topic: choppers subject: power electronics(ee-309) class: 5 th sem. (ee) presented by: er. ram singh...

Punjab EDUSAT Society 1

Topic: ChoppersSubject: Power Electronics(EE-309)

Class: 5th sem. (EE)

Presented By:Er. Ram Singh(Asstt. Prof.) Deptt. Of EE

BHSBIET Lehragaga


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.


Types of Choppers

Step-down choppers. Step-up choppers.

In step down chopper output voltage is less than input voltage.

In step up chopper output voltage is more than input voltage.


Principle Of Step-down Chopper

V

i0

V 0

C hopper

R

+


• A step-down chopper with resistive load. • The thyristor in the circuit acts as a switch.• When thyristor is ON, supply voltage appears

across the load • When thyristor is OFF, the voltage across the

load will be zero.


V d c

v 0

V

V /R

i0

Id c

t

t

tO N

T

tO F F


verage value of output or load voltage.

verage value of output or load current.

Time interval for which SCR conducts.

Time interval for which SCR is OFF.

Period of switching

dc

dc

ON

OFF

ON OFF

V A

I A

t

t

T t t

or chopping period.

1 Freq. of chopper switching or chopping freq.f

T


Average Output Voltage

.

duty cycle

ONdc

ON OFF

ONdc

ON

tV V

t t

tV V V d

T

tbut d

t


2

0

Average Output Current

RMS value of output voltage

1 ON

dcdc

ONdc

t

O o

VI

RtV V

I dR T R

V v dtT


2

0

2

But during ,

Therefore RMS output voltage

1

.

.

ON

ON o

t

O

ONO ON

O

t v V

V V dtT

tVV t V

T T

V d V


2

2

Output power

But

Output power

O O O

OO

OO

O

P V I

VI

R

VP

R

dVP

R


Effective input resistance of chopper

The output voltage can be varied by

varying the duty cycle.

idc

i

VR

I

RR

d


Principle Of Step-up Chopper

+

V OV

C hopper

CLOAD

DLI

+


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

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

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

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


• When the chopper is off, the inductor current I

is forced to flow through the diode D and load

for a period, tOFF.

• The current tends to decrease resulting in

reversing the polarity of induced EMF in L.

• Therefore voltage across load is given by:-


. ., O O

dIV V L i e V V

dt


• A large capacitor ‘C’ connected across the

load, will provide a continuous output voltage

• Diode D prevents any current flow from

capacitor to the source.

• Step up choppers are used for regenerative

braking of dc motors


Expression For Output VoltageAssume the average inductor current to be

during ON and OFF time of Chopper.

Voltage across inductor

Therefore energy stored in inductor

= . .

Where

When Chopper

period of chopper.

is ON

ON

ON

I

L V

V I t

t ON


(energy is supplied by inductor to load)

Voltage across

Energy supplied by inductor

where period of Chopper.

Neg

When Chopper

lecting losses, energy stored in inductor

is OFF

O

O OFF

OFF

L V V

L V V It

t OFF

L

= energy supplied by inductor L


Where

T = Chopping period or period

of switching.

ON O OFF

ON OFFO

OFF

OON

VIt V V It

V t tV

t

TV V

T t


1

1

1

1

Where duty cyle

ON OFF

OON

O

ON

T t t

V Vt

T

V Vd

td

T


For variation of duty cycle ' ' in the

range of 0 1 the output voltage

will vary in the range O

O

d

d V

V V


Control Strategies

• The output dc voltage can be varied by the

following methods.

1. Pulse width modulation control or constant

frequency operation.

2. Variable frequency control


Pulse Width Modulation

• tON is varied keeping chopping frequency ‘f’ &

chopping period ‘T’ constant.

• Output voltage is varied by varying the ON

time tON


Pulse Width Modulation

V 0

V

V

V 0

t

ttO N

tO N tO F F

tO 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 varied over a wide range.

• This method produces harmonics in the output and for

large tOFF load current may become discontinuous


Variable Frequency Control

v 0

V

V

v 0

t

t

tO N

tO N

T

T

tO F F

tO F F


Performance Parameters• The thyristor requires a certain minimum

time to turn ON and turn OFF. • Duty cycle d can be varied only between a

min. & max. value, limiting the min. and max. value of the output voltage.

• Ripple in the load current depends inversely on the chopping frequency, f.

• To reduce the load ripple current, frequency should be as high as possible.


Example 1

• A Chopper circuit is operating on TRC at a frequency of 2 kHz on a 460 V supply. If the load voltage is 350 volts, calculate the conduction period of the thyristor in each cycle.


3

460 V, = 350 V, f = 2 kHz

1Chopping period

10.5 sec

2 10

Output voltage

dc

ONdc

V V

Tf

T m

tV V

T


3

Conduction period of thyristor

0.5 10 350

4600.38 msec

dcON

ON

ON

T Vt

V

t

t


Example 2

• Input to the step up chopper is 200 V. The output

required is 600 V. If the conducting time of thyristor

is 200 sec. Compute

– Chopping frequency,

– If the pulse width is halved for constant frequency

of operation, find the new output voltage.


6

200 , 200 , 600

600 200200 10

Solving for

300

ON dc

dcON

V V t s V V

TV V

T t

T

T

T

T s


6

6

Chopping frequency

1

13.33

300 10Pulse width is halved

200 10100

2ON

fT

f KHz

t s


6

6

Frequency is constant

3.33

1300

Output voltage =

300 10200 300 Volts

300 100 10

ON

f KHz

T sf

TVT t


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

C hopper

FW D

+

v 0

v 0

i0

i0

LOAD

V


• When chopper is ON, supply voltage V is connected

across the load.

• When chopper is OFF, vO = 0 and the load current

continues to flow in the same direction through the

FWD.

• The average values of output voltage and current

are always positive.

• Class A Chopper is a first quadrant chopper


• Class A Chopper is a step-down chopper in

which power always 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 A Chopper.


O utpu t cu rren t

T hyristo rgate pu lse

O utpu t vo ltage

ig

i0

v 0

t

t

ttO N

T

C H O N

FW D C onduc ts


Class B Chopper

V

C hopper

+

v 0

v 0

i0

i0

L

E

R

D


• When chopper is ON, E drives a current through

L and R in a direction opposite to that shown in

figure.

• During the ON period of the chopper, the

inductance L stores energy.

• When Chopper is OFF, diode D conducts, and part

of the energy stored in inductor L is returned to

the supply.


• Average output voltage is positive.

• Average output current is negative.

• Therefore Class B Chopper operates in second

quadrant.

• 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.


O utpu t cu rren t

D con d u cts C h o pp e r

con d u cts

T hyristo rgate pu lse

O utpu t vo ltage

ig

i0

v 0

t

t

t

Im in

Im ax

T

tO NtO F F


Expression for Output Current

min

For the initial condition i.e.,

During the interval diode 'D' conduc

at 0

The solution of the ab

ts

voltage equation

ove equation is obtained

along similar lines as in s

is given by

OO

O

LdiV Ri E

dt

i t I t

tep-down chopper

with R-L load


min

max

max min

During the interval chopper is ON voltage

equation is g

1 0

At

1

0

iven by

OFF OFF

R Rt tL L

O OFF

OFF O

R Rt tL L

OO

V Ei t e I e t t

R

t t i t I

V EI e I e

R

LdiRi E

dt


max

max

min

min max

Redefining the time origin, at 0

The solution for the stated initial condition is

1 0

At

1ON ON

O

R Rt tL L

O ON

ON O

R Rt tL L

t i t I

Ei t I e e t t

R

t t i t I

EI I e e

R


Class C Chopper

V

C hopper

+

v0

D 1

D 2C H 2

C H 1

v 0i0

i0

L

E

R


• Class C Chopper is a combination of Class A and Class B Choppers.

• 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 is zero.


• 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


• 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 ate pu lseof C H 2


O utput cu rren t

O utput vo ltage

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


Class D Chopper

V+ v 0

D 2

D 1 C H 2

C H 1

v 0

i0

L ER i0


• 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 the load.

• When CH1 and CH2 are turned OFF, the load current

continues to flow in the same direction through load,

D1 and D2 , due to the energy stored in the inductor L.

• Output voltage vO = - V .


• Average load voltage is positive if chopper ON

time is more than 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.




O utpu t curren t

O utpu t vo ltage

Average v0

ig 1

ig 2

i0

v 0

V

t

t

t

t

C H ,C HO N1 2 D 1,D 2 Conducting


G ate pu lseo f C H 2

G ate pu lseo f C H 1

O utpu t curren t

O utpu t vo ltage

Average 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

i0L ER

C H 2 C H 4D 2 D 4

D 1 D 3C H 1 C H 3

+


Four Quadrant Operation

v 0

i0

C H - C H O NC H - D C onducts

1 4

4 2

D D2 3 - C onductsC H - D C onducts4 2

C H - C H O NC H - D C onducts

3 2

2 4

C H - D C onductsD - D C onducts

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 output voltage 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.


• 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 voltage

vO = V.

• Therefore the chopper operates in second

quadrant as vO is positive but iO is negative.


Voltage Commutated Chopper

• Voltage Commutated choppers are widely used in high power circuits where load fluctuation is not large.

• This chopper is also known as – Parallel capacitor turn-off chopper – Impulse Commutated chopper– Classical chopper.


LOAD

L

C

IL

L S

V S

+

_

+

_

T 2

T 1

D 1

a

biC

iT 1

vO

+

_

FW D


• To start the circuit, capacitor ‘C’ is initially charged with polarity (with plate ‘a’ positive) by triggering the thyristor T2.

• Capacitor ‘C’ gets charged through VS, C, T2 and load.

• As the charging current decays to zero thyristor T2 will be turned-off.

• With capacitor charged with plate ‘a’ positive the circuit is ready for operation.

• Assume that the load current remains constant during the commutation process


• For convenience the chopper operation is divided into five modes:-– Mode-1– Mode-2– Mode-3– Mode-4– Mode-5


Mode-1 Operation

LOADL

C

IL

L S

V S

+

_

+

_

T 1

D 1

V C iC


• 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


Capacitor Discharge Current

sin

1Where

& Capacitor Voltage

cos

C

C

Ci t V t

L

LC

V t V t


Mode-2 Operation

LOAD

C

L S

V S+

_+

_

T 2

V C

IL

IL


• 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.


C

Circuit turn-off time is given by

Where is load current.

t depends on load current, it must be designed

for the worst case condition which occur at the

maximum value of load current and mini

CC

L

L

V Ct

I

I

mum

value of capacitor voltage.


• Capacitor recharges back to the supply voltage (with plate ‘a’ positive).

• This time is called the recharging time and is given by:-


Sd

L

V Ct

I


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


r C dt t t


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


Mode-3 Operation

LOAD

C

L S

V S

+_

+

_

T 2V S

FW D

IL

IL


• FWD starts conducting and the load current

decays.

• The energy stored in source inductance LS is

transferred to capacitor.

• Hence capacitor charges to a voltage higher

than supply voltage, T2 naturally turns off.


The instantaneous capacitor voltage is

sin

Where

1

SC S L S

S

S

LV t V I t

C

L C


Mode-4 Operation

LOAD

C

L S

V S

+_

+

_

D 1

LFW D

IL

V C


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

• 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

IL

FW D


• Both thyristors are off and the load current

flows through the FWD.

• This mode will end once thyristor T1 is fired


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

t

t

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

ic

0Ip

iT1

0

I L


t

t

t

Vo l ta g e a c r o s s T 1

O u tp u t Vo l t a g e

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

tctd

v T1

V c

0v o

V s c+ V

V s

v c

V c

-V c


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 high frequency 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.


THANKS

topic: choppers subject: power electronics(ee-309) class: 5 th sem. (ee) presented by: er. ram singh...

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