Shantilal Shah Engineering College

SUBJECT : POWER ELECTRONICS

TOPIC:DC TO DC CONVERTER

(CHOPPERS)

Branch : Electrical

Sem : 5th

Prepared By :

NO. NAME ENROLLMENT NO

1 MER VISHAL GIRISHBHAI 130430109031

2 PATEL BHAVIN 130430109039

3 SINGH SUDHIR 130430109052

GUIDED BY :

PROF. J.K.CHAVDA

3

DC-DC Converters Convert a fixed DC Source into a Variable DC Source

DC equivalent to an AC transformer with variable turns ratio

Step-up and Step-down versions

APPLICATION

Motor Control

Voltage Regulators

4

Step-down Operation• Switch SW is known as a “Chopper”

• Use BJT, MOSFET, or IGBT

• Close for time t1

• VS appears across R

• Open for time t2

• Voltage across R = 0

• Repeat

• Period T = t1 + t2

5

Waveforms for the Step-Down Converter

6

Average Value of the Output Voltage1

1

0

0

11

1

1

t

a O

t

a S

a S S

a S

V v dtT

V V dtT

tV V ft V

T

V kV

7

Average Value of the Load Current

1

a Sa

V kVI

R R

T period

tk dutycycle

T

f frequency

8

RMS Value of the output voltage

1

22

0

1

22

0

1

1

kT

O O

kT

O S

O S

V v dtT

V V dtT

V kV

9

If the converter is “lossless”, Pin = Pout

0

2

0

2

2

1

1

1

kT

in O

kT

Oin

Sin

Sin

P v idtT

vP dt

T R

VP kT

T R

VP k

R

10

Modes of Operation

• Constant – frequency operation

• Period T held constant, t1 varied

• Width of the pulse changes

• “Pulse-width modulation”, PWM

• Variable -- frequency operation

• Change the chopping frequency (period T)

• Either t1 or t2 is kept constant

• “Frequency modulation”

G1

G2

VS

C

A2

T2

K2

L

A

D

K

AF

KF

FD

RL

LL

DC CHOPPER USING THE SCR

CIRCUIT DIAGRAM

Working Principle For convenience the

chopper operation is divided into five

modes.

1. Mode-1

2. Mode-2

3. Mode-3

4. Mode-4

5. Mode-5 T1

30V

V MODE -1 OPERATION

G1

G2

VS

C

A2

T2

K2

L

A

D

K

AF

KF

FD

RL

LL

»»»

»»

»»»»

»»

»»»»

»»

»

»»»»

»»

»»

V

30V

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

T1

30V

VMODE -2 OPERATION

G1

G2

VS

C

A2

T2

K2

L

A

D

K

AF

KF

FD

RL

LL

»»»

»»

»»»»»»

»»»»

»»

»»

»»

V

30V

• 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.

• Capacitor recharges back to the

supply voltage (with plate ‘a’

positive).

T1

30V

VMODE -3 OPERATION

G1

G2

VS

C

A2

T2

K2

L

A

D

K

AF

KF

FD

RL

LL

»»»

»»

»»»»»»

»»»»

»»

»»

»»

V

30V

FWD starts conducting and

the load current decays.

• Hence capacitor charges to a

voltage higher than supply

voltage, T2 naturally turns

off.

T1

30V

VMODE -4 OPERATION

G1

G2

VS

C

A2

T2

K2

L

A

D

K

AF

KF

FD

RL

LL

»»»

»»

»»»»

»»

»»»»

»»

»»

»»

V

30V

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 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

T1

30V

V

MODE -5 OPERATION

G1

G2

VS

C

A2

T2

K2

L

A

D

K

AF

KF

FD

RL

LL

»»

»»

»

V

30V

»»

»

»» Both thyristors are off

and the load current

flows through

the FWD.

This mode will end once

thyristor T1 is fired.

T1

THANKFUL TO YOU !!