Chapter-5 - Yolakavediasir.yolasite.com/resources/Chapter-5-Power... · Web viewIn SMPS, switching transistor is used, which turn On and Turn Off periodically at a high frequency.

Chapter-5Power Supplies

Power distribution Computers require steady and noise free power. But many times in office or

house or factories, the power line in which computers are connected, to the same line some other instruments are also connected. On and off of such instruments like air conditioners, Xerox machines, vacuum cleaners, heaters and other motor driven applications may take large amount of current during on and off. Hence, it produces a large voltage variation or fluctuations. The voltage variations can cause voltage spikes from 800 to 1000V, which can damage computer.

In the power line, the main thing is earthling, which should be proper and the power difference between phase, neutral and earthling is as shown:

For connections of computer, always 3-pin plugs and sockets are used and use of 2-pin plugs and sockets should be avoided, as there is no facility of earthling. The wire length which supplies power should be as minimum as possible. If the length of power cord is more, the resistance will be high, which will reduce the power to computer. Hence, extension cords should be avoided. All the instruments which are having on/off cycling and which take large current should be connected to separate line or phase. Electrical daisy chain connection should be avoided.

Power protection Power protection systems as the name implies protects the equipment from the

effect of surge power and power failure. The power surge or spikes can destroy the computer equipment and loss of power can result in data loss.

The computer power supplies should be such that it provides protection from higher than normal voltage and current and should provide power line noise filtering.

The automatic shut down of computer during power disturbance is a built-in function of most high quality power supply.

Power supplies should stay within operating specifications and continue to run a system if any of these power line disturbances occur:

Voltage drop to 80v for up to 2 seconds Voltage drop to 70v for up to 0.5 seconds Voltage surge of up to 143v for up to 1 second

IBM also states that neither their power supplies nor systems will be damaged by the following occurrences:

Full power outage Any voltage drop (brownout) A spike of up to 2,500v

Other than power supplies, there are some devices used for protection of power to the systems, those are: -

1) Surge suppressor2) Line conditioner3) Backup power supply

Power Supplies A power supply (sometimes known as a power supply unit) Power Supply is a

device or system that supplies electrical energy or other types of energy to an output load or group of loads.

The power supply converts the alternating current (AC) to the regulated direct current (DC) needed by the personal computer.

A computer power supply typically is designed to convert 110-240 V AC power from the mains, to several low-voltage DC power outputs for the internal components of the computer. The most common computer power supplies are built to conform to the AT and ATX form factor.

In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord , socket for Monitor Supply Cord(optional) and the cooling fan.

There are three types of power supply:1. Linear power supply2. Switch mode power supply (SMPS)3. Uninterrupted power supply (UPS)

Linear Power Supply A simple AC powered linear power supply usually uses a transformer to convert

the voltage from the wall outlet (mains) to a different, usually a lower voltage.

Fig. shows block diagram of linear power supply, it constitutes a transformer, rectifier, filter and regulator (series or shunt type). Power supply converts AC to DC voltages. The rectifier converts the AC voltage at the transformer output into DC voltage. The regulator maintains a constant output voltage in spite of the variations in the input voltage and load current.

First a transformer is used to “down-convert” the AC line voltage to a smaller peak voltage Vm, which is usually around 2-3 Volts larger than the ultimately desired DC output.

Filter

A diode circuit rectifies the AC signal, producing a waveform with large DC component. A capacitor filter bank is then used to “smooth” or “filter” the rectified sinusoid. It rectifies the AC voltage at first to pulsating DC. Then a capacitor smooths out part of the pulses giving a type of DC voltage. The smaller pulses remaining are known as ripple.

Under normal loading conditions there is always some residual periodic variation or “ripple” in the filtered signal. If the application requires very low ripple and constant DC output over a wide range of loading conditions, then active regulation is required to further reduce or eliminate this residual ripple.

Finally, depending on the requirements of the load, a linear regulator may be used to reduce the ripple sometimes also allowing for adjustment of the output to the desired but lower voltage.

Many versions of linear power supply used by circuit designers are adjustable up to 30 volts and up to 5 amperes output.

Linear power supplies are well known for their extremely good line and load regulation; low output voltage ripple and almost negligible RFI/EMI.

Switch mode Power Supply(SMPS) A switched-mode power supply, switch-mode power supply, or SMPS, is an

electronic power supply unit that incorporates a switching regulator. Linear power supply uses a linear regulator. The linear regulator uses a

transistor biased in its active region to specify an output voltage; but in SMPS actively switches a transistor between full saturation(ON)and full cutoff (OFF) at a high rate.

In SMPS, the active device that provides regulation is always operating in a switched mode i.e. it is operated either in cut off or in saturation.

The input DC is chopped at a high frequency (10 kHz to 100 kHz) using an active device and the converter transformer.

The transferred chopped waveform is rectified and filtered. A sample of the output voltage is used as a feedback signal for drive circuit for

the switching transistor to achieve the regulation. The use of feedback mechanism to change the output voltages as per the

requirement of the load. Hence SMPS is more efficient than linear power supply.

In SMPS, switching transistor is used, which turn On and Turn Off periodically at a high frequency.

There is no power loss due to stabilization, because there is no power dissipation across the transistor, whether it is ON or OFF.

The switching rate is of the order of120 kHz.

The bridge rectifier converts the AC input voltage into pulsating DC voltage. The filter removes ripple at the output voltage ie it convert pulsating DC to Pure DC.

The switching transistor converts the DC voltage into square wave AC. The step down transformer is used to step down the voltage. This AC square wave is rectified by a full wave rectifier. The DC output which ripples from the rectifier is filtered by a filter. The DC output voltage is compared with a reference voltage by the comparator. The error voltage at the comparator output is fed to the pulse width modulator,

where a DC voltage is chopped at a high frequency. The optoisolator provides isolation for the loop against any pick up from the

main path. Optoisolator connect feedback path optically and it is electrically isolated.

The output of the optoisolator is amplified by the driver stage. The driver output controls the switching transistor.

Specification of SMPS

Voltage AC Frequency Current (Amp.)Normal MIN MAX HZ (3HZ) Maximum220V 180V 259V 50/60 75A at 180V

DC output specification:

Voltage VDC Current (amp) ToleranceNormal Min Max Min Max+5 4.8 5.25 2.3 15 5%-5 -4.6 -5.50 0 0.3 10%+12 11.25 -12.6 0.4 4.2 5%-12 -10.92 -13.2 0 0.25 10%

Most of compatible power supplies have power rating between 90 to 250 watts output for continuous operation.

Comparison between linear and switched mode power supply: Comparison factor Linear power supply SMPSSize Large SmallWeight Heavy LightEfficiency Poor, vary from 25 - 50% Good, vary from 65 - 80%EMI Low HighRipple Low, less than 5mv High, around 25mvOperating cost High LowNoise level Low LowMains performance Bulky SmallFilter capacity Bulky MediumTransformer core Laminated iron Ferrite coreAC I/P voltage Typically 200 - 240V 140 - 270VRegulation Less than 1% Less than 0.3%

AT and ATX Power Supply Early PCs using the PC/XT, AT, Baby AT and LPX (Low Profile eXtension)form

factors all use a mechanical switch to turn the computer on and off. Newer form factors, starting with the ATX/NLX, and including the SFX(Small

Form Factor) and WTX(Workstation Technology Extended), have changed the way the power supply is turned on and off.

Instead of using a physical switch, these systems are turned on By a signal from the motherboard telling the power supply what to do.

The versions popular power supply form factors are:1. AT2. ATX/NLX

.(Advanced Technology )AT Style SMPS:

AT(Advanced Technology) style computer cabinets had a front side power button that was connected to SMPS of computer system.

The wires were either soldered to the power button or blade connectors were used.

The AT-style SMPS provides a DC output on two 6-pin connectors(J1 and J2) and four (two)(MOLEX) 4-pin connectors.

The two, 6-pin connector carry DC power connections to the motherboard. It carries +5 V, -5 V, +12 V and -12 V voltages and a PGS signal as shown in Fig.

The POWER GOOD signal is a special flag to the CPU, are stable and of the POWERindicating that the output voltages usable by the CPU. In the absence GOOD signal CPU remains reset.

ATX / NLX(New Low Profile eXtended) Style SMPS: The ATX form factor is sometimes called “ATX/NLX” form factor. An ATX power supply does not directly connect to the system power button,

allowing the computer to be turned off via software. Older AT power supplies had two similar connectors that could be accidentally

switched, usually causing short-circuits and irreversible damage to the motherboard. ATX used one large, keyed connector instead, making a reversed connection very difficult.

ATX / NLX provides five DC voltages +5V, —5 V, +12 V, —12 V and +3.3 V, through a 20-pin connector. It has three special signals

PS-ON 5-VSB PW-OK

Voltage +12V This voltage is used on some types of serial port circuits, whose amplifier

circuits require both -12V and +12V. It is not needed on some newer systems, and even on older ones not very much

is used, because the serial ports require little power. Most power supplies provide it for compatibility with older hardware, but usually

with a current limit of less than 1A.

Voltage +5V A now archaic voltage, —5V was used on some of the earliest PCs for floppy

controllers and other circuits used by ISA bus cards.

It is usually provided, in small quantity (generally less than 1A), for compatibility with older hardware.

Voltage 0V Zero volts are the ground of the PC electrical system, also sometimes called

common or earth. The ground signals provided by the power supply are used to complete circuits

with the other voltages. They provide a plane of reference against which other voltages are measured.

Voltage 3.3V The newest voltage level provided by modern power supplies, it was introduced

with the ATX form factor and is now found on the ATX/NLX, SFX and WTX form factors.

It is not found in Baby AT or older form factors. It is used to run newer CPUs, as well as some types of system memory, AGP video cards, and other circuits.

Power good signal When the power supply first starts up, it takes some time for the components to

get “up to speed” and start generating the proper DC voltages that the computer needs to operate.

Before this time, if the computer were allowed to try to boot up, strange results could occur since the power might not be at the right voltage.

It can take a half-second or longer for the power to stabilize, and this is an eternity to a processor that can run half a billion instructions per second

To prevent the computer from starting up prematurely, the power supply puts out a signal to the motherboard called “Power Good” (or “Power Good”, or “Power OK”, or “PWR OK” and so on) after it completes its internal tests and determines that the power is ready for use. Until this signal is sent, the motherboard will refuse to start up the computer.

Soft power (power On and 5V standby signals)(PS-0N) Newer form factors, starting with the ATX/NLX, and including the SFX and WTX,

have changed the way the power supply is turned on and off. Instead of using a physical switch, these systems are turned on by a signal from the motherboard

telling the power supply what to do. In turn, the motherboard can be told to change this signal under software

control. This is what allows Windows to shut the power down to a PC, or what allows such features as turning a PC on from a button on the keyboard. This feature is called “Soft Power” and the signal that controls the power supply is called “Power On or alternately, “PS On” or “PS_On”.

The answer is the other “Soft Power” signal, which is called “+5 V Standby” (or “÷5VSB”, or “5VSB” etc.) This signal is the same output level as the regular ÷5 V lines from the power supply, but is independent of the other provided voltages and is always on, even when the rest of the power supply is turned off.

A small amount of current on this wire is what allows the motherboard to control the power supply when it s off.

It also permits other activities that must occur while the PC is off, such as enabling wake up from sleep mode, or allowing the PC to be turned on when activity is detected on a modem (“Wake on Ring”) or network card (“Wake on LAN”).

Power Supply CharacteristicsThe electrical characteristics of the power supply describe the quality of the power supply’s outputs, and its ability to handle special situations such as disruptions or disturbances to its input power, or variations in the loads the power supply drives.Following are the power supply characteristics

Wattage: The total maximum output of the power supply in watts. It is the nominal,

total ,maximum output power of all the voltages that the power supply provides.

Computer power supplies are rated based on their maximum output power. Typical power ranges are from 200 W to 500W.

Efficiency: The efficiency of an entity in electronics and electrical engineering is defined as

useful power output divided by the total electrical power consumed. The efficiency of SMPS is 70-85%.

Regulation: The ability of a SMPS to maintain an output voltage within specified limits under

varying input voltages and output loads.Ripple:

Also sometimes called “AC Ripple” or “Periodic and Random Deviation (PARD)” or simply “Noise”. The power supply of course produces DC outputs from AC input.

However, the output is not “pure” DC. There will be some AC components in each signal, some of which are conveyed through from the input signal, and some of which are picked up from the components in the power supply.

Typically these values are very small, and most power supplies will keep them within the specification for the power supply form factor.

Ripple values are usually given in terms of millivolts peak-to-peak (mV “Peak-to peak” refers to measuring the AC voltage from its

negative maximum to its positive maximum. Lower numbers are better.

Load regulation: Sometimes it is called voltage load regulation. This specification refers to the

ability of the power supply to control the output voltage level as the load on the power supply increases or decreases.

The voltage of a DC power source tends to decrease as its load increases, and vice-versa.

Load regulation is usually expressed as a “+/-“ percentage value for each of the voltages the power supply delivers. 3% to 5% are typical; 1% is quite good. (The -5V and -12 V signals usually are no better than ÷ 5% even on very good units; there’s no point bothering getting them better than that since they are low-current and mostly unused anyway)

Line regulation:

The complement of load regulation, this parameter describes the ability of the power supply to control its output levels as the level of the AC input voltage varies from its minimum acceptable level to its maximum acceptable level.

Again, a value for each output level is usually specified as a “+/- “ percentage. +/-1% to 2% is typical.

Power Problem

Blackout : A blackout is a complete loss of electrical power where voltage and current drop

to almost zero. Blackouts are usually caused by physical interruption in the power line due to

accidental damage by a person or act of nature. A loss of AC wills invariably shutdown the computer in a millisecond. Loosing

power may cause the loss of valuable data, reduction in productivity. In extremely rare cases, a sudden and complete power loss can corrupt file structure and damage files.

Protection against blackout is to save work regularly.Brownouts:

The under voltage condition also called brownout or sag. The high load items like air conditioners, welding machine, motor etc. draw too much current that the AC voltage level drops.

When AC voltage falls outside of the tolerable range, the power supply will fall out of regulation, resulting in intermittent system operation.

Due to this system hang, random memory errors occur or file may be lost or corrupted on the hard drive.

Surge : Surges are small over voltage conditions that take place over relatively long

periods (usually more than 1 second). To regulate power to a desired level, excess energy must be switched (in SMPS) or dissipated away (in linear power supply). In either case, excessive voltage creates overheating in the supply; and therefore damages the power supply.

Power problem often occurs too quickly to measure without very specialized power monitoring equipment. But, there are some situation that may generate power problems as follows:

1. The lights tend to flicker or periodically vary in intensity.

2. There are frequent or regular errors in data transmission between network nodes.

3. PC hangs or reboot for no logical reason.4. CMOS RAM periodically looses its contents or becomes corrupted.5. The monitor display flicker or waves.

Spikes : A spike is a large over voltage condition that occurs in the

milliseconds. Lightning strikes and high-energy switches can cause spikes on the AC line.

Heavy equipment like drill machine, grinders, welding equipments etc. can produce power spikes.

If the PC is on the same AC line as that heavy equipment, the spikes can damage the PC-SMPS.

While some SMPS are designed with a surge suppression component like capacitor and metal oxide Varistors, spike that passes through these components can damage the SMPS or pass through it to damage many portions of the motherboard. Spikes may be of high enough frequency to introduce RFI-like problems.

Symptoms of Power Problems: Computers are sensitive to their power environment. Anyone who has ever had a computer toasted by a lightning strike or who has

lost a morning’s work to a sudden blackout knows that all too well. But even with the increased awareness of the need to protect computers from power problems, many people still believe their vulnerability is limited to the occasional storm or utility outage.

There are two unfortunate realities of the electronic age; the utility companies simply cannot provide the clean, consistent power demand by sensitive electronics, and you are responsible for the health and safe operation of your equipment.

The following are the symptoms of power problem:1. Flickering lights2. Errors in data transmissions between nodes3. Unexplained component lockup4. Premature component failure5. Hard Drive crashes6. Corrupt or loss of data in CMOS and other EPROM chips7. System devices behave erratically when too many are turned on8. Frequently aborted modern transfer9. Wavering monitor screens10. Disc Drive writes errors

Most common SMPS problems: The following are the common SMPS problems: No output, fuse blown - shorted switch mode power transistor and other

semiconductors or open fusible resistors. Supply dead, fuse not blown - bad startup circuit (open startup resistors), open

fusible resistors (due to shorted semiconductors), bad controller components. One or more outputs out of tolerance or with excessive ripple at the line

frequency (50/60 Hz) or twice the line frequency (100/120 Hz) - dried up main filter capacitor(s) on rectified AC input.

One or more outputs out of tolerance or with excessive ripple at the switching frequency ( of kHz typical) dried up or leaky filter capacitors on affected outputs.

Audible whine with low voltage on one or more outputs shorted semiconductors, faulty regulator circuitry resulting in over voltage crowbar kicking in, faulty overvoltage sensing circuit or SCR, faulty controller.

Periodic power cycling, tweet-tweet, flub-flub, blinking power light - shorted semiconductors, faulty over voltage or over current sensing component bad controller.

Bad solder connections are a possibility as well since there are usually large components in these supplies and soldering to their pins may not always be perfect.

An excessive load can also result in most of these symptoms or may be the original cause of the failure. And do not overlook the trivial: a line voltage select switch in the wrong position or between positions (possibly by accident when moving the supply, particularly with PCs), or damaged.

Power Protection DevicesPower protection systems as the name implies protects the equipment from the effect of surge power and power failure. The power surge or spikes can destroy the computer equipment and loss of power can result in data loss. The computer power supplies should be such that it provides protection from higher than normal voltage and current and should provide power line noise filtering. The automatic shut down of computer during power disturbance is a built-in function of most high quality power supply. Power supplies should stay within operating specifications and continue to run a system if any of these power line disturbances occur:

Voltage drop to 80v for up to 2 seconds Voltage drop to 70v for up to 0.5 seconds Voltage surge of up to 143v for up to I second

IBM also states that neither their power supplies nor systems will be damaged by the following occurrences:

Full power outage Any voltage drop (brownout) A spike of up to 2,500v

Other than power supplies, there are some devices used for protection of power to the systems, those are: -

4) Surge suppressor5) Line conditioner6) Backup power supply

Surge suppressor or spike-guard or spike protector This is the simplest form of power protection device available. This devices

inserted between the system and the power line. This device can absorb high voltage transients (short period voltage height)

produced by on and off of high power equipment or by line fluctuations or by lightning strikes.

Fig.Diagram of Spike Supressor

Surge protector uses metal-oxide-varistors (MOV) that can clamp or shunt the voltages above threshold level.

Metal oxide varistors are designed to accept voltages as high as 6000 volts and divert any voltage above 200 volts to ground and then surges get suppressed.

Some surge suppressors have status indicators that let you know when a surge is very large enough to blow (damage) the varistor.

Another good feature of surge suppressor is a built-in circuit breaker that resets when high voltage spike comes.

Phone Line Surge Protectors In addition to protecting the power lines, it is critical to provide protection to

your systems from any phone lines that are connected. If you are using a modem or fax board that is plugged into the phone system,

any surges or spikes that travel the phone line can potentially damage your system.

In many areas, the phone lines are especially susceptible to lightning strikes, which is the largest cause of fried modems and any computer equipment attached to them.

Several companies manufacture or sell simple surge protectors that plug between your modem and the phone line. These inexpensive devices can be purchased from most electronics supply houses.

Line conditioner In addition to high voltage and high current conditions, other problems can be

reduction in the power-line voltage i.e. voltage may go down than the normal operating voltage and result in brown out because of high current applications running on the same line. Other form electrical noise other than simple voltage surge or spike migth be on the power line such as radio frequency interference or electrical noise caused by motor or other inductive load , the voltage may be reduced.

Remember two things when you wire together digital devices (such as computers and their peripherals):

Any wire can act as an antenna and will have voltage induced in it by nearby electromagnetic fields, which can come from other wires, telephones, CRTS, motors, fluorescent fixtures, static discharge, and, of course, radio transmitters.

Digital circuitry also responds with surprising efficiency to noise of even a volt or two, making those induced voltages particularly troublesome. The electrical wiring in your building can act as an antenna and pick up all kinds of noise and disturbances.

A line conditioner can handle many of these types of problems. A line conditioner is designed to reduce the problems as mentioned, it filters the power ,bridge brownout ,suppress high voltage and current condition and it acts as a buffer between power line and the system.

It is a type of active device, which is functioning continuously rather than passive device that activates only when the surge is present. A line conditioner contains a transformer, capacitor and other circuitry that temporarily can bridges out or reduces the above mentioned problems

Protection Devices To run a computer system properly, requires a steady power supply with clean

and noise free power. It achieves through the protection devices. Computers and other peripherals are very sensitive devices. Interference or fluctuations in the power supplied to these devices can cause

crashes of computers and lighting boards, and in some cases permanent damage.

Thus, it is desirable to use some sort of protection on the power line when plugging in these devices.

Surge Suppressor: A surge suppressor also called a “surge protector” is a device inserted in the

alternating current (AC) utility line and/or telephone line to prevent damage to electronic equipment from voltage “spikes” called transients. A typical surge suppressor is a small box with several utility outlets, a power switch, and a 3-wire cord for plugging into a wall outlet.

The effective AC utility voltage is 230 volts; the peak voltage is on the order of plus-or-minus 240 to 270 volts at a frequency of 50 hertz. But transients, who arise from various causes, commonly reach peak levels of several hundred volts. These pulses are of short duration, measured in microseconds (units of 106 second), but at that time, they can cause hardware to malfunction. The worst type of transient occurs when lightning strikes in the vicinity (it is not necessary for a power line to be directly hit). Such a “spike” can peak at thousands of volts and cause permanent damage to equipment.

A surge suppressor prevents the peak AC voltage from going above a certain threshold such as 200 volts. Semiconductor devices are used for this purpose. The power line is effectively short-circuited to electrical ground for transient pulses exceeding the threshold, while the flow of normal 50-Hz current is unaffected. For the suppressor to work, a 3-wire AC power connection must be used.

Surge suppressors should be used with all semiconductor-based electronic and computer hardware, including peripherals such as printers, monitors, external disk drives, and modems. But the suppressor should not be relied upon to provide protection against lightning-induced transients. The safest procedure, inconvenient though it be, is to ensure that all susceptible hardware is plugged into the suppressor box, and to unplug the suppressor’s main power cord when the equipment is not in use if you live in a thunderstorm-prone area

Circuit Breaker1. Actuator lever: used to manually trip and reset the circuit breaker. Mso

indicates the status of the circuit breaker (On or Off’tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the on position. This is sometimes referred to as “free trip” or “positive trip” operation.

2. Actuator mechanism : forces the contacts together or apart.3. Contacts allow current to flow when touching and break the flow of current

when moved apart.4. Terminals5. Bimetallic strip6. Calibration screw : allows the manufacturer to precisely adjust the trip

current of the device after assembly.7. Solenoid8. Arc divider / extinguisher

Fig.Circuit Breaker

Working The basic circuit breaker consists of a simple switch, connected to either a

bimetallic strip or an electromagnet. The hot wire in the circuit connects to the two ends of the switch. When the switch is flipped to the on position, electricity can flow from the

bottom terminal, through the electromagnet, up to the moving contact, across to the stationary contact and out to the upper terminal.

The electricity magnetizes the electromagnet. Increasing current boosts the electromanget’s magnetic force, and decreasing

current lowers the magnetism.

Uninterrupted Power Supplies (UPS) UPS is a device connected between the primary power source and a

computer to provide uninterrupted electrical flow.

UPS is also known as an uninterruptible power source or a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when primary power is not available.

An alternative power source, usually a set of batteries, used to power a computer system if the normal power service is interrupted or falls below acceptable levels.

A UPS can often supply power for just long enough to let you shutdown the computer in an orderly fashion. It is not designed to

support long-term operations. Sometimes lighting, storms will kill or destroy the power for just a second,

which is enough to erase the data in computer’s temporary memory. Thus the total loss of power can be avoided with battery — based power

systems. Such systems are called Uninterrupted Power Supply (UPS). A UPS generally protects a computer against four different power

problemsVoltage surges and spikes: Times when the voltage on the line is greater than it should be.Voltage sags : Times when the voltage on the line is less than it should be.Total power failure : Times when a line goes down or a fuse blows somewhere on the grid or in the buildingFrequency differences : Times when the power is oscillating at something other than 50 Hertz.Need of UPSThere are various common power problems, for avoiding these problems we need a UPS. They are as follows1. Power failure : When primary power supply is not available, then electrical

equipment like computer, printer, switches, servers are stop working.2. Voltage Sag: If there is a variation in primary power supply i.e. under-voltage:

Causes flickering of lights, its effects in the computer system.3. Voltage Spike : If there is a variation in primary power supply i.e. over-

voltage, spike or peak: Causes wear or acute damage to computer systems.4. Under-voltage (brownout) : When primary power supply line voltage is low

for an extended period of time: Causes overheating in electronic parts, so it reduces age of components or damage frequently.

5. Over-Voltage : If primary power line voltage increased suddenly for an extended period of time:

6. Causes light bulbs to fail or it effects on SMPS of computer system.7. Line Noise If there are distortions on input primary power line, it superimposed

on the power waveform and it creates a electromagnetic interference.8. Frequency Variation : If there is a deviation from the nominal frequency (50

or 60 Hz) of primary power line then it effects to SMPS of computer system.9. Switching Transient : If there is instantaneous under voltage (notch) on input

primary line for few nanoseconds: It may create problem for HDD or computer memory to memory loss, data error, data loss and component stress.

10.Harmonic Distortion : If multiples of power frequency superimposed on the power line waveform then it effects on excess heating in wiring a fuses.

Block Diagram of Online UPS

The UPS s block diagram consists of following functional blocks.1.AC mains section containing filter, transformer and rectifier.2.Inverter and filter.3.Battery charger circuit and battery.4.Static switch/contactor.

Detail study of Section UPS are known as online backup supplies because they continuously function

and supply power to the computer. In the UPS, the system operates from a battery with a voltage inverter, which converts 12V DC from battery to 230V AC. A battery charger is connected to the line, which keeps the battery charge at a rate equal to or greater than the rate at which the power is consumed.

When main power is disconnected, UPS continues functioning undisturbed as a charging function is stopped. In UPS, to switch from main to battery, almost 0sec time is required because when the UPS is running with battery, no switching takes place hence there is no power disturbance. The battery then starts discharging at the rate at which power is consumed b the system. When the power in the line returns, the battery starts charging and now inverter converts the DC, which is coming from rectification of main AC and hence battery is disconnected. The switching from battery and main is done with the help of high-speed diode.

If in the specification of UPS switching time exists, then the unit cannot be a true UPS , however, that a good SPS with a ferroresonant transformer can virtually equal the performance of a true UPS at a lower cost. Because of a UPS's almost total isolation from the line current, it is unmatched as a line conditioner and surge suppresser. The best UPS systems add a ferroresonant transformer for even greater power conditioning and protection capability. This type of UPS is the best form of power protection available.

The addition of a ferroresonant transformer improves a unit's power conditioning and buffering capabilities. Good units have also an inverter that produces a true sine wave output; the cheaper ones may generate a square

wave. A square wave is an approximation of a sine wave with abrupt up-and-down voltage

The cost of UPS varies with both i.e. the length of time it can continue and the amount of power, which it can supply. Figure shows the block diagram of UPS. The different blocks are: -1) Step-down transformer

The mains voltage (230V – 50HZ AC) is stepped down to a low voltage with the help of step-down transformer, which is supplied to rectifier.

2) RectifierThe input voltage is rectified with the help of bridge rectifier i.e. AC voltage is converted to DC and the ripple present in DC is filtered out with the help of electronic filter. Hence, rectifier provides pure DC at its output.

3) RegulatorRegulator is to provide constant DC without ripple and fluctuation i.e. if in any case voltage at output increases or decreases the regulator will keep the output constant irrespective of any fluctuation. Usually, switched mode regulator is used instead of linear regulator because of its high efficiency.

4) InverterThe DC output of regulator is fed to inverter, the function of inverter is to convert DC into AC. The inverter circuit is designed such that the output will be 230V AC, which acts as AC supply for computers power supply.

5) Battery charger Battery charger is used to charge battery at a constant rate when main supply

is on. When AC supply is available this section charges the battery through a battery

charger circuit. The battery charging circuit converts input AC supply to the desired DC levels

and charges the battery. This section has a special protection to prevent overcharging of batteries. Also

this circuit employs and 5CR controlled converter that charges the battery with constant current supply.

The batteries can be ordinary 12 V / 10 A car batteries or maintenance free batteries.

(6)Static switch / contactor:In the event of power failure the inverter is connected to the load with the help of static contactor switches

OperationWhen the line voltage is present the battery is charged to its full value through

a charger circuit to its full value through battery charger. When the supply is off, the charging action of battery is stopped and battery provides DC supply to the regulator which in turn converted to AC through an inverter. The high speed switching diode is used to provide isolation between the battery and AC main supply.

Types of UPS

There are two distinct types of UPS

1. Off-line (standby UPS) and2. Line-interactive (also called on-line or continuous UPS).

It is widely believed that there are only two types of UPS systems, namely standby UPS and online UPS.These two commonly used terms do not correctly describe many of the UPS systems available.The most common design approaches are as follows

• Line Interactive• Standby-Ferro• Double Conversion On-Line Standby• Delta Conversion On-Line

Standby UPS / Off-Line UPS: The Standby UPS is the most common type used for Personal Computers. In the

block diagram illustrated in Fig., the transfer switch is set to choose the filtered AC input as the primary power source (solid line path), and switches to the battery / inverter as the backup source should the primary source fail.

When that happens, the transfer switch must operate U switch the load over to the battery / inverter backup power source (dashed path). The inverter only starts when the power fails, hence the name “Standby.”

The main benefits of this design have high efficiency, small in size, and low cost per unit. If we use proper filter and surge circuitry, then these design provides adequate noise filtration and surge suppression.

A off-line UPS runs the computer off of the normal utility power until it detects a problem. At that point, it very quickly (in five milliseconds or less) turns on a power inverter and runs the computer off of the UPS’s battery. A power inverter simply turns the DC power delivered by the battery into 230-volt, 65-Hertz AC power.

In this type of UPS, the battery is charged when AC mains are on and as soon as AC mains are off, the battery discharges and supplies power to the PC as shown in Fig.

Standby power supply (OFF Line UPS)

Fig. Diagram of Offline Ups Stand by power supply is also called as offline device or offline UPS. It

functions only when normal power is disturbed. An SPS has special circuit that can sense the AC line voltage and current, if it detects a loss of power on a line, the control circuitry of SPS switches over to standby battery and power inverter.

The power inverter converts the battery power to 230V AC power, which is then supplied to the system. If the switching of SPS is not fast enough than the computer unit may shut down or may reboot. A truly outstanding SPS has additional to its circuit a ferroresonant transformer, a large transformer with the capability to store a small amount of power and deliver it during the switch time

SPS units also may or may not have internal line conditioning of their own; most cheaper units place your system directly on the regular power line under normal circumstances and offer no conditioning.

The addition of a ferroresonant transformer to an SPS gives it additional regulation and protection capabilities due to the buffer effect of the transformer. SPS devices without the ferroresonant transformer still require the use of a line conditioner for full protection

Depending upon the quality and the power output capacity, the rates of SPS may vary. Now days SPS with microprocessor controlled control circuitry are available.

Advantages of Online UPS: It provides isolation between main supply and load. Since inverter is always ON, the quality of load voltage is free from distortion. All the disturbances of supply such as blackout, brownouts, spikes etc are

absent in the output. Voltage regulation is better Transfer time is practically zero since inverter is always ON.

Disadvantages of Online UPS: Overall efficiency of UPS is reduced since inverter is always ON. The wattage of the rectifier is increased since it has to supply power to inverter

as well as charge battery. Online UPS is costlier than other Uninterruptible Power Supply Systems.

Applications of Online UPS:1. Induction motor drives and similar other motor control applications.2. Intensive care units, medical equipments.

Advantages of Offline UPS: Offline UPS has high efficiencies, since charger is not continuously on. The power handling capacity of charger is reduced. Offline UPS are not very costly. Internal control is simpler in offline Uninterruptible Power Supply.

Disadvantages of Offline UPS: Since offline UPS provides mains supply when it is present, the output contains

voltage spikes, brownouts, blackouts. There is finite transfer time from mains to inverter when mains supply fails. Output of offline Uninterruptible Power Supply is not perfectly reliable.

Applications of Offline UPS:1. Computers, printers, scanners etc use offline UPS.2. Emergency power supplies, EPABX.

Comparison between Online and Offline UPS:Sl No parameter Online UPS Offline UPS

1  Inverter  Always ON  Turned On when  mains fails

2  Rectifier cum charger

 Supplies power to inverter as well  as charges battery

 Charges only  battery

3  Output  waveform  Sine wave  Quasi square wave4  Harmonic  distortion  Low  High5  Efficiency  Low  High6  Load  Isolated from supply  Not isolated from  Supply7  Cost  High  Low8 backup Time Less Time More Time

10 Switching Time Almost Zero Finite Time or some time needed

11 Switching Component High Speed Diode Electromechanical Swtich

(relay)

Note : Were Ever Diagrams are not Drawn Draw which are explain in Lecture