Computer Power Supply, Power Adaptor, UPS and Power packs

There are two broad categories of power supplies:

• Linear regulated power supply
• switched mode power supply (SMPS)

Linear regulated power supply

A DC Power Supply Unit (commonly called a PSU) deriving power from the AC mains (line)
supply and performs a number of tasks:
1. It changes (in most cases reduces) the level of supply to a value suitable for driving the load
circuit.
2. It produces a DC supply from a pure AC wave.
3. It prevents any AC from appearing at the supply output.
4. It will ensure that the output voltage is kept at a constant level, independent of changes in:
a. The AC supply voltage at the supply input.
b. The Load current drawn from the supply output.
c. Temperature.

To perform the above tasks, the PSU has four main stages:
 Transformer:
 Rectifier
 Filters
 Regulators
Power supplies in recent times have greatly improved in reliability but, because they have to
handle considerably higher voltages and currents than any or most of the circuitry they supply,
they are often the most susceptible to failure of any part of an electronic system. Modern
power supplies have also increased greatly in their complexity, and can supply very stable
output voltages controlled by feedback systems. Many power supply circuits also contain
automatic safety circuits to prevent dangerous over voltage or over current situations.

The transformer stage must be able to supply the current needed by the load. If too small a
transformer is used, it is likely that the power supply's ability to maintain full output voltage at
full output current will be reduced. With too small a transformer, the losses will increase
dramatically as full load is placed on the transformer.

Transformers and Rectifiers

In a basic power supply the input power transformer
has its primary winding connected to the mains (line)
supply. A secondary winding, electro-magnetically
coupled but electrically isolated from the primary is
used to obtain an AC voltage of suitable amplitude,
and after further processing by the PSU, to drive the
electronics circuit it is to supply.
The transformer stage must be able to supply the current needed. If too small a transformer is
used, it is likely that the power supply's ability to maintain full output voltage at full output
current will be impaired. With too small a transformer, the losses will increase dramatically as
full load is placed on the transformer.

As the transformer is likely to be the most costly item in the power supply unit, careful
consideration must be given to balancing cost with likely current requirement. There may also
be a need for safety devices such as thermal fuses to disconnect the transformer if overheating
occurs, and electrical isolation between primary and secondary windings, for electrical safety.

Rectifier Stage
The rectifier AC line voltage to pulsating DC voltage.

Half Wave Rectification

A single silicon diode may be used to obtain a DC voltage from the AC input. This method is
cheap but is only suitable for fairly non-demanding
uses (light loads). The DC voltage produced by the
single diode is less than with the other systems,
limiting the efficiency of the power supply, and the
amount of AC ripple left on the DC supply is generally
greater.

The half wave rectifier conducts on only half of each cycle of the AC input wave, effectively
blocking the other half cycle, leaving the output wave shown in Fig. 1.1.2. As the average DC
value of one-half cycle of a sine wave is 0.637 of the peak value, the average DC value of the
whole cycle after half wave rectification will be 0.637 divided by 2, because the average value of
every alternate half cycle where the diode does not conduct, will of course be zero. This gives
an output of: 0.318 V peak

This figure is approximate, as the amplitude of the half cycles for which the diode conducts will
also be reduced by about 0.6V due to the forward voltage drop (the depletion layer p.d.) of the
silicon rectifier diode. This additional voltage drop may be insignificant when large voltages are
rectified, but in low voltage power supplies where the AC from the secondary winding of the
mains transformer may be only a few volts, this 0.6V drop across the diode junction may have
to be compensated for, by having a slightly higher transformer secondary voltage.

Half wave rectification is not very efficient at producing DC from a 50Hz AC input. In addition
the gaps between the 50 diode output pulses make it more difficult to remove the AC ripple
remaining after rectification.

Full Wave Rectification

If a transformer with a centre tapped secondary winding is used, more efficient full wave
rectification can be used. The centre-tapped secondary produces two anti-phase outputs. If
each of these outputs is ‘half wave rectified’ by one of the two diodes, with each diode
 conducting on alternate half cycles, two pulses of
current occur at every cycle, instead of once per cycle
in half wave rectification. The output frequency of the
full wave rectifier is therefore twice the input
frequency. This effectively provides twice the output
voltage of the half wave circuit, Vpk x 0.637 instead of
Vpk x 0.318 as the ‘missing’ half cycle is now rectified,
reducing the power wasted in the half wave circuit.
The higher output frequency also makes the smoothing of any remaining AC ripple easier.
Although this full wave design is more efficient than the half wave, it requires a centre tapped
(and therefore more expensive) transformer.

The Bridge Rectifier

The full wave bridge rectifier uses four diodes arranged in a bridge circuit to give full wave
rectification without the need for a centre-
tapped transformer. An additional
advantage is that, as two diodes (effectively
in series) are conducting at any one time,
the diodes need only half the reverse
breakdown voltage capability of diodes used
for half and conventional full wave
rectification. The bridge rectifier can be built
from separate diodes or a combined bridge
rectifier can be used.
The current paths on positive and negative half cycles of the input wave are shown in Fig. a and
Fig. b below. It can be seen that on each half cycle, opposite pairs of diodes conduct, but the
current through the load remains in the same polarity for both half cycles.

Filter Circuits
A typical power supply filter circuit is divided into two parts,
the reservoir capacitor and the low pass filter. Each of these
parts contributes to removing the remaining AC pulses, but in
different ways. The figure shows an electrolytic capacitor
used as a reservoir capacitor, so called because it acts as a
temporary storage for the power supply output current. The
rectifier diode supplies current to charge a reservoir capacitor on each cycle of the input wave.
The reservoir capacitor is a large electrolytic, usually of several hundred or even a thousand or
more microfarads, especially in mains frequency PSUs. This very large value of capacitance is
required because the reservoir capacitor, when charged, must provide enough DC to maintain a
steady PSU output in the absence of an input current; i.e. during the gaps between the positive
half cycles when the rectifier is not conducting.

During each cycle, the rectifier anode

AC voltage increases towards Vpk. At
some point close to Vpk the anode
voltage exceeds the cathode voltage,
the rectifier conducts and a pulse of
current flows, charging the reservoir
capacitor to the value of Vpk.

Once the input wave passes Vpk the rectifier anode falls below the capacitor voltage, the
rectifier becomes reverse biased and conduction stops. The load circuit is now supplied by the
reservoir capacitor alone (hence the need for a large capacitor).

Even though the reservoir capacitor has large value, it discharges as it supplies the load, and its
voltage falls, but not by very much. At some point during the next cycle of the mains input, the
rectifier input voltage rises above the vo1tage on the partly discharged capacitor and the
reservoir is re-charged to the peak value Vpk again.

AC Ripple
The amount by which the reservoir capacitor discharges on each half cycle is determined by the
current drawn by the load. The higher the load current, the more the discharge, but provided
that the current drawn is not excessive, the amount of the AC present in the output is much
reduced. Typically the peak-to-peak amplitude of the remaining AC (called ripple as the AC
waves are now much reduced) would be no more than 10% of the DC output voltage.

The DC output of the rectifier, without the reservoir capacitor, is either 0.637 Vpk for full wave
rectifiers, or 0.317 Vpk for half wave. Adding the capacitor increases the DC level of the output
wave to nearly the peak value of the input wave.

To obtain the least AC ripple and the highest DC level it would seem sensible to use the largest
reservoir capacitor possible. There is a snag however. The capacitor supplies the load current
for most of the time (when the diode is not conducting). This current partly discharges the
capacitor, so all of the energy used by the load during most of the cycle must be made up in the
very short remaining time during which the diode conducts in each cycle.

The charge (Q) on a capacitor depends on the amount of current (I) flowing for a time (t). Q = It.
Therefore the shorter the charging time, the larger current the diode must supply to charge it. If
the capacitor is very large, its voltage will hardly fall at all between charging pulses; this will
produce a very small amount of ripple, but require very short pulses of much higher current to
charge the reservoir capacitor. Both the input transformer and the rectifier diodes must be
capable of supplying this current. This means using a higher current rating for the diodes and
the transformer than would be necessary with a smaller reservoir capacitor.

There is an advantage therefore in reducing the value of the reservoir capacitor, thereby
allowing an increase in the ripple present, but this can be effectively removed by using a low
pass filter and regulator stages between the reservoir capacitor and the load.

Low Pass Filters

Although a useable power supply can be made using only a reservoir capacitor to remove AC
ripple, it is usually necessary to also include a low pass filter and/or a regulator stage after the
reservoir capacitor to remove any remaining AC ripple and improve the stabilisation of the DC
output voltage under variable load conditions.

Either LC or RC low pass filters can be

used to remove the ripple remaining
after the reservoir capacitor. The LC
filter shown is more efficient and gives
better results than the RC filter but for
basic power supplies, LC designs are less
popular than RC, as the inductors
needed for the filter to work efficiently at 50 need to be large and expensive laminated or
toroidal core types. However modern designs using switch mode supplies, where any AC ripple
is at much higher frequencies, much smaller ferrite core inductors can be used. The low pass
filter passes low frequency, in this case DC (0Hz) and blocks higher frequencies, whether 50Hz
or 100Hz in basic circuits or tens of kHz in switch mode designs.

(Computer PSU) typically is designed to convert 230 V AC power from the mains to usable low-
voltage DC power for the internal components of the computer. The most common computer
power supplies are built to conform with the ATX form factor. The most recent specification of
the ATX standard is version 2.2, released in 2004. This enables different power supplies to be
interchangeable with different components inside the computer.
Regulator
Regulator: so a voltage regulating stage is necessary, done by a zener diode or by a voltage
regulator integrated circuit. After this stage the output is true DC voltage

Factors to consider when selecting a power supply

Voltage rating • Current rating • Power requirements
• Line regulation - describes the ability of the power supply to maintain its output voltage
constant irrespective of change in input source of power supply. A value for each output level is
usually specified as a "±" percentage. ± 1% to 2% is typical.
• Load regulation - refers to the ability of the power supply to maintain constant output voltage
irrespective of the change in load. The voltage of a DC power source tends to decrease as its
load increases, and vice-versa. Better power supplies do a better job of smoothing out these
variations. 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.
• Nature of input (AC) • Usage of computer system • Spikes and surges in the power
• Efficiency - Efficiency is defined as useful power output divided by the total electrical power
consumed. Efficiency of SMPS is 70-85%
• Linearity • Frequency of operation.
• Rated Wattage - To operate different components of PC, power supply must generate rated
power. The generated power by power supply as per requirement of system is called rated
wattage.
Typical power ranges are 200W to 500W. General-use computers require 130–205 watts. –
Servers and high-performance workstations require 350 - 500 watts.

Switched Mode Power Supply

1. AC line voltage is first cleaned by removing Electromagnetic Interferences that may be
introduced by external noise 2. EMI filter remove noise - AC input 3. Bridge rectifier and pi filter
convert AC to DC and remove ripples 4. Unregulated DC fed as input to switching regulator . 5.
It will select buck or boost principle based on desired output voltage level 6. The series of
square wave pulses produced by switching regulator are isolated and then filtered to produced
regulated DC output voltage. 7. To maintain desired voltage level the actual compare with
reference voltage 8. If difference found by error amplifier 9. It gives signal to PWM controller
10.PWM controller then adjust the ON period of switch so as to maintain the desired output
voltage.
Advantages of SMPS:-
• SMPS is of Smaller size , lighter in weight and possesses higher efficiency because of its
high frequency operation
• SMPS are less sensitive to input voltage variations.
• Lower heat generation
Disadvantages:-
• It is costly and more complex than linear regulators.
• SMPS has higher output ripple and its regulation is worst
• Switching regulators generate electromagnetic and radio frequency interference noise
due to high switching current.
• To control radio frequency noise required the use of filters on both input and output of
SMPS

Power Supply form factor

The form factor of the power supply refers to its general shape and dimensions of the PSU. The
form factor of the power supply must match that of the case that it is supposed to go into, and
the motherboard it is to power.

Early Pc using PC/XT,AT,baby AT and LPX form factors all use mechanical switch to turn
computer on and off. The AT-style is found on older computers and earlier Pentium systems.
The ATX-style (current technology) is found on Pentium II and later systems. You should
compare the existing power supply with the new one before replacing it.

AT style
The AT form factor is the first modern form factor to be widely used. AT (Advanced Technology)
was released in 1984 by IBM. AT style computer cases had a power button that was directly
connected to system PSU. AT style SMPS provides DC output on two 6-pin connectors and four
4-pin connectors. The six-pin connector carry dc power connections to the motherboard. It
carries +5V,-5V,+12V,-12V voltages and PGS(power good signal ). The power good signal is a
special flag to the CPU, indicating that the output voltages are stable and usable by the CPU. In
the absence of power good signal CPU remains reset.

ATX /NLX style SMPS style

The ATX (for Advanced Technology Extended) form factor
was created by Intel in 1995. It was the first big change in
computer case and motherboard design in many years.

•-12V: used on some types of serial port circuits, whose

amplifier circuits require both –12v and +12V. It is not
needed on some newer systems. Older system use it rarely.
• Serial port require little power. • Most power supplies
provide it for compatibility with older hardware. +12V Drive
motors and dynamic RAM, CMOS RAM.
Use of output voltages of SMPS.
•-5V:- Used on some of earliest PCs for floppy controllers and other circuits used by ISA cards
• +5V :-All Logic chips TTL or CMOS.
• 0V :- Zero volts is the ground of the Pc’s electrical system(common earth) .The ground signals
are provide by the power supply are used to complete circuits with the other voltages. It
provide plane of reference against which other voltages are measured.
• +3.3V :- Not used in baby AT. Newest voltage level provided by modern power
supplies ,Introduced with ATX form factor ,now found on the ATX/NLX,SFX,WTX form factors.
Used to run newer CPUs, system memory , AGP video cards

Power Good Signal :- To prevent the computer from starting up prematurely, the power supply
puts out a signal to the motherboard called ‘PWR OK’ after it completes its internal tests and
determine that the power is ready for use. Until this signal is sent, motherboard will refuse to
start up the computer.
 5VSB:- – Power always on, even when the rest of the power supply is turned off. – A
small amount of current on this wire that allows the motherboard to control the power
supply when it is off. – It also permit activities that occur while PC is off – Enabling wake
up and sleep mode – Wake on LAN or ring network
Hold (or Hold-up) Time:
This is the amount of time the power supply will keep producing its output, if it loses its input.
• A typical value is about 20 milliseconds • It is also important to compare against the switch
time of a UPS . • The hold time should be considerably greater than the switch time to reduce
the chances of problems

Power Failures or Power Problems

Power failures can have internal or external causes. External failures, which are more common,
include:
–Surges (increase in the voltage source, small over voltage conditions for a short time)
–Spike (large over voltage condition measured in nanoseconds)
–Sags (under voltage condition)
–Brownouts (sag longer than 1 second)
–Blackouts (complete power failure)

Blackout
A Blackout is complete loss of electric power where voltage and current drops to almost zero.
Caused by physical interruption in the power line due to accidental damage by a person or act
of nature. Loss of AC will shut down the computer in few millisecond. Losing power may cause
the loss of valuable data, reduction in productivity, corrupt file structure, damage file.
Protection against blackout is to save work regularly.

Brownouts or Sag
The under voltage condition called as brownout or sag. • The high load items like air
conditioners, welding machines, motors draw so much current that AC line voltage drops. •
Results in intermittent system operation, can also damage the power supply. • System hang,
random memory errors occur. • Files may be lost or corrupted on the hard drive

Surge
Surges are small over voltage conditions that take place over relatively long periods. (more than
1 second) • Excessive voltage creates overheating in the supply and damages the power supply.

Spikes
A spike is a large over voltage condition that occurs in the milliseconds • Lightening strikes and
high-energy switches can cause spike in AC line. • Heavy equipment's like drill machines ,
grinders , welding equipment can produce power spike. • Spike can damage the PC-SMPS

Symptoms Of Power Problems

• Flickering lights. • Errors in data transmissions between nodes. • Unexplained component
lockup. • Premature component failure. • Hard disk crashes. • Corruption or loss of data in
CMOS and other EPROM circuits. • System devices behave erratically. • Frequently aborted
modern transfer. • Waving monitor lines. • Disk drive write errors.

Common SMPS problems

Bad solder connections • Excessive load • Low voltage on one or more outputs • Supply dead,
fuse blown- shorted switched mode power transistor and other semiconductors, open fusable
resistors, other bad parts • Supply dead, fuse not blown- bad startup circuit • One or more
outputs out of tolerance or with excessive ripple at switching frequency – leaky filter capacitors
on affected output. • Periodic power cycling, blinking of power light- shorted semiconductors

Protection Devices
Surge suppressor • A surge protector (or surge suppressor) is a device designed to protect
electrical devices from voltage spikes. • It is a small block with several utility outlet, a power
switch and a 3 wire cable for plugging. • A surge protector attempts to regulate the voltage
supplied to an electric device by either blocking or by shorting to ground voltages above a safe
threshold.

Circuit Breaker
• A circuit breaker is an automatically-operated electrical switch designed to protect an
electrical circuit from damage caused by overload or short circuit. • Its basic function is to
detect a fault condition and, by interrupting continuity, to immediately discontinue electrical
flow. • Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be
reset (either manually or automatically) to resume normal operation. • Circuit breakers are
made in varying sizes, from small devices that protect an individual household appliance up to
large switchgear designed to protect high voltage circuits feeding an entire city.

UPS protects system from power problems

Voltage Surges and spikes •Voltage sags •Total power failure •Frequency difference
Why do you need a UPS
Power failure • Voltage sag • Voltage spike • Brownout • Over-voltage • Line noise •
Frequency variation • Switching transient • Harmonic distortion.

UPS working
AC Main Section
-receives AC supply, filter it with line
filter and rectifies it desired level for
further circuits. • Inverter and Filter -
works with and without power - with
power , delivers constant 230v , 50Hz
output to load. - without power ,this
takes 12v DC from battery, convert it
into 230v , 50Hz with the help of
inverter given to output load.

Battery and Battery Charger:

- with ac supply , charges battery through battery charger. - battery charger circuit convert
input AC to the desired DC level and charges the battery. • Static switch / contractor In event
of power failure the inverter is connected to the load with the help of static contractor
switches.

UPS – There are three major types of UPS system configuration:

 Online double conversion
 Line-interactive
 Offline (also called standby and battery backup)

The standby UPS (OFF line)

The standby UPS is the most common type used for desktop computers. In the block diagram,
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 to 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." High efficiency,
small size, and low cost are the main benefits of this design. With proper filter and surge
circuitry, these systems can also provide adequate noise filtration and surge suppression.
Advantages
Lower in cost compared to on-line UPS

The line interactive UPS

The line interactive UPS, illustrated in the figure, is the most common design used for small
business, Web, and departmental servers.
In this design, the battery-to-AC power
converter (inverter) is always connected to the output of the UPS. Operating the inverter in
reverse during times when the input AC power is normal provides battery charging. When the
input power fails, the transfer switch opens and the power flows from the battery to the UPS
output. With the inverter always on and connected to the output, this design provides
additional filtering and yields reduced switching transients when compared with the standby
UPS topology. In addition, the line interactive design usually incorporates a tap-changing
transformer. This adds voltage regulation by adjusting transformer taps as the input voltage
varies. Voltage regulation is an important feature when low voltage conditions exist, otherwise
the UPS would transfer to battery and then eventually down the load. This more frequent
battery usage can cause premature battery failure. However, the inverter can also be designed
such that its failure will still permit power flow from the AC input to the output, which
eliminates the potential of single point failure and effectively provides for two independent
power paths. High efficiency, small size, low cost and high reliability coupled with the ability to
correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5 kVA
power range.

The double conversion on-line UPS

This is the most common type of UPS above 10 kVA. The block diagram of the double
conversion on-line UPS, illustrated in figure below, is the same as the standby, except that the
primary power path is the inverter instead of the AC main..

In the double conversion on-line design, failure of the input AC does not cause activation of the transfer
switch, because the input AC is charging the
backup battery source which provides power
to the output inverter. Therefore, during an
input AC power failure, on-line operation
results in no transfer time. Both the battery
charger and the inverter convert the entire
load power flow in this design. This UPS
provides nearly ideal electrical output
performance. But the constant wear on the
power components reduces reliability over
other designs. Also, the input power drawn
by the large battery charger may be non-
linear which can interfere with building power wiring or cause problems with standby generators.

Factors to consider when choosing power adaptors for Laptop computers

The voltage and wattage of an adapter needs to match the required voltage of the laptop. The
alternative adapter can also be compatible to charge your laptop. In fact, there are many alternative
adapters with a higher CP value than the original adapter, and a Second factor to consider is the safety
of the adapter.