ACKNOWLEDGEMENT

This is great honor for us to thank those who have helped to make this project possible. First

of all we would like to pay our deepest respect and honor to our instructor, Md. Aziz Ul Huq

sir for giving us the opportunity to work on this project under his supervision. His support,

guidance and encouragement from the initial stage to the end have enabled us to understand

the concept behind this project work.

We are also thankful to the lab assistants for their guidance and support.

Finally, all the thanks to Almighty Allah that we have come to this stage so far.

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AbstractAn inverter circuit, by using Square Wave Modulation (SWM) switching schemes is

developed to run AC utilities. Inverter is a circuit that converts a DC source to an AC source.

DC is one type of energy that is found in solar panels and can be stored in batteries for usage

in future. Semiconductor device, Metal Oxide Field Effect Transistor (MOSFET) is used as

switch in full bridge (H-Bridge) inverter configuration using Bipolar voltage switching .

Driver circuit for MOSFET is also very important as it is used to interface between controls

circuits (Low voltage part ) and inverter (High voltage part). Micro controller chip is used to

generate modulating signals. At the end of this project, the SWM output signal is developed

from the Micro-controller and applied to the MOSFET driver and the inverter .

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INTRODUCTION

Electricity is the major source of power for country's most of the economic activities. But in

Bangladesh, we have been suffering for the electricity crisis for a long time. To reduce this

problem, there are some alternative ways which can help in this purpose. This report focuses

on DC to AC power inverters, which aim to efficiently transform a DC Power source to a

high voltage AC source, similar to power that would be available at an electrical wall outlet.

Inverters are used for many applications, as 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 car

battery to run a laptop, TV or cell phone. 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 square wave modulation. Another method to complete the

desired outcome would be to first convert the low voltage DC power to AC, and then use a

transformer to boost the voltage to 220 volts. This project focused on the first method

described and specifically the transformation of a high voltage DC source into an AC output.

Generally these kinds of projects like solar panels are mostly dependent on the DC

appliances. Here, DC sources are converted to the AC source by using an inverter. For that

reason, our objective is to design a square wave inverter which can be used in the Solar Home

System at an affordable cost.

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OBJECTIVESThe Objectives of our project is to design an inverter that is drived by battery and can be

used to operate AC loads while minimizing the conventional inverter cost and complexity

using Micro- controller. Our system’s main properties are –

Generation of a square wave signal from a DC rechargeable battery.

Reduction of circuit’s complexity by using micro-controller to generate modulating

signals.

METHODOLOGY

In this project, we have used Battery as DC source. In this Inverter, we have used 8

MOSFETs switches. This MOSFETs are used to convert the DC source to AC source.

Besides we have used gate drivers to conduct those MOSFET switches . To generate

modulating signals, we have implemented micro-controller where two modulating signals are

used to run those MOSFETs switches . To get 220 v AC , we have used a step – up

transformer . We use a battery charging circuit to charge the Battery. We also use a control

circuit to isolate the inverter portion from the line source.

Figure 1: Block Diagram of the System

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DC source

Power Circuit Transformer

Load

Gate Drive

Micro controller

GENERAL BLOCK DIAGRAMThis block diagram shows that we are giving DC supply as input and we are getting the AC

signal with square wave form.

Fig 2: Wave form Conversion

INVERTERInverter or power inverter is a device that converts the DC sources to AC sources. Inverters

are used in applications such as adjustable-speed ac motor drivers; uninterruptible power

supplies (UPS) and ac appliances run from an automobile battery.

Power inverters produce one of the three different types of wave output:

Square Wave

Modified Square Wave (Modified Sine Wave)

Pure Sine Wave (True Sine Wave)

The three different wave signals represent three different qualities of power output. Square

Wave inverters result in power delivery that is efficient for running low power devices.

Square wave inverters were the first type of inverters made.

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MOSFET DRIVERSWhen utilizing N-Channel MOSFETs to switch a DC voltage across a load, the drain

terminals of the high side MOSFETs are often connected to the highest voltage in the system.

This creates a difficulty, as the gate terminal must be approximately 10V higher than the

drain terminal for the MOSFET to conduct . Often , integrated circuit devices known as

MOSFET drivers are utilized to achieve this difference through charge pumps or

bootstrapping techniques. These chips are capable of quickly charging the input capacitance

of the MOSFET quickly before the potential difference is reached, causing the gate to source

voltage to be the highest system voltage plus the capacitor voltage, allowing it to conduct. A

diagram of an N-channel MOSFET with gate, drain, and source terminals is shown in below

Figure.

Fig 3: N-channel MOSFET’s

There are many MOSFET drivers available to power N-Channel MOSFETs through level

translation of low voltage control signals into voltages capable of supplying sufficient gate

voltage. Advanced drivers contain circuitry for powering high and low side devices as well

as N and P-Channel MOSFETs. In this design, all MOSFETs are N-Channel due to their

increased current handling capabilities. To overcome the difficulties of driving high side N

Channel MOSFETs, the driver devices use an external source to charge a bootstrapping

capacitor connected between Vcc and source terminals. The bootstrap capacitor provides gate

charge to the high side MOSFET. 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

name bootstrap component refers to this process and how the MOSFET acts as if it is

“pulling itself up by its own boot strap”.

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Microcontroller

CD4047 is a low power IC capable of operating in either in astable or monostable mode.

Here it is wired in astable mode. It works by charging a capacitor (C2) through a resistor

(RV1) as in every astable multivibrators. Variable resistor (RV1) is provided for adjusting the

output frequency to exact 50Hz. The time period of the oscillation is given by the relation

T = 4.40*R*C. For detailed working of the IC. CD4047 has two outputs (pins 10 and 11)

which are complementary to each other. These square wave pulses are generated by this two

pin which is connected to the gate terminal of MOSFET’s. The gate voltage is work as

switching device whether the MOSFETs are on or off. The LM7808 voltage regulator is used

to provide constant 8V for the IC.

Fig 4: CD4047 chip

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Implementing the Design

This is a full diagram of our project where all the components are visible and how the

components are connected . Here we divided our total circuit into 4 portion . In working

Procedure session all the circuits are described.

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Circuit description

Battery Charging Circuit

Fig 6: Battery Charging Diagram

In battery charging circuit we use a step down transformer (rating: 220v – 18v). A bridge

rectifier is connected in the secondary terminal of transformer. A capacitor is connected in

parallel and a resistor of 1Ω 10w is connected in series. Then a DC rechargeable battery is

connected in Parallel . Transformer primary terminal is connected with line voltage.

Control Circuit

Fig 7: Control Circuit

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In control circuit two 6v relay is operated by a LM7808 voltage regulator IC. A 22Ω resistor

are connected each coil of relay. The relay contact at 6 v. In Inverted mood the relays are in

normal condition, one terminal is normally closed and another one is normally open. When

relay operates the close terminal becomes open and the open one is closed. In that time the

Inverter portion isolate from the total circuit. Then the line voltage goes to load.

Logic Circuit

Fig 8: Logic Circuit

Logic circuit is used to control the gate voltage at the gate of MOSFETs. A LM7808 voltage

regulator IC is used to operate the chip. Two outputs ( pin 10 and pin 11) are used for

modulated signal which passes the square waveform to the gate of the MOSFETs.

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Power circuit

Fig 9: Power Circuit

We use 8 MOSFETs to generate the power. 4 in series which are connected with another

four. Each MOSFETs source is connected with a 5A fuse. If more than 5A current is passes

through the MOSFETs the fuse blown up and protect the MOSFETs from damaged. All the

drain terminal are connected to the transformer primary coil(12v-0-12v) side. The centre

terminal is connect in battery positive terminal. A 25A fuse is used to protect the whole

power circuit from damaged. In secondary terminal of transformer we got the 220v AC

signal.

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Wave Form

We made this inverter for 20mS where our frequency is 50Hz. Every half cycle is worked for

10mS. We got square wave in the output of microcontroller chip, by filtering the wave signal

We got quasi square wave form.

Fig 10: Wave form of micro controller

Voltage spikesAnother complication of the fairly high harmonic content in the output of square wave

inverters is that appliances and tools with a fairly inductive load impedance can develop

fairly high voltage spikes due to inductive - back EMF. These spikes can be transformed back

into the circuit where they have the potential to damage the MOSFETs and their driving

circuitry. For this reason that many inverters have a pair of high-power zener diodes

connected across the MOSFETs the zeners conduct heavily as soon as the voltage rises

excessively, protecting the MOSFETs from damage. Or there are transistors with build in

diode to protect from these back voltages.

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Circuit Overview

Frequency stability

Although most appliances and tools designed for mains power can tolerate a small variation In supply frequency, they can malfunction, overheat or even be damaged if the frequency changes significantly. Examples are electromechanical timers, clocks with small synchronous motors, turntables in older. And many reel-to-reel tape recorders. To avoid such problems, most DC-AC inverters include circuitry to ensure that the inverter’s output frequency stays very close to the nominal mains frequency: 50Hz, or 60Hz. in some inverters this is achieved by using a quartz crystal oscillator and divider system to generate the master timing for the MOSFET drive pulses. Others simply use a fairly stable oscillator with R-C timing, fed via a voltage regulator to ensure that the oscillator frequency doesn’t change even if the battery voltage varies quite widely in our project we programmed IC which is called PIC to give me SPWM with frequency 50Hz.

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EfficiencyIt is not possible to convert power without losing some of it (it's like friction). Power is lost

in the form of heat. Efficiency is the ratio of power out to power in, expressed as a

percentage. If the efficiency is 90 percent, 10 percent of the power is lost in the inverter. The

efficiency of an inverter varies with the load. Typically, it will be highest at about two thirds

of the inverter's capacity. This is called its "peak efficiency." The inverter requires some

power just to run itself, so the efficiency of a large inverter will below when running very

small loads in a typical home, there are many hours of the day when the electrical load is

very low. Under these conditions, an inverter's efficiency may be around 50 percent or less.

Because the efficiency varies with load, don't assume that an inverter with 93 percent peak

efficiency is better than one with 85 percent peak efficiency. If the 85 percent efficient unit is

more efficient at low power levels, it may waste less energy through the course of a typical

day.

Further scope

We are currently two using 12V 7wh batteries to derive this converter. Its capacity is about

300W now. It can run over half an hour to serve that much load that stated before. By adding

more batteries in parallel or liquid cell large batteries we can enhance the backup time much

more than it is now. We can also add a solar panel and also can make an charging circuit to

convert solar energy to DC power to charge its batteries.

Conclusion

The main aim of our project work we have achieved that is converting the DC voltages into

AC voltage . We were successful to have output of 300watt at the frequency of 50 Hz .By

using this , we derived a tungsten bulb of 100 watt and a fan of 60 watt . By achieving this

success , we are quite confident to apply this experience for our daily appliances through

using the input as Photovoltaic source at cheap cost .

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