subject : computer hardware and maintenance (17428) · 2015-02-13 · subject : computer hardware...

Subject : Computer Hardware and Maintenance (17428)

Compiled By : Ms. M.S.Karande (I/c HOD-IF) of 14

Power Supplies

5



A Power supply (Power supply unit or PSU) is a device or system that supplies electrical or other types of energy to an output load or group of loads.

Power supply provides the alternating current (AC) and the direct current (DC).

There are three types of power supply

1. Linear power supply(LMPS) 2. Switch Mode Power Supply(SMPS)

Switch Mode Power Supply (SMPS)

A switched power supply is an electronic power supply unit that incorporates a switching regulator

Working of SMPS

Input Rectifier and Filter Stage

The process of converting AC to DC is called Rectification. SMPS converts AC to DC

The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor.

If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage(~120 VAC) range and as a straight rectifier when operating on the high voltage (~240 VAC) range.

If an input range switch is not used then a full wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of dc voltages that will be produced by the rectifier stage.

Input Rectifier & Filter

Inverter Chopp

er

Output Transfor

mer

Output rectifier

and filter

Chopper Controller

Main

input

DC

output



Inverter Chopper Stage

The inverter “Chopper” stage converts DC, whether directly from the input or from the rectifier and filter stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20kHz to make it inaudible to humans.

The output voltage is optically coupled to the input and thus very tightly controlled

Output transformer

If the output is required to be isolated from the input as is usually the case in mains power supplies the inverted AC is used to drive the primary winding of a high frequency transformer. This converts the voltage up or down to the required output level on its secondary winding. The output transformer in the block diagram serves this purpose.

Output Rectifier and Filter

If a DC output is required the AC output form the transformer is rectified. For output voltages above 10 volts or so, ordinary silicon diodes are commonly used. The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.

Chopper Controller

A feedback circuit monitors the output voltage and compares it with a reference voltage which is set manually or electronically to the desired output.

If there is an error in the output voltage the feedback circuit compensates .This part of power supply is called switching regulator. The Chopper controller performs this function.



Power Signal Description and Pinout of AT and ATX (Power Supply Form Factor)

The power supply form factors are

1. AT 2.ATX/NLX

AT Style SMPS



a) At style computer cases had a power button that was directly connected to the system computer power supply(PSU)

b) The wires were soldered to the power button making it difficult to replace the power supply if failed

c) The AT-style SMPS provides DC output on two 6-pin connectors

d) The two 6-pin connector carry DC power connections to the motherboard. It carries +5V,-5V,+12V and -12V voltages and a PGS signal(power Good Signal)

e) The PGS is a special flag to the CPU, indicating that the output voltages are stable and usable by the CPU. In absence of the PGS signal CPR remains reset .

2. ATX/NLX Style SMPS



a) The ATX form factor is called the ATX/NLX form factor

b) An ATX power supply does not directly connect to the system power button allowing the computer to be turned off via software

c) 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.

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

i) PS-ON

ii) 5 VSB

iii) PW-OK

e) The PS-On is an input from motherboard to SMPS only when this signal is low the SMPS outputs should be ON. Otherwise the output should be OFF. The signal can be generated by software

f) The 5 VSB is a standby voltage which supplies power to special circuits even when the SMPS and the System are off

g) The PW-OK is a power good signal.



Power Supply Characteristics

1. Wattage : The total, maximum output of the power supply in watts. It is 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 200W to 500W.

2. Efficiency : The efficiency of an entity in electronics and electrical engineering is defines as useful power output divided by the total electrical power consumed. The efficiency of SMPS is 70-85%

Efficiency= Useful Power Output

Total Electrical Power Consumed

3. Regulation : The ability of a SMPS to maintain an output voltage within specified limits under varying of input voltage and output load.

a. Load Regulation: Sometimes 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 decreased. The voltage of a DC power source tends to decrease as its load increases and vice-versa. Load regulation is usually expressed as a “+/-“ % value for each of the voltages the power supply delivers. 3% to 5% are typical 1% is quite good

b. Line Regulation: The complement of load regulation, this parameter describes the ability of the power supply to control its output levels 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 a typical.

4. Ripple : Also 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 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 (m Vpp)

[Peak to Peak refers to measuring the AC voltage from its negative maximum to its positive maximum. Lower numbers are better.]



Power Problems:

Blackouts: a) A blackout is complete loss of electric power where voltage and current drop to

almost 0 b) Blackout are usually caused by physical interruption in the power line due to

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

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

Brownouts (Sag): a) The under voltage condition also called brownout or sag. The high load items like

air conditioners, welding machine, motor etc draw to much current that the AC voltage level drops.

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

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

Surge: Surge 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 LMPS). In either case excessive voltage creates overheating in the supply and therefore damages the power supply.

Spikes: a) 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. b) Heavy equipment like drill machine, grinders, welding equipment etc. can

produce power spikes. c) If the PC is on the same AC line as that heavy equipment the spikes can damage

the PC-SMPS. Symptoms of Power Problems:

1. Flickering Lights 2. Premature Component Failure 3. Hard Drive Crashes 4. The PC stalls, crashes, or reboots for no apparent reason. 5. You suffer chronic or frequent component failures (for example, modems

don't seem to last long). 6. You suffer chronic or frequent hard drive failures or file access problems. 7. The CMOS RAM or modem NVRAM periodically looses its contents or

becomes corrupted. 8. The PC behaves erratically when other high-energy devices are turned on. 9. The modem regularly looses its connection, or fails data transfers. 10. The monitor display flickers or waves. 11. You encounter frequent or chronic write errors to disks.

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.

Surge Suppressor:

Surge suppresses, are simple and relatively inexpensive devices that are designed to absorb high-voltage transients produced by lightning and other high-energy equipment.

Protection is accomplished by clamping (or shunting) voltages above a certain level (usually above 200 volts).

In the most common type of surge protector, a component called a metal oxide varistor, or MOV, diverts the extra voltage.

MOVs(Metal Oxide Varistor) are often included that can respond quickly and clamp voltages as high as 6000 volts. However, powerful surges such as direct lightning strikes can blow right through an MOV.

Also, MOVs degrade with each spike. Once they have passed a number of surges, they are destroyed and must be replaced. There is no way to know whether or not the MOV is working, so there is no way to really tell if a surge suppresser is actually protecting the system.



Many protectors show a neon lamp or LED that goes out when the MOV has blown or when the protector is no longer active. Good suppressers also incorporate a circuit breaker rather than a fuse. This feature is a great convenience, because a circuit breaker can be reset, whereas many fused units must be disassembled to replace the fuse (or the unit must be replaced outright).

Modems and fax boards are also susceptible to damage from spikes present on everyday telephone lines. When recommending protection schemes, do not forget to include telephone line spike protection as well.

AC Line Surge Suppressors Phone Line Surge Suppressors

Protects against telephone line lightning strike surges. Built into a power line surge suppressor

Circuit Breaker Its purpose was to protect lighting circuit wiring from accidental short-circuits and overloads.



The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is

usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs. Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel. The design includes the following components: 1. Actuator lever - used to manually trip and reset the circuit breaker. Also 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 when touching and break the current when moved apart. 4. Terminals 5. Bimetallic strip 6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly. 7. Solenoid (In physics, the term refers specifically to a long, thin loop of wire, often wrapped

around a metallic core, which produces a uniform magnetic field in a volume of space (where some experiment might be carried out) when an electric current is passed through it. A solenoid is a type of electromagnet when the purpose is to generate a controlled magnetic field.) 8. Arc divider / extinguisher

http://en.wikipedia.org/wiki/Physics

http://en.wikipedia.org/wiki/Metal

http://en.wikipedia.org/wiki/Magnetic_field

http://en.wikipedia.org/wiki/Electric_current

http://en.wikipedia.org/wiki/Electromagnet



Uninterrupted Power Supply(UPS) :

An UPS provides a back up power supply when there is a power failure from AC mains.

There are two types of UPS :

1. Standby UPS/ Off-line UPS

Working :

Most Common type used for Personal Computers.

Here the primary power source is the AC from the mains and the secondary power source is the UPS Battery.

When the primary power source fails, the transfer switch must operate to switch the load over to the battery power source. The Inverter only starts when the power fails.

The transfer switch is set to choose the filtered AC input as the primary power source (dark line) and switches to the battery /inverter as the backup source should the primary source fail.

If the primary source fails then the transfer switch must operate to switch the load over to the battery /inverter backup power source. The inverter only starts when the power fails hence the name Standby

A power inverter simply turns the DC power delivered by the battery into 230 volt 65 Hz AC power.

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.

Features

Off-line power source.

Batteries are continuously charged.

When power is interrupted, batteries switch into the current path



Voltage Inverter converts the battery voltage to AC.

Some AC power line conditioning may be provided.

Advantages :

- High efficiency, small size and low cost are the main advantages

- With proper filter and surge circuitry these systems can also provide adequate noise filtration and surge suppression.

Disadvantages:

- High switching is involved in off-line UPS and power will be available till the battery discharges.

- It is useful when there is a good power supply

- Spikes may be generated due to switching

2. On-Line UPS



- An on line UPS continuously powers the protected load from its reserves (batteries).

- In the online UPS the primary power source is the UPS’s battery and AC mains is the secondary power source.

- It continuously powers the protected load from the batteries.

- It provides protection against all types of power problems in addition to the power failure. So it is also known as power conditioner and a line conditioner.

- Under Normal operation, the online UPS is always running through the battery using its inverter while the line power run the battery charger.

- Here there is no transfer time is needed in the event of a power failure. Since there is no switching under normal operation the voltage spikes re not generated.

- Here the transfer switch will act as backup in the event that the inverter fails due to some of internal problem.

Features

Provides complete power conditioning.

Uses batteries as backup power source.

Voltage Inverter converts the battery DC voltage to AC voltage.

The PC runs off the UPS inverter all the time.

Primary and secondary power passes through the inverter, so no switching is required.

When AC power is interrupted, the UPS continues to power the PC with its battery.

Advantages

Practical power range 500 to 3000 VA

High degree of power conditioning

Very high efficiency design

Instantaneous switching times

subject : computer hardware and maintenance (17428) · 2015-02-13 · subject : computer hardware...

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