stray capacitance can be charged up as quickly as possible. The best way to do this is to use a
dedicated MOSFET driver chip.
Basically, there are two fundamental categories for gate drivers ICs. These are high side and low
side drivers. High side means the source of MOSFETs of the power element can float between
ground and high voltage power rail. A typical connection of the gate driver IC IR2110 is shown
in Appendix E. Low side means the source of the MOSFET is always connected to ground. For
the gate drivers to operate as a bootstrap circuit, the Vbs voltage is used to provide the supply to
the high side driver circuitry of the gate driver. Vbs is the voltage difference between the Vb and
Vs pins on the gate driver IC. The bootstrap capacitor provides gate charge to the high side
MOSFETs. As the switch begins to conduct, the capacitor maintains a potential difference,
rapidly causing the MOSFET to further conduct, until it is fully on. The supply to the gate driver
26 needs to be in the range of 1OV to 20V to ensure that the gate driver fully enhances the power
MOSFETs.
One of the most popular and cost effective drive circuit for driving MOSFETs is a bipolar, noninverting totem-pole driver as shown in Figure 2.15.
VCC
12V
VCC
Q2
2N3904
2
R1
100Ω
1
4
Q1
R2
3
6.8Ω
IRF740
Q3
2N3906
0
Figure 2.15 Totem-pole gate driver circuit
A low value resistor is inserted between the MOSFET driver and the MOSFET gate terminal.
This is to dampen down any ringing oscillations caused by the lead inductance and gate
capacitance which can otherwise exceed the maximum voltage allowed on the gate terminal. It
also slows down the rate at which the MOSFET turns on and off. This can be useful if the
intrinsic diodes in the MOSFET do not turn on fast enough.
Like all external drivers, this circuit handles the current spikes and power losses making the
operating conditions for the PWM controller more favourable. They can be and should be placed
right next to the power MOSFET they are driving. That way the high current transients of
driving the gate are localized in a very small loop area, reducing the value of parasitic
inductances. Even though the driver is built from discrete components, it needs its own bypass
27 capacitor placed across the collectors of the upper npn and the lower pnp transistors. Ideally
there is a smoothing resistor or inductor between the bypass capacitor of the driver and the
microcontroller output for increased noise immunity. The interesting property of the bipolar
totem-pole driver is that the two base-emitter junctions protect each other against reverse
breakdown. Furthermore, assuming that the loop area is really small and RGATE is negligible,
they can clamp the gate voltage between VBIAS+VBE and GND-VBE using the base-emitter diodes
of the transistors. Another benefit of this solution, based on the same clamp mechanism, is that
the npn-pnp totem-pole driver does not require any Schottky diode for reverse current protection.
2.4.3 Power Losses Conduction losses are usually the most significant in power electronics design. Conduction loss
is due to a finite resistance between the drain and the source; hence it is calculated by
(2.13)
The second most significant cause of power loss is switching losses. As previously mentioned,
these losses occur due to the finite switching times. These losses can be estimated by assuming
that the currents and voltages in Figure 2.14 increase or decrease linearly. By doing so the losses
are simply calculated by
(2.14)
Where f is the switching frequency and Ts is the total of the transition rise and fall times. There
are also significant losses in the reverse recovery of the device. When the device turns off; the
current reverses in direction for a short time called the reverse-recovery time (trr) before falling
to zero. The reverse recover time in MOSFETs is fairly slow because the reverse recovery
current must flow through the body diode, which is intrinsic to the MOSFET. The power loss
due to reverse recovery effects can be estimated by
(2.15)
Where VSD is the drain to source diode forward voltage drop, Irr is the reverse recovery current,
and f is the switching frequency. The only other loss that may be experienced is gate drive
losses. These are fairly insignificant when compared to the other losses and so are neglected.
28 2.5 Pulse Width Modulation (PWM)
Modulation techniques are used to control the state of switches in power electronics circuits.
Pulse width modulation (PWM) is the method of choice to control modern power electronics
circuits. The basic idea is to control the duty cycle of a switch such that a load sees a controllable
average voltage. To achieve this, the switching frequency (repetition frequency for the PWM
signal) is chosen high enough that the load cannot follow the individual switching events.
Variation of duty cycle in the PWM signal to provide a DC voltage across the load in a specific
pattern will appear to the load as an AC signal, or can control the speed of motors that would
otherwise run only at full speed or off. This is further explained in this section. The pattern at
which the duty cycle of a PWM signal varies can be created through simple analog components,
a digital microcontroller, or specific PWM integrated circuits.
Analog PWM control requires the generation of both reference and carrier signals that feed into a
comparator which creates output signals based on the difference between the signals. The
reference signal is sinusoidal and at the frequency of the desired output signal, while the carrier
signal is often either a saw-tooth or triangular wave at a frequency significantly greater than the
reference. When the carrier signal exceeds the reference, the comparator output signal is at one
state, and when the reference is at a higher voltage, the output is at its second state. This process
is shown in Figure 2.16[1]
Figure 2.16 Triangular carrier wave and PWM signal for 80% duty cycle
29 The spectrum of a typical PWM signal with 25% duty cycle with a switching frequency of 10
kHz is shown in Figure 2.17[1]. The DC magnitude of 25% is clearly visible. The harmonics are
multiples of the carrier frequency. The lowest harmonic is located at 10 kHz. Due to the
switching speed of modern power semiconductors, the carrier frequency can be chosen
sufficiently high that the harmonics can be easily filtered with capacitors and inductors of small
size.
Figure 2.17 Spectrum of a PWM signal with 25% duty cycle
In order to source an output with a PWM signal, transistor or other switching technologies are
used to connect the source to the load when the signal is high or low. Full or half bridge
configurations are common switching schemes used in power electronics. Full bridge
configurations require the use of four switching devices and are often referred to as H-Bridges
due to their orientation with respect to a load. Modern power electronics controllers are rapidly
moving toward digital implementation. Typical solutions consist of microcontrollers or DSPs.
The digital implementations have 8 to 12 bits of resolution. If more than one channel is present,
the PWM signals can be left, right, or center aligned. To be center aligned, up–down counters are
used, which count up to their maximum count and then back to zero before starting the next
cycle. The maximum count (2bits−1) is determined by the number of stages (bits) the digital
counter has. In a digital PWM modulator each counter has an associated period register.
30 The content of this register determines the maximum count at which the counter resets. If this
number is less than the maximum count (2bits−1), the repetition (switching) frequency is
increased and the resolution of the duty cycle is decreased for a given clock speed. It is often
important to make the correct trade-off between the switching frequency and the resolution.
The advantage of hardware support for PWM generation is that the processor typically only
needs to access any registers if the duty cycle is to be changed, since the period is typically only
initialized once upon program start-up. It should also be mentioned that the duty cycle registers
are typically “double buffered,” meaning that an update of a duty cycle does not need to be
synchronized with the current state of the counter. In double-buffered systems, the new duty
cycle will only be chosen once the previous period is completed to avoid truncated PWM signals.
If necessary, a software override can disable this feature.
2.5.1 Bi­Polar PWM Bi-polar PWM refers to the technique of switching a full-bridge inverter in complementary pairs
(A+, B- and A-, B+) so that only two possible output voltages are available: Vd and –Vd. This
method produces an output from a full bridge inverter that is the same as the waveform expected
from a half bridge inverter – but with the power handling advantages.
2.5.2 Uni­Polar PWM This method of PWM allows transitions in the output between Vo and zero and –Vo and zero. The
figure 2.18 [2], clearly shows how a uni-polar PWM signal is generated. The voltages specified
on this diagram are with reference to the full bridge circuit.
31 Figure 2.18: Sinusoidal Uni-Polar PWM
The main reason that uni-polar switching is preferred over bi-polar switching is that it effectively
doubles the switching frequency and at the same time cuts maximum voltage transitions in half.
A simple analysis of Figure 2.18 shows that the voltages VAN and VBN are 90º out of phase with
each other. The switching frequency harmonic components of VAN and VBN however have the
same phase. Thus, switching frequency harmonics disappear from the output voltage. The
sidebands of the switching frequency harmonic component also disappear, as does the dominant
harmonic at twice the switching frequency. Clearly then, uni-polar switching provides a
significant reduction in EMI.
32 2.6 Digital Control A microcontroller is a single-chip computer that is specifically manufactured for embedded
computer control applications. These devices are very low-cost and can be used very easily in
digital control applications. For example, the PIC16F877 is an 8-bit, 40-pin microcontroller with
the following features:
Most microcontrollers have the built-in circuits necessary for computer control applications. For
example, a microcontroller may have A/D converters so that the external signals can be sampled.
They also have parallel input–output ports so that digital data can be read or output from the
microcontroller. Some devices have built-in D/A converters and the output of the converter can
be used to drive the plant through an actuator such an amplifier. Microcontrollers may also have
built-in timer and interrupt logic. Using the timer or the interrupt facilities, we can program the
microcontroller to implement the control algorithm accurately.
Microcontrollers have traditionally been programmed using the assembly language of the target
device. As a result, the assembly languages of the microcontrollers manufactured by different
firms are totally different and the user has to learn a new language before being able program a
new type of device. Nowadays microcontrollers can be programmed using high level languages
such as BASIC; PASCAL or C. High-level languages offer several advantages compared to the
assembly language:
•
It is easier to develop programs using a high-level language.
•
Program maintenance is much easier if the program is developed using a high-level
language.
•
Testing a program developed in a high-level language is much easier.
•
High-level languages are more user-friendly and less prone to making errors.
•
It is easier to document a program developed using a high-level language.
However, high-level languages have some disadvantages. For example, the length of the code in
memory is usually larger when a high-level language is used, and the programs developed using
the assembly language usually run faster than those developed using a high-level language.
The software requirements in a control computer can be summarized as follows:
•
The ability to read data from input ports.
33 •
The ability to send data to output ports.
•
Internal data transfer and mathematical operations.
•
Timer interrupts facilities for timing the controller algorithm.
All of these requirements can be met by most digital computers, and, as a result, most computers
can be used as controllers in digital control systems. The important point is that it is not justified
and not cost-effective to use a minicomputer to control the speed of a motor, for example. A
microcontroller is therefore much more suitable for this kind of control application.
One of the important features of the control algorithms is that once they have been started they
run continuously until some event occurs to stop them or until they are stopped manually by an
operator. It is important to make sure that the loop is run continuously and exactly at the same
times, that is, exactly at the sampling instants.
This is called synchronization and there are several ways in which synchronization can be
achieved in practice, such as:
•
using polling in the control algorithm
•
using external interrupts for timing
•
Using timer interrupts
•
Using an external real-time clock.
A timer is peripheral whose basic function related to the measurement or generation of timebased events. Timer usually measure time relative an internal clock of the microcontroller,
although some may be clocked by an external source. Timer units can provide such functions as
generating periodic interrupts; input capturing, output comparing, measuring pulse width, and
performing PWM functions. Some advanced timers are capable of executing small programs.
Some type of serial unit is included on microcontrollers to allow the CPU to communicate bitserially with external devices. Using a bit serial format instead of bit-parallel format requires
fewer I/O pins to perform the communication function. This makes it less expensive, but slower.
34 CHAPTER THREE 3.0 DESIGN AND IMPLEMENTATION The main objective was to design and implement a low cost inverter to illustrate that the control
principles and the theory behind it are sound and can be achieved. With this in mind, the
following sections explain the decisions made during the design and implementation, with
reference to the literature discussed earlier.
3.1 DC­DC Converter Design The DC-DC converter was designed to provide a steady DC voltage of 240V to allow the
implementation of the inverter stage. The topology chosen for implementing the converter was a
multilevel switched capacitor (MMCCC) design based on the Makowski charge pump DC-DC
converter.
This topology was chosen because of several reasons. The design is modular in structure and
hence can be designed to achieve any conversion ratio. Each modular block has one capacitor
and three transistors leading to three terminal points. The schematic diagram of a 4-stage
Makowski charge pump cell, which has 6 capacitors, is shown in Figure 3.1
35 Q17
IRF740
C5
C8
V2
12 V
Q18
IRF740
4
9
10
Q24
IRF740
Q20
IRF740
Q1
IRF740
12
1uF
1uF
Q22
IRF740
7
C3
C2
1uF
Q23
IRF740
6
11
C1
1uF
1uF
Q19
IRF740
C4
1000uF
Load
30Ω
Vout
Q21
IRF740
3
1
2
Q25
Q26
Q27
Q28
IRF740
IRF740
IRF740
IRF740
0
Figure 3.1 A 4-stage MMCCC Dc-Dc converter
In addition, this topology can achieve high frequency operation, low input/output current ripple,
and low ON-state voltage drop and bidirectional power flow management. Compared to other
capacitor-clamped dc-dc converters it is more compact, reliable, better component utilization as
well simple switching scheme with only two switching states. The simpler switching scheme
enables high-speed operation for the multilevel switched capacitor converter. It is the maximum
conversion ratio that can be attained from a switched capacitor dc-dc converter by using
capacitors. In other words, for a specified conversion ratio, the Makowski charge pump requires
the least number of capacitors and switches.
The next question was how many modules were needed to achieve the 240V required. Since the
design requires N-1 modules for N conversion ratio, 20 modules would be required to achieve
the desired voltage considering the voltage drop across the MOSFETs.
3.1.1 Capacitor Selection Two types of capacitors are popular in switched-capacitor (SC) DC-DC converter applicationsintegrated capacitors and external ceramic capacitors. The integrated capacitors are convenient
for low power applications as they help create an ultra compact final product. However, due to
the higher power level in the project, ceramic capacitors were chosen. The selection of capacitors
depend on both the capacitance and the output ripple current. Unlike other multilevel
inverter topologies, such as flying capacitor multilevel inverters, in which the capacitor for the
36 various levels have to be calculated separately, the MMCCC converter is modular in nature and
allows for the use of capacitors of the same value in each module. Electrolytic capacitor used in
each module is rated 1µF, 300V while the output capacitor is rated 1000µF, 300V. The reason
for choosing an output capacitor of large capacitance value was so as to achieve a smooth
constant DC voltage desired to be fed to the inverter stage.
3.1.2 MOSFET Selection The selection of MOSFETs was determined by the current requirements and the voltage ratings
as well as closely comparing their RDS(on) , power dissipation and switching characteristics.
Lower voltages ratings and higher current capabilities tend to drive down the conduction losses
through reduced RDS(on) values, however, the switching losses are increased due to large gate
charges and parasitic capacitances. Thus, a compromise has to be made between low conduction
losses or low switching losses. In addition, the device must be rated above the maximum output
voltage of the converter. As a result of the above consideration, fairly low cost MOSFETs was
found to be made by Fairchild Semiconductor called IRF740. This MOSFET is 10A, 400V with
a fairly low RDS(on) of 0.48Ω and good switching performance.
3.1.3 Gate Drive Power MOSFETs have considerable gate capacitances that must be charged beyond their
threshold voltage for MOSFET to turn ON. The output voltage from the ATtiny26L is 2.3V and
this is not sufficient to turn on the MOSFET.As a result, a gate-driver is required to provide the
gate with high enough currents to charge the gate capacitances within the time required to reduce
switching losses. Two options were available to supply the required gate current. One of the
methods was to use a gate drive IC and the other is to use an interface circuit such as totem-pole
type arrangement using an npn and pnp transistor to achieve the required current. Whereas the
gates drive ICs would have been ideal because of its capabilities and simplicity they were not
available on time. The author settled for a simple interface gate driver circuit as shown in figure
3.2
37 VCC
Q2
12V
VCC
IRF740
R2
10kΩ
2
Signal from Microcontroller
R1
3
Q1
1
BC107
10kΩ
0
Figure 3.2 Gate driver circuit
3.2 DC­AC Inverter Design The DC-AC Inverter design chosen was built around a standard full-bridge topology due entirely
to its high power handling capabilities and the ability to provide bi-polar PWM and thus
effectively double the switching frequency of the output. The input voltage is provided by the
DC-DC stage and is assumed to be 240V. This circuit was designed as shown in figure 3.3
3
Q2
Q1
PWM Signal 1
5
4
IRF740
Vdc
Load
1
PWM Signal 2
2
Q3
Q4
IRF740
IRF740
7
6
0
Figure 3.3 H-bridge inverter circuit
38 PWM Signal 2
IRF740
PWM Signal 1
The inverter circuit is an H-bridge consisting four N-channel Enhancement MOSFETs. The
MOSFETs used are International Rectifies IRF740, which can hold a maximum drain to source
voltage of 400V and can pass a maximum current of 10A. The MOSFET can switch with a
maximum switching frequency of about 16MHz.Since the manufacturer integrated an inverse
parallel diode to the MOSFETs there was no need for external inverse parallel diodes although
they can be incorporated for added safety.
The full-bridge inverter was chosen as the inverting output stage for a number of reasons. It is
preferred over a half-bridge inverter because for an equivalent input voltage, the full-bridge
inverter can provide twice the output voltage. As [2] states, this implies that for equivalent power
output, the output current is halved. The full-bridge inverter is also significantly more
controllable.
Because of the on-state resistance of the MOSFET there will be a conduction loss. The onresistance isn't the only cause of power dissipation in the MOSFET. Another source occurs when
the MOSFET is switching between states. For a short period of time, the MOSFET is half on and
half off. The longer that the MOSFET is in the state where it is neither on nor off, the more
power it will dissipate. Because of this power dissipation to avoid the MOSFETs being damaged
by heat, heat sinks are used.
3.3 Control Circuit To generate the SPWM signal an Atmel ATtiny26L microcontroller was used. The ATtiny26L is
a low voltage, high-performance, low-power AVR 8-bit Microcontroller. The device is
manufactured using Atmel's high density non volatile memory technology and is compatible
with the industrial standard MCS-5 1 instructions set. It also has two 8-bit timers that deliver the
function used in this application. By combining a versatile 8 bit CPU with Flash on a monolithic
chip, the ATtiny26L is powerful microcomputer which provides a highly flexible and cost
effective solution to many embedded control applications. The choice of switching frequency
was a difficult one in that switching losses and component sizes decrease with switching
frequency – at the expense of noise limitations and faster switching devices. This decision was
made easier by the wide range of high speed MOSFETs available.
Some of the features of the Atmel ATtiny26L that make it suitable for this kind of tasks include:
•
Data and Non-volatile Program Memory
39 – 2K Bytes of In-System Programmable Program Memory Flash
Endurance: 10,000 Write/Erase Cycles
– 128 Bytes of In-System Programmable EEPROM
Endurance: 100,000 Write/Erase Cycles
– 128 Bytes Internal SRAM
– Programming Lock for Flash Program and EEPROM Data Security
•
Peripheral Features
– 8-bit Timer/Counter with Separate Prescaler
– 8-bit High-speed Timer with Separate Prescaler
2 High Frequency PWM Outputs with Separate Output Compare Registers
Non-overlapping Inverted PWM Output Pins
– Universal Serial Interface with Start Condition Detector
– 10-bit ADC
– On-chip Analog Comparator
– External Interrupt
•
I/O and Packages
– 20-lead PDIP/SOIC: 16 Programmable I/O Lines
– 32-lead QFN/MLF: 16 programmable I/O Lines
•
Operating Voltage
– 4.5V - 5.5V Speed
– 0 - 16 MHz
Since two square waves were required to generate the 10 KHz signal for switching in the DC-DC
converter stage and a 50Hz signal for the inverter stage and the two must run concurrently, the
use interrupts was the best solution to better achieve the desired waveforms. Atmel ATtiny26
microcontroller has built-in interrupt systems with their timer input modules. Instead of
continuously polling for a flag, a microcontroller performs other tasks and relies on its interrupt
system to detect the programmed event. The task of computing the period and the frequency is
the same as that of polling method, except that the microcontroller will not be tied down
constantly checking the flag, increasing the efficient use of the microcontroller resources.
40 CHAPTER FOUR 4.0 ANALYSIS AND RESULTS 4.1 Calculated Efficiency To calculate the efficiency, the power loss which occurs mainly through each MOSFET is
calculated and then the result multiplied by the number of MOSFETs used in the project. In
addition, the power supply losses are included in determining the overall efficiency of the
inverter. The losses can be summarised as follows:
Total MOSFET conduction loss
(4.1)
Total switching losses
(4.2)
Total power supply losses
(4.3)
Total Losses
8.12w
(4.4)
Therefore for 100W inverter this translates to an overall efficiency of:
(4.5)
4.2 Simulation Results Simulations were performed using Multisim Software from National Instruments (NI) Inc.
4.2.1 DC­DC converter stage Simulation of four modules of the DC-DC converter stage gave an output of between 46.92V and
49.98V which is around the expected 48V.
41 Figure 4.1 Four Modules Simulation result of the Dc-Dc converter stage
4.3Testing of the gating signal The output of the ATtiny26L from the output port was tested and the waveforms obtained from
the oscilloscope were as follows:
1. 50 Hz Gating Signal
42 2. 10 kHz Signal
4.4 Prototype Results The prototype of the system outlined above was implemented and tested both on breadboard as
shown in figure 4.2.The figure consist of the microcontroller power supply unit, the
microcontroller, the MOSFET driver circuit and the Dc-dc converter stage. Testing of both
stages showed that the hardware design was valid and the following results were noted.
43 Figure 4.2 The prototype circuit
The Dc to Dc stage was tested using four modules and an output of 46.8V was obtained. This
value was quite acceptable since the expected output was 48V. The difference in the two values
can be attributed to the losses in circuit components and the accuracy of the converter power
supply unit.
The inverter stage was tested using a load of 30Ω and the input being the output of the converter
stage. A square wave output, though not perfect was obtained with a peak to peak value of
45.6V.
44 CHAPTER FIVE 5.0 CONCLUSION AND FUTURE WORK 5.1 Conclusion The main objective of this project was to design and build a 12Vdc to 240Vac inverter for solar
energy. Attempts have been made to develop a low cost transformer-less inverter using switched
capacitor circuits in the Dc-Dc step up converter stage and a H-Bridge design in the inversion
stage. Whereas the complete implementation was not achieved, the prototype that was built
showed that the design principles of hardware and control software were valid. The method used
to control the switches, sinusoidal PWM, was very accurate since the switching signal obtained
was as desired. This justifies the choice of the microcontroller used.
5.1 Future Work Despite the achievements of the project, further work is possible to increase efficiency of the
inverter while reducing cost even further. These include:
I.
Improve the switching schemes. II.
PWM techniques using microcontroller could be implemented to produce a pure sinusoidal output instead of the H‐Bridge configuration. 45 APPENDIX A: PWM ATtiny26L Program .INCLUDE "tn26def.inc"
; Universal register definition
.DEF mp = R16
.def mp1=r17
; Counter for timer timeouts, MSB timer driven by software
.DEF z1 = R24
; Reset-vector to address 0000
rjmp
main
; The interrupt-vector commands, 1 Byte each:
reti
reti
reti
reti
reti
rjmp
tc0i
; Timer/Counter 0 Overflow, my jump-vector!
reti
reti ;
reti
reti
46 reti
reti
; Interrupttc0i:
in
r1,SREG
inc
z1
; save the content of the flag register
; increment the software counter
cpi z1,39
;compare contents of z1 with 39 to check for overflow
brne cont
;branch to subroutine cont if register z1 is not equal to 39
in mp,PORTA
com mp
; complement the data at port A
out PORTA,mp
ldi z1,0
; output PWM 50% signal at port A
;clear register z1
cont:
out SREG,R1
reti
; restore the initial value of the flag register
; Return from interrupt
; The main program starts here
main:
ldi
mp,LOW(RAMEND)
out
SP,mp
;Initiate Stack pointer
; for the use by interrupts and subroutines
; Software-Counter-Register reset to zero
47 ldi
mp,0
; z1 cannot be set to a constant value, so we set mp
mov
z1,mp
; to zero and copy that to R0=z1
; and set the seconds to zero
; Prescaler of the counter/timer = 1, that is 1 MHz= 1µs
ldi
mp,0x01
;Initiate Timer/Counter 0 Prescaler
out
TCCR0,mp
; to Timer 0 Control Register
; Port B is 50Hz PWM port
ldi
mp,0xFF
; all bits are output
out
DDRB,mp
; to data direction register B
out
DDRA,mp
; to data direction register A
ldi mp,0b10101010
out PORTA,mp
; initialize the value at output port A pins
out PORTB,mp
; initialize the value at output port B pins
; enable interrupts for timer 0
ldi
mp,$02
; set Bit 1
out
TIMSK,mp
; in the Timer Interrupt Mask Register
; enable all interrupts generally
Sei
; enable all interrupts by setting the flag in the status-
register
48 loop:
; =============================
; delay loop generator
;
50 cycles:
; ----------------------------; delaying 48 cycles:
ldi R17, $10
;move 16 to register R17
Wait: dec R17
brne Wait
;
; ----------------------------; delaying 2 cycles:
nop
nop
; =============================
in mp1,PORTB
com mp1
out PORTB,mp1
rjmp loop
; complement the data at port B
; output PWM 50% signal at port B
;Repeat forever unless the interrupt flag is raised
49 APPENDIX B: Microcontroller Power Supply Circuit The ATtiny26L requires a stable input voltage,
of +5V. A simple power supply circuit was
constructed using AN7805 voltage regulator. The AN7805 voltage regulator connection was as
shown in the figure
Pinout of the 7805 regulator IC
•
1. Unregulated voltage in
•
2. Ground
•
Regulated voltage out
50 APPENDIX C: ATtiny26 Datasheet C.1 Pin Configuration 51 C.2 Pin Description 52 C.3 ATtiny26 Block Diagram 53 C.4 ATtiny26 Timer/Counters Prescaler 54 Appendix D: IRF740 Datasheet 55 56 57 58 REFERENCES [1] N. Mohan, W. Robbins and T. Undeland, Power Electronics - Converters,
Applications and Design (2nd Ed), Wiley and Sons, New York, 1995.
[2] Rashid, M. H., Power Electronics: Circuits, Devices and Application (2nd Ed), Prentice-Hall,
Englewood Cliffs, 1993.
[3] Fang Lin Luo and Hong Ye, Advanced DC/DC converters, CRC Press LLC, 2000 N.W.
Corporate Blvd., Boca Raton, Florida, 2004.
[4] F. H. Khan, L. M. Tolbert, “A Multilevel Modular Capacitor Clamped DC_DC Converter,”
IEEE Industry Applications Society (IAS) Conference, Oct. 2006, pp. 966-973.
[5] F. H. Khan, L. M. Tolbert, “A 5-kW Multilevel DC-DC Converter for Future Hybrid Electric
and Fuel Cell Automotive Applications,” IEEE Industry Applications Society (IAS) Conference,
2007.
59