Chapter 22 - Pearson

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Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 22.
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Power Electronics


Introduction


Bipolar Transistor Power Amplifiers


Classes of Amplifier


Four
-
layer Devices


Power Supplies and Voltage Regulators

Chapter 22

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Introduction


Amplifiers that produce voltage amplification or
current amplification also produce power
amplification


However, the term
power amplifier

is normally
reserved for circuits whose main function is to deliver
large amounts of power


These can be produced using FETs or bipolar
transistors, or using special purpose devices such as
thyristors

and
triacs

22.1

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Bipolar Transistor Power Amplifiers


When designing a power amplifier we normally

require a low output resistance so that the circuit can
deliver a high output current


we often use an
emitter
-
follower


this does not produce voltage gain but has a
low
output resistance


in many cases the load applied to a power amplifier is
not simply resistive but also has an
inductive

or
capacitive

element

22.2

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Current sources and loads


when driving a reactive load we need to
supply

current
at some times (the output acts as a
current source
)


at other times we need to
absorb

current (the output
acts as a
current sink
)

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the circuit above is a
good

current source but a
poor

current sink (stored charge must be removed by
R
E
)


an alternative circuit using
pnp
transistors

(below) is a
good

current sink but a
poor

current source


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Push
-
pull amplifiers


combining these
circuits can produce
an arrangement that
is both a good current
source and a good
current sink


this is termed a

push
-
pull amplifier

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Driving a push
-
pull stage

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Distortion in push
-
pull amplifiers

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Improved push
-
pull output stage arrangements

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Amplifier efficiency


an important consideration in the design of power
amplifiers is efficiency




efficiency determines the power dissipated in the
amplifier itself


power dissipation

is important because it determines
the amount of waste heat produced


excess heat may require heat sinks, cooling fans, etc.

supply
the
from

absorbed

power
load

the

in
dissipated
power
Efficiency

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Classes of Amplifier


Class A


active device conducts for complete cycle of input signal


example shown here


poor efficiency

(normally less

than 25%)


low distortion

22.3

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Class B


active devices conducts

for half of the complete

cycle of input signal


example shown here


good efficiency

(up to 78%)


considerable distortion

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Class AB


active devices conducts

for more than half but

less than the complete

cycle of input signal


example shown here

(with appropriate
R
bias
)


efficiency depends on bias


distortion depends on bias

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Class C


active devices conducts

for less than half the

complete cycle of

input signal


example shown here


high efficiency

(approaching 100%)


gross distortion

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Class D


in class D amplifiers the active devices are switches
and are either ON or OFF


an ideal switch would dissipate no power


since either the current or the voltage is zero


even real devices make good switches


amplifiers of this type are called
switching amplifiers

or
switch
-
mode amplifiers


efficiency is very high

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Four
-
layer Devices


Although transistors make excellent switches, they
have limitations when it comes to switching high
currents at high voltages


In such situations we often use devices that are
specifically designed for such applications


These are
four
-
layer devices


these are not transistors, but have a great deal in
common with bipolar transistors

22.4

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The thyristor


a four
-
layer

device with a

pnpn
structure


three terminals:

anode, cathode

and gate


gate is the

control input

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Thyristor operation


construction

resembles two

interconnected

bipolar transistors


turning on T2

holds on T1


device then

conducts until

the current goes

to zero

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Use of a thyristor in

AC power control


once triggered the device

conducts for the remainder

of the half cycle


varying firing time

determines output power


allows control from 0
-
50%

of full power


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Full
-
wave power

control using thyristors


full
-
wave control

required two devices


allows control from

0
-
100% of full power


requires two gate

drive circuits


opto
-
isolation

often

used to insulate

circuits from AC supply


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The triac


resembles a bidirectional

thyristor


allows full
-
wave control

using a single device


often used with a

bidirectional trigger

diode (a
diac
) to produce

the necessary drive pulses


this breaks down at a

particular voltage and fires the triac

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A simple lamp
-
dimmer using a triac

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Power Supplies and Voltage Regulators


Unregulated DC power supplies

22.5

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Regulated DC power supplies

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Voltage regulators

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Switch
-
mode

power supplies


uses a switching

regulator


output voltage is

controlled by the

duty
-
cycle of the

switch


uses an averaging

circuit to ‘smooth’

output

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An
LC

averaging circuit

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Using feedback in a switching regulator

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Key Points


Power amplifiers are designed to deliver large amounts of
power to their load


Bipolar circuits often use an emitter follower circuit


Many power amplifiers use a push
-
pull arrangement


The efficiency of an amplifier is greatly affected by its class


While transistors make excellent switches, in high power
applications we often use special
-
purpose devices such as
thyristors or triacs


A transformer, a rectifier and a capacitor can be used to
form a simple unregulated supply


A more constant output voltage can be produced by adding
a regulator. This can use linear or switching techniques