5-1

Chapter 5

Diode Rectifiers

5-2

Diode Rectifier Block Diagram

• Uncontrolled utility interface (ac to dc)

5-3

A Simple Circuit

• Resistive load

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 (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

5-6

Single-Phase Diode Rectifier Bridge

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

5-7

Diode-Rectifier Bridge Analysis

• Two simple (idealized) cases to begin with

5-8

Redrawing Diode-Rectifier Bridge

• Two groups, each with two diodes

5-9

Waveforms with a purely resistive load and a purely dc current at the output

• In both cases, the dc-side voltage waveform is the same

5-10

Diode-Rectifier Bridge Input Current

• Idealized case with a purely dc output current

5-11

Diode-Rectifier Bridge Analysis with AC-Side Inductance

• Output current is assumed to be purely dc

5-12

Understanding Current Commutation

• Assuming inductance in this circuit to be zero

5-13

Understanding Current Commutation (cont.)

• Inductance in this circuit is included

5-14

Current Commutation Waveforms

• Shows the volt-seconds needed to commutate current

5-15

Current Commutation in Full-Bridge Rectifier

• Shows the necessary volt-seconds

5-16

Understanding Current Commutation

• Note the current loops for analysis

5-17

Rectifier with a dc-side voltage

5-18

DC-Side Voltage and Current Relationship

• Zero current corresponds to dc voltage equal to the peak of the input ac voltage

5-19

Effect of DC-Side Current on THD, PF and DPF

• Very high THD at low current values

5-20

Crest Factor versus the Current Loading

• The Crest Factor is very high at low values of current

5-21

Diode-Rectifier with a Capacitor Filter

• Power electronics load is represented by an equivalent load resistance

5-22

Diode Rectifier Bridge

• Equivalent circuit for analysis on one-half cycle basis

5-23

Diode-Bridge Rectifier: Waveforms

• Analysis using MATLAB

5-24

Diode-Bridge Rectifier: Waveforms

• Analysis using PSpice

5-25

• Analysis using PSpice

Input Line-Current Distortion

5-26

Line-Voltage Distortion

• PCC is the point of common coupling

5-27

• Distortion in voltage supplied to other loads

Line-Voltage Distortion

5-28

Voltage Doubler Rectifier

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

5-29

A Three-Phase, Four-Wire System

• A common neutral wire is assumed

5-30

Current in A Three-Phase, Four-Wire System

• The current in the neutral wire can be very high

5-31

Three-Phase, Full-Bridge Rectifier

• Commonly used

5-32

Three-Phase, Full-Bridge Rectifier: Redrawn

• Two groups with three diodes each

5-33

Three-Phase, Full-Bridge Rectifier Waveforms

• Output current is

assumed to be dc

5-34

Three-Phase, Full-Bridge Rectifier: Input Line-Current

• Assuming output current to be purely dc and zero ac-side inductance

5-35

Three-Phase, Full-Bridge Rectifier

• Including the ac-side inductance

5-36

3-Phase Rectifier: Current Commutation

• output current is assumed to be purely dc

5-37

Rectifier with a Large Filter Capacitor

• Output voltage is assumed to be purely dc

5-38

Three-Phase, Full-Bridge Rectifier

• THD, PF and DPF as functions of load current

5-39

Crest Factor versus the Current Loading

• The Crest Factor is very high at low values of current

5-40

Three-Phase Rectifier Waveforms

• PSpice-based analysis