Transcript UNIT 7 - WordPress.com

Subject Name: LINEAR INTEGRATED CIRCUITS
Subject Code: 10EC46
Prepared By: M.PRIYADHARSINI
Department: ECE
Date: 6/03/2015
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UNIT 7
VOLTAGE REGULATORS
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TOPIC DETAILS
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Introduction
Voltage Regulation
Line Regulation
Load Regulation
Series Regulator
Shunt Regulator
IC Voltage Regulator
Switching Regulator
INTRODUCTION
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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.
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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.
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A more reliable method of obtaining DC power is to transform, rectify, filter
and regulate an AC line voltage.
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A power supply can by broken down into a series of blocks, each of which
performs a particular function.
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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
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LINE REGULATION
Line regulation: A change in input (line) voltage does not significantly affect the output
voltage of a regulator (within certain limits)
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LINE REGULATION
Line regulation: A change in input (line) voltage does not significantly affect the output
voltage of a regulator (within certain limits)
• Line regulation can be defined as the percentage change in the output voltage for a
given change in the input voltage.
• Line regulation calculated using the formula as
Line regulation = (Δ VOUT/ Δ Vin)*100
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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)
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LOAD REGULATION
Load regulation can be defined as the percentage change in the output
voltage from no-load (NL) to full-load (FL).
Load regulation = (VNL – VFL/ VFL)*100
where:
VNL = the no-load output voltage
VFL = the full-load output voltage
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TYPES OF REGULATORS
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.
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SERIES REGULATORS
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Control element in series with load between input and output.
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Output sample circuit senses a change in output voltage.
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Error detector compares sample voltage with reference voltage →
causes control element to compensate in order to maintain a
constant output voltage.
OP-AMP SERIES REGULATORS
Control Element
VREF
Error Detector
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Sample
Circuit
OP-AMP SERIES REGULATORS
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The resistor R1 and R2 sense a change in the output voltage and
provide a feedback voltage.
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The error detector compares the feedback voltage with a Zener
diode reference voltage.
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The resulting difference voltage causes the transistor Q1 controls
the conduction to compensate the variation of the output voltage.
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The output voltage will be maintained at a constant value of:

R1 
Vo  
1  R 
VZ
2 

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IC REGULATORS
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Regulation circuits in integrated circuit form are widely used.
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Their operation is no different but they are treated as a single device with
associated components.
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These are generally three terminal devices that provide a positive or negative
output.
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Some types have variable voltage outputs.
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A typical 7800 series voltage regulator is used for positive voltages.
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The 7900 series are negative voltage regulators.
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These voltage regulators when used with heatsinks can safely produce current
values of 1A and greater.
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The capacitors act as line filtration.
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IC REGULATORS
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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.
Fixed-Positive Voltage Regulator:
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The series 78XX regulators are the three-terminal devices that provide a fixed
positive output voltage.
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FIXED REGULATORS
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An unregulated input voltage Vi is filtered by a capacitor C1 and connected to the
IC’s IN terminal.
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The IC’s OUT terminal provides a regulated +12 V, which is filtered by capacitor C2.
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The third IC terminal is connected to ground (GND)
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FIXED VOLTAGE REGULATORS
Positive-Voltage Regulators in the 78XX Series
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IC Part
7805
7806
7808
7810
7812
7815
7818
7824
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OUTPUT VOLTAGE
+5V
+6V
+8V
+10V
+12V
+15V
+18V
+24V
FIXED VOLTAGE REGULATORS
Fixed-Negative Voltage Regulator
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The series 79XX regulators are the three-terminal IC regulators that provide a fixed
negative output voltage.
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This series has the same features and characteristics as the series 78XX regulators
except the pin numbers are different.
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FIXED VOLTAGE REGULATORS
Negative Voltage regulators in the 79XX Series
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IC Part
7905
7906
7908
7909
7912
7915
7918
7924
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OUTPUT VOLTAGE
-5V
-6V
-8V
-10V
-12V
-15V
-18V
-24V
SWITCHED MODE POWER SUPPLY
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Switching regulation is the technique by which an unregulated power is efficiently
converted to regulated load power through the use of controlled switching
devices.
Switching voltage regulators are used where it is required to supply a stable
output voltage from a varying input voltage with a minimum power loss even
under varying load conditions.
A simple switching regulator consists of a control switch (transistor), a low pass
filter and sampling network etc..
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SWITCHED MODE POWER SUPPLY
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In the circuit S is a transistor used as a control switch. It is switched between
saturation and cut-off.
During ‘ON’ mode the switch is in saturation mode with negligible voltage drop
across the collector and emitter terminals of the switch where as in ‘OFF’ mode
the switch is in cut-off mode with negligible current through the collector and
emitter terminals.
On the contrary the voltage-regulating switch, in a linear regulator circuit, always
remains in the active region.
Due to this current is not passed continuously through the transistor and hence
the power dissipation is reduced to minimum.
The transistor is switched ON and OFF by an electronic pulse width modulator.
The filter circuit smoothens the output wave form and the sampling network (R1R2) controls the duty cycle such that the output voltage remains constant.
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IC VOLTAGE REGULATORS
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There are basically two kinds of IC voltage regulators:
• Multipin type, e.g. LM723C
• 3-pin type, e.g. 78/79XX
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Multipin regulators are less popular but they provide the greatest flexibility and
produce the highest quality voltage regulation
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3-pin types make regulator circuit design simple
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IC VOLTAGE REGULATORS
o The LM723 has an equivalent circuit
that contains most of the parts of
the op-amp voltage regulator
discussed earlier.
o It has an internal voltage reference,
error amplifier, pass transistor, and
current limiter all in one IC package.
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IC VOLTAGE REGULATORS-HIGH VOLTAGE CONFIGURATION
Design equations:
Vo 
Vref ( R1  R2 )
R2
R1 R2
R3 
R1  R2
Rsens 
0.7
I max
Choose R1 + R2 = 10 kW,
and Cc = 100 pF.
External pass transistor and
current sensing added in the circuit
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To make Vo variable,replace R1 with a pot.
IC VOLTAGE REGULATORS-LOW VOLTAGE CONFIGURATION
Design equations:
R 2 Vref
R1  R 2
Vo 
I L (max) 
I short
IC 723 With external pass
transistor and foldback
current limiting
R4Vo  0.7( R4  R5 )
R5 Rsens
0.7( R4  R5 )
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R5 Rsens
Rsens 
0.7Vo
I short (Vo  0.7)  0.7 I L (max)
Under foldback condition:
0.7 R L (R 4  R 5 )
Vo ' 
R 5 R sen s  R 4 R L
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