# Power electronics Uncontrolled Rectifiers - Diode Rectifiers

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22-Jan-2018Category

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Diode (Uncontrolled) Rectifiers

ER. FARUK BIN POYEN

ASST. PROFESSOR

DEPT. OF AEIE, UIT, BU

mailto:[email protected]

Contents:

Classification of Rectifiers

Performance Parameters of Rectifiers

1 Half Wave Rectifier

1 Full Wave Rectifier Centre Tapped

1 Full Wave Bridge Rectifier

3 Uncontrolled Rectifier Classification

3 Half Wave Rectifier

3 Full Wave 6 Pulse Mid Point Rectifier

3 Full Wave Bridge Rectifier

3 Full Wave 12 Pulse Rectifier

2

Classification of Rectifiers based on Control:

The converter circuit which converts AC to DC is called a Rectifier.

The rectifier circuit using diodes only is called an Uncontrolled rectifier circuit.

All rectifiers are broadly categorized into three sections.

1. Controlled Rectifier - It has only thyristors. NO diodes

2. Half Controlled Rectifier - It has thyristor + diodes

3. Uncontrolled Rectifier - Only diodes

Control here means controlling when to start rectification and when to stop.

3

Classification of Uncontrolled Rectifiers:

Single Phase Half Wave Uncontrolled Rectifier (with R load, RL load and RL with FD)

Single Phase Full Wave Uncontrolled Rectifier.

1. Centre Tapped (Mid Point) Rectifier

2. Bridge Rectifier

Three Phase Full Wave Uncontrolled Rectifier.

1. 3 Half Wave Rectifier

2. 3 Mid Point 6 Pulse Rectifier

3. 3 Bridge Rectifier

4. 3 12 Pulse Rectifier

4

Uncontrolled Rectifiers Parameter Comparison:

Parameters Half-wave Centre tapped

Full-wave Bridge

No of Diodes 1 2 4

Max. Efficiency 40.6% 81.2% 81.2%

Peak Inverse Voltage VM 2VM VM

Average Current/Diode Idc Idc/2 Idc/2

Vdc (no load) Vm/ 2Vm/ 2Vm/

Output Frequency f 2f 2f

Transformer Utilisation Factor 0.287 0.693 0.812

Ripple Factor 1.21 0.48 0.48

Form Factor 1.57 1.11 1.11

Peak Factor 2 2 2

5

Single Phase Half Wave Rectifier:

During each positive half cycle of the AC sine wave, the diode is forward biased as the

anode is positive with respect to the cathode resulting in current flowing through the

diode.

Since the DC load is resistive (resistor, R), the current flowing in the load resistor is

therefore proportional to the voltage (Ohms Law), and the voltage across the load resistor

will therefore be the same as the supply voltage, V s (minus V f), that is the DC voltage

across the load is sinusoidal for the first half cycle only so V out = V s.

During each negative half cycle of the AC sinusoidal input waveform, the diode is

reverse biased as the anode is negative with respect to the cathode.

6

Single Phase Half Wave Rectifier:

Therefore, NO current flows through the diode or circuit. Then in the negative half cycle

of the supply, no current flows in the load resistor as no voltage appears across it so

therefore, V out = 0

7

Single Phase Half Wave Rectifier (R Load): 8

. 0 =1

2 0() =

2 0

=

=1

2 02 2()

1 2=

2 0 12

2()

1 2=

2

. 0 =0

=

=

=

2

=

=

2

2= 0.707

Single Phase Half Wave Rectifier (RL Load):

Current I 0 continues to flow even after source voltage V S is negative because of the

presence of inductance L in load.

After + ve half cycle, diode remains ON, so ve half cycle appears across load current

until I 0 decays to zero at t = .

When I 0 = 0 at t = ; V L = 0, V R = 0 and V S appears as reverse bias across diode D.

At , diode voltage V D jumps from 0 to V M sin where > .

Here = is the conduction angle of the diode.

9

Single Phase Half Wave Rectifier (RL Load):

. 0 =1

2 0 sin =

21 cos

. 0 =0

=

21 cos

10

Single Phase Half Wave Rectifier (RL with FD):

Performance is improved by connecting FD across the load.

FD prevents o/p voltage from becoming ve.

The load current waveform is more smooth and load performance is better.

System efficiency is improved as energy from L is transferred to R through FD.

. 0 =1

2 0 sin =

; . 0 =

11

1 Full Wave Rectifier Centre Tapped

Also called Mid point rectifier.

The turns ration from each secondary to primary is taken as unity for simplicity.

When A is +ve w.r.t mid point O, D1 conducts for radians.

When B is +ve w.r.t mid point O in the next half cycle, D2 conducts for the other

radians.

Peak Inverse Voltage (PIV) for both D1 and D2 is 2 V S and hence it is called 1 2

pulse diode rectifier.

12

1 Full Wave Rectifier Centre Tapped

. 0 =1

0 sin =

2

. 0 =0

=

2

=1

02 2()

1 2=

2=

=

=

= = 2

=.

.= 1

13

1 Full Wave Bridge Rectifier

On the positive half cycle of transformer secondary supply voltage, diodes D1 and D2

conduct, supplying this voltage to the load.

On the negative half cycle of supply voltage, diodes D3 and D4 conduct supplying this

voltage to the load.

It can be seen from the waveforms that the peak inverse voltage of the diodes is only V m

The average output voltage is the same as that for the centre - tapped transformer full-

wave rectifier.

=

. 0 =2

; = 2

14

1 Full Wave Bridge Rectifier

. =1

2 0 sin =

;

= 2

=1

2 0

2 sin2 1 2=

2

15

3 Uncontrolled Rectifier

3 Rectifier offers the following advantages:

1. Higher o/p voltage for a given i/p voltage.

2. Lower amplitude ripples i.e. output voltage is smoother.

3. Higher frequency ripples simplifying filtering.

4. Higher overall efficiency.

16

3 Uncontrolled Rectifier: Classification

They are generally of four types.

1. 3 Half wave rectifier.

2. 3 Mid point 6 Pulse rectifier.

3. 3 Bridge rectifier.

4. 3 12 Pulse rectifier.

17

3 Half Wave Rectifier:

It uses a 3 transformer with primary in delta and secondary in star connection.

D1, D2 and D3 have common connected cathode to common load R and all diodes are

oriented in different phases and therefore called as Common Cathode Circuit.

18

3 Half Wave Rectifier:

The rectifier element connected to the line at the highest +ve instantaneous voltage can

only conduct and pulsates between V max and 0.5 V max.

It is called 3 3 pulse rectifier as the o/p is repeated thrice in every cycle of V s.

The ripple frequency (f r) of the o/p voltage is

=

; = . ,

= .

The ON diode connects its most +ve source terminal to the other two diode cathodes

keeping the other diodes OFF.

The sudden switchover from one diode to another is called commutation.

Each diode conducts for 120 intervals.

Delta connection provides path for triplen (odd multiples of the 3rd harmonic) harmonic

currents stabilizing the voltage on star secondary.

19

3 Half Wave Rectifier:

0(.) =

sin

= 0.827 = 0.477 ; = 3;

0(.) =

sin

= 0.827 ; (.) =

0(.)

RMS value of load current

() = 1

2

+1

2sin

2

1 2

= 0.408 ; = 3

=2

21=

2

321= 0.177

= = 3 = 1.732

20

3 Mid Point 6 Pulse Rectifier:

A rectifier with more number of pulses will provide a smoothed out curve giving

improved performance and lesser ripples.

Delta primary, Star secondary transformer is used here.

The secondary of each pulse is in two halves.

The mid point of all the secondary's are connected to form the neutral (n).

Six phase supplies are a1, c2, b1, a2, c1 and b2 terminals.

Phase voltages are 120 apart from V a1, V b1 and V c1 and likewise for others.

V a1 V a2, V b1 V b2, V c1 V c2 are 180 apart.

Adjacent voltages are 60 apart.

Diode that senses the highest +ve anode voltage conducts with a periodicity of 60 .

21

3 Mid Point 6 Pulse Rectifier: 22

3 Mid Point 6 Pulse Rectifier:

. =1

3 3

2 3 sin =

3

=1

3 3

2 32 2()

1

2=

3

2

3

sin 240sin 120

2

1

2= 0.9558

= = 2 0

2 = 0.95582

3

2 1 2

= 0.0408

=

0=

0.0408

3= 4.3%

=

0=

0.9558

3= 1.009

=

0=

3

2

1

(0.9558)2= 99.82%

23

3 Bridge Rectifier:

Two series diodes are always conducting while four diodes are blocking.

One of the conducting diodes is odd numbered while the other is even numbered.

Each diode conducts for 120 .

Current flows out from the most +ve source terminal through an odd numbered diode

through the load followed by the even numbered diode and then back to the most ve

source terminal.

Output has less ripples and the diodes are numbered in accordance to their conductance.

The bridge uses both the +ve and ve halves of the i/p voltage.