Session 5 Rectifiers

8/3/2019 Session 5 Rectifiers

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M. S. Ramaiah School of Advanced Studies, Bangalore

PEMP ESE505

Day -5

Rectifiers

Session Speaker

Stanley J.


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Session Objectives

To understand the operation and characteristic

of diodes To understand the performance parameters of

diode rectifier

To study the effect of load inductance on theload current


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Session Topics

Single phase half wave rectifiers

Single phase half wave rectifiers with RL load Single phase full wave rectifiers

Single phase Full wave rectifier with RL Load

Bridge rectifier

Bridge rectifier with RL load

Three phase bridge rectifier


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Rectifier

A rectifier is a circuit that converts an ac signal

into a unidirectional signal. The half-wave rectifier is the simplest power

electronics circuit that is used for low-cost

power supplies for electronics like radios


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Single-Phase Half-wave Rectifier

During the positive half-cycle of the input voltage,diode D1 conducts and the input voltage appearsacross the load.

During the negative half-cycle of the input voltage,the diode is a blocking condition and the outputvoltage is zero


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Waveforms for input voltage and current


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2 2

2

21 1

dc dc dc ac rms rms

dcac rms dc

ac

rms ac

dc dc

rms

dc

P V I P V I

P V V VP

V VFF RF V V

V RF FFV

Output DC and AC power

Efficiency of rectifier

Form Factor

Ripple Factor


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22 2

1

2

1 1

1 1

( )

cos

1

cos cos

dc

s s

s s s

s s

s S S

s S S

S peak

S

PTUF DF

V I

I I I HF

I I

V I IPF

V I I

ICF

I

Transformer utilization

Factor

Harmonic Factor

Power Factor

Crest Factor


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Half-wave Rectifier with RL Load

Due to inductive load, the conductive period of

diode D1 will extend beyond 180o until the

current becomes zero The average Vl of the inductor is zero


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The average output voltage and output current is

( )0

( )

1sin

2

1 cos2

1 cos2

o dc m

m

mo dc

V V d

V

VIR


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Full-Wave Rectifier (center-tapped)

There is no dc current flowing through thetransformer, there is no dc saturation problem of thetransformer core


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The average output voltage is

( )0

22sin2

mo dc m

VV V d

The rms value of the output voltage and

current is

2 2

( )0

( )

2sin

2 2

2

mo rms m

mo rms

VV V d

VI

R


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Full-Wave Bridge Rectifier

During the positive half cycle of the input voltage, thepower is supplied to the load through diodes D1 andD2.

During the negative half cycle, diode D3 and D4Conduct.


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Bridge Rectifier (motor load)


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Bridge Rectifier (RL load)


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( / )1

22 1

2 sin 0

2 sin

tan ( / )

oo s o

R L t o

di L Ri E V t for i

dt

V Ei t A eZ R

Z R L L R

Case 1: Continuous conduction, Io >0

Case 2: Discontinuous conduction, Io =0


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( / )

( / )( / )

0

2 2sin

1

0 0

R L t

o R L

V Ei t e

Z e R

for t and i



( / )( / )

0

1

tan

2 2sin sin

0

sin , ( ) 0

sin sin 0cos cos

R L t

o

o

m

V E V E i t e Z R Z R

for i

E At t i t V

x xe


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PEMP ESE505Boundary of continuous and

discontinuous (single-phase)


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Multiphase Rectifiers


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Three-Phase Bridge Rectifier


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Waveforms and Conduction Times


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3-Phase Bridge Rectifier (RL Load)

Figure 3.15

22 sin

3 3

2 sin 0

ab ab

oo ab o

v V t for t

di L Ri E V t for i

dt


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Three-Phase Bridge Rectifier (RL load)

( / )

1

22 1

2 sin 0

2sin

tan ( / )

oo ab o

R L t abo

di L Ri E V t for i

dt

V Ei t A eZ R

Z R L L R


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Three-Phase Bridge (RL load)

( / )( / 3 )

( / )( / 3 )

0

sin 2 / 3 sin / 32sin

1

/ 3 2 / 3 0

R L t abo R L

V Ei t e

Z e R

for t and i



( / )( /3 )

( / )( /3 )

( )2tan

( )2tan

0 ( .3.47)

sin 2 /3 sin /320

1

sin 2 /3 sin /3( ) cos

21

o

R L

ab

R L

ab

Setting I in Eq

eV E

Z e R

eEx

Ve


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PEMP ESE505Boundary of continuous and

discontinuous (three-phase)

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DC Filters

Filters can be used to smooth out the dc outputvoltage of the rectifier and these are known as

dc filters. The DC filters are usually of L, C and LC type

PEMP ESE505


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AC Filters

Due to rectification action, the input current of therectifier contains harmonics and ac filter is used to

filter out some of the harmonics from the supplysystem.

AC filter is normally of LC type.

PEMP ESE505


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Single-Phase Rectifier with C-Filter

PEMP ESE505


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Single-Phase Rectifier with C-Filter

( )

( )

( )

( )

( )

4

2 2 4 2

1

2(4 1)

mo dc m

e

r pp mrms ripple

e

rms ripple

o dc e

VV V

fRC

V VV

fRC

VRF

V fRC

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Output Voltage with C-Filter

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Summary

Diode rectifiers convert an ac voltage to a dcvoltage.

The output voltage contain ripples which can

be removed by filters. Depending on time constant of the load, the

load current may be discontinuous.

Line inductances cause overlap and reduce the

effective dc output voltage.

Session 5 Rectifiers

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