Download Voltage

Subject Code: 2131006
Electronics & Communication Department
KIRC
Introduction
Batteries
are often shown on a schematic diagram as the
source of DC voltage but usually the actual DC voltage
source is a power supply.
There
are many types of power supply. Most are designed
to convert high voltage AC mains electricity to a suitable low
voltage supply for electronics circuits and other devices.
 A more reliable method of obtaining DC power is to
transform, rectify, filter and regulate an AC line voltage.
 A power supply can be broken down into a series of
blocks, each of which performs a particular function.
Power Supplies
 For example a 5V regulated supply
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Transformers convert AC electricity from one voltage to
another with little loss of power through inductively
coupled electrical conductors.
Transformers work only with AC voltage
Step-up transformers increase voltage.
Step-down transformers reduce voltage.
 Electrical device that converts alternating current (AC) to
direct current (DC) with a process known as rectification
Full-wave bridge rectifier
 Produces full-wave varying DC.
 A bridge rectifier can be made using four individual diodes,
but it is also available in special packages containing the
four diodes required.
 Voltage regulator ICs are available with
fixed (typically 5, 12 and 15V) or variable
output voltages.
• 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 as shown in the diagram on the next page.
 Zener diodes are rated by their breakdown voltage Vz and
maximum power Pz (typically 400mW or 1.3W).
• Zener diode regulator
Voltage Regulation
 Line regulation is the maintenance of a nearly constant output
voltage when the input voltage varies.
 When the ac input (line) voltage of a power supply changes,
an electronic circuit called a regulator maintains a nearly
constant output voltage
How well a regulator performs line regulation can be determined
by the formula below.
Line Regulation =
 Vout 

100%
 Vin 
Voltage Regulation
Load regulation is the maintenance of a nearly constant
output voltage when the load varies.
When the amount of current through a load changes due
to a varying load resistance, the voltage regulator must
maintain a nearly constant output voltage across the load.
How well a regulator performs load regulation can be
determined by the formula below.
Load Regulation =
 VNL  VFL 

100%
 VFL 
Types of Regulator
 The fundamental classes of voltage regulators are linear
regulator and switching regulators.
Two basics types of linear regulator are :
•
Series Regulator
•
Shunt Regulator
Basic Linear Series Regulator
 In series regulation the control element is in series with
the input and output.
Simple series voltage regulator and block diagram
Basic op-amp series regulator
 The control element is a transistor in series with the load.
 The output sample circuit senses a change in the output voltage.
The error detector compares the sample voltage with a reference
voltage (zener diode)
Control is achieved by controlling the voltage across Q1
 Regulating
Action
The resistive voltage divider formed by R2 and R3 senses any
changes in the output voltage.
 When the output voltage tries to decrease, a proportional voltage
decrease is applied to the op-amp’s inverting input by the voltage
divider.
 Zener diode sets the reference voltage for the non-inverting input of
the op-amp. Any changes in the output are fed back to the inverting
input of the op-amp.
 The difference voltage (error voltage) is amplified and the op-amp’s
output voltage is increased.
 This increase is applied to the base of Q1 – causing the emitter
voltage to increase until the voltage to the inverting input equals the
reference voltage.
 The regulated output voltage of the series regulator is :
Vout
 R2 
 1  VREF
 R3 
 Short-Circuit or Overload Protection
 Overload protection for a series regulator protects the control element in
the case of a short.
 Also known as constant-current limiting.
 The load current through R4 produces a voltage from base to emitter of Q2
 When IL reaches a predetermined maximum value, the voltage drop
across R4 is sufficient to forward – biased the base – emitter junction of Q2 –
causing it to conduct.
 When load current exceeds the predetermined level, Q2 diverts current
from the base of Q1- reducing the load current through Q1 – preventing any
additional of load current.
 Regulator with Fold-Back Current Limiting
 Fold-back current limiting allows operation up to peak load current
(high current regulator)
 With a shorted output the current is dropped to a lower value
(folded back) to prevent the overheating the device.
 The voltage drop developed across R4 by the load current must not
only overcome the base-emitter voltage – but VR5 + VBE must be
overcome before Q2 conducts to limit current.
Basic Linear Shunt Regulator
 In shunt regulation the control element is in parallel
with input and output.
Simple shunt voltage regulator and block diagram
 The control element is a transistor, Q1 parallel (shunt)
with the load and a resistor R1 in series with the load.
 The operation same as series, except that regulation is
achieved by controlling the current through Q1.
Basic op-amp shunt regulator with load resistor
 Regulating Action
When the output voltage tries to decrease due to a change in
input voltage or load current, the attempted decrease is sense
by R3 and R4 - applied to the op-amp’s non-inverting input.
The difference voltage increases the op-amp’s output voltage - and increasing the collector voltage, Vc – keeping the output
nearly constant.
Basic Switching Regulator
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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.
Basically, a switching regulator passes voltage to the load
pulses, which are then filtered to provide a smooth dc voltage.
With switching regulators 90% efficiencies can be achieved.
 Step
– Down Configuration
 With the step-down (output voltage is less than the input voltage)
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.
 Since Q1 is either on or off, the power lost in the control element is
relatively small
Basic step – down switching regulator
 When the on-time is
increased, capacitor charges
more – increasing the output
voltage.
When the on-time is
decreased, the capacitor
discharges more – decreasing
the output voltage.
 The inductor smoothes the
fluctuations of the output
voltage caused by the
charging and discharging.
Switching regulator waveform
 Step – Up Configuration
 The step-up configuration works much the same as the step-down.
 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.
 Voltage – Inverter Configuration
 With the voltage-inverter configuration the output voltage
output is of opposite polarity of the input.
 This is achieved by VL forward-biasing 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.
Basic inverting switching regulator
IC Regulator
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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 are have variable voltage outputs.
Connection of a three-terminal voltage regulator IC to a load
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A typical 78XX series of IC regulators – three-terminal
devices that provide a fixed positive output voltage.
A typical 79XX series of IC regulators – three-terminal
devices that provide a fixed negative output voltage
Almost all applications of regulators require the device be
secured to a heat sink to prevent thermal overload.
The capacitors act as line filtration.
 Adjustable Positive Linear Voltage Regulator
 Adjustable IC regulators are available with either positive or
negative output.
 They can be set to produce a specific voltage by way of an
external reference voltage divider network.
 Vout can be varied from 1.2V to 37V depending on the resistor
values.
The LM317 three-terminal adjustable positive voltage regulator
 The External Pass Transistor
 To increase the current capability of an IC regulator an
external pass transistor can be used in order to prevent
thermal load condition.
A 78XX-series three-terminal regulator with an external pass transistor
 Current Limiting
 The external transistor must be protected from
excessive current.
 An additional current limiting circuit (Qlim and Rlim ) – to
protect Qext from excessive current and possible burn out.
Regulator with current limiting
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 than linear
making them ideal for low voltage high current
applications.
 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.