5-1

Chapter 5

Diode Rectifiers

• Basic rectifier concepts• Single-Phase diode bridge rectifiers• Voltage-Doubler rectifiers

5-2

Diode Rectifier Block Diagram

• Uncontrolled utility interface (ac to dc)

5-3

A Simple Circuit (R Load)

• Resistive load

VdxxVvR

0

)sin(2

1

5-4

A Simple Circuit (R-L Load)

• Current continues to flows for a while even after the input voltage has gone negative

5-5

A Simple Circuit (R-L Load)

0AreaArea

011

0)0()3(1

1

3

1

1

0

3

0

)3(

)0(

BA

dtvL

dtvL

itididtvL

didtvL

dt

diLv

t

t

L

t

L

t ti

i

L

L

L

5-6

A Simple Circuit (Load has a dc back-emf)

• Current begins to flow when the input voltage exceeds the dc back-emf

• Current continues to flows for a while even after the input voltage has gone below the dc back-emf

5-7

Single-Phase Diode Rectifier Bridge

• Large capacitor at the dc output for filtering and energy storage

5-8

Diode-Rectifier Bridge Analysis

• Two simple (idealized) cases to begin with: (a) R load (b) current load

R load I load

5-9

Waveforms with a purely resistive load at the output

RMSRMSd VVV

dxxVv 9.0222

)sin(1

0

5-10

Diode-Rectifier Bridge Input Current

• Idealized case with a purely dc output current

oddh /

evenh 0

9.022

1

1

hII

III

ssh

dds

THD=48.43%

DPF=1.0 PF=DPF x Is1/Is=0.9

5-11

Diode-Rectifier Analysis with AC-Side Inductance

• Output current is assumed to be purely dc

5-12

Understanding Current Commutation

• Assuming inductance to be zero

5-13

Understanding Current Commutation #2

• Assuming inductance to be zero

5-14

Understanding Current Commutation #3

• Assuming inductance to be zero

5-15

Understanding Current Commutation #4

• Inductance is included

5-16

Current Commutation Waveforms

vd

vL

is

u

Id

5-17

Current Commutation Waveforms

V

ILu

ILuVA

uVtdtVA

ILdiLtdtV

diLtdtVdt

diLtVv

d

du

u

u

I

d

u

L

d

1cos

)cos1(

)cos1()()sin(

)()sin(

)()sin(

)sin(

0

00

5-18

Average voltage <Vd>

dd

dRMS

u

d

RMSRMSd

IL

v

IL

V

tdtVtdtVv

VVV

tdtVv

2

245.0

)()sin(2

1)()sin(

2

1

45.021

)()sin(2

1

00

0

When L=0

With finite L

Reduction in average output voltage

5-19

Current Commutation in Full-Bridge Rectifier

5-20

Current Commutation in Full-Bridge Rectifier

5-21

Current Commutation Waveforms

V

ILu

ILuVA

uVtdtVA

ILdiLtdtV

diLtdtVdt

diLtVv

d

du

u

u

I

I

d

u

L

d

d

21cos

2)cos1(

)cos1()()sin(

2)()sin(

)()sin(

)sin(

0

0

5-22

Average voltage <Vd>

dd

dRMS

u

d

RMSRMSd

IL

v

IL

V

tdtVtdtVv

VVV

tdtVv

2

29.0

)()sin(1

)()sin(1

9.0222

)()sin(1

00

0

When L=0

With finite L

Reduction in average output voltage

5-23

Conclusions

Average output voltage drops with

1. increased current

2. increased frequency

3. Increased L

Load Regulation is a major consideration in most rectifier systems because

• voltage changes with load (IL)

5-24

Diode-Rectifier with a Capacitor Filter

• Power electronics load is represented by an equivalent load resistance

5-25

Diode-Bridge Rectifier: Waveforms

• Analysis using PSpice

V=I/C T/2

5-26

Voltage Doubler Rectifier

• In 115-V position, one capacitor at-a-time is charged from the input.

input

input