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KOFORIDUA POLYTECHNIC
SCHOOL OF ENGINEERING
DEPARTMENT OF ELECTRICAL/ELECTRONIC
PROJECT TOPIC:
DESIGN AND CONSTRUCTION OF A PURE SINE
WAVE D.C / A.C POWER INVERTER
BY:
KORANTENG EBENEZER
INKOOM ROMEO
This project is presented to the Department of Electrical/Electronic Engineering – Koforidua Polytechnic,
Koforidua – Ghana in partial fulfillment for the award of an HND, June 2011. No. 06/2011
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CHAPTER ONE
1.0 Background of the study
Power supply is a need for every sphere of human endeavor and since the time of revolution be
used for industrial and technological advancement of the world.
The consistent supply of power has been the major drawback to the supplier and the consumer of
power. Power failure, surges, and variation of voltage supply just to mention a few, necessitates
a reliable power supply, if not a standby.
Cost, space, and convenience are a few factors considered when choosing an appropriate power
supply. This project aims at providing a standard, affordable, and cost effective power supply
inverter to supplement the existing power supply.
An inverter is an electronic device that converts electrical energy of a Direct Current (D.C) form
into that of Alternating Current (A.C) form. Inverters come in various shapes and sizes, power
efficiency, purpose, etc. it is used as a standby power supply for laptops, sound systems,
microwaves, motors, fridges, etc.
1.1 Statement of the problem
Most electrical appliances need constant power supply to function effectively. Power failures,
surges, and low voltages cause damage or destruction of electrical appliances. As a result,
industries, offices, students, teachers, as well as every consumer of power supply faces
difficulties in using their electrical appliances when their power supply is disrupted which hinder
the progress of their activities or performances at work. This project aims at addressing these
problems.
1.2 Objectives of the study
This project is aimed at conversion Direct (D.C) voltage to the Alternating (A.C) voltage (220V
at 50Hz A.C) applicable to any domestic electrical appliance.
Also this inverter will serve as a backup supply to electrical appliances when the main source of
power supply outage or unstable. It is also designed with cost effective, efficiency, and reliability
in mind.
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1.3 Hypothesis.
The following research questions will be addressed at the end of this project.
i. What is the benefit of inverters in its field of applications?
ii. What are the types of inverters?
iii. What are the advantages of the various types of inverters over the others?
iv. What are the components of the inverter?
v. What are the criteria used in selecting these components?
vi. How reliability is inverter?
vii. Is this type of inverter cost effective than the existing ones in the market?
1.4 The Scope.
To design and construct an inverter of 500VA, 50Hz as backup or power supply for electrical
appliances, with the aid of converting a D.C input from a battery to an A.C output by using pulse
width modulation and effective output filtering(Harmonic Filtering)with a negligible switching
noise.
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CHAPTER TWO
2.0 Literature Review
This report focuses on Power Inverters (DC to AC); a device which efficiently transforms a DC
power source to a 240/220 voltage AC source, similar to power that would be available at an
electrical socket outlet. Inverters are used for many applications. In situations where low voltage
DC sources such as batteries, Solar panels or fuel cells must be converted so that devices can
run-off of AC power. One example of such a situation would be converting electrical power
from a battery to run a laptop, TV or cell Phone. [6]
The method, in which the low voltage DC power is inverted, is completed in two steps. The first
being the conversion of the low voltage DC power to a high voltage DC source, and the second
step being the conversion of the high DC source to an AC waveform using pulse width
modulation. Another method to complete the desired outcome, would be to first convert the low
voltage DC to AC and then use a transformer to step up to the required voltage. This project
focused on the second method described. There are Different DC/AC inverters on the market
today which are essentially in two different forms of AC outputs: modified sine wave and pure
sine wave. A modified sine wave can be seen as more of a square wave than a sine wave; it
passes the high DC voltage for specified amounts of time so that the average power and Root
Mean Square (RMS) voltage are the same as if it were a sine wave. [1]
These types of inverters are much cheaper than pure sine wave inverters and therefore are
attractive alternatives. Pure sine wave inverter on the other hand, produces a sine wave output
identical to the power coming out of an electrical socket outlet. These devices are able to run
more sensitive devices that a modified Sine wave may cause damage to such as: laser printers,
laptop computers, power tools, digital clocks, fridges, fans, medical equipment and other
inductive loads. This form of AC power also reduces audible noise in devices such as
fluorescent, Lights and runs inductive loads, like motors, faster and noiseless due to the low
harmonic distortion.
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Figure 2.1: The complete block diagram-inverter unit
2.1 Pulse width modulation (PWM)
In switching mode power suppliers (SMP) such as inverters, the most predominant mode of
switching is by the use of pulse width modulation (PWM) to control the switching devices.
In electronic power converters and motors, PWM is used extensively as means of powering
alternating current (A.C) devices with an available direct current source (D.C) for advance
D.C/A.C conversion. Variation of duty cycle in the PWM signal to provide a D.C voltage across
the load in a specific pattern will appear to the load as an A.C signal. The pattern
At which the duty cycle of PWM signal varies, can be through simple analog components, a
digital microcontroller or specific PWM integrated circuits. [5]
The SG3524 PWM integrated circuit (I.C) is capable of producing duty cycle which can be used
for a wide range of switching applications. It consists of two complementary outputs which can
be used for driving push-pull configuration. Any analog/logic operation liable to the production
of output pulse is done internally by the I.C. The figure below shows the pin diagram of the
SG3524 PWM.
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Figure 2.2: Pin diagram of SG3524 I.C
In PWM, the reference signal is sinusoidal ant at frequency of the desired output signal whiles
the carrier is often either a saw tooth or triangular wave at a frequency significantly greater than
the reference. These two signals are fed into a comparator. When the carrier signal exceeds the
reference, the comparator output signal is at one state and when the reference signal exceeds the
carrier, the output is at its second state. This process is shown below.
Figure 2.3: Pulse Width Modulation[5]
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 bridge, half bridge or
push-pull configurations are used commonly in power electronic. Full bridge configurations
require the use of four switching devices and are often called H-Bridge.
2.2 The oscillator.
The frequency of oscillation is set by the resistor (RT) and a capacitor (CT), connected to the pins
6 and or respectively of the SG3524 I.C. The I.C has an in built oscillator circuit whose time
constant can only be varied outside by either varying R1 or C1 to oscillate the circuit at the
required frequency. The formula for calculating the output frequency is given by:
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FO = K
R1 C1
Where K, is the constant due to the design parameter of the oscillating circuit. Mean while there
are various means of creating oscillation signals for analog PWM. Phase shift oscillator,
triangular wave oscillators, wein bridge oscillators and bubba oscillators are just to mention a
few. [8] The SG3524 I.C makes it cheaper and easy to design an oscillator with only C1 and R1
to serve the same purpose as any of the aforementioned.
A typical output wave form produced by the SG3524 I.C is shown below.
Figure 2.4: Wave form produced by SG3524 I.C
2.3 The H-Bridge configuration.
An H-Bridge or full bridge converter is a switching configuration composed of four switches in
an arrangement that resembles an H. There are variations of the H-Bridge converter such as (P
and P), (N and N), and (P and N) of which the simplest to work with is the P and N. By
controlling different switches in the bridge, a positive, negative or zero potential voltage can be
placed across a load. When this load is a motor, these states correspond to forward, reverse and
off. The use of an H-Bridge configuration to drive a motor is shown below.
Figure 2.5: H-Bridge Configuration using N-Channel MOSFETs[2]
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As shown in Figure 5 the H-Bridge circuit consists of four switches corresponding to high side
left, high side right, low side left, and low side right. There are four possible switch positions that
can be used to obtain voltages across the load. These positions are outlined in Table 1. Note that
all other possibilities are omitted, as they would short circuit power to ground, potentially
causing damage to the device or rapidly depleting the power supply.
Table 2.1: Valid H-Bridge Switch States
High Side Left High Side Right Low Side Left Low Side Right Voltage Across Load
On Off Off On Positive
Off On On Off Negative
On On Off Off Zero Voltage
Off Off On On Zero Voltage
The switches used to implement an H-Bridge can be mechanical or built from solid state
transistors. Selection of the proper switches varies greatly. The use of P-Channel MOSFETs on
the high side and N-Channel MOSFETs on the low side is easier and simple because with this
approach, the need for complex FET driver is truncated as it would have been the case for N to N
type or P to P type MOSFETS H-Bridge. [5]
2.4 MOSFET Drivers/Buffer
When connecting the gate of a FET to the oscillator, it is often advisable to consider the
matching conditions between the gate input and the oscillator output. The FET is design for
effective switching at high input impedance. But the output of the oscillator is designed for low
output impedance for better performance. The MOSFET buffer drives the gate of the MOSFET
with total isolation between the two stages while keeping their working parameters at constant.
There are various types of MOSFET drivers made of transistors, logic gates and specific
integrated circuits. With this type of H-Bridge configuration, simple transistors circuit can be
used to drive the switches. [5] A typical arrangement of the transistors for MOSFET drive is
shown in the figure below.
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Figure 2.6: MOSFET Drive using bipolar transistors
(A Class B push-pull amplifier)
2.5 Circuit protection and Snubber [5]
One of the major factors in any electronic device is its ability to protect itself from surges that
could damage the circuitry. In the case of the inverter, inductive loads can cause special
problems because an inductor cannot instantly stop conducting current, it must be dampened or
diverted so that the current does not try to flow through the open switch. If not dampened the
surges can cause trouble in the MOSFETs used to produce the output sine wave; when a
MOSFET is turned off the inductive load still wants to push current through the switch, as it has
nowhere else to go. This action can cause the switch to be put under considerable stress, the high
dV/dt, dI/dt, V and I associated with this problem can cause the MOSFETs to malfunction and
break. To combat this problem snubber circuits can reduce or eliminate any severe voltages and
currents. Composed of simply a resistor and capacitor placed across each switch it allows any
current or voltage spikes to be suppressed by critically dampening the surge and protecting the
switch from damage. The snubber can become more effective by the addition of a zener diode so
that any large current surge the resistor capacitor snubber cannot handle gets passed through to
ground by the zener diode. [4]
The diagram in Figure 7 shows a simple representation of an inductive load (L) over a switch
representation, Figure 8 and Figure 9 show how Snubber can be implemented so that a surge will
be suppressed.
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Figure 2.7: Inductive Load Circuit[5]
Figure 2.8: Inductive Load Circuit with Snubber[5]
Figure 2.9: Inductive Load Circuit with Snubber and Zener Diode [5]
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2.6 Filtering
Filters come in many different packages, with many different advantages and disadvantages. For
example, a digital filter is easily reconfigurable and can have almost any frequency; an active
filter can be made to have a very sharp edge at the cut off, resulting in enormous reduction in
noise and very little attenuation of signal. These, however, require Opamps. Opamps capable of
filtering a 120V RMS sine wave exist, but are expensive and lossy, since the Opamp must be
able to source hundreds of watts, and must be very large to do so without burning. Digital filters
have a similar drawback and, designed with TTL and CMOS technology, can only work with
small signals. Lastly we come to a passive filter. Generally large in size and very resistive at low
frequencies, these filters often seem to have more of a prototyping application, or perhaps use in
a device where low cost is important, and efficiency is not. Given these choices, an application
such as a high power sine inverter is left with only one viable option: the passive filter. This
makes the design slightly more difficult to accomplish. Noting that passive filters introduce
higher resistance at lower frequencies (due to the larger inductances, which require longer
wires), the obvious choice is to calculate the values to allow the passage of the required output
frequency. In this case, the secondary windings of the transformer must be taking into account
since the transformer forms part of the filtering network.
The figure below shows the low-pass filter selected for the output of the inverter.
Figure 2.10: A Passive low-pass filter
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2.7 Feedback / Voltage Regulation
A voltage regulator is an electrical regulator designed to automatically maintain a constant
voltage level. A voltage regulator may be a simple “feed forward” design or may include
negative feedback control loops. It may use an electromechanical mechanism or electronic
components. Depending on the design, it may be used to regulate one or more A.C or D.C
voltages. Electronic voltage regulators are found in devices such as computer power supply,
where they stabilize the D.C voltage used by the processor and other elements. In an electric
power distribution system, voltage regulators may be installed at a substation or along
distribution lines so that all consumers receive steady voltage independent of how much power is
drawn from the lines.
The SG3524 (PWM) I.C provides a reference voltage that can be compared with a fraction of the
total output as a feedback voltage to regulate the output power in a close loop manner.
The figure below show the closed loop system for the voltage regulation. [12]
Figure 2.11: Closed-loop operation for voltage regulation
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CHAPTER THREE
3.0 METHODOLOGY
The construction of the pure sine wave inverter can be complex when taught of as a whole but
when broken up into smaller divisions; it becomes much easier to manage. The following
sections detail each specific part of the project as how each section is constructed and the
interaction of each section with other blocks to obtain a 220V sine wave power Inverter. [5]
An analogue integrated circuit, a discrete component, a MOSFET bridge, a transformer, a low
pass filter, and a MOSFET driver are the components necessary to generate a 60Hz 220V sine
wave across a load. The block diagram shown in figure 1, in Chapter two (2).
Shows the various parts of the project that would be addressed. The control circuit comprise of a
dual output PWM IC within built error amplifier, on chip reference voltage, programmable
oscillator, pulse steering flip flop, two uncommitted output transistors, a high gain comparator,
current limiting and shutdown circuitry. [3]
With the appropriate biasing of the IC a PWM signal is fed to gates of the FET via the class B
push-pull buffer/drive circuit. The PWM signals are fed into these MOSFET to switch in an H-
Bridge configuration inducing an emf in the primary winding of the output step-up transformer.
From here the signals at output of the transformer (modified sine wave) sent through a low pass
LC filter to remove all harmonics and undesired waveform so that the signal can be smoothen to
deliver a pure sine wave. [5]
Some specific operation, calculations, constructions and resulting output waveforms for each
part will be discussed in detailed in the following sections.
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Table 3.1: List of materials and description/values.
Component
number
Description/
Value
R1 2.2K Ω
R2 2.2K Ω
R3 1.5K Ω
R4 1.5K Ω
R5 100K Ω
R6 10K Ω /VR
R7 15K Ω
R8 15K Ω
R9 1K Ω
R10 1K Ω
R11 1K Ω
R12 1K Ω
R13 48Ω
R14 2.3K Ω
R15 2.3K Ω
R16 1K Ω
R17 1K Ω
R18 3.3K Ω
C1 1μf 50V
C2 0.17μf
ceramic
C3 100μf
C4 1μf 250V
ceramic
C5 0.1μf 50V
ceramic
C6 0.1μf 50V
ceramic
C7 0.1μf 50V
ceramic
C8 0.1μf 50V
ceramic
U1 SG3524N
T1 13H
T2 8V:250V /
1:32 500W
Q1 C945
Q2 C945
Q3 FSJ9160
Q4 FSJ9160
Q5 IRFP150N
Q6 IRFP150N
S1 62A
CATRIDGE
S2 SWITCH
D1-4 IN4001
FUSE HOLDER
IC SOCKET DOP 16
I3A SOCKET
10mm CABLE
flexible
BATTERY
TERMINALS(2)
PC BOARD
Housing
3.1 Pulse Width Modulation Controller.
The control circuit is built around an SG3524 PWM regulator IC. It can perform the functions of
output voltage sensing and correction, voltage to pulse width conversion, a stable reference
voltage, an oscillator, over current no protection and power switch drivers. It also include a
shutdown and a compensate circuitry.
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3.1.1 The Oscillator
The oscillator sets the frequency of operation of the supply and generates a saw tooth waveform
for the D.C to pulse width converter. The timing components CT and RT determine the
frequency of operation. [3] The time constant for the rate of oscillation can be calculated by the
relation as follows:
T = CT RT…………… 3.0
Where T = time constant
Parameters of the circuit.
CT = Timing Capacitor
RT = Timing Resistor
Hence the frequency of operation is given by:
F= K
T =
K
CT RT …………………3.1
Where K depends on the design parameter of the circuit, and from data sheet K=1.18.
F = 1.18
CT RT
For a frequency of approximately 60Hz, let RT equal to 100KΩ then
60 = 1.18
CT(100x103)
CT= 1.18
60x100x103 =
1.18
6x106 =
10−6
6
CT = 1.96x10-7
CT=0.196µf
Then RT =100kΩ, CT = 0.17µf, F= 60Hz
3.1.2 The voltage error amplifier (High Gain Comparator)
The voltage error amplifier amplifies the difference between the ideal reference voltages (in this
case about 5V) and the sensed output voltage presented by the feedback elements. The error
amplifiers output represents this error between the reference and the actual output multiplied by
the high D.C offset. [5] This error signal is then presented to the D.C-to-pulse width converter,
which produces a pulse train whose duty cycle represents this error signal. This pulse train is
then presented to a digital flip-flop that steers the pulses alternately between two output drivers.
The output drivers themselves are uncommitted transistors which is where both the emitters and
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collectors of the output transistors brought out of the IC for external biasing- pins 11, 12, 13, and
14.
3.2 Biasing the controller IC
Biasing the IC is controlling the voltage and current at the various pins for optimum
performance. In most applications where DC to AC conversion is required the IC can be wired as
shown in the figure 3.0 below, to produce PWM signal of approximately 45% duty cycle,
alternately at both outputs to switch enough power for a given load. [9]
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3.3.0 The Output
The output transistors (within the IC) are biased externally in the common collector mode. In this
mode the transistors inherit enough power to drive subsequent circuits and also attain good
output stability. Collector A and B are connected directly to the supply voltage. Emitter resistors,
R5 and R6, are connected to Emitter A and B respectively, as shown in the figure 3.1, to couple
to the output of the next stage. [5]
R4 is a current limiting resistor which ensures that the current flowing through the IC is
controlled to a moderate level for the IC to function properly. Capacitor C3 is used as decoupler
to filter out any undesired signal that may interfere with the D.C supply voltage to the oscillator.
Assuming the output network is in the figure 3.1. An approximate value of good resistance value
can be selected for R4, R5, and R6.
.
From applications not for the SG3524 the collector emitter voltage VCE =40Vmax
Collector leakage current =150μA at 40V saturation current IC =50 m A at 12V
Saturation both transistors are switched on.
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With reference to the data sheet, i.e. appendix D: the other branch is negligible, since that branch
take a very minute current for the logic operation in the IC. [4]
At saturation I1 = 50mA, I2 = 50mA but this will cause the IC to heat which may break down the
output transistor if care is not taken.
10% of the saturation current allows the IC to run at extremely cool temp without fan.20% can
also be used provided the max current rating is not excelled.
Now 10% of 50mA = 0.1×50mA =5mA.
From the equivalent circuit
I = I5 +I0 ………..3.2
= 5mA + 5mA
= 10mA
R = 𝑉
I ………..3.3
= 12V
5mA
=2.4 ×10³Ω =2.4KΩ
2% of this resistance for current limiting, R4, gave
R4 =0.02 ×2.4 ×10³
= 48Ω
Now 2.4 k Ω - 48Ω
2.4 × 10³ - 48
= 2352Ω ≈ 2.3kΩ
But R5 and R6 are in parallel, meaning that the total combination of R5 and R6 is given by
R = (𝑅5 × R6)
(R5 + R6) ………3.4
But, R5 = R6
Therefore, R = (R5)²
2R5 =
R5
2
Both transistors do not switch on at once. Considering one at a time R5 = 2352Ω
≈ 2.3kΩ
R4 =48Ω, R5=R6=2.3kΩ
To filter signals at the operating frequency (50-60) Hz of the power supply to the oscillator, C3 is
calculated to damp or attenuate all signals within the desire frequency, i.e. below 50Hz
Fc = 50 Hz
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= 1
2πR4C3… … .3.5
C3= 1
2πFcR4
C3 = 1
2 X 3.142 X 50 X 48
C3= 1
15081.6
=6.6 X 10-5
≈66×10⁻6
=66μf
Therefore 66µf or any value above 66μf is better. With reference to section 3.1.2 where the
reference voltage VF (pin 16) is connected to a compiling resistor R3. A feedback voltage VFB
will be fed into the infringing input (pin 1, - IN) via a potentiometer R2 and resistor R1, R1 and
R3 can be any value between 21Ω to 10kΩ. R1 is there to further limit the current flowing from
the output into the inverting input. Potentiometer R2 allows the adjustment of the error voltage to
adjust the amplitude of the output pulse to the FET drivers. [1]
The positive and the negative sense terminals m(pin 4 and pin 5 respectively) and the shut down
terminal (pin 10) are shorted to ground, to avoid any interference from stray electric charges,
since the operations of those terminals will not employed. The timing capacitor C1 is connected
between ground and CT (pin 6) and the timing resistor R7 is connected to pin 7 and ground. [2]
The compensating capacitor; from the datasheet can be any value from 0.01 μ f to 1 μ f. At this
state where the negative feedback is not connected the output pulses will be at the maximum.
With switch S1, closed the output wave form of the current are given in figure 3.3
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3.3.1 The Drive/Buffer amplifier
The buffer circuit is made up of an NPN transistor biased in the common emitter configuration as
shown in figure 3.2.1. [5] This circuit has the qualities of low input impedance to match the
PWM IC and high output impedance to effectively couple to the gates of the output FET
switches. These attributes make the circuit useful to the coupling of the IC to gates of the FET
switches without impedance mismatching, since FETs have very high input impedance and the
IC has low output impedance.
Considering the pulse signal to be at its max with respect to ground, when pulse goes positive,
the base-emitter junction of the NPN transistor will be forward biased turning on current from
the supply through the collector to the emitter causing the output voltage to drop zero. When the
signal goes negative the emitter-base junction will be reverse biased making the transistor to cut-
off causing the output voltage to reach maximum. This operation will cause the output voltage
waveform to be amplified as the opposite of the input one – inverted output. This waveform is
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used in driving the MOSFETs to conduct alternatively. R7 is a pull up resistor used to drive
positive current to the gate of the MOSFET. By nature MOSFETs does not required a high gate
current to operate. [3]
According to Ohms Law:
I = V
R ……….3.6
Taking R7 to be 15kΩ, where BT=V=12v
I = 12𝑉
15𝑘Ω
I =0.8mA
This current is quite enough to fully turn the FET on. When the output of the IC goes positive,
the NPN transistor will turn on; this in turn causes the output of the drive amplifier to be negative
to turn the P- MOSFET to turn on. For effective operation of the drive circuit,
R8 = 𝑅7
10
R8 = 15𝑘
10
Therefore, R8 = 1.5kΩ
The 0.8mA is then the gate current for the FETs as well as collector current for the drive
transistor.
This is just to establish the fact that the collector current is so small, hence a transistor chosen for
the design can be any value suitable for 12V and the current of 0.8mA.
With reference from datasheet transistor such as C945 can be suitable for such topology.
3.3.2 The H-Bridge configurations and the output switches (MOSFETS)
The full-bridge connecter is the last of the power transformers to isolate PWM topologies. Like
the other double ended regulators, its transformers flux is driven in both the positive and negative
polarities. Its performance with respect to output power is significantly improved over that of
the half bridge connecter. This is because the balancing capacitors are replaced with another pair
of half bridge style power switch identical to the first pair. This time two of the four power
switches are turned on simultaneously. [5] During one conduction cycle either (1) the upper left
and lower right left and lower right left switches or 2 the upper right and lower left switches are
turned on. Each associated pair of power switches conduct on alternate cycle. This places the
full output voltage across the primary winding. This effectively doubles the maximum power
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handling capacity of the topology over the half – bridge (see figure 3.2.1 and figure 3.2.2).
The full bridge regulator topology is used in applications where output powers of 300W or more
kilowatts are required.
3.2.3 The Power MOSFETS
The power MOSFETS are quickly gaining popularity in the switching power supply (SMP) field
and used as test power switches. Power MOSFET technology has matched greatly in recent
years and excelled the performance of the bipolar power transistors. Power MOSFETs now
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switch approximately 10 times faster than their bi-polar counterparts when driven by fixed base
drive methods. MOSFETs have also attained saturation voltages very comparable to those of the
bipolar transistors. This makes the power MOSFET the better choice for switching power
supplies in the majority of applications. The power MOSFET is an isolated –gate, voltage driven
device. That means that it takes less average current to drive the gate of a MOSFET. In order to
find a suitable complementary pair of MOSFETs to match the 500W power design a simple
analysis of the H-bridge network is made as follows. [5]
From the figure shown above, the battery supply BT is 12V.When gate 1,Q1, is on while gate
2,Q2, is off, transistor D and transistor A will turn on, assuming the transistors to turn on fully
without losses then a maximum current will flow from +BT through transistor A to load
transformer to transistor D to ground.
Then IC = Power Input
Voltage Input ……….3.7
= 500W
12V = 41.67 A
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This 41.67A being the max drain current ID. Taking a half way through the bridge it can be
deduced that when the two transistor A and B are both off, the potential voltage across them can
be seen as shown in figure 3.8
Thus VDS = +BT
2 ………3.8
= 12V
2= 6V
But in most applications the VDS for power switches is considered less than twice the value of the
calculated VDS. The drain to source voltage VDS required for this topology is any value greater
than 2(VDS)
Thus VDS > 2 X (the calculated VDS)
VDS > 2 X 6V
VDS > 12V
This is because the back emf from the load transformer to the output switches is normally 2X
(the supply voltage) at full load conditions and this could break the transistor down if not taken
into consideration. Heat dissipation, which is a product of the ID² and the terminal resistance of
the FET i.e.
Heat loss = ID² × R terminal ……..3.9
For max performance R (internal) should be around 0.001 to 0.05 ranges.
Hence from this analysis, a MOSFET with the follow parameters is considered.
VDS =100 or 60v
IG = above 42A
R drop = less than 0.03Ω
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From the datasheet 1RFP 140 and 1R FP9140 were considered choices for this topology.
3.3 The Output filter
In the actual sense the output wave form from the bridge across load has a pattern us shown
below.
In order to improve this waveform into a pattern similar to a sine wave a low- pass passive filter
is designed. One of the characteristics low-pass filter is their ability to smoothing the square
edges in a wave into a curve like pattern.
The design is for 60Hz frequency and below shows the calculations.
Let C =1μf, f = 60Hz
f = 1
2π√LC ……………..3.10
√𝐿𝐶 = 1
2πf = LC =
1
2πf2
LC = 1
(2πf)2C
LC = 1
(2×3.142×60)2×10−6
= 106
377.04
= 2652 H
Most of this total inductance will be accommodated within the output transformer, while only a
few of about 0.5% connected external to complete the T-network. Here the primary windings of
the transforms part of the filter network. But since the inductance of the transformer cannot
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easily be estimated a try and error method is adopted to choose either L or C for better
performance. (See fig 3.10)
3.4 The Output Transformer
In this configuration, the transformer provides D.C isolations between the input lines and the
output. It also performs a voltage step-up function to transform the low voltage A.C output to the
required 220V A.C level.
In order to estimate the appropriate turn ratio of the output transformer, it is necessary to
estimate for the actual output voltage from the switches without the transformer.
The output wave form the switches without the transformer, is given as shown in figure 3.1, the
switching network so discussed forms a full bridge buck convertor. Ideally the RMS voltage
across the primary winding for every half cycle will be
VP = VB + √𝐷 …………3.11
Where D is the duty cycle of the switching circuit, and VBt is the battery voltage supplied to the
inverter.
From the datasheet D = 45%
Thus VP = 12√0.45
= 8. 0498 V
= 8 V (RMS)
That means the output voltage from the bridge will be swinging between +8V and – 8V.
Considering figure 3.3.3, it is obvious that the filter network will cause some voltage drops and
as results the actual voltage across the primary windings of the transformer will be below 8V as
estimated. To compensate for this voltage loss an output voltage of value little above 220V is
used in calculating for the turns ration so that the output power will not be affected by the filter
network as well as the 1R loss incurred by the switching transistors.
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Now, in the transformer principle,
(Input Power) = (Output Power)
Vp
Vs=
Np
Ns………………3.12
Where VP = Primary Voltage (8V)
Vs = secondary voltage (taken to be 250v)
NP = primary turns
Ns = secondary turns.
Thus N = Vs
Vp =
250V
8V = 31.25V≈ 32
In this case a transformer of 1:32 must be selected for the output.
3.5 The Feedback Network. (Voltage Regulation)
The voltage regulation is implemented by a simple close loop in a negative feedback system. The
output of the inverter (thus the untransformed output) is full wave rectified to produce a
pulsating D.C. The D.C. pulse feedback into the inverting input terminal of the PWM IC via a
high resistance potentio-meter to smoothly adjust a set point for the output voltage. The fraction
of the output feeding the I C is compared with the reference voltage at the non-inverting input to
regulate the PWM output to maintain stability, irrespective of load type and to respond to load
demand. The value of the potentio-meter is chosen to draw a very little current. The figure 3.5
shows the circuit arrangement of the feedback network, not that the 1k resistors were chosen to
limit the current flowing into the rectifier circuit so that diodes of lower rating and size could be
used. In this sense a glass diode or any related diode is a considerable choice.
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Assuming the output power to be 500watts then the current drawn by the feedback network will
be.
I = Vp − 2 (Vd)
10K +2(1K) ……….3.13
I = 8 − 2 (0.7)
12K
I= 0.00055 = 0.55mA
This result shows clearly that the feedback network has no significant effect on the output power,
due to its higher resistance.
3.6 Protection
The protection of the circuit is divided into two peculiar parts, the snubber circuit as discussed in
chapter one and a protective fuse. The fuse being used to protect the transistors when it happens
that a higher current is drawn than the current can handle. From the design point of view a
cartridge fuse is selected for easy installation and replacement.
To estimate for the best fusing action, a simple calculation involving the fusing factor, fusing
current and current rating of the fuse are to be considered as:
Fusing Factor = Fusing Current
Current Rating …..3.14
For a cartridge fuse the fusing factor is between 1.25 and 1.75, averagely we have
1.25 + 1.75
2=
3
2
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= 1.5
The current rating of the inverter is given as
Iin =Power Input
Voltage Input… … .3.15
Iin =500W
12V
= 41.67𝐴 ≈ 42𝐴.
Therefore the minimum current that will cause the fuse to blow –fusing current will be
Fusing Current = Fusing Factor × Current Rating ……….3.16
= 1.5 × 41.67A
= 62.5 A
This means the 42A fuse will deteriorate when a current of 62.5A passes through it.
CHAPTER FOUR
4.0 Results and Analysis
After a systematic arrangement of the various sections in the circuit discussed in the previous
chapter, for the construction of the D.C to A.C converter, the following results where obtain after
some analysis.
The output wave form after feeding the circuit with a 12V D.C car battery that was displayed on
an oscilloscope is shown in the figure below. And also a recorded digital multimeter reading of
the RMS output voltage on no load was recorded as 117V.
Upon many checks, it was realized that the amplitude of the Gate signal are far below the
specified firming point due to the effect of the feedback loop.
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Several experiments made to check the output voltage when the converter is on load is recorded
and tabulated below:
Load Type Ratings(W) Output voltage(V) Percentage Variation
(%)
Filament
Bulb
100 213 -0.3
Table Fan 75 225 2.3
Computer 150 218 0.91
Table 4.1: Output voltage when inverter is on load.
From the various output voltages recorded, it clearly shows that the output transistors are relaxed
when there is no load on the output and also save the duration of the battery bank. Generating of
heat by the transistor was very moderate, indicating efficient switching as a result of lower I2R
losses as presumed in the previous chapter.
CHAPTER FIVE
5.0 Conclusion
In this project many sections and configurations of converting a direct current – voltage (i.e.12V
D.C.) to an alternating current – voltage (i.e.220V A.C) were discussed. In the designing time,
funding, availability of components, were considered.
Simple approaches and simplified formulae were implemented breaking down the hostilities
associated with the converting of D.C to A.C. Also demystifying the various operations involved
in the process of D.C to A.C. converting, appliances such as fans, television and computers at
destinations where the availability stretch of the national electricity grid could be taught of as a
night mare, without any problem.
The designing and construction of a sine wave 220V A.C inverter per the aims of the project was
achieved. Given that the output gave 213V and an efficiency of 97%.
5.1 Recommendation
The expectation of this project was to design a power inverter with an output of a pure sine wave
form. Although the goals were met successfully, it was realized upon critical analysis that the
results obtained had some losses in the filtering process as a result of harmonic content in the
output signals Of the H-Bridge network.
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Therefore it is recommended that further research could be conducted in looking into the
reducing of the harmonic content for a perfect system output.
6.0 References
6.1 Books
1. Brown(1990),practical switching power supply design
2. Brian Scanddan, Electrical installation work,4th edition
3. Hart, D. (1997). Introduction to Power Electronics. Upper Saddle River, NJ: Prentice
Hall. International Rectifier. (2006). AN978HV
4. Singn, M. D, power electronics,2nd edition
6.2 Journals
5. Current Transmission Systems Technical Review Paper. Retrieved December 15, 2006
from.pdf.
6. Floating MOS_Gate Driver ICs. Retrieved November 10, 2006, from pdf.
7. International Rectifier. (2006). IR2110 High and Low Side Driver. Retrieved November
10, 2006, from pdf.
8. Jim Doucet, Dan Eggleston, Jeremy Shaw, DC/AC Pure Sine Wave Inverter, pdf.
9. Trace Engineering. (April 9, 1999). Modified Sine wave and Sine wave Waveforms.
Retrieved December6, 2006 from pdf.
6.3 Web pages
10. http://www.donrowe.com/inverters/puresine_600.html.
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11. http://inventors.about.com/library/inventors/blstanley.html.
12. http://www.4lots.com/browseproducts/GoPower600WattInverter.html.
13. http://www.powerdesigners.com/InfoWeb/design_center/articles/PWM/pm.shtm.
14. http://www.datasheetcatalog.org/data
15. http://www.electrosurplus.com/sh-index
16. http://www.ir.theicstock.com
7.0 Appendix-A: The complete circuit diagram of the project
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7.1 Appendix- B: The component parts list and cost analysis
Number Compone
nt number
Description Unit
cost
¢
1 R1 2.2K Ω 10p
2 R2 2.2K Ω 10p
3 R3 1.5K Ω 10p
4 R4 1.5K Ω 10p
5 R5 100K Ω 10p
6 R6 10K Ω VR 10p
7 R7 15K Ω 10p
8 R8 15K Ω 10p
9 R9 1K Ω 10p
10 R10 1K Ω 10p
11 R11 1K Ω 10p
12 R12 1K Ω 10p
13 R13 48Ω 10p
14 R14 2.3K Ω 10p
15 R15 2.3K Ω 10p
16 R16 1K Ω 10p
17 R17 1K Ω 10p
18 R18 3.3K Ω 10p
19 C1 1μf 50V 10p
20 C2 0.17μf
ceramic
10p
21 C3 100μf 50p
22 C4 1μf 250V
ceramic
1.00
23 C5 0.1μf 50V
ceramic
10p
24 C6 0.1μf 50V
ceramic
10p
25 C7 0.1μf 50V
ceramic
10p
26 C8 0.1μf 50V
ceramic
10p
27 U1 SG3524N 6.00
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28 T1 13H 2.00
29 T2 8V:250V /
1:32 500W
50.0
0
30 Q1 C945 20p
31 Q2 C945 20p
32 Q3 FSJ9160 4.00
33 Q4 FSJ9160 4.00
34 Q5 IRFP150N 4.00
35 Q6 IRFP150N 4.00
36 S1 62A
CATRIDGE
3.00
37 S2 SWITCH 1.00
38 D1-D4 IN4001 1.00
39 FUSE
HOLDER
2.00
40 IC
SOCKET
DIP 16 1.00
41 SOCKET I3A 3.00
42 CABLE
flexible
10mm 5.00
43 BATTERY
TERMINAL
(2)
4.00
44 PC
BOARD
2.00
45 Housing &
Accessories
50.0
0
Labour
Cost
60
Total Cost 240
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7.2 Appendix-C: The IRFP150N Data sheet
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7.3 Appendix-D: The SG3524N Data sheet
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