chandresh chopper

05/01/2023 chandresh kumar 1

DC TO DC CONVERTER

Submitted to :Er. Shimi S.L(Assistant Professor) Department of Electrical Engineering , NITTTR Chandigarh

Presentation by:Chandresh kumar (141503)ME-MOD-IC-2014Department of Electrical engineering NITTTR Chandigarh


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 (as a switch) 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 AI AttT t t

or chopping period.1 Freq. of chopper switching or chopping freq.fT


Average Output Voltage

.

duty cycle

ONdc

ON OFF

ONdc

ON

tV Vt t

tV V V dT

tbut dt


2

0

Average Output Current

RMS value of output voltage

1 ON

dcdc

ONdc

t

O o

VIR

tV VI 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 VT T

V d V


2

2

Output power

But

Output power

O O O

OO

OO

O

P V IVIR

VPRdVP

R


Effective input resistance of chopper

The output voltage can be varied by varying the duty cycle.

idc

i

VRIRRd


Principle Of Step-up Chopper

+

V OV

C hopper

CLOAD

DLI

+


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

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

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

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


• The current tends to decrease resulting in

reversing the polarity of induced EMF in L.

• Therefore voltage across load is given by:-

• When the chopper is off, the inductor current I

is forced to flow through the diode D and load

for a period, tOFF.

. ., O OdIV V L i e V Vdt


• 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 Voltage

Assume 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 tt 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 VT t


1

1

11

Where duty cyle

ON OFF

OON

O

ON

T t t

V VtT

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

dd 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 Controlv 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.


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 utput curren t

Thyristo 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

Thyristorgate 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 obtainedalong 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 tR

t t i t I

V EI e I eR

Ldi Ri Edt


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 tR

t t i t I

EI I e eR


Class C Chopper

V

C hopper

+

v 0

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.


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

current t 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 lseo f C H 2

G ate pu lseof C H 1

O utput curren t

O utpu t 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 2O 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.


G ate pulseof C H 2

G ate pu lseof C H 1

O utpu t curren t

O utpu t voltage

Average v0

ig 1

ig 2

i0

v 0

V

t

t

t

t

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


G ate pulseo f C H 2

G ate pulseo f C H 1

O utpu t curren t

O utput 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

v0

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 onduc ts

1 4

4 2

D D2 3 - ConductsC H - D C onduc ts4 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.


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