CHAPTER 5

SMPS

Power Supply
 Chapter Overview
• Power Supplies
1) LINEAR POWER
SUPPLY
2) SMPS
3)UPS
• Power Supply
Problems
• Connectors
• UPS : Types
• Protection devices.
 Computer Power Supply
Unit
(Computer PSU) typically is designed to convert
110 V or 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..
 Selection of power supply
• Voltage rating
• Current rating
• Power requirements
• Line regulation
• Load regulation
• Nature of input (AC)
• Usage of computer system
• Spikes and surges in the power
• Efficiency
• Linearity
• Frequency of operation.
 Linear Power Supply

• Power supply convert AC to DC voltages.

Waveforms found on a linear power supply
Transformer:
Transformer used to
convert the voltage
from mains to a
different ,usually lower
voltage. (Step down)

Rectifier:
Then rectification is done by
a set of diodes, transforming
this AC voltage into
pulsating voltage DC
(bridge rectifier)
Filter: The next step is
filtering, which is done by
an electrolytic capacitor,
transforming this
pulsating voltage into
almost DC
DC obtained after the
capacitor oscillates a little
bit (Ripple)
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
SWITCHED MODE POWE SUPPLY
DC
SMPS REGULATED
OUTPUT
DC –DC CONVERSION + ISOLATION
V0

BASE
GATE
DRIVER

Vref
 Explanation
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:-
• 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.
• 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
Power Supply form factor.
• Early Pc using
PC/XT,AT,baby AT
and LPX form
factors all use
mechanical switch
to turn computer on
and off.
 Power Supply Sizes
• Power supply sizes are based on the type of
case and motherboard connections.
• 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.
• On –off power control circuit. Not the button
on ATX boards is built into the MB, on AT style
it comes from the PS.
 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
 ATX 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.
 Interior view of an ATX
switched-mode power supply:
• A - Bridge rectifier

• B - Input filter
capacitors

• C - Transformer

• D - Output filter coil

• E - Output filter
capacitors
Power Supply Connectors
Connections to Peripheral
Hardware
 Splitter
• A splitter increases the number of
connections.
AT style SMPS.

• AT style computer cases had a power

button that was directly connected to
system PSU.
• The wires are soldered to the power
button
• AT style SMPS provides DC output on
two 6-pin connectors and four 4-pin
connector.
• 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 Power supply.
 ATX/NLX style SMPS
20 pin ATX power supply
connector
If the motherboard
requires 3.3V
current over 12A or
+5V current over
24A. 24 pin power
connector is used.
 Use of output voltages of
SMPS.
• -12V :
used on some types of serial port circuits,whose
amplifier circuits require both –12v and +12V.
• 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
Use of output voltages of SMPS
• 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
 Power supply characteristic

• Rated Wattage :
To operate different components of PC, power
supply must generate rated power. The
generated power by power supply as per
requirment 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.
• Efficiency
Efficiency is defined as useful power output divided
by the total electrical power consumed.
Efficiency of SMPS is 70-85%
• Regulation
The ability of SMPS to maintain an output voltage
constant as per the rated value irrespective of change
in any other parameter.
Load Regulation:
• Sometimes called voltage load regulation.

• This specification 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.
Line Regulation:
• This parameter describes the ability of the power
supply to maintain its output voltage constant
irrespective of change in input source of power
supply.

• Again, a value for each output level is usually

specified as a "+/-" percentage. +/- 1% to 2% is
typical.
Ripple
• Also sometimes called "AC Ripple" or "Periodic and Random
Deviation (PARD)" or simply "Noise".

• Periodic noise in constant output voltage of power supply is

called as ripple or ripple factor

• The power supply produces DC outputs from AC input. The

output isn't "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 (mVp-p).
• "Lower numbers are better.
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
• 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)
 Power Problems
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 shutdown the
computer in few millisecond.
• Loosing 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
Power Problems
• 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
 Power Problems
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.
 Power Problems
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
1. 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 cord 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.
2. 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.
 Circuit breaker
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
 Circuit breaker
5. Bimetallic strip - A bimetallic strip is used to
convert a temperature change into mechanical
displacement.

6. Calibration screw - allows the manufacturer to

precisely adjust the trip current of the device
after assembly.

7. Solenoid - long, thin loop of wire, often wrapped

around a metallic core, which produces
a magnetic field when an electric current is
passed through it. Solenoids are important
because they can create controlled magnetic
fields and can be used as electromagnets .

8. Arc divider / extinguisher

 UPS protects system from
power problems
•Voltage Surges and spikes
•Voltage sags
•Total power failure
•Frequency difference
 Need OF UPS
• Power failure
• Voltage sag
• Voltage spike
• Brownout
• Over-voltage
• Line noise
• Frequency variation
• Switching transient
• Harmonic distortion.
General UPS working
 BLOCK DIAGRAM OF UPS
• 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.
Standby UPS(OFF LINE UPS)
ON LINE UPS
Advantage of OFF LINE UPS
• Lower in cost compared to on-line
UPS

Disadvantage of Stand By UPS

High switching is required otherwise

there is possibility that cut in power
and reboot the system.
Advantages Of on-line ups

• Switch not involved , avoids resetting

of PC and spike generation.

• Computer is isolated from AC line

problems.

• Provide protection by breaking down

and resetting the power.
Disadvantages of on-line UPS
• It is costlier than off-line UPS.

• It generates more heat.

• UPS batteries need to be replaced.

• UPS running its inverter all the time

results in a lower efficiency.
 Comparison
Off-line UPS On-line UPS
• Simplest and least • Complex and
expensive. expensive
• Battery is charged • Battery is
when Ac mains are continuously
on, when AC mains charged.,delivers DC
are Off, battery power to inverter for
discharges and converting To AC and
supplies power to PC. supplying To PC.
• Switching occur. • Switching not occurs.
• Not at high speed, • At high speed , avoid
resetting may occur resetting
• Spikes are generated. • Spikes not generated
 Advantages Of UPS over
normal power supply.
• Provides power backup.
• Surge protection.
• Isolation and proper shielding of power.
• Short-circuit protection.
• Stabilities the power.
• Maintains constant 230V, 50Hz.
• Power conditioning.
• Allows you enough time to save data.
• Avoids data loss.
• Protect OS from corruption .
• Can be controller by OS called smart UPS.
 USB
• Peripherals to be connected using single
standardized interface socket
• Improve plug and play capabilities (without
rebooting)
• Low power consumption without external power
supply
• Connect computer peripherals
• Possible to install and remove devices without
opening computer case.
• The Universal Serial Bus gives you a single,
standardized, easy-to-use way to connect up to
127 devices to a computer.
 Cables:
• Maximum length of standard USB cable is 5 meters.
• Primary reason for this limit is the maximum allowed Round
trip delay of about 1500ns.
• If USB device does not answer to host commands within
the allowed time the host considers command to be lost.
• Maximum delay caused by single cable turns out to be 26
ns.
Sr. No
• USB Name
2.0 specification Cable
states that delayColor
must beDescription
less than
5.2ns per meter.
1 VCC RED +5V
• Inside a USB cable: There are two wires for power –
• +5 2 D-
volts (red) and ground (black)GREEN DATA-
and a twisted pair (Green
and white) of wires to carry the data. The cable is also
3
shielded. D+ WHITE DATA+
4 GND BLACK GROUND
 USB Features
• Host :- The computer acts as the host.
• Multiple devices :- Up to 127 devices can connect to the host, either
directly or by way of USB hubs.
• USB Cable length :- Individual USB cables can run as long as 5 meters;
with hubs, devices can be up to 30 meters away from the host.
• Transfer rate:-With USB 2.0,the bus has a maximum data rate of 480
megabits per second.
• Ease of installation :- A USB cable has two wires for power (+5 volts and
ground) and a twisted pair of wires to carry the data.
• Power allocation:- USB controller in PC detects the presence or absence
of the USB device and does allocation of electrical power.
• On the power wires, the computer can supply up to 500 milliamps of power
at 5 volts.
• Low-power devices (such as mice) can draw their power directly from the
bus.
• High-power devices (such as printers) have their own power supplies and
draw minimal power from the bus.
• Hubs can have their own power supplies to provide power to devices
connected to the hub.
• Hot Swappable :- USB devices are hot-
swappable, meaning you can plug them into the
bus and unplug them any time.
• Hot Pluggability :- without powering off a PC
and Plug and play feature in the BIOS together
with intelligence in the USB device takes care of
detection , device recognition and handling.
• Hub architecture
• Power saving : -Many USB devices can be put to
sleep by the host computer when the computer
enters a power-saving mode.
• Supports for wide range of peripherals.
 RS 232 interface
• Developed by Electronics Industries Association
(EIA)
• RS – recommended Standards
• Serial communication is the most simplistic form
of communication between two devices.
• One important aspect of RS-232 is that it is an
asynchronous form of communication.
• Asynchronous communication is important
because it is efficient; if no data needs to be
sent, the connection is “idle.” No additional CPU
overhead is required for an idle serial line.
 Voltage levels of RS232

• Logical 1 – Marking estate –indicate negative

level - OFF
• Logical 0 – space estate – indicate positive level -
ON