PJ HND KDUA POLY

PROJECT WORK BELONGING TO, INKOOM ROMEO & KORANTENG EBENEZER (KOFORIDUA POLYTECHNIC, 2011)

COPY RIGHT RESERVED……

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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



26

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.



27

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.



28

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



29

= 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.



30

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.



31

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.

http://www.wholesalesolar.com/pdf.folder/Download%20folder/sine_modsine.pdf



32

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

http://www.datasheetcatalog.org/data

http://www.electrosurplus.com/sh-index

http://www.ir.theicstock.com/



33

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



34

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



- 35 -

7.2 Appendix-C: The IRFP150N Data sheet



- 36 -

7.3 Appendix-D: The SG3524N Data sheet



- 37 -

Documents

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