Uncontrolled AC to DC Converters

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Power Electronics

Lecture 2: Uncontrolled AC to DC Converters (Diode Rectifiers)

Lecturer: Dr. Mahdi Dargahi K. Semester 2, 2015 Curtin University University,, Perth, Australia

Power Electronics Lecture 2

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DIODE RECTIFIERS Introduction 1- Single phase half-wave diode rectifier

2- Battery charger

3- Single phase full wav wave e diode rectifier

4-Multiphase half-wave rectifier

5-Three-phase full wave bridge rectifier

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DIODE RECTIFIERS Block Diagram

 AC DC



Uncontrolled utility interface (ac to dc)

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DIODE RECTIFIERS Main issues in diode rectifiers

 AC DC Single-phase

Resistive Inductive Resistive-inductive

Three-phase

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R load)

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R load) Example:  Find average and rms values Example: of the output voltage and FF, RF and PF for the following circuit:

/

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R load) The average output voltage V dc is defined as:

The frequency of the source is f  is  f  =  = 1/T  1/T  and  and w = 2π f . Thus

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R load) The rms (root mean square) value of a periodic waveform is defined as:

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R load) The form factor (FF) is defined as the ratio of the root-mean- square value (heating component) of a voltage or current cur rent to its average average value:

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The ripple factor (RF), which is a measure of the ripple content, is defined as: where V ac is the effective (rms) value of the ac component of load voltage v o.

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R load) Rectification Rectification efficiency:  P  dc

 P 

ac

 

(0.3 (0.31 18V  ) 

 R

( 0. 5V  ) 

 R m

( 0. 5V  ) m

Power factor:

2

m

(0.3 (0.31 18V  ) 

2

m

2

2 

40.5%

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DIODE RECTIFIERS How can we calculate the Power Factor (PF)?

v s(t ) is(t )

is(t )

a

v s(t ) b

Linear System

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DIODE RECTIFIERS How can we calculate the Power Factor (PF)?

v s(t ) is(t )

is(t )

a

v s(t ) b

Nonlinear System

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R load) Diode voltage drop

The conduction angle c is that part of the cycle during which the diode is conducting

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R-L load)

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

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R-L load)

Example:  Find the average output Example: voltage of the following circuit assuming the conduction angle of degree:

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R-L load)

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DIODE RECTIFIERS Single phase half-wave half-wave diode rectifier (R-L load)



Compare the load current waveform with the one in the previous circuit. (without freewheeling diode)



Average output voltage depends on the load parameters.

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DIODE RECTIFIERS Battery charger with a resistive current limiter R

D1

Diode D1 is turned off when vs <  E  at  at

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DIODE RECTIFIERS Battery charger with an inductiv inductive e current limiter Current begins to flow when the input voltage exceeds Ed. •

Current continues to flows for a while even even after the input voltage has gone below Ed. •

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DIODE RECTIFIERS Single phase full-wave diode rectifier with center-tapped transformer, (R load)

-

+

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DIODE RECTIFIERS Single phase full-wave diode rectifier with center-tapped transformer, (R load) The average output is:

The rms value of the output voltage is:

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DIODE RECTIFIERS Single phase full-wave diode rectifier with center-tapped transformer, (R load) Rectification efficiency:

Full-wave

half-wave

 P  dc

 P  ac

 

(0.3 (0.31 18V  ) 

 R

( 0. 5V  ) 

 R m

( 0. 5V  ) m

Power factor:

2

m

(0.3 (0.31 18V  ) 

2

m

2

2 

40.5%

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DIODE RECTIFIERS Single phase full-wa full -wave ve diode bridge rectifier

Two Two simple (idealized) cases (pure resistive and inductive loads)

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DIODE RECTIFIERS Single phase full-wa full -wave ve diode bridge rectifier

Output voltage and current waveforms with a purely resistive load

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DIODE RECTIFIERS Single phase full-wa full -wave ve diode bridge rectifier Output voltage and current waveforms waveforms with a purely inductive load

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

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DIODE RECTIFIERS Single phase full-wa full -wave ve diode bridge rectifier

Input current in the idealized case

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DIODE RECTIFIERS Design of filter to limit the ripple in a single phase bridge rectifier Draw v o here D1

Full Rectified

D3

+ C

v s

V o

D2

D4

v s(t ) Draw iS  here

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DIODE RECTIFIERS Design of filter to limit the ripple in a single phase bridge rectifier Full Rectified

Vo

V r(pp)

v s(t ) In practice, if the peak-to-peak ripple voltage is small, it can be approximated as:

1

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DIODE RECTIFIERS Design of filter to limit the ripple in a single phase bridge rectifier

D1

Full Rectified

D3

Vo

+ C

v s

V o

D2

D4

v s(t )

Typical harmonic content for a

is(t )

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DIODE RECTIFIERS Example of applying passive PFC (power factor correction) filter 64

Rectifier with AC-side inductor is(t )

    )     A     ( D1

0

D3

+ C

v s

   e     d    u    t     i     l    p    m     A

V o

D2

15

D4

-64

is(t ) La

D1

D3

    )     A     (

+ 0

C

v s

V o

D2

D4

   e     d    u    t     i     l    p    m     A

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DIODE RECTIFIERS Voltage Doubler Rectifier

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

When the switch is in the 115 V position and the line voltage of 115V, each capacitor gets charged to approximately the peak of the AC input voltage. The output voltage is the sum of voltages across C1 and C2. •

The capacitor C1 is charged through the diode D1 during the positive half-cycle of the input AC voltage and C2 is charged through D2 during the negative half-cycle.

•

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DIODE RECTIFIERS Voltage Doubler rectifier

The positive half-cycle of the input AC voltage

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DIODE RECTIFIERS Voltage Doubler rectifier

The negative half-cycle of the input AC voltage

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DIODE RECTIFIERS  A three-phase, three-phase, four-wire system system

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DIODE RECTIFIERS Multiphase Star Rectifier

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DIODE RECTIFIERS Multiphase Star Rectifier

V1  has the highest magnitude compared to the other phases. Thus during this period D1 conducts and the rest of diodes are reverse biased. DC value of the output voltage for the case of 6-phase star rectifier:

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DIODE RECTIFIERS Multiphase Star Rectifier V2 >V1 and other phases, thus during this period, D1 is turned off and D2 is turned on.

RMS value of the output voltage for the case of 6-phase star st ar rectifier:

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DIODE RECTIFIERS  A three-phase half-wave half-wave rectifier rectifier with pure inductive inductive load

When D1  is off, VD=V1-V2 or V1-V3 which is a line voltage

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier rectifier with resisti resistive ve load

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier: rectifier: Redrawn Redrawn

 Two groups with three diodes each

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier: rectifier: Redrawn Redrawn

 Two groups with three diodes each

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DIODE RECTIFIERS  A three-phase, full-bridge rectifier: rectifier: Redrawn Redrawn

 Two groups with three diodes each

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DIODE RECTIFIERS Understanding Current Commutation

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DIODE RECTIFIERS Understanding Current Commutation

 There is a short circuit for a short time and the current commutates from one leg to another one.

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DIODE RECTIFIERS Understanding Current Commutation

 Shows the volt-seconds needed to commutate current

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