unit 1-dc power supply

7/28/2019 UNIT 1-DC Power Supply

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UNIT 1

LINEAR DC POWERSUPPLY


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THE IMPORTANCE OF DC POWERSUPPLY Electronic equipment are using an active devices like diodes,

transistors, and others. An active devices required d.c voltage

to operate.

Electronic equipment that requires a high d.c voltage uses a lotof dry cells. Therefore it is not economical.

The electricity supplied to the public through the sockets at

homes and buildings are in the form of a.c voltage and the

value of these a.c voltage are high (1 phase = 240 V, 3 phase =

415 V)


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BLOCK DIAGRAM OF DC POWER SUPPLY

Tansformer

to increase / decrease the voltage alternating current.

Rectifiers

convert the alternating current voltage to the voltage pulse.

Filterconvert the voltage pulse to the voltage at rippled or preferably the voltage

at pure.

Regulator

help to reduce to zero or at least to a minimal value.

Voltage dividers

divide the voltage according to the circuit.

A.C voltage D.C voltageTRANSFORMER RECTIFIER FILTER REGULATOR VOLTAGE

DIVIDER

Figure 1.1 : Block Diagram For Dc Power Supply


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TRANSFORMER to increase / decrease the voltage alternating current

Step-up transformer increases the voltage while decreasing the

current.

Step-down transformer decreases the voltage while increasing the

current

Figure 1.2 : Diagram and type oftransformer


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RECTIFIER Introduction

Most devices or electronic systems require a dc power

supply to operate.

Generally many source of power that supplied to the houseis a.c, so to get the d.c voltage, we use a rectifier circuit.

Rectifier to be discussed are: -

half-wave rectifier full-wave rectifier bridge rectifier.


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HALF-WAVE RECTIFIER CIRCUIT

Operation.

1. During the positive cycle of the input signal, the diode D is in

forward bias condition. D acts as a switch (close condition) and

the current can pass through the circuit. The value of theVoltage drop on the RL is equal to the magnitude of the

positive cycle of the input signal. ( if we ignore the voltage drop

on the diode)

VmVk

t

D

RL

t

VmVm

Figure 1.3 : Half wave rectifier and theoutput


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2. During the negative cycle of input signal, the diode D in reverse

biased condition. D acts as a switch ( open condition) so the

current can not pass through the circuit. So, the value of

Voltage drop in the RL during the negative cycle is empty.

3. When we connect the oscilloscope across the RL we will find

the output waveform is the same as Figure 1.3

Output VoltageOutput voltage for half-wave rectifier circuit produced only at

positive cycle of the current. Since the current through the

diode and the diode voltage drop is 0.7V (assuming silicon

diode), the output voltage is: -

frequency

The frequency output signal is equal to the input frequency.

Vk = Vm - 0.7V


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FULL-WAVE RECTIFIER CIRCUIT

When the a.c voltage provided on the circuit, the end of M and

N on the transformer will be positive and negative alternately .

When the input voltage is positive cycle , the terminal M is

positive, G= 0 (the earth) and N is negative.

Diodes D1 to be forward biased while diode D2 to be reverse

biased. Current flows along the M, D1, C, A, B, G.

A positive cycle of the wave will be produced at the load RL

M

N

C

A

B

G

D1

D2 RL

Figure 1.4 Full wave rectifier circuit


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When the input voltage is negative cycle , the terminal M is

negative, G = 0 (the earth) and N be positive.

Diode D2 become forward biased while the diode D1 become

reverse bias. Current flows along the N, D2, C, A, B, G. Apositive cycle of the wave will be produced at the load RL.

When we analyze the operation the output should be as shown

in Figure 1.5:Vm

VMG

VNG

t

t

t

Vk

t

Figure 1.5: Output for full wave rectifier circuit


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Output VoltageOutput voltage of full wave rectifier circuit resulting in two

cycles. Since in a cycle of current through a diode and the

voltage drop on the diode is 0.7V (assuming silicon diode), the

output voltage is: -

Frequency

Output signal frequency is twice the input frequency.

Vk = VM-G - 0.7V


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Figure 1.7 : Electron f low fo r pos i t ive half -cycles

Time

Time

Figure 1.8 : Electron f low for n egative hal f -cycles


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Self study Clarify the application of bridge rectifier in a power supply unit.

Discuss the operation of integrated circuit bridge rectifier

a. Data sheet of a bridge rectifier IC

b. Pin configuration and assignments c. Application of bridge rectifier IC


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FILTER

Rectifier

circuit

Filter

circuit

V

t

V

t

V

t

A.C Voltage Ripple voltage

Figure 1.9: The shape of waveform before and after filtering

Introduction

The main task of the filter is to convert the pulsed of a.cvoltage to a rippled d.c voltage or preferably the pure d.c

voltage and free from pulses of a fixed value as can be

obtained from the battery.

Among the circuits that commonly used filters are: -

C filter

RC filter

LC filters

filter


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C FILTER /CAPACITOR FILTERFilter is performed by a large value electrolytic capacitor

connected across the DC supply to act as a reservoir,

supplying current to the output when the varying DC voltage

from the rectifier is falling.

Filter CircuitC1 RL Vk

ID

Figure 1.10: filter circuit with electrolytic capacitor


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The diagram below shows the unsmoothed varying DC and the

smoothed DC

The capacitor charges quickly near the peak of the varying DC,and then discharges as it supplies current to the output.

Vm

20

t

15

10

5

0 62 84 10

15Vp-p

Vk

20

15

10

5

0t

62 84 10A

B

C

P

Q

R

X

Figure 1.11: Output for c filter


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A filter is not perfect due to the capacitor voltage falling a little

as it discharges, giving a small ripple voltage. For many

circuits a ripple which is 10% of the supply voltage is

satisfactory and the equation below gives the required value for

the filter with capacitor.

Where: I [Amps], C [F], f [Hz] and Vr [V]

Note: The ripple voltage shouldn't be higher than 10% of Vs.

A larger capacitor will give less ripple. The capacitor value

must be doubled when filtering half-wave DC.

fC

IVr

.2


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Time constant RLC1

Vk

20

15

5

0t

62 84 10A

B

C

X

Z

C1

0.1uFC2

1uF

Vr

Time constant RLC2

10

Ex 1: What should be the Capacitance to appropriately filter an

24Vpp AC signal, 60Hz and 1A?

F695060Hz12V0,12

1A

fVr2

IC

Figure 1.12 : Ripple voltage if using different capacitor C1 and C2


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RC FILTER

RC filter is a circuit that we added after the capacitor filter. RC filters are produced by placing a resistor in series with the

load (RL) and a capacitor in parallel with the load (RL). Resistor

(R) will drain ripple voltage to a smaller value. Capacitor C2 will

filtering the remaining ripple voltage.

However, this RC filter has led to some disadvantages wherethe value of the d.c voltage that across RL will also be

diminished to a lower value.

C1Rectifie

r circuitC2 RL Vk

R

Figure 1.13 : RC Filter


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FILTER filter acts to overcome the problems generated by the RC

filter.To built filter we can replace the resistor in RC filter withthe inductor (L).

Inductor will only reduce the value of a ripple a.u. voltage.

Inductor has a low resistance to d.c but also provide a high

impedance to the a.c .

Thus, the d.c voltage output is not declining when across the

RL, but the ripple a.c voltage will be reduced significantly when

cross L.

C1Rectifier

C2 RL Vk

L

Figure 1.14 : Filter Circuit


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LC FILTER By combining the advantages of series inductance and parallel

capacitor, LC filters have been produced. This filter also acts as

a low pass filter (low-pass filter).

Rectifier C2 RL Vk

L

Figure 1.15 : LC Filter Circuit


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REGULATOR An electrical regulator designed to automatically maintain a

constant voltage level.

A voltage regulator may be a simple "feed-forward" design or

may include negative feedback control loops. It may use an

electromechanical mechanism, or electronic components.

Depending on the design, it may be used to regulate one or

more AC or DC voltages.

Electronic voltage regulators are found in devices such as

computer power supplies where they stabilize the DC voltages

used by the processor and other elements.


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Regulation percentage:

where

VNL = no-load voltageVfl = full load voltageThere are three typical voltage regulator circuit is used: -

voltage regulator zener diode transistor series voltage regulator voltage regulator integrated circuit (78XX series)

% pengaturanV V

VNL FL

FL

100

%min

pengaturanVmak V

Vmak

100


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ZENER DIODE VOLTAGEREGULATOR Zener diodes will operate as a voltage regulator in reverse

bias.

Zener diode has its own specialties which it can regulate

voltage if operating in the zener region.

To operate in the zener region, the input voltage must begreater than the zener voltage and the load resistance does

not cause the zener current equal to zero

Rectifier FilterDz

R

RL

Figure 1.16 : zener diode voltage

regulator


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SERIAL TRANSISTOR VOLTAGEREGULATOR

Transistors connected in series with the load will control the input

voltage allowed to the output. The output voltage are sampled by a

circuit which supplies a feedback voltage (compared to the reference

voltage ).

Referring to figure , if the output voltage decreases, increase in VBE

causes the transistor to produce more current, this current will

increase the output voltage and maintaining the output voltage

The zener diode will act as a reference voltage. The same process

occurs if the output voltage increases. The transistor will reduce thecurrent value, causing the output voltage and maintaining the output

RRectifie

rFilter

Q1

Dz

RL

Figure 1.17: Serial Transistor Voltage Regulator


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VOLTAGE REGULATOR-INTEGRATED CIRCUITS (SERIESLM78XX) The 78xx (sometimes LM78xx) is a family of self-contained

fixed linear voltage regulator integrated circuits.

The 78xx family is commonly used in electronic circuitsrequiring a regulated power supply due to their ease-of-use and

low cost.

For ICs within the family, the xxis replaced with two digits,

indicating the output voltage (for example, the 7805 has a 5 voltoutput, while the 7812 produces 12 volts).

The 78xx lines are positive voltage regulators: they produce a

voltage that is positive relative to a common ground.


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Rectifier Filters

LM7405

1

2

3

C1

Vk

C2

Figure 1.18: Voltage regulator- Integrated Circuits


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VOLTAGE DIVIDER CIRCUIT A voltage divider (also known as a potential divider) is a simple

linear circuit that produces an output voltage (Vout) that is a

fraction of its input voltage (Vin).

Voltage division refers to the partitioning of a voltage amongthe components of the divider.

A simple example of a voltage divider consists of two resistors

in series or a potentiometer(adjus table resis tors ). It is

commonly used to create a reference voltage or to get a low

voltage signal proportional to the voltage to be measured


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COMPLETE LINEAR POWERSUPPLY CIRCUITS Figure 1.20 shows a schematic diagram of a simple power

supply unit which includes full wave rectifier, filter and voltage

regulator.

C1 C2

L

Dz

RL

R

Filter Regulator Voltage DividerTransformer

Rectifier

A Linear

OutputVoltage

M

N

C

G

D1

D2

unit 1-dc power supply

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