# chopper 2002

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choppers_ all u need to knowTRANSCRIPT

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

1

Chapter 3

DC to DC CONVERTER(CHOPPER)

General Buck converter Boost converter Buck-Boost converter Switched-mode power supply Bridge converter Notes on electromagnetic compatibility

(EMC) and solutions.

DC-DC Converter (Chopper)

DEFINITION: Converting the unregulated DC input to a controlled DC output with a desired voltage level.

General block diagram:

LOAD

Vcontrol(derived from

feedback circuit)

DC supply(from rectifier-filter, battery,fuel cell etc.)

DC output

APPLICATIONS: Switched-mode power supply (SMPS), DC

motor control, battery chargers

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

3

Linear regulator Transistor is

operated in linear (active) mode.

Output voltage

The transistor can be conveniently modelled by an equivalent variable resistor, as shown.

Power loss is high at high current due to:

TLo RIP2=

TLo RIV =

+

VoRL

+ VCE IL

MODEL OF LINEARREGULATOR

RT

EQUIVALENTCIRCUIT

Vs

RL

+ VCE IL

VsVo

+

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

4

Switching Regulator Power loss is zero

(for ideal switch): when switch is

open, no current flow in it,

when switch is closed no voltage drop across it.

Since power is a product of voltage and current, no losses occurs in the switch.

Power is 100% transferred from source to load.

Switching regulator is the basis of all DC-DC converters

+

Vo

RL

+ VCE IL

MODEL OF LINEARREGULATOR

EQUIVALENT CIRCUIT

Vs

RL

IL

VsVo+

(ON)closed

(OFF)open

(ON)closed

DT T

OUTPUT VOLTAGE

Vo

SWITCH

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

5

Buck (step-down) converter

Vd

L

D C RL

S+

Vo

Vo

+

CIRCUIT OF BUCK CONVERTER

CIRCUIT WHEN SWITCH IS CLOSED

CIRCUIT WHEN SWITCH IS OPENED

Vo

+

iL

Vd D RL

S

Vd D RL

S

+ vL

+ vL

iL

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

6

Circuit operation when switch is turned on (closed)

Diode is reversed biased. Switch conducts inductor current

This results in positive inductor voltage, i.e:

It causes linear increase in the inductor current

odL VVv =

==

dtvL

i

dtdiLv

LL

LL

1

Vd VD

+ vL -

C RL

+

Vo

VdVo

Vo

closedopened

closedopened

t

DT Tt

iLmin

iLmaxIL

vL

iL

iL

+

S

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

7

Operation when switch turned off (opened)

Because of inductive energy storage, iL continues to flow.

Diode is forward biased

Current now flows through the diode and

oL Vv =

Vd

+ vL -

C RL

+

Vo

VdVo

Vo

closedopened

closedopened

t

DT Tt

iLmin

iLmaxIL

vL

iL

iL

S

D

(1-D)T

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

8

Analysis for switch closed

( ) DTL

VVi

LVV

DTi

ti

dtdi

i

i

LVV

dtdi

dtdiL

VVv

odclosedL

odLLL

L

L

odL

LodL

=

===

=

==

Figure From

linearly. increase must Therefore

constant. tive-posi a is of vative

-deri thesince :Note

oltage,inductor v The

IL

iL max

DT T

iL

Vd Vo

vL

t

t

iL min

closed

iL

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

9

Analysis for switch opened

( ) TDLVi

LV

TDi

ti

dtdi

ii

LV

dtdi

dtdiLVv

oopenedL

oLLL

LL

oL

LoL

)1(

)1(

Figure From

linearly. decreasemust constant, tive

-nega a is of vative-deri thesince :Note

opened,switch For

=

==

=

===

IL

iL max

DT T

iL

Vd Vo

vL

t

t

iL min

opened

iL

(1 D)T

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

10

Steady-state operation

( ) ( )

do

so

sod

openedLclosedL

L

L

DVV

TDLVDT

LVV

ii

i

i

=

=

=+

0)1(

0

:i.e zero, is period oneover of change theisThat cycle.next theof begining

at the same theis cycle switching of endat the that requiresoperation state-Steady

iL Unstable current

Decaying current

Steady-state current

t

t

t

iL

iL

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

11

==

+=

+=+=

===

LfD

RViII

LfD

RV

TDL

VR

ViII

RVII

oL

L

o

ooLL

oRL

2)1(1

2

:current Minimum

2)1(1

)1(21

2

:current Maximum

Rin current Average currentinductor Average

min

max

L

Average, Maximum and Minimum inductor current

IL

Imax

Imin

iL

iL

t

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

12

Continuous current operationiL

Imax

Imin t0

min

min

min

min

be bechosen is Normally operation. of mode continous ensure

current toinductor minimum theis This2

)1(

02

)1(1

,0 operation, continuousFor

2)1(1

2

analysis, previous From

LL

RfDLL

LfD

RV

I

LfD

RViII

o

oL

L

>>

=

==

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

13

Output voltage ripple

2

2

8)1(

factor, ripple theSo,8

)1(8

82221

:formula area triangleuse figure, From

LCfD

VVr

LCfD

CiTV

iTiTQ

CQVVCQCVQ

iii

o

o

Lo

LL

ooo

RLc

==

==

=

=

===+=

iRiLL

iC

iLiL=IR

imax

imin

0

0Vo

Vo/R+

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

14

Design procedures for Buck

Vd(inputspec.)

SWITCH

f = ?D = ?TYPE ?

D

L

Lmin= ?L = 10Lmin

Cripple ?

RLPo = ?Io = ?

Calculate D to obtain required output voltage.

Select a particular switching frequency: preferably >20KHz for negligible acoustic

noise higher fs results in smaller L, but higher device

losses. Thus lowering efficiency and larger heat sink. Also C is reduced.

Possible devices: MOSFET, IGBT and BJT. Low power MOSFET can reach MHz range.

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

15

Design procedures for Buck

Determine Lmin. Increase Lmin by about 10 times to ensure full continuos mode.

Calculate C for ripple factor requirement.

Capacitor ratings: must withstand peak output voltage must carry required RMS current. Note RMS

current for triangular w/f is Ip/3, where Ip is the peak capacitor current given by iL/2

Wire size consideration: Normally rated in RMS. But iL is known as

peak. RMS value for iL is given as:

22

, 32

+= LLRMSL iII

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

16

Examples of Buck converter

A buck converter is supplied from a 50V battery source. Given L=400uH, C=100uF, R=20 Ohm, f=20KHz and D=0.4. Calculate: (a) output voltage (b) maximum and minimum inductor current, (c) output voltage ripple.

A buck converter has an input voltage of 50V and output of 25V. The switching frequency is 10KHz. The power output is 125W. (a) Determine the duty cycle, (b) value of L to limit the peak inductor current to 6.25A, (c) value of capacitance to limit the output voltage ripple factor to 0.5%.

Design a buck converter such that the output voltage is 28V when the input is 48V. The load is 8Ohm. Design the converter such that it will be in continuous current mode. The output voltage ripple must not be more than 0.5%. Specify the frequency and the values of each component. Suggest the power switch also.

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

17

Boost (step-up) converter

Vd

L D

C

RL

S

Vd

L D

CRLS

Vd

LD

C RLS

+ vL

+

Vo

+ vL -

Vo

+

CIRCUIT OF BOOST CONVERTER

CIRCUIT WHEN SWITCH IS CLOSED

CIRCUIT WHEN SWITCH IS OPENED

Vo

+

iL

Power Electronics and Drives (Version 2): Dr.

Zainal Salam

18

Boost analysis:switch closed

Vd

L D

CS

+ vL

iL

+vo

( )LDTVi

LV

dtdi

DTi

ti

dtdi

LV

dtdi

dtdiLVv

dclosedL

dL

LLL

dL

LdL

=

=

==

==

=

DT T

iL

vL

CLOSED

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