A Report on Mosfet Based

1. INTRODUCTION

Most of the UPS available in the market use frequency ranging from

100Hz to 50KHz.The regulation of output voltage is done using the pulse width

modulation technique, which produces a quasi-square waveform output from the

inverter transformer. Such an output waveform produces lots of noise, which is not

desirable. This voltage waveform can drive a computer, but not the tube light, fan,

TV, VCR etc. properly. The advantage offered by this UPS is that its changeover

period is quite low, so that the computer or any other sensitive load is not

interrupted during mains failure.

EPS (emergency power supply) of various brands, providing 50Hz square-

wave output, can drive the computer, tube light, TV, VCR, fan etc., but

considerable noise is produced from the EPS or the load. Another drawback of an

EPS is that its changeover period is relatively high, so the computer may get reset

or continuity of the play mode of the VCR may get interrupted on mains failure.

Advantage offered by our UPS over Conventional UPS

1. Its changeover period is quite low (< 1 millisecond) as switching speed of

MOSFET is high.

2. It produces less noise.

3. It is smaller in size.

4. It occupies less space.

5. It has longer life.

6. It has high efficiency.

7. These devices will not last as long as would on pure sine wave inverters.

For the money the modified square wave inverter will cost more in the long

run by reduced efficiency and possible damage to appliances.

1.1 Advantages of MOSFET over conventional Transistor:

1. MOSFET is a unipolar device, depending only on majority charge carriers,

Transistor is bipolar device

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2. It has high input resistance(10^10 to 10^15 ohms) due to reverse bias at

input whereas transistor has forward bias at input

3. It is a voltage controlled device and not current controlled like transistor

4. It is less noisy than transistors because no junctions are present (so partition

noise is absent).

5. It is less affected by radiation

6. It has very high power gain

7. It has better thermal stability

1.2 Circuit:

1. When mains is present and is within the specified limits, the same is fed to

the load. At the same time, battery is charged.

2. If mains voltage goes below 170 volts (or mains power fails) or above 270

volts, system changes over from mains to back-up mode.

3. In the back-up mode, battery voltage of 12V DC is converted into 230V

AC and applied to the load within 1 millisecond.

4. However, if battery voltage drops below 10V DC, or output voltage goes

below 225V AC, there will be a visual and audible indication of low-

battery state.

5. After the end of back-up time, system switches off automatically, due to

activation of battery’s deep discharge cut-out circuit, which reduces the

power consumption from the battery to a negligible value (only 90mA).

1.3 UPS:

It is abbreviated as uninterruptible power supply or uninterruptible power

source.

It is an electrical apparatus that provides emergency power to a load. When the

input power source i.e., utility mains fails. The on-battery runtime of most

uninterruptible power sources is relatively short—5–15 minutes. It allows

sufficient time to bring an auxiliary power source on line, or to properly shut down

the protected equipment. UPS units range in size from units designed to protect a

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single computer without a video monitor (around 200 VA rating) to large units

powering entire data centers, buildings, or even cities.

Fig 1.1 : Emergency Power Supply System

1.4 EPS:

Emergency power systems are a type of system, which may include lighting,

generators, fuel cells and other apparatus, to provide backup power resources in a

crisis or when regular systems fail. They find uses in a wide variety of settings

from residential homes to hospitals, scientific laboratories, data

centers, telecommunication equipment and modern naval ships. Emergency power

systems can rely on generators, deep cycle batteries, flywheel energy storage

or hydrogen fuel cells for supply of energy.

1.5 Quasi Square Wave:

Quasi Square Wave or Modified Square Wave actually means the same

thing. Modified square wave is the actual wave form. Quasi Square Wave is a

marketing term used by many low cost inverter manufactures. 

Inverters convert Direct Current DC to Alternating Current AC. Old obsolete

inverter technology created square wave output. As stated by the name the wave

form is square not sinus as required to have pure sine wave AC. Modified square

wave has a step or dead space between the square waves. This reduces the

distortion or harmonics that causes problems with electrical devices. Modified

Square Wave inverters will work fine for pure resistive loads, like lamps or

heaters. It will also work well with pure inductive loads, like universal motors in

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mixers and blenders. If these devices have electronic speed control this could be

damaged. Devices that have transformers in their power supplies, Microwave

Ovens, TVs, Computers etc. will run hotter and less efficient. These devices will

not last as long as would on pure sine wave inverters. For the money the modified

square wave inverter will cost more in the long run by reduced efficiency and

possible damage to appliances.

Fig 1.2: Quasi square wave

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2. COMPONENTS IN THE CIRCUIT:

2.1 Resistor:

A resistor is a two-terminal electronic component that produces

a voltage across its terminals that is proportional to the electric current through it in

accordance with Ohm's law:

V = IR

The primary characteristics of a resistor are the resistance, the tolerance, the

maximum working voltage and the power rating. The power dissipated by a

resistor (or the equivalent resistance of a resistor network) is calculated using the

following:

2.2 Capacitor:

A capacitor (formerly known as condenser) is a passive electronic

component consisting of a pair of conductors separated by a dielectric (insulator).

When there is a potential difference (voltage) across the conductors, a

static electric field develops in the dielectric that stores energy and produces a

mechanical force between the conductors. An ideal capacitor is characterized by a

single constant value, capacitance, measured in farads. This is the ratio of

the electric charge on each conductor to the potential difference between them.

Capacitors are widely used in electronic circuits for blocking direct current while

allowing alternating current to pass, in filter networks, for smoothing the output

of power supplies, in the resonant circuits that tune radios to

particular frequencies and for many other purposes.

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2.3 Diode:

Fig 2.1: Diode

Diode is a two-terminal electronic component that conducts electric

current in only one direction. The most common function of a diode is to allow an

electric current to pass in one direction (called the diode's forward bias direction)

while blocking current in the opposite direction (the reverse direction). This

unidirectional behaviour is called rectification.

2.4 Transistor:

A transistor is a semiconductor device used to amplify and

switch electronic signals. It is made of a solid piece of semiconductor material,

with at least three terminals for connection to an external circuit. A voltage or

current applied to one pair of the transistor's terminals changes the current flowing

through another pair of terminals. Because the controlled (output) power can be

much more than the controlling (input) power, the transistor

provides amplification of a signal. 

Fig 2.2: Transistor schematic diagram

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2.5 Relay:

A relay is an electrically operated switch. Many relays use an

electromagnet to operate a switching mechanism mechanically, but other operating

principles are also used. Relays are used where it is necessary to control a circuit

by a low-power signal (with complete electrical isolation between control and

controlled circuits), or where several circuits must be controlled by one signal.

2.6 Transformer:

A transformer is a device that transfers electrical energy from one circuit to

another through inductively coupled conductors—the transformer's coils. A

varying current in the first or primary winding creates a varying magnetic flux in

the transformer's core and thus a varying magnetic field through

the secondary winding. This varying magnetic field induces a

varying electromotive force (EMF) or "voltage" in the secondary winding. This

effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the

secondary winding and electrical energy will be transferred from the primary

circuit through the transformer to the load. In an ideal transformer, the induced

voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp),

and is given by the ratio of the number of turns in the secondary (Ns) to the number

of turns in the primary (Np) as follows:

Fig 2.3: Step Up Transformer

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2.7 MOSFET:

Fig 2.4 : symbol of MOSFET

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-

FET, or MOS FET) is a device used for amplifying or switching electronic signals.

In MOSFETs, a voltage on the oxide-insulated gate electrode can induce

a conducting channel between the two other contacts called source and drain. The

channel can be of n-type or p-type , and is accordingly called an n-MOSFET or a

p-MOSFET (also commonly n-MOS, p-MOS).

Fig 2.5: Structure of MOSFET

When a voltage is applied across a MOS structure, it modifies the

distribution of charges in the semiconductor. If we consider a P-type

semiconductor (with NA the density of acceptors, p the density of holes; p = NA in

neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates

a depletion layer by forcing the positively charged holes away from the gate-

insulator/semiconductor interface, leaving exposed a carrier-free region of

immobile, negatively charged acceptor ions (see doping (semiconductor)). If VGB is

high enough, a high concentration of negative charge carriers forms in an inversion

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layer located in a thin layer next to the interface between the semiconductor and

the insulator.

2.7.1 MOSFET as a Switch:

MOSFET switches use the MOSFET channel as a low–on-resistance

switch to pass analog signals when on, and as a high impedance when off. By

applying a suitable drive voltage to the Gate of an MOSFET the resistance of

the Drain-Source channel can be varied from an "OFF-resistance" of many

hundreds of kΩ's, effectively an open circuit, to an "ON-resistance" of less than

1Ω, effectively a short circuit. We can also drive the MOSFET to turn "ON" fast or

slow, or to pass high currents or low currents. This ability to turn the power

MOSFET "ON" and "OFF" allows the device to be used as a very efficient switch.

Fig 2.6: MOSFET as Switch

2.8 NE555 Timer:

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Fig 2.7: NE555 Timer

The 555 Timer IC is an integrated circuit (chip) implementing a variety

of timer and multi-vibrator applications. The 555 has three operating modes:

1. Mono-stable mode: in this mode, the 555 functions as a "one-shot".

Applications include timers, missing pulse detection, bounce free switches,

touch switches, frequency divider, capacitance measurement, pulse-width

modulation (PWM) etc.

2. Astable - free running mode: the 555 can operate as an oscillator. Uses

include LED and lamp flashers, pulse generation, logic clocks, tone

generation, security alarms, pulse position modulation, etc.

3. Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the

DIS pin is not connected and no capacitor is used. Uses include bounce free

latched switches, etc.

Pin Name Purpose

1 GND Ground, low level (0 V)

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2 TRIG OUT rises, and interval starts, when this input falls below 1/3 VCC.

3 OUT This output is driven to + V CC or GND.

4 RESET A timing interval may be interrupted by driving this input to GND.

5 CTRL "Control" access to the internal voltage divider (by default, 2/3 VCC).

6 THR The interval ends when the voltage at THR is greater than at CTRL.

7 DIS Open collector output; may discharge a capacitor between intervals.

8 V+, VCC Positive supply voltage is usually between 3 and 15 V.

Table 2.1: Pin configuration of NE555 Timer

Fig 2.8: Pin Diagram of NE555 Timer

In this project, NE555 timer is used as square wave oscillator that is in astable mode.

2.8.1 Astable mode:

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Fig 2.9: Standard 555 Astable Circuit

In astable mode, the '555 timer ' puts out a continuous stream of rectangular

pulses having a specified frequency. Resistor R1 is connected between VCC and the

discharge pin (pin 7) and another resistor (R2) is connected between the discharge

pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common

node. Hence the capacitor is charged through R1 and R2, and discharged only

through R2, since pin 7 has low impedance to ground during output low intervals of

the cycle, therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on the values of R1,

R2 and C:

[7]

The high time from each pulse is given by

And the low time from each pulse is given by

Where R1 and R2 are the values of the resistors in ohms and C is the value of the

capacitor in farads.

To achieve a duty cycle of less than 50% a diode can be added in parallel with

R2 towards the capacitor. This bypasses R2 during the high part of the cycle so

that the high interval depends only on R1 and C1.

2.9 7473 dual JK flip flop:

In digital circuits, a flip-flop is a term referring to an electronic circuit (a

bistable multivibrator) that has two stable states and thereby is capable of serving

as one bit of memory.

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A flip-flop is usually controlled by one or two control signals and/or a gate

or clock signal. The output often includes the complement as well as the normal

output.

In JK flip-flop the combination J = 1, K = 0 is a command to set the flip-flop;

the combination J = 0, K = 1 is a command to reset the flip-flop; and the

combination J = K = 1 is a command to toggle the flip-flop, i.e., change its output

to the logical complement of its current value. Setting J = K = 0 will hold the

current state.

The characteristic equation of the JK flip-flop is:

and the corresponding truth table is:

JK Flip Flop operation [8]

Characteristic table Excitation table

J K Qnext Comment Q Qnext J K Comment

0 0 Qprev hold state 0 0 0 X No change

0 1 0 Reset 0 1 1 X Set

1 0 1 Set 1 0 X 1 Reset

1 1 Qprev Toggle 1 1 X 0 No change

Table 2.2 : Operation of JK Flip-flop

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7473 contains two independent JK flip-flops with clear inputs. Clear inputs allow us to

reset the flip-flop at any time.

Fig 2.10 : Pin Configuration of 7473

2.10 7812 voltage regulator:

Fig 2.11: 7805 voltage regulator

The heart of the voltage conversion circuit is a small integrated circuit type

7805 which has three legs. The 78xx series voltage regulators are easily obtained

and are quite cheap. The xx part signifies the voltage e,g 7812, 7815. In our case

we need the 7805 which gives a regulated voltage of 5 volts DC from an

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unregulated supply which must be over 5 volts. There are several versions. We

need the 1 amp version.

78xx series ICs do not require any additional components to provide a constant,

regulated source of power, making them easy to use, as well as economical, and

also efficient uses of circuit board real estate. By contrast, most other voltage

regulators require several additional components to set the output voltage level, or

to assist in the regulation process. Some other designs can require not only a large

number of components but also substantial engineering expertise to implement

correctly as well.

78xx series ICs have built-in protection against a circuit drawing too much

power. They also have protection against overheating and short-circuits, making

them quite robust in most applications. In some cases, the current-limiting features

of the 78xx devices can provide protection not only for the 78xx itself, but also for

other parts of the circuit it is used in, preventing other components from being

damaged as well.

2.11 DB107 Bridge rectifier:

A diode bridge rectifier is a device for converting AC, or alternating current

electricity into DC, or direct current. AC electricity goes from positive to negative

constantly, whereas DC always flows in the same direction. AC electricity is more

efficient to transmit and easier to convert into different voltages, so power is

transmitted from the station to your house as AC. However, most appliances need

DC current to run. A diode bridge rectifier is the most common type of power

supply used to provide this current.

The diode bridge is the heart of the diode bridge rectifier. Diodes work like

electric one-way valves, only letting a positive or negative charge through,

depending on the direction in which they are wired. The wire coming from the

transformer has two ends. Because the current is AC, when one end of the wire is

negative, the other is positive. 

Each end of the wire is attached to two diodes which shunt the electricity into

positive and negative DC wires. So if end A has a negative charge and end B has a

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positive charge, the diode between end A and the negative DC wire opens while

the diode between end B and the positive wire opens. When the electricity switches

direction, the diode between end A and the positive wire opens and the diode

between end B and the negative wire opens.

2.12 LM358N Dual Op-amp:

An Operational amplifier ("op-amp") is a DC-coupled high-gain electronic

voltage amplifier with a differential input and, usually, a single-ended output.[1] An

op-amp produces an output voltage that is typically hundreds of thousands times

larger than the voltage difference between its input terminals.[2]

Fig 2.12 : differential amplifier

The amplifier's differential inputs consist of a   input and a   input, and

ideally the op-amp amplifies only the difference in voltage between the two, which

is called the differential input voltage. The output voltage of the op-amp is given

by the equation,

where   is the voltage at the non-inverting terminal,   is the voltage at the

inverting terminal and AOL is the open-loop gain of the amplifier. (The term "open-

loop" refers to the absence of a feedback loop from the output to the input.) The

magnitude of AOL is typically very large—10,000 or more for integrated circuit op-

amps—and therefore even a quite small difference between   and   drives the

amplifier output nearly to the supply voltage. This is called saturation of the

amplifier. 

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Fig 2.13: Pin configuration of LM358N

3. DESCRIPTION OF THE CIRCUIT

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Fig 3.1: circuit diagram

3.1 PARTS OF THE CIRCUIT

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1. Inverter Control Circuit

2. Power Output stage

3. Charger Circuit

4. Under and Over Voltage cut-out Circuit

5. Low Battery Indicator

6. Battery Deep Discharge Cut-out circuit

3.1.1 Inverter Control Circuit:

1. It uses the basic square-wave ( astable multivibrator) oscillator employing

IC 555

2. Works with 5.1V supply voltage derived from 12V battery by using 5.1V

zener ZD3 in series with a resistance.

3. It is designed for a frequency of 100 Hz, which can be varied above or

below 100 Hz using pre-set PR1.

Fig 3.2 : Inverter control circuit

The frequency ‘f’ of astable multivibrator is given by the relationship.

f= 1.44/(Ra+2Rb)C

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Where Rb = In-circuit resistance of pre-set PR1

If Ra=220 ohms and R =15 kilo-ohms, then f=100 Hz

4. The output of the astable multivibrator is given to pin 5 of the bistable

multivibrator wired around IC 7473, which produces the two 50Hz square-

wave outputs at its pins 8 and 9 with a phase difference of 180degrees

between the two.

5. One of the outputs is coupled to the base of transistor T1 through diode D1

and series current-limiting resistor R3, while the second output is given to

the base of transistor T2 through diode D2 and series resistor R4.

6. Transistors T1 and T2 act as MOSEFT drivers.

3.1.2 Power Output Stage:

1. The collector of transistor T1 is connected to the gates of MOSFETs M1

through M3 (referred to as bank 1), while that of transistor T2 is connected

to the gates of MOSFETs M4 through M6 (referred to as bank 2).

2. MOSFETs M1 through M3 are connected in parallel gates of MOSFETs

M1 through M3 and those of MOSFETs M4 through M6 are made

common.

3. Similarly, drains and sources of MOSFETs in each bank are paralleled.

4. Drains of MOSEFTs of one bank are connected to one extreme taping of

9-volt primary of the inverter transformer X1, and that of the MOSEFTs of

the second bank are connected to the other extreme 9-volt taping of the

same transformer.

5. Centre tap of the primary is directly connected to the positive terminal of

12V, 7Ah battery. Capacitor C2 is connected across the secondary of the

inverter transformer, either directly or via inductor L1 (wound on the same

core as ex-tension of secondary winding), using sine/square slide switch

SW2.

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Fig 3.3: Power output stage

6. When mains power fails, relay gets de-energised and12V battery supply is

fed to the control circuit through top contacts of the relay to produce

square-wave outputs at pin Nos. 8 and 9 of IC 7473 with a frequency of 50

Hz. At any instant, if voltage at pin 8 of IC2is +5V, the voltage at pin 9

ofIC2 is 0V, and vice versa. Therefore, when transistor T1 conducts,

transistor T2 is cut-off, and vice versa. When transistor T1 conducts, the

voltage at collector of transistor T1 drops to 0.7 V, and therefore

MOSFETs of bank 1 remain cut off while collector of transistor T2 is at

5V. Thus, MOSFETs of bank 2 conduct and the current flows through one-

half of inverter transformer X1 primary. During the next half cycle, vice-

versa happens.

7. In this way, two banks of the MOSFETs conduct alternately to produce

230V AC, 50Hz across the secondary of the inverter transformer X1.

Inductance L1 and capacitor C2 on the secondary side produce a waveform

approaching a sine-wave.

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3.1.3 Charger Circuit:

1. This circuit comprises step-down transformer X2, followed by rectifier,

regulator, and double changeover 12V relay RLY. Mains supply of 230V

AC is applied across the primary of the transformer through triac BT136.

2. The gate of the triac is connected to the output of over-/under-voltage cut-

off circuit. As long as the mains voltage is between 170 and 270 volts, +5V

is provided to the gate of the triac and hence it conducts. If AC mains

voltage goes out of the above-mentioned limits, the gate voltage falls to 0.7

volt and the triac does not conduct.

3. When traic conducts, 16-0-16V AC voltage is developed across the

secondary of X2. It is converted into DC voltage by the diodes D3 and D4,

and the rectified output is given to the input of the 12V regulator

7812(IC3). The output of the regulator is connected across the relay coil

through series resistor R7.

4. When mains voltage is within the range of 170-270 volts, relay activates. In

this mode, mains voltage is directly routed to the load through N/O contacts

(lower) of relay RLY and 4-amp rated contact breaker (CB)

5. LED2 indicates that the system is on mains.

6. At the same time, the rectified voltage from diodes D3' and D4‘ is made

available through N/O contacts (upper) of relay RLY for charging the

battery via charging resistance R.

7. If the mains supply fails or goes out of the range of 170-270 volts, relay

de-energises and the battery supply of 12V is connected to the inverter

circuit through N/C contacts (upper). The volt-age developed by the

inverter goes to the output socket of UPS through the N/C contacts via 4-

amp CB.

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3.1.4 Under and Over Voltage Cut-out Circuit:

1. The 230V AC main is stepped down to 12V AC, using transformer X3. It is

rectified by the bridge rectifier and filtered by two Pi section filters to

reduce the level of ripple voltage. The filtered DC voltage is applied to dual

op-amp IC5 (used as dual comparator).

2. The reference voltage for the comparators is developed across zener diode

ZD4, which is connected to the filtered DC positive rail via resistor R9.

3. Even if the AC mains voltage varies between 170V and 270V, the voltage

across zener ZD4 remains constant at 5.1 volts. The cathode of zener diode

ZD4 is connected to the inverting input of the comparator IC5(b) and non-

inverting input of the comparator IC5(a)

.

Fig 3.4: under and over voltage cut-out circuit

4. Pre-set PR2 can be used to vary the inverting terminal voltage of the

comparator IC5 (a) above and below the reference voltage of 5.1V.

Similarly, non-inverting input of the comparator IC5 (b) can also be varied

above and below the 5.1V reference voltage applied to the inverting input

of IC5 (b), using pre-set PR3.

5. Pre-set PR2 is adjusted such that when AC mains voltage goes below 170

volts, the voltage at inverting input of comparator IC5 (a) goes below 5.1

volts, so that its output goes high. As a result, transistor T3 conducts and its

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collector voltage (connected to the gate of triac TR) drops to 0.7 volt, and

hence the triac cuts off. These causes relay RLY to de-energise and the

system changes over to back-up mode of operation. Glowing of LED3

indicates the under-voltage condition.

6. Similarly, pre-set PR3 is adjusted to operate in similar way when mains

voltage goes above 270V AC. The over-voltage indication is shown by

glowing of LED4.

7. Diodes D6 and D7 act as 2-input wired-OR gate for combining the outputs

from the two comparators and prevent the output of one comparator going

into the output terminal of other comparator.

8. Zener diode ZD6 is used to limit the gate voltage of the traic to 5.1 volts.

3.1.5 Low Battery Indicator:

1. This circuit is wired around op-amp µA741 (IC6), which functions as a

comparator here.

2. Battery voltage is applied across pins 7 and 4. Voltage at non-inverting

input of IC6 is maintained constant at 5.1 volts by zener diode ZD5 and

series resistor R17. Voltage at the inverting input of IC6 can be varied

above and below 5.1 volts using pre-set PR4. Pre-set PR4 is adjusted in

such a way that if battery voltage goes below 10V, the voltage at inverting

input goes below 5.1 volts, so that output voltage at pin 6 of IC6 goes high

(about 10 V). Hence, LED5 glows and produces intermittent sound from

the buzzer, indicating low-battery status.

3.1.6 Battery Deep Discharge Cut out Circuit:

1. If the UPS system keeps operating in the inverter mode, the battery voltage

will drop eventually to prohibitively low level (say, 5 volts). If such

condition occurs frequently, the life of the battery will be considerably

reduced. To remove this drawback, it is necessary to use battery deep-

discharge cut-out circuit. If battery voltage goes below 9.5 volts, this circuit

will cause the UPS to shut down, which prevents the battery from further

discharge.

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2. This circuit is also built around op-amp µA741 (IC7) working as a

comparator. Voltage at non-inverting input of IC is 5.1 volts, which is kept

constant by zener diode ZD6 and resistor R20. Pre-set PR5 is adjusted in

such a way that if battery voltage goes below 9.5 volts, IC7 output would

go high to turn on SCR. Once SCR conducts, the supply voltage for control

circuit drops to near 0V. As a result, the control circuit is unable to produce

gate drive pulses for the two MOSFET banks and the inverter stops

producing AC output.

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4. PROTECTION CIRCUITS

1. Reverse Battery Protection

2. No-Load Protection

3. Over-heating Protection

4. Protection from Damage to load due to Spikes

4.1 Reverse battery protection:

A 16A to 20A diode (D8) in conjunction with fuse F2 provides reverse battery

protection in case battery is connected with reverse polarity. In case of reverse

polarity, fuse F2 will blow and battery supply to the circuit will be immediately

switched off.

4.2 Protection against no-load:

An optional circuit for ‘no load‘ condition, during which the output voltage

may shoot up to 290V AC or more The rectifier and filter used are identical to that

of under voltage or over-voltage protection circuit, while the comparator circuit is

identical to over-voltage comparator. And hence, no separate explanation is

required to be included. The output of the circuit is connected to the gate of SCR1.

4.3 Spike suppression:

Since TRIAC TR is connected in series with the primary of charging

transformer X2 and gate voltage is obtained from the under-/over-voltage cut-out, a

spike is treated on par with the over-voltage (>270V) condition. If mains voltage

spike goes above 270V AC, gate voltage of triac TR becomes 0.7 volt, and hence

triac does not conduct. As voltage across the coil of relay RLY is zero, the relay is

de-energised and system changes over to back-up mode during voltage spike

period. Thus the load is protected from the voltage spikes in the mains.

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5. FLEXIBILITY

1. Increase Back Up Time.

2. Square/Sine-wave Output Selection

3. Increase Output Power.

5.1 Back-up time:

Using a single battery of 12V, 7Ah with a load (100 to 120W) comprising

computer along with colour monitor, the back-up time is 10 to 15 minutes with

square-wave output (half with sine-wave output). With a battery of 12V, 180Ah, the

back-up time is 4 to 5 hours with square-wave output (2 to 2.5 hours with sine-wave

output).

5.2 Charging resistance:

For 12V/7Ah battery, charging resistance Rc should be 10 ohms/20 watts so

that the battery will not be heated during charging. Similarly, for 12V/90Ah battery,

charging resistance Rc should be 4.7 ohms/25 watts, and for 12V/180Ah battery,

3.3 ohms/30 watts.

5.3 Square/Sine-wave output selection:

The selection of sine wave or square wave is done using slide switch SW2.

In square-wave position, capacitor C2 is directly shunted across 230V terminal of

secondary transformer X1, while in sine-wave position, coil L1 (extension of 230V

secondary marked 600V) ia added in series with capacitor C2 to resonate at 50Hz.

The power consumption from the battery increases in sine-wave output position of

switch SW2, indicating comparative values shown in the graphs.

5.4 Output power:

Using six MOSFET’s (three per bank) with proper heat sinks with inverter

transformer of 16 amp primary rating, the EPS cum UPS provides power upto250

watts. Using the same circuit, if we use ten MOSFET’s (five per bank) and inverter

transformer of 32 amp primary rating, the power of the UPS cum EPS can be

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increased to 500 watts or 625VA.

The transformer ratings as mentioned are applicable for square-wave

output. The transformer rating will be 25 percent higher in case of sine-wave

output. Also the number of MOSFET’s per bank should also be considerably

higher.

5.5 Applications:

It can be used

1. Domestically to protect computers etc.

2. In data centre’s.

3.  in telecommunication equipment or other electrical equipment.

4. In any place where an unexpected power disruption could cause

injuries, fatalities, serious business disruption and/or data loss.

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6. CONCLUSION

Our project overcomes the disadvantages of the conventional UPS and EPS

which are available in the market. Here we have presented a prototype which can

further be converted for large scale use. The advantage of our UPS is that its

changeover period is quite low due to the usage of MOSFETs instead of

conventional transistors. It has the capacity to replace the inverters being used

presently for household purposes since it produces sine wave output with very low

noise level. Also has an advantage of when the mains is present and is within the

specified limits, the same is fed to the load. At the same time battery is charged. If mains

voltage goes below 170V or above 280V system changes over from mains to backup

mode. As the cost of the MOSFET based switches is less, the cost decreases and is more

efficient than the conventional ups.

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FUTURE SCOPE

Due to the low power consumption by MOSFET and due to the low change

over time of the system the conventional ups is getting replaced by MOSFET

based ups in the market. The IGBT based ups which has low power consumption

compared to the MOSFET, but it cannot compete the switching speed of the

MOSFET.

So MOSFET based ups are coming into existence in the market.

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REFERENCES

1. Electronics For You magazine, October 2010, Pg.no: 157

2. http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/index.html

3. http://www.educypedia.be/electronics/javaanalogsemi.htm

4. http://en.wikipedia.org/wiki/78xx

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