UNIT V POWER SUPPLIES AND ELECTRONIC DEVICE TESTING
Linear mode power supply - Rectifiers - Filters - Half-Wave Rectifier Power Supply Full-Wave Rectifier Power Supply - Voltage regulators: Voltage regulation - Linear
series, shunt and switching Voltage Regulators - Over voltage protection - BJT and
MOSFET – Switched mode power supply (SMPS) - Power Supply Performance and
Testing - Troubleshooting and Fault Analysis, Design of Regulated DC Power Supply.
1
Voltage Regulators
 A voltage regulator is a device designed to keep the output voltage of a
power supply as nearly as constant as possible.
 A voltage regulator is designed to automatically ‘regulate’ voltage level. It
basically steps down the input voltage to the desired level and keeps that
in that same level during the supply. This makes sure that even when a
load is applied the voltage doesn’t drop.
2
Voltage Regulators
A voltage regulator is used for the following reasons:•
To regulate or vary the output voltage of the circuit.
•
To keep the output voltage constant at the desired value inspite of
variations in the supply voltage or in the load current.
Voltage regulators -Applications
•
Computers
•
Alternators
•
Power generator plants
3
Voltage Regulators
 Voltage regulators may be classified as electromechanical or electronic. It
can also be classified as AC regulators or DC regulators.
 There are mainly two types of voltage regulators:
•
Linear voltage regulators
•
Switching voltage regulators
 Linear voltage regulator is the easiest type of voltage regulators. It is
available in two types, which are compact and used in low power, low
voltage systems.
4
Regulator - Zener diode regulator
 For low current power supplies - a simple voltage regulator can be
made with a resistor and a zener diode connected in reverse.
 Zener diodes are rated by their breakdown voltage Vz and maximum
power Pz (typically 400mW or 1.3W)
5
Voltage Regulation
Two basic categories of voltage regulation are:

line regulation

load regulation
 The purpose of line regulation is to maintain a nearly constant output
voltage when the input voltage varies.
 The purpose of load regulation is to maintain a nearly constant
output voltage when the load varies
6
Line Regulation
Line regulation: A change in input (line) voltage does not significantly affect the output voltage of a
regulator (within certain limits)
7
Line Regulation
 Line regulation can be defined as the percentage change in the
output voltage for a given change in the input voltage.
 VOUT 

Line regulation  
 V
 100%
IN


Δ means “a change in”
 Line regulation can be calculated using the following formula:

VOUT / VOUT   100%
Line regulation 
VIN
8
Load Regulation
Load regulation: A change in load current (due to a varying RL) has practically no effect on the output
voltage of a regulator (within certain limits)
9
Load Regulation
 Load regulation can be defined as the percentage change in the
output voltage from no-load (NL) to full-load (FL).
 VNL  VFL
Load regulation  

VFL

Where:
VNL = the no-load output voltage
VFL = the full-load output voltage


 100%

10
Load Regulation
 Sometimes power supply manufacturers specify the equivalent
output resistance (Rout) instead of its load regulation.
 RFL equal the smallest-rated load resistance, then VFL:
VFL

RFL
 VNL 
 ROUT  RFL



11
Load Regulation
Rearrange the equation:
 ROUT  RFL
 VFL 

RFL



VNL


 ROUT  RFL
VFL 

RFL

Load regulation 
VFL


  VFL

 100%
 ROUT  RFL

Load regulation  
 1
 100%


RFL


 ROUT 

Load regulation  
 R
  100%
FL 

12
Types of Regulator
 Fundamental classes of voltage regulators are linear regulators and
switching regulators.
 Two basic types of linear regulator are the series regulator and the
shunt regulator .
 The series regulator is connected in series with the load and the
shunt regulator is connected in parallel with the load.
13
Series Regulator Circuit
 Control element in series with
load between input and output.
 Output sample circuit senses a
change in output voltage.
 Error detector compares sample
voltage with reference voltage
causes
control
element
to
compensate in order to maintain
a constant output voltage.
14
Op-Amp Series Regulator
Control Element
VREF
Error Detector
Sample Circuit
15
Op-Amp Series Regulator
 The resistor R1 and R2 sense a change in the output voltage and
provide a feedback voltage.
 The error detector compares the feedback voltage with a Zener
diode reference voltage.
 The resulting difference voltage causes the transistor Q1 controls
the conduction to compensate the variation of the output voltage.
 The output voltage will be maintained at a constant value of:

R1 

Vo  
1

V
Z


R2 

16
Transistor Series Regulator
 The transistor Q1 is the series control element.
 Zener diode provides the reference voltage.
17
Transistor Series Regulator
 Since Q1 is an NPN transistor, Vo is found as:
VBE  VZ  Vo
 The response of the pass-transistor to a change in load resistance as
follows:




If load resistance increases, load voltage also increases.
Since the Zener voltage is constant, the increase in Vo causes VBE to
decrease.
The decrease in VBE reduces conduction through the pass- transistor, so
load current decreases.
This offsets the increase in load resistance, and a relatively constant
load voltage is maintained
18
Shunt Regulator Circuit
 The unregulated input voltage
provides current to the load.
Some of the current is pulled
away by the control element.
 If the load voltage tries to
change due to a change in the
load
resistance,
the
sampling
circuit provides a feedback signal
to a comparator.
19
Shunt Regulator Circuit
The resulting difference voltage then
provides a control signal to vary the
amount of the current shunted away
from the load to maintain the regulated
output voltage across the load.
20
Op-Amp Shunt Regulator
21
Op-Amp Shunt Regulator
 When the output voltage tries to decrease due to a change in input
voltage or load current caused by a change in load resistance, the
decrease is sensed by R1 and R2.
 A feedback voltage obtained from voltage divider
R1 and R2 is
applied to the op-amp’s non-inverting input and compared to the
Zener voltage to control the drive current to the transistor.
 The current through resistor RS is thus controlled to drop a voltage
across RS so that the output voltage is maintained.
22
Transistor Shunt Regulator
 The control element is a transistor, in
parallel
with
the
load.
While,
the
resistor, RS, is in series with the load.
 The operation of the transistor shunt
regulator is similar to that of the
transistor series regulator, except that
regulation is achieved by controlling the
current through the parallel transistor
23
Transistor Shunt Regulator
 Resistor
RS
drops
the
unregulated voltage depends on
current supplied to load RL.
 Voltage across the load is set
by zener diode and transistor
base-emitter voltage.
 If RL decrease, a reduced drive
current to base of Q1 
shunting less collector current.
 Load current, IL is larger,
maintaining
the
regulated
voltage across load.
24
Transistor Shunt Regulator
 The output voltage to the load is:
Vo  VL  VZ  VBE
 Voltage across the load is set by the Zener diode voltage and the
transistor base-emitter voltage.
 If the load resistance decreases, the load current will be larger at a
value of:
VL
IL 
RL
 The increase in load current causes the collector current shunted by
the transistor is to be less:
IC  I S  I L
 The current through RS:
IS
Vi  VL

RS
25
Switching Regulator

The switching regulator is a type of
regulator
circuit
which
its
efficient
transfer of power to the load is greater
than series and shunt regulators because
the transistor is not always conducting.

The
switching
regulator
passes
voltage to the load in pulses, which then
filtered to provide a smooth dc voltage.
26
Switching Regulator
 The switching regulator is more efficient than the linear series or
shunt type.
 This type regulator is ideal for high current applications since less
power is dissipated.
 Voltage regulation in a switching regulator is achieved by the on and
off action limiting the amount of current flow based on the varying
line and load conditions.
 With switching regulators 90% efficiencies can be achieved.
27
Switching Regulator-Step-Down Configuration
 With the step-down (output is
less
than
the
input)
configuration
the
control
element Q1 is pulsed on and off
at variable rate based on the
load current.
 The pulsations are filtered out
by the LC filter.
28
Switching Regulator-Step-up configuration
 The difference is in the placement of the inductor and the fact
that Q1 is shunt configured.
 During the time when Q1 is off the VL adds to VC stepping the
voltage up by some amount.
29
Switching Regulator-Voltage-Inverter configuration
 Output voltage is of opposite polarity
of the input.
 This is achieved by VL forwardbiasing reverse-biased diode during
the off times producing current and
charging the capacitor for voltage
production during the off times.
 With
switching
regulators
90%
efficiencies can be achieved.
30
IC Voltage Regulators
 Regulation circuits in integrated circuit form are widely used.
 Their operation is no different but they are treated as a single device
with associated components.
 These are generally three terminal devices that provide a positive or
negative output.
 Some types have variable voltage outputs. A typical 7800 series voltage
regulator is used for positive voltages. The 7900 series are negative
voltage regulators.
 These voltage regulators when used with heat sinks can safely produce
current values of 1A and greater. The capacitors act as line filtration.
31
IC Voltage Regulators
 Several types of both linear (series and shunt) and switching
regulators are available in integrated circuit (IC) form.
 Reference source
 Comparator amplifier
 Control device
 Overload protection
 Generally, the linear regulators are three-terminal devices that
provides either positive or negative output voltages that can be
either fixed or adjustable.
32
Fixed Voltage Regulator
 The fixed voltage regulator has an unregulated dc input voltage Vi
applied to one input terminal, a regulated output dc voltage Vo from
a second terminal, and the third terminal connected to ground.
33
Fixed Voltage Regulator
Fixed-Positive Voltage Regulator
 The series 78XX regulators are
the three-terminal devices that
provide a fixed positive output
voltage.
34
Fixed Voltage Regulator
 An unregulated input voltage Vi is
filtered by a capacitor C1 and
connected to the IC’s IN terminal.
 The IC’s OUT terminal provides a
regulated +12 V, which is filtered
by capacitor C2.
 The third IC terminal is connected
to ground (GND)
35
Fixed Voltage Regulator
Positive-Voltage Regulators in the 78XX Series
IC Part
Output Voltage (V)
Minimum Vi (V)
7805
7806
+5
+6
7808
+8
+10.5
7810
7812
+10
+12
+12.5
+14.5
7815
+15
+17.7
7818
7824
+18
+24
+21.0
+27.1
+7.3
+8.3
36
Fixed Voltage Regulator-Negative Voltage Regulator

The series 79XX regulators are the three-terminal IC
regulators that provide a fixed negative output voltage.

This series has the same features and characteristics as the
series 78XX regulators except the pin numbers are different.
37
Fixed Voltage Regulator
Negative-Voltage Regulators in the 79XX Series
IC Part
Output Voltage (V)
Minimum Vi (V)
7905
7906
-5
-6
7908
-8
-10.5
7909
7912
-9
-12
-11.5
-14.6
7915
-15
-17.7
7918
7924
-18
-24
-20.8
-27.1
-7.3
-8.4
38
Fixed Voltage Regulator –Adjustable Voltage Regulator

Voltage regulators are also available in circuit configurations
that allow to set the output voltage to a desired regulated value.

The LM317 is an example of an adjustable-voltage regulator, can
be operated over the range of voltage from 1.2 to 37 V.
39
Summary
 Voltage regulators keep a constant dc
output despite input voltage or load
changes.
 The two basic categories of voltage
regulators are linear and switching.
 The two types of linear voltage regulators
are series and shunt.
 The three types of switching are step-up,
step-down, and inverting.
 Switching regulators are more efficient
 IC regulators are available with fixed
positive or negative output voltages or
variable
negative
or
positive
output
voltages.
 Both linear and switching type regulators
are available in IC form.
 Current capacity of a voltage regulator
can be increased with an external pass
transistor.
than linear making them ideal for low
voltage high current applications
40
Fixed Voltage Regulator
 Power supply convert alternating current to the direct (DC) current mainly
convert 110-240v AC
 Three types of power supply:
 Linear power supply
 Switched mode (SMPS stands for Switch Mode Power Supply).
 Uninterrupted (UPS) power supply
 This receives 230V,AC and translates it into different DC levels such as
+5V, -5V, +12V, -12V.
41
Linear power supply
 Transformer convert the line AC voltage to a smaller peak voltage
 Rectifies AC signal produces large waveforms , capacitor filter is used to
filter the rectified wave which contain small pulses (ripple).
 Depending on the requirements regulator adjust the output voltage
 Good line and load regulation lower output voltage ripples.
42
Linear power supply
43
Operation
 The power supplies used in computers are switched mode
power supplies.
 The primary power received from AC mains is rectified and
filtered as high-voltage DC.
44
Switched Mode Power Supply (SMPS)

The disadvantages of LPS such as lower efficiency, the
need for large value of capacitors to reduce ripples and heavy
and
costly
transformers
etc.
are
overcome
by
the
implementation of Switched Mode Power Supplies.

The working of SMPS is simply understood by knowing
that the transistor used in LPS is used to control the voltage
drop while the transistor in SMPS is used as a controlled
switch.
45
SMPS
SMPS stands for Switch Mode Power
Supply which is an electronic power
supply with switching regulator.
 This
receives
230V
AC
and
translates it into different DC levels
such as +5V, -5V, +12V, -12V.
 It is switched to a high frequency
approximately 10 to 100 KHz by a
bipolar transistor and fed to the
primary side (P) of a step-down
transformer.
 Uses feedback mechanism
46
SMPS SCHEMATIC DIAGRAM
47
48
Input Stage
The AC input supply signal 50 Hz is given directly to the rectifier
and filter circuit combination without using any transformer. This
output will have many variations and the capacitance value of the
capacitor should be higher to handle the input fluctuations. This
unregulated dc is given to the central switching section of SMPS.
49
Switching Section
A fast switching device such as a Power transistor or a MOSFET
is employed in this section, which switches ON and OFF according to the
variations and this output is given to the primary of the transformer
present in this section. The transformer used here are much smaller and
lighter ones unlike the ones used for 60 Hz supply. These are much
efficient and hence the power conversion ratio is higher.
50
Output Stage
The output signal from the switching section is again rectified
and filtered, to get the required DC voltage. This is a regulated
output voltage which is then given to the control circuit, which is a
feedback circuit. The final output is obtained after considering the
feedback signal.
Control Unit
This unit is the feedback circuit which has many sections
51
SMPS working
 Convert AC to DC voltage with rectifier which is unregulated DC voltage
sent it to filter
 Inverter convert DC to AC with help of power oscillator.
 Output transformer inverts AC voltage up to down to the required
output level.
 Output rectifier and filter : AC output from transformer is rectified.
 For lower voltage uses silicon/schottky diodes used and smoothing the
rectified output by using filter.
52
The SMPS is mostly used where switching of voltages is not at all a
problem and where efficiency of the system really matters.
SMPS circuit is operated by switching and hence the voltages vary
continuously.
The switching device is operated in saturation or cut off mode.
The output voltage is controlled by the switching time of the feedback
circuitry.
Switching time is adjusted by adjusting the duty cycle.The efficiency of
SMPS is high because, instead of dissipating excess power as heat, it
continuously switches its input to control the output.
53
SMPS working

This reduces the amount of the voltage passed through the
transformer.
 So the output voltage will be maintained normally.
 Then it is sent to the output of the power supply.
 A sample of this output is sent back as feedback signal for regulation.
54
Types of SMPS
SMPS is the Switched Mode Power Supply circuit which is designed for
obtaining the regulated DC output voltage from an unregulated DC or AC
voltage. There are four main types of SMPS such as

DC to DC Converter

AC to DC Converter

Fly back Converter

Forward Converter
55
Types of SMPS

The AC to DC conversion part in the input section makes the
difference between AC to DC converter and DC to DC converter. The
Fly back converter is used for Low power applications.

Also there are Buck Converter and Boost converter in the
SMPS types which decrease or increase the output voltage
depending upon the requirements. The other type of SMPS include
Self-oscillating fly-back converter, Buck-boost converter, Cuk,
Sepic, etc.
56
DC-DC Converter
The power received from AC mains is rectified and filtered as
a high-voltage DC. This high voltage DC voltage is then switched and
fed to the step-down transformer at the primary side. At the
secondary side of the step-down transformer the rectified and
filtered output is collected which is ultimately sent as the output to
power supply.
57
Forward Converter
Irrespective of if the transistor is conducting or not the
choke carries the current in forward converter. The diode inside the
transistor carries the current during the OFF period to support the
energy flow through the load. During the On period, the choke stores
the energy and also passes a part of the energy to the output load.
58
Flyback Converter
In a Flyback converter, during the On period of the switch
magnetic field of the inductor stores energy. When the switch is in the
open state the energy is emptied into the output voltage circuit. The
Duty cycle in the Flyback converter is determined by the output
voltage.
59
Self-Oscillating Flyback Converter
It is based on the Flyback principle. During conduction, a current
through the transformer primary starts to ramp up linearly with the
slope Vin/Lp. Due to the voltage induced in the feedback winding and
the secondary winding the fast recovery rectifier start to operate in
reverse biased and hold the conducting transistor ON.
The core begins to saturate once the current reaches its peak
value. The result is a sharp rise in current is not supported by the fixed
base drive supported by feedback windings. Hence, the switching begins
to come out of saturation.
60
Advantages

The efficiency is as high as 80 to 90%

Less heat generation; less power wastage.

Reduced harmonic feedback into the supply mains.

The device is compact and small in size.

The manufacturing cost is reduced.

Provision for providing the required number of voltages.
61
Disadvantages
The noise is present due to high frequency switching.
The circuit is complex.
It produces electromagnetic interference
62
Applications
 Motherboard of computers
 Motor vehicles
 Mobile phone chargers
 Consumer electronics
 HVDC measurements
 Laptops
 Battery chargers
 Security systems
 Central power distribution
 Space stations
63
Power Supply Characteristic
Wattage
The total, maximum output of the power supply in watts, Typical power
ranges are from 200W to 500W.
Efficiency
Efficiency=Useful Power Output / Total Electrical Power Consume.
Regulation
The ability of a SMPS to maintain an output voltage within specified
limits under varying of input voltage.
64
Power Supply Characteristic

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.

Load Regulation: Sometimes called voltage load regulation. This
specification refers to the ability of the power supply to control the
output voltage level

Line Regulation: The complement of load regulation, this parameter
describes the ability of the power supply to control its output levels
65
Power Supply problems
Brownouts (Sag):
Blackouts
This is complete loss of electric
The under voltage condition The
power where voltage and current drop
high load items like air conditioners,
to
physical
welding machine, motor etc draw to
interruption in the power line due to
much current that the AC voltage
accidental damage by a person or act
level drops. power supply will fall out
of nature, loss of AC will invariably
which
shutdown the computer,loss of data,
system operation. file may be lost or
reduction productivity, corrupt file
corrupted on the hard drive.
0,usually
caused
by
resulting
in
intermittent
structure and damage files.
66
Power Supply problems
Surge: Small over voltage conditions that take place over relatively long
periods and regulate power to a desired level excess energy must be
switched (in SMPS).
Spikes: A spike is a large over voltage condition that occurs in the
milliseconds. high energy switches can cause spikes on the AC line.
Example equipment like drill machine, grinders, welding equipment etc.
can produce power spikes.
67
Symptoms Supply problems
i.
Flickering Lights
ii.
Premature Component Failure
iii.
Hard Drive Crashes
iv.
The PC stalls, crashes, or reboots for no apparent reason
v.
You suffer chronic or frequent hard drive failures or file access problems.
vi.
The CMOS RAM or modem NVRAM periodically looses its contents or becomes
corrupted.
vii. The PC behaves erratically when other high-energy devices are turned on.
viii. The modem regularly looses its connection, or fails data transfers.
ix. The monitor display flickers or waves.
68
Protection Devices
Surge Suppressor: Simple and relatively inexpensive devices, designed to
absorb high-voltage transients produced by lightning and other highenergy equipment. Device inserted in AC to avoid spikes. Avoids peak AC
voltage. Protection is accomplished by clamping (or shunting) voltages
above a certain level (usually above 200 volts). Metal oxide varistor, or
MOV, diverts the extra voltage.
Circuit Breaker : Its purpose is to protect lighting circuit wiring from
accidental short-circuits and overloads.
69