Table of Contents
1.0 Basics
1.1 Power
1.2 Transformers
1.3 Rectifiers
1.4 Filters
1.5 Regulators
Assignment Questions
For Further Research
An Introduction to Communications Systems
i
_____ Notes _____
1.0 Basics
Some Definitions
http://www.glossarist.com/glossaries/technology/electricity.asp
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Power (P) is the rate at which energy is used. The most common unit is the watt.
One watt is the use of one joule of energy per second. One joule is the amount
of energy required to lift one kilogram by one meter.
Power is the product of voltage and current.
P  I V
Volt (V or E) It is the unit of potential difference or electromotive force in the
MKS (meters-kilograms-second) system equal to the potential difference
between two points for which 1 coulomb of electricity will do 1 joule of work in
going from one point to another. It is the potential difference required to make a
current of one ampere flow through a resistance of one ohm.
Ampere (A) it is the unit of electric current in the MKS system defined in terms
of the force of attraction between two parallel current carrying conductors. It is
equal to one coulomb of charge flowing across a surface in one second. One
ampere is the current flowing through one ohm of resistance at one volt
potential.
Coulomb is a standard unit of charge equal to 6.24 x 1018 electrons and is
equal to one ampere second.
Alternating Current (AC) is one that periodically reverses direction.
Direct Current (DC) is one that travels in only one direction.
Ohm is a measure of electrical resistance.
Ohm’s Law E  I  R
In the 18th century, scientist thought that electrical current was a sort of fluid
where both positive and negative charges moved. Consequently, circuit theory
initially defined current as the movement of the positive charge. When it was
discovered that electrical current was the movement of negative charge, it was
too late to rewrite the technical literature. As a result, circuit theory continues to
use conventional (positive) current flow.
An Introduction to Power Supplies
1-1
Introduction
_____ Notes _____
Basic Power Supply
A power supply consists of four principle parts:
1. Transformer
2. Rectifier
3. Filter
4. Regulator
A simple regulated DC power supply converts the AC voltage from the power
utility, to a constant DC voltage. This process requires four unique stages:
Voltage transformation (step-up or step-down)
AC rectification (half-wave of full-wave)
Filtering (capacitors and perhaps inductors)
Regulation (discrete circuit or IC)
AC
Mains
Transformer
Rectfier
Filter
Regulator
Output
Some switched-mode power supplies dispense with the transformer entirely.
1.1
Power
http://www.howstuffworks.com/power.htm
In small power supply applications, the main source of power comes from the
national power grid. This power is most often created by water or steam driven
generators. The term hydroelectric power is generally reserved to describe power
generated by falling water. However, since coal, oil, gas, and nuclear power
plants utilize water to drive steam turbines, even they may properly be called
hydro electric. In any case, the power source is referred to as the ‘mains’.
Most simple power supplies receive their input power from a standard 120-volt
circuit consisting of three wires. It is interesting to note that in the electrical
trade, it is called a two-wire circuit since the ground wire is not counted.
Black wire
switch.
this carries the power and is fitted with a circuit breaker or
White wire this is the neutral or ‘return’ wire in the circuit. Its voltage is
generally very close to ground potential.
Green wire this is the ground wire and is not used to carry power. It is a
safety feature that prevents electrocution in the event of a circuit failure.
In home wiring, this is a bare copper wire, and is connected to the copper
cold water pipe where it enters the building.
Electric power is generally distributed as AC (alternating current). This is
because it is easy to convert to other voltages by means of transformers.
However, most electronic devices require a source of DC (direct current) power.
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An Introduction to Power Supplies
Introduction
_____ Notes _____
Consequently, many types of power supplies isolate the mains from the regulated
circuit power by means of transformers.
Power from a generating station is distributed over a 3-phase circuit. This simply
means that there are three power conveying wires, each carrying a sinusoidal
voltage displaced by 120o from each other. This offers many advantages.
If the load on each phase is equal (balanced) then the return or neutral wires can
be combined, and amazingly enough, this neutral wire will carry no current. In
some cases, the neutral wire can be dropped however, it is needed if the phase
currents become unbalanced.
Since power follows the sinusoidal nature of the signal, heavy machinery will
use all 3-phases. This provides a reasonably constant torque since there will be
six power maximums for every motor shaft revolution. Three-phase circuits also
power large buildings.
In domestic applications, only 1-phase is distributed. This phase is inverted so
that 2 single phase lines separated by 180o enter the home. This provides a
double size voltage for ovens and dryers.
AC Voltage
Several different schemes are used to describe the magnitude of sinusoidal
voltages:

Peak

Average

RMS (transformers are specified in rms)
A d’Arsonval movement responds to the average value and is the number
obtained if the signal were smoothed out. The average value of any sinusoid is
naturally zero however; the average value of a fully rectified sinewave is 63.6%
of the peak value.
The RMS (root mean square) value is the DC equivalent which would create the
same amount of heat in a load, as the AC signal. The RMS value of a sinusoid is
70.7% of the peak value.
For a full-wave rectified sinusoid:
Vp 
 Vav
2
 2 Vrms
When using an AC voltmeter, it is important to understand which unit is being
measured. Unless a meter specifically says true RMS, the reading should not be
taken as a reliable RMS value. Most meters measure the rectified average or are
calibrated to read RMS for sine waves only.
An Introduction to Power Supplies
1-3
Introduction
_____ Notes _____
Derivations
Average Value of a Half-Wave Sinusoid

Vav


0
V pk sin t dt
2
V pk cost  0



2

V pk cos(0)  cos( ) 
2

V pk 1  1
2
V pk

Average Value of a Full-Wave
Vav 
2V pk

RMS Value of a Half-Wave Sinusoid

Vrms 

V
0
2
pk
sin 2 t dt
2
V pk
2
RMS Value of a Full-Wave
Vrms 
V pk
2
1.2 Transformers
A transformer consists of insulated wires wound around a magnetic flux
conductor, such as an iron loop. It is used to alter or transform one AC voltage
into another, and to provide electrical isolation from the power mains.
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An Introduction to Power Supplies
Introduction
_____ Notes _____
The transformer input winding is called the primary and the output winding is
the secondary.
It is interesting to note that the ratio of winding voltage to number of turns is the
same on both sides of the transformer. Thus by changing the number of turns, the
input voltage can easily be stepped up or stepped down to the secondary.
This principle can be extended to any number of windings. In most cases, there
is only one primary, but there may be several secondaries.
One of the most common transformers used in power supply design is the centertapped transformer. Each of the secondary windings can generally be considered
as a separate winding.
An ideal transformer has the following current and voltage relationships:
v1 v 2 v3



n1 n2 n3
i1 n1  i2 n2  i3 n3    0
z1
n1 
2

z2
n2 
2

z3
n3 2

From these equations, we note that transformers can also be used to perform
impedance matching. This is feature is used in audio, telephony, video, and RF
applications.
An Introduction to Power Supplies
1-5
Introduction
_____ Notes _____
All electronic components consume power. A typical power transformer has an
overall efficiency of 85-95%. Several factors contribute to loss including:
winding resistance, and eddy currents.
A more complete equivalent circuit for a transformer resembles:
A real transformer has series winding resistance and inductance, shut
conductance and capacitance, and inter-winding capacitance. In many power
supply applications, these additional complications can be ignored, however in
signaling, telecommunications, and switch mode applications they must be
considered.
1.3
Rectifiers
A diode rectifier is often used to convert the AC waveform at the secondary, to a
DC waveform.
Diodes
As a first order approximation, diodes act like a voltage polarity (or current
direction) switch. They will tolerate only a reverse (or negative polarity) voltage
and a forward current.
This characteristic curve suggests that the diode does not consume power since
the there is no voltage drop across it when it is conducting current.
Real diodes are not perfect rectifiers. They exhibit both junction drop and bulk
resistance. In fact, there is a voltage drop across the diode when it conducts. This
drop is approximately 0.7 volts for most silicon diodes.
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An Introduction to Power Supplies
Introduction
_____ Notes _____
If the circuit draws significant current, the power rating of the diode must be
considered. However, an actual diode exhibits yet another characteristic called
bulk resistance.
Under heavy load current conditions, the voltage drop across the diode can
increase by as much as 2.5 volts.
An even more detailed examination of the diode reveals that it is even more
complex than this third approximation suggests, since a real diode does not have
such an abrupt change in its characteristic curve. Furthermore, if the reverse
voltage exceeds the maximum reverse bias diode rating, the diode will
breakdown and likely self-destruct.
When a power supply if first turned on, there is an initial surge current which
charges the filter capacitors. This current is often at least twice the rated power
supply load current.
Some general rules for selecting rectifier diodes:
Only use diodes specifically designed for power supply applications.
Always calculate the diode power dissipation by multiplying the maximum
peak current by 0.7 volts and then double the result as a safety margin.
The minimum reverse bias breakdown voltage should be at least 1.4 x the
peak input voltage for a full-wave rectifier and 2.8 x for a half-wave
rectifier circuit.
The surge current is often twice the maximum load current.
There are three basic types of rectifier circuits:

Half wave rectifier

Full-wave bridge rectifier

Center-tapped full wave rectifier
An Introduction to Power Supplies
1-7
Introduction
_____ Notes _____
Half-Wave Rectifier
Approximation Formulas
vout( pk )  vsec( pk )  0.7
vout( av) 
Full-Wave Bridge Rectifier
vout( pk )

Approximation Formulas
vout( pk )  vsec( pk )  1.4
Center-Tapped Full-Wave Rectifier
Approximation Formulas
Center-Tapped Bridge Rectifier
Approximation Formulas
The center-tapped circuit has less loss than the bridge rectifier since the current
on each half-cycle passed through only one diode. However, it does require a
larger transformer since the two secondary windings only conduct current on
alternate half-cycles.
A center tapped bridge circuit is used to create a dual polarity supply.
1.4
Filters
Most power supplies create a constant output voltage. Even variable supplies are
adjusted to some fixed value and then left. The output of a rectifier however is
not constant. There is a huge ripple voltage, which must be smoothed or filtered.
Filtering generally requires reactive components. Capacitors placed across the
load will tend to reduce the ripple voltage, while inductors placed in series with
the load will tend to reduce the ripple current.
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An Introduction to Power Supplies
Introduction
_____ Notes _____
Capacitive Filter
Half-Wave Rectifier (add a capacitor)
Approximation Formulas
vripple 
vout( pk )
f RC
f  60 Hz
vout( av)  vout( pk ) 
Full-Wave Rectifier
v ripple
2
Approximation Formulas
vout( pk )  vsec( pk )  1.4
v ripple 
vout ( pk )
2 f RC
f  60 Hz
C  2.4
I Load
vout( av)  vout( pk ) 
VRMS ripple
v ripple
2
The required size of the filter capacitor (in fd) can be approximated by:
C  2.4
I Load
VRMS ripple
Where:
C is in fd.
ILoad is in ma.
RC discharge is assumed.
A closer approximation can be made by using the transformer Schade curves. In
any case, ripple voltage increases as the load current increases.
Care must be exercised when selecting filter capacitors. Power supplies
generally use large electrolytic capacitors as filters. It is very important to
never exceed the voltage rating or to reverse the polarity of electrolytic
capacitors.
Although the voltage across the capacitor is relatively constant, the current
through it is not. The ripple currents are typically 2 – 3 times the DC load
current. The capacitor supplies the load current when the rectifier circuit current
falls. It also accepts the excess rectifier current when it exceeds the load current
demands. The current in the filter capacitor is therefore an alternating current
(AC) since it reverses direction.
An Introduction to Power Supplies
1-9
Introduction
_____ Notes _____
5Spice simulation
A free spice program is available from http://www.5spice.com/ . Using this
program, the voltage and current waveforms may be observed.
Full Wave Bridge and Filter
Voltage Waveforms
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An Introduction to Power Supplies
Introduction
_____ Notes _____
Current Waveforms
Capacitive-Inductive Filter
Ripple Factor
The significance of ripple in the output is somewhat dependant on the average
output voltage. The lower the average value, the more important it is to reduce
ripple.
ripple factor 
1.5
rms value of the AC component in the output
average value
Regulators
An Introduction to Power Supplies
1 - 11
Introduction
_____ Notes _____
The typical dropout voltage (minimum VCE across the series pass transistor) of
this type regulator is about 0.6 volts at full load and may be as low as 50 mV at
minimum load current. It is therefore used in battery-powered circuits however;
the ground pin current is relatively high since it is the load current divided by the
gain of the PNP transistor.
For high load current applications, the gain in the feedback loop must be
increased. This increases the maximum load current and reduces the required
ground pin current.
The disadvantage of this circuit is that device requires a minimum voltage drop
of 2.5 – 3.0 volts across the series pass transistor in order to maintain regulation.
Voltage Reference
As noted above, the regulator must contain an internal voltage reference. The
simplest of these uses a zener diode.
Zener Diode
The forward bias curve of the zener is the same as an ordinary diode and is
generally of little interest however; the reverse bias breakdown region is quite
unique. The most common reverse breakdown voltages range from 3.3 to 75
volts, and the power ratings are typically ¼, ½, 1,5, and 10 watts.
The electrical equivalent circuit of a zener consists of a small ‘voltage source’ in
series with a bulk resistance. The diode is a passive device, but when biased in a
conduction region, has a well-defined voltage across it, as if it had an internal
regulated voltage.
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An Introduction to Power Supplies
Introduction
_____ Notes _____
Simple Zener Diode Regulator
Because the Zener diode has an internal bulk resistance, the output voltage will
not be exactly equal to the Zener voltage. The exact output reference voltage will
be a function of the supply current.
There must be no loading on the reference output in order for this circuit to
function properly. If this is the case, then all of the current from the supply
passes through the Zener diode. The output voltage is given by:
VO  VZ  I Z RZ
 V  VBZ
 VZ   CC
 R  RZ

VZ

From this we observe that changes in VCC will cause changes in the output
reference voltage.
Constant Current Zener Diode Reference
In order to minimize voltage variations caused by the input voltage variations
and internal bulk resistance, the zener should be driven by a constant current
source (or sink).
An Introduction to Power Supplies
1 - 13
Introduction
_____ Notes _____
Since the voltage VBE is essentially independent of the supply voltage, the
voltage across the resistor is constant. Therefore, the current through it is
constant. The zener current will be slightly higher since it must also supply
transistor base current.
The reference voltage created by this circuit is only slightly sensitive to change
in the supply voltage and temperature.
Improved Zener Diode Reference
An improved variation of this circuit is:
The main disadvantage with this circuit is that it requires a supply voltage in
excess of 9 volts.
Bandgap Voltage Reference Source
The bandgap reference circuit is the most common integrated zener reference
source.
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An Introduction to Power Supplies
Introduction
_____ Notes _____
These voltage sources cannot drive significant load currents. As a result, they
must be buffered from the rest of the circuit. This is accomplished by means of
operational amplifiers.
Zener Diode Regulator (with series pass transistor)
A zener diode cannot regulate a great deal of current. To overcome this
limitation. a series pass element, such as a transistor can be added to form a
simple high current regulator:
R = the value needed supply the minimum zener current at minimum input
voltage (check the data sheet). Care must be taken to not exceed the power
dissipation rating of the diode.
The disadvantage of this circuit is that the zener current is not constant, it is
dependant on the input voltage.
Generic Op Amp Circuit
The operational amplifier is probably the singe most versatile analog device
imaginable. It has three very important characteristics:
The impedance at the inverting and non-inverting inputs is essentially
infinite (thus drawing no current).
The output impedance (when feedback is employed) is essentially zero.
When used as a linear device, the voltage an the inverting and non-inverting
input terminals is equal.
An Introduction to Power Supplies
1 - 15
Introduction
_____ Notes _____
Many regulator ICs bring the op amp inputs to pins on the package, namely: noninverting input, inverting input, and reference input. This allows an external
network to control the actual final output voltage.
Differential Op Amp Circuit
This circuit amplifies the difference between two voltages.
Non-Inverting Op Amp Circuit
This circuit acts as a simple amplifier. If the input signal is AC, the op amp must
be powered from a split voltage power supply.
Inverting Op Amp Circuit
This circuit amplifies and inverts the polarity of a voltage. In order to do this, the
op amp needs a split voltage power supply.
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An Introduction to Power Supplies
Introduction
_____ Notes _____
Op Amp Reference Buffer
An operational amplifier can be used to isolate a reference voltage from loading
effects and provide a simple means of obtaining any arbitrary reference voltage.
Besides being able to provide significant load currents, the buffer output voltage
can be programmed by adjusting the resistor ratio. These amplifiers are powered
from the non-regulated part of the power supply; the rectified and filtered input
voltage.
An Introduction to Power Supplies
1 - 17
Introduction
_____ Notes _____
Assignment Questions
Quick Quiz
1.
A d’Arsonval movement responds to the [peak, average, rms] current value.
2.
When calculating wattage, the [peak, average, rms] current and voltage
value must be used.
3.
All rms meters are true rms meters. [True, False]
4.
The voltage drop across a rectifying diode is constant. [True, False]
5.
Good power supply designs maximizes the ripple current. [True, False]
6.
[Electrolytic, Mica, Ceramic] capacitors are polarized.
7.
Regulator feedback must be [increased, decreased] as the load current
increases.
Analytical Questions
1.
What transformer turns ratio is needed to produce an average of 12 volts
from a 120 volt rms source?
2.
What is the maximum current that can be drawn from a 5:1 transformer
supplied from a 15-amp circuit?
3.
What transformer turns ratio is needed to match a 50 ohm source to a 600
ohm load?
Composition Questions
To answer these questions, it will be necessary to do some research.
1.
Find the technical definition for:
Coulomb
Farad
Henry
Inductance
Capacitance
Permeability
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An Introduction to Power Supplies
Introduction
_____ Notes _____
Permittivity
Conductivity
2.
What are the four principle parts of any power supply?
3.
What is the difference between power and VA?
4.
Derive the formula for the average value of a full-wave rectified sinusoid.
5.
Derive the formula for the RMS value of a full-wave rectified sinusoid.
6.
Given a series pass zener diode regulator, with the following characteristics:
15  Vin  20
I Load  1 amp
Vout  12 volts
1 ma  I zener  10 ma
Determine the following:
a) Voltage and minimum power rating of the zener diode
b) Size and minimum power rating of the resistor
c) Maximum power dissipation of the transistor.
An Introduction to Power Supplies
1 - 19
Introduction
_____ Notes _____
For Further Research
Power
http://www.iclei.org/efacts/hydroele.htm
http://www.bchydro.com/powersupply/power_generation/
http://users.owt.com/chubbard/gcdam/html/hydro.html
http://hydropower.inel.gov/
http://www.world-nuclear.org/
http://www.opg.com/ops/H_how.asp
http://www.tva.gov/power/
Diode Rectifiers
http://www.tpub.com/neets/book7/24j.htm
http://ocean.phys.boun.edu.tr/~arif/exercise/intro.html
http://www.americanmicrosemi.com/tutorials.htm
http://www.oldcrows.net/~patchell/archives/idealdiode.html
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An Introduction to Power Supplies