choppers

Power Electronics

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DC CHOPPERS

1

Introduction• Chopper is a static device.• A variable dc voltage is obtained from a constant dc voltage

source.• Also known as dc-to-dc converter.• Widely used for motor control.• Also used in regenerative braking.• Thyristor converter offers greater efficiency, faster response,

lower maintenance, smaller size and smooth control.

•22Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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Choppers are of Two Types

Step-down choppers. In step down chopper output voltage is less than input

voltage.

Step-up choppers. In step up chopper output voltage is more than input

voltage.

3Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Principle Of Step-down Chopper

V

i0

V0

Chopper

R

+

•44Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

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• A step-down chopper with resistive load.

• The thyristor in the circuit acts as a switch.

• When thyristor is ON, supply voltage appears acrossthe load

• When thyristor is OFF, the voltage across the load willbe zero. 5

Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

V d c

v 0

V

V /R

i0

I d c

t

t

t O N

T

t O F F


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4


v e r a g e v a l u e o f o u t p u t o r l o a d v o l t a g e .

v e r a g e v a l u e o f o u t p u t o r l o a d c u r r e n t .

T i m e i n t e r v a l f o r w h i c h S C R c o n d u c t s .

T i m e i n t e r v a l f o r w h i c h S C R i s O F F .

P e r i o d o f s w i t c h i n g

d c

d c

O N

O F F

O N O F F

V A

I A

t

t

T t t

o r c h o p p i n g p e r i o d .

1 F r e q . o f c h o p p e r s w i t c h i n g o r c h o p p i n g f r e q .f

T


A v e r a g e O u t p u t V o l t a g e

.

d u t y c y c l e

O Nd c

O N O F F

O Nd c

O N

tV V

t t

tV V V d

T

tb u t d

t

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2

0

A v e r a g e O u t p u t C u r r e n t

R M S v a l u e o f o u t p u t v o l t a g e

1 O N

d cd c

O Nd c

t

O o

VI

RtV V

I dR T R

V v d tT


2

0

2

B u t d u r i n g ,

T h e r e f o r e R M S o u t p u t v o l t a g e

1

.

.

O N

O N o

t

O

O NO O N

O

t v V

V V d tT

tVV t V

T T

V d V

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2

2

O u t p u t p o w e r

B u t

O u t p u t p o w e r

O O O

OO

OO

O

P V I

VI

R

VP

R

d VP

R


E f f e c t i v e i n p u t r e s i s t a n c e o f c h o p p e r

T h e o u t p u t v o l t a g e c a n b e v a r i e d b y

v a r y i n g t h e d u t y c y c l e .

id c

i

VR

I

RR

d

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Methods Of Control• The output dc voltage can be varied by the following

methods.– Pulse width modulation control or constant

frequency operation.– Variable frequency control.

13

Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

Pulse Width Modulation• tON is varied keeping chopping frequency ‘f’ & chopping

period ‘T’ constant.• Output voltage is varied by varying the ON time tON


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V 0

V

V

V 0

t

tt O N

t O N t O F F

t O F F

T


Variable Frequency Control• Chopping frequency ‘f’ is varied keeping either tON or tOFF

constant.

• To obtain full output voltage range, frequency has to be variedover a wide range.

• This method produces harmonics in the output and for largetOFF load current may become discontinuous

16

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v 0

V

V

v 0

t

t

t O N

tO N

T

T

t O F F

tO F F


Step-down ChopperWith R-L Load

V

i 0

V 0

C h o p p e r

R

LF W D

E

+


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• When chopper is ON, supply is connected across load.• Current flows from supply to load.• When chopper is OFF, load current continues to flow in the

same direction through FWD due to energy stored in inductor‘L’.

• Load current can be continuous or discontinuous dependingon the values of ‘L’ and duty cycle ‘d’

• For a continuous current operation, load current variesbetween two limits Imax and Imin

• When current becomes equal to Imax the chopper is turned-offand it is turned-on when current reduces to Imin. 19

O u t p u tv o l t a g e

O u t p u tc u r r e n t

v 0

V

i 0

I m a x

I m i n

t

t

t O N

T

t O F F

C o n t i n u o u sc u r r e n t

O u t p u tc u r r e n t

t

D i s c o n t i n u o u sc u r r e n t

i 0

20Pro. T.K. Anantha Kumar, E&E Dept., MSRIT

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Principle Of Step-up Chopper

+

VOV

Chopper

CLOAD

DLI

+


• Step-up chopper is used to obtain a load voltage higher thanthe input voltage V.

• The values of L and C are chosen depending upon therequirement of output voltage and current.

• When the chopper is ON, the inductor L is connected acrossthe supply.

• The inductor current ‘I’ rises and the inductor stores energyduring the ON time of the chopper, tON.


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• When the chopper is off, the inductor current I is forced to flowthrough the diode D and load for a period, tOFF.

• The current tends to decrease resulting in reversing the polarityof induced EMF in L.

• Therefore voltage across load is given by

. . ,O O

d IV V L i e V V

d t


• A large capacitor ‘C’ connected across the load, will provide acontinuous output voltage

• Diode D prevents any current flow from capacitor to thesource.

• Step up choppers are used for regenerative braking of dcmotors.


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Expression For Output Voltage


A s s u m e t h e a v e r a g e i n d u c t o r c u r r e n t t o b e

d u r i n g O N a n d O F F t i m e o f C h o p p e r .

V o l t a g e a c r o s s i n d u c t o r

T h e r e f o r e e n e r g y s t o r e d i n i n d u c t o r

= . .

W h e r e

W h e n C h o p p e r

p e r i o d o f c h o p p e r .

i s O N

O N

O N

I

L V

V I t

t O N


( e n e r g y i s s u p p l i e d b y i n d u c t o r t o l o a d )

V o l t a g e a c r o s s

E n e r g y s u p p l i e d b y i n d u c t o r

w h e r e p e r i o d o f C h o p p e r .

N e g

W h e n C h o p p e r

l e c t i n g l o s s e s , e n e r g y s t o r e d i n i n d u c t o r

i s O F F

O

O O F F

O F F

L V V

L V V I t

t O F F

L

= e n e r g y s u p p l i e d b y i n d u c t o r L

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W h e r e

T = C h o p p i n g p e r i o d o r p e r i o d

o f s w i t c h i n g .

O N O O F F

O N O F FO

O F F

OO N

V I t V V I t

V t tV

t

TV V

T t


1

1

1

1

W h e r e d u t y c y l e

O N O F F

OO N

O

O N

T t t

V Vt

T

V Vd

td

T

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Classification Of Choppers

Choppers are classified as Class A Chopper Class B Chopper Class C Chopper Class D Chopper Class E Chopper


Class A Chopper

V

Chopper

FWD

+

v0

v0

i0

i0

LOAD

V


Class A Chopper is a first quadrant chopper .

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• When chopper is ON, supply voltage V is connected across theload.

• When chopper is OFF, vO = 0 and the load current continues toflow in the same direction through the FWD.

• The average values of output voltage and current are alwayspositive.

• Class A Chopper is a step-down chopper in which poweralways flows form source to load.

• It is used to control the speed of dc motor.• The output current equations obtained in step down chopper

with R-L load can be used to study the performance of Class AChopper. 31Prof. T.K. Anantha Kumar, E&E Dept., MSRIT

O u tp u t c u rre n t

T h y r is to rg a te p u lse

O u tp u t v o l ta g e

i g

i 0

v 0

t

t

tt O N

T

C H O N

F W D C o n d u c ts


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Class B Chopper

V

Chopper

+

v0

v0

i0

i0

L

E

R

D


Class B Chopper operates in second quadrant

• When chopper is ON, E drives a current through L and R in adirection opposite to that shown in figure.

• During the ON period of the chopper, the inductance L storesenergy.

• When Chopper is OFF, diode D conducts, and part of theenergy stored in inductor L is returned to the supply.

• Average output voltage is positive and average output currentis negative.

• In this chopper, power flows from load to source.• Class B Chopper is used for regenerative braking of dc motor.• Class B Chopper is a step-up chopper.


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O u tp u t c u rre n t

Dc o n d u c ts C h o p p e r

c o n d u c ts

T h y r is to rg a te p u lse

O u tp u t v o lta g e

i g

i 0

v 0

t

t

t

I m in

Im a x

T

tO NtO F F


Class C Chopper

V

Chopper

+

v0

D 1

D 2CH 2

CH 1

v0i0

i0

L

E

R


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• Class C Chopper is a combination of Class A and Class BChoppers.

• For first quadrant operation, CH1 is ON or D2 conducts.• For second quadrant operation, CH2 is ON or D1 conducts.• When CH1 is ON, the load current is positive.• The output voltage is equal to ‘V’ & the load receives power

from the source.• When CH1 is turned OFF, energy stored in inductance L forces

current to flow through the diode D2 and the output voltage iszero.


• Current continues to flow in positive direction.• When CH2 is triggered, the voltage E forces current to flow in

opposite direction through L and CH2 .• The output voltage is zero.• On turning OFF CH2 , the energy stored in the inductance

drives current through diode D1 and the supply• Output voltage is V, the input current becomes negative and

power flows from load to source.


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• Average output voltage is positive• Average output current can take both positive and negative

values.• Choppers CH1 & CH2 should not be turned ON simultaneously

as it would result in short circuiting the supply.• Class C Chopper can be used both for dc motor control and

regenerative braking of dc motor.• Class C Chopper can be used as a step-up or step-down

chopper.


G a te p u ls eo f C H 2


O u tp u t c u r re n t

O u tp u t v o l ta g e

ig 1

ig 2

i0

V 0

t

t

t

t

D 1 D 1D 2 D 2C H 1 C H 2 C H 1 C H 2

O N O N O N O N


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Class D Chopper

V+ v 0

D 2

D 1 C H 2

C H 1

v 0

i 0

L ER i 0


• Class D is a two quadrant chopper.• When both CH1 and CH2 are triggered simultaneously, the

output voltage vO = V and output current flows through theload.

• When CH1 and CH2 are turned OFF, the load current continuesto flow in the same direction through load, D1 and D2 , due tothe energy stored in the inductor L.

• Output voltage vO = - V .


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• Average load voltage is positive if chopper ON time is morethan the OFF time

• Average output voltage becomes negative if tON < tOFF .• Hence the direction of load current is always positive but load

voltage can be positive or negative.






A v e ra g e v 0

ig 1

ig 2

i0

v 0

V

t

t

t

t

C H ,C HO N1 2 D 1 ,D 2 C o n d u c t in g


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A v e ra g e v 0

ig 1

ig 2

i0

v 0

V

t

t

t

t

C HC H

1

2

D , D1 2


Class E Chopper

V

v 0

i 0L ER

C H 2 C H 4D 2 D 4

D 1 D 3C H 1 C H 3

+


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Four Quadrant Operationv 0

i 0

C H - C H O NC H - D C o n d u c ts

1 4

4 2

D D2 3- C o n d u c tsC H - D C o n d u c ts4 2

C H - C H O NC H - D C o n d u c ts

3 2

2 4

C H - D C o n d u c tsD - D C o n d u c ts

2 4

1 4


• Class E is a four quadrant chopper• When CH1 and CH4 are triggered, output current iO flows in

positive direction through CH1 and CH4, and with outputvoltage vO = V.

• This gives the first quadrant operation.• When both CH1 and CH4 are OFF, the energy stored in the

inductor L drives iO through D2 and D3 in the same direction,but output voltage vO = -V.


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• Therefore the chopper operates in the fourth quadrant.• When CH2 and CH3 are triggered, the load current iO flows in

opposite direction & output voltage vO = -V.• Since both iO and vO are negative, the chopper operates in

third quadrant.• When both CH2 and CH3 are OFF, the load current iO continues

to flow in the same direction D1 and D4 and the output voltagevO = V.

• Therefore the chopper operates in second quadrant as vO ispositive but iO is negative.


Effect Of Source & Load Inductance

• The source inductance should be as small as possible to limitthe transient voltage.

• Also source inductance may cause commutation problem forthe chopper.

• Usually an input filter is used to overcome the problem ofsource inductance.


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• The load ripple current is inversely proportional to loadinductance and chopping frequency.

• Peak load current depends on load inductance.• To limit the load ripple current, a smoothing inductor is

connected in series with the load.


V

i0

v 0

C h o p p e r

F W D

+

LOAD


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Mode-1 Operation

LOADL

C

I L

L S

V S

+

_

+

_

T 1

D 1

V C i C


• Thyristor T1 is fired at t = 0.• The supply voltage comes across the load.• Load current IL flows through T1 and load.• At the same time capacitor discharges through T1, D1, L1, & ‘C’

and the capacitor reverses its voltage.• This reverse voltage on capacitor is held constant by diode D1.


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C a p a c i t o r D i s c h a r g e C u r r e n t

s i n

1W h e r e

& C a p a c i t o r V o l t a g e

c o s

C

C

Ci t V t

L

L C

V t V t

Mode-2 Operation

LOAD

C

L S

V S+

_+

_

T 2

V C

I L

I L


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• Thyristor T2 is now fired to commutate thyristor T1.• When T2 is ON capacitor voltage reverse biases T1 and turns if

off.• The capacitor discharges through the load from –V to 0.• Discharge time is known as circuit turn-off time.



CC

L

V Ct

I

• Circuit turn-off time is given by:

• Where IL is a load current• tc is depends on load current, it must design for the

worst case condition which occur at the maximumvalue the load current and minimum value of loadvoltage

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• Capacitor recharges back to the supply voltage (with plate ‘a’ +ve).• This time is called the recharging time and is given by

• The total time required for the capacitor to discharge and rechargeis called the commutation time and it is given by

• At the end of Mode-2 capacitor has recharged to VS and the freewheeling diode starts conducting.


Sd

L

V Ct

I

r C dt t t

Mode-3 Operation

LOAD

C

L S

V S

+_

+

_

T 2V S

F W D

IL

IL


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sinSC S L S

LV t V I t

C

Mode-4 Operation

LOAD

C

L S

V S

+_

+

_

D 1

LF W D

IL

V C


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• Capacitor has been overcharged i.e. its voltage is above supplyvoltage.

• Capacitor starts discharging in reverse direction.• Hence capacitor current becomes negative.• The capacitor discharges through LS, VS, FWD, D1 and L.• When this current reduces to zero D1 will stop conducting and

the capacitor voltage will be same as the supply voltage


Mode-5 Operation

LOAD

I L

F W D

• Both thyristors are off and theload current flows through theFWD.

• This mode will end oncethyristor T1 is fired.


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C a p a c i t o r C u r r e n tI L

t

t

I p C u r r e n t t h r o u g h T 1

i c

0I p

i T 1

0

I L


t

t

t

V o l t a g e a c r o s s T 1

O u t p u t V o l t a g e

C a p a c i t o r V o l t a g e

t ct d

v T 1

V c

0v o

V s c+ V

V s

v c

V c

- V c


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Disadvantages• A starting circuit is required and the starting circuit should be

such that it triggers thyristor T2 first.• Load voltage jumps to almost twice the supply voltage when

the commutation is initiated.• The discharging and charging time of commutation capacitor

are dependent on the load current and this limits highfrequency operation, especially at low load current.


• Chopper cannot be tested without connecting load.• Thyristor T1 has to carry load current as well as resonant

current resulting in increasing its peak current rating.


choppers

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