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Step-Down

Switching

Step-up Up & Down(Negative)

Choppers

Buck Boost Buck-boost

Up & Down(Negative)

Cuk

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),.....(II

VV

Tt

kCycleDuty )o(as

s

oon49485

).....()VV(V

VLIT

f ttperiodSw itching

asa

s

s505

121

).....(Lf V

)VV(VIcurrentripplepeaktoPeaks

asa 515

).....(CTI

VvoltageripplepeaktoPeak c 5358

io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

is=iL

ONMode 1

Control

v c

e L

iL ,I Lis ,I s

ic ,I cio ,I a

Control

v c

e L

iL ,I Lis ,I s

ic ,I cio ,I a

io,Iaic,Ic

IL

OFFMode 2

io,Iaic,Ic

IL

OFFMode 2

Equation required for DesigningBuck Converter

)57.5....(161 2 Lf

k C c)56.5....(2)1(&tan

f Rk Lcapacitor ceinducCritical c

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).....(kII

VV

Tt

CycleDuty )o(as

s

aon

63511

).....()VV(V

VLIT


SaS

a

s665

121

).....(Lf V)VV(V

IcurrentripplepeaktoPeaks

SaS675

).....(fC

kIVvoltageripplepeaktoPeak ac 715

Equation required for DesigningBoost Converter

)73.5....(2 LR

k C c)72.5....(2)1(&tan

f Rk k Lcapacitor ceinducritical c

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).....(kII

VV

Tt

CycleDuty)o(a

s

s

aon6351

1

).....()VV(V

VLIT


SaS

a

s665

121

).....(Lf V

)VV(VIcurrentripplepeaktoPeaks

SaS 675

).....(fC

kIVvoltageripplepeaktoPeak ac 715

Equation required for DesigningBuck-Boost Regulators

)89.5....(2 R f

k C c)88.5....(2)1(&tan

f Rk Lcapacitor ceinducCritical c

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Controlvc

e L

iL,ILis ,Is

ic,Icio,Ia

IGBT

io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

IL

OFFMode 2

FIGURE 5.16 Buck regulator with continuous i L

Mode 1-When transistor is switched ON at t=0.

The current flowsthrough L, C & load

Mode 2-When transistor isswitched OFF at t=t 1. The

current flows throughDiode, C & loadt

ONMode

OFF

ModeON

t=0 t=t1

o s L V V v

o L V v 0

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V

t

VL=e L

Vs

LVV

dtdi osL ?

dtdiL

LV

LV oo0

kT T

ONON OFF

kT T

dtdiLeLL

VL

Current in the inductor

will not change thedirection

Voltage in the inductor will change the

direction (polarity)

osL VVv

oV

ILIL

I1

I2

(1-k)T

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I

t

VL=e L

Vs

IC

kT T

ONON OFF

kT T

VC

Current in the Capacitor changes its direction

I1-Ia

I2-Ia

(1-k)T

Voltage in the capacitor will not change

direction (polarity)

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Figure 5.16

io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

IL

OFFMode 2

io,Iaic,Ic

IL

OFFMode 2

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Figure 5.18 Buck_BoostFigure 5.18 Buck_Boost

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Figure 5.19 Ck regulator

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BUCK BOOST BUCKBOOST

io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

IL

OFFMode 2

io,Iaic,Ic

IL

OFFMode 2

asL VVv aL Vv 0

LL

L vdt

diLe

LVV

tI as

1

)VV(LI

tas

1

)VV(VVLI

f ttT

asa

s

sS

121

aVLI

t 2

io,Iaic ,Ic

is=iL

ONMode 1

io,Iaic ,Ic

is=iL

ONMode 1

2t

ILVV oa

1tI

Ldtdi

LVS asL VVv

LVV

tI

as1

)VV(LI

tSa

2SVLI

t1

1tI

LVS

SVLI

t1

2tI

LVa

aVLI

t 2

LV

tI a

2

2

0884

1

T

C fC

I

C

IT

dt

I

CV 1 1

0

1

0

11t

a

t

aCC C

tI

ICdtICV 1 1

0

1

0

11t

a

t

aCC C

tI

ICdtICV

)VV(VVLI

f ttT

SaS

a

sS

121

aS

Sa

sS VV

)VV(LIf

ttT1

21

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)50.5....()( a sa

s

V V V V L I

T

)51.5....()(

L f V

V V V I

s

a sa

)53.5....(8 C

T I V c

)57.5....(161

2 Lf k

C c

)56.5....(2

)1( f

Rk Lc

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S

a

in

out

V V

V V k cycle Duty

S onon f t T t

k .

off on

on

S

a

t t

t

V

V k

k

Vo/V

S

k t t

t

off on

on

1

1

k

k

t t

t

off on

on

1

k

Vo/VS

1

k

-Vo/VS

1

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Figure 5.23Power Factor conditioning of diode Rectifiers

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Its percent voltage ripple is 100%.

The output voltage with such a high ripple content may be satisfactory for electric heaters, light dimming circuits, etc., it is certainly not suitable for theoperation of amplifiers and other circuits requiring almost constant dcvoltage.

The high voltage ripple can be controlled by placing a capacitor across theload.

The capacitor is large enough so that its voltage does not have anynoticeable change during the time the switch is off.

Somewhat better circuit can be developed by including an inductor, which is

in series with the switch when the switch is on (closed), to limit the current inrush.

However, this creates another problem. Since the current in the inductor cannot change suddenly, we have to provide at least one more switch, sucha freewheeling diode, to provide a path for the inductor current when the

switch is off (open).

Please Read - Switching Regulator

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In summary, a good dc-to-dc converter may have, aninductor, a capacitor, and a freewheeling diode, andan electronic switch.

The placement of these elements in a circuit dictatesthe performance of the circuit.

The three configurations that utilize these circuitelements are:

a) Boost Converter (raising the output voltage, step-upapplication), andb) Buck-Boost Converter (lowering or raising the output

voltage, step-down or step up application).c) Buck Converter (lowering the output voltage, step-down

application),

Please Read - Switching Regulator

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o Calculate D to obtain required output voltage.

o Select a particular switching frequency (f) and device preferably f>20KHz for negligible acoustic noise higher f s results in smaller L and C. But results in higher

losses. Reduced efficiency, larger heat sink. Possible devices: MOSFET, IGBT and BJT. Low power

MOSFET can reach MHz range.

Please Read - Switching Regulator Basic Design Procedure

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o Calculate L min . Choose L>>10 L mino 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 I p/3, where I p is the peak capacitor current given by i L/2.

o

Wire size consideration:Normally rated in RMS. But i L is known as peak. RMSvalue for i L is given as :

Please Read - Switching Regulator Basic Design Procedure

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1. 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.

2. Design a buck converter such that the outputvoltage is 28V when the input is 48V. The load is8Ohm. Design the converter such that it will be in

continuous current mode. The output voltageripple must not be more than 0.5%. Specify thefrequency and the values of each component.Suggest the power switch also.

Please Do it - Practice the following Problems

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t

IC

ONON OFF

kT T

VC

(1-k)T

IL

IS

iO


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io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

is=iL

ONMode 1

io,Iaic,Ic

IL

OFFMode 2

io,Iaic,Ic

IL

OFFMode 2

io,Iaic ,Ic

is=iL

ONMode 1

io,Iaic ,Ic

is=iL

ONMode 1

OOFF

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For good performance without needing detailed design: For most applications, for up to about 100 W, go with the CCM[continuous current] flyback. Over 100 W, line-powered andisolated, use either the isolated Cuk topology or a half-bridge, controlled as a PFC [power factor corrector].

-- Dennis Feucht, power elect. Designer (article is in list of URLs)

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Relevant URLs / Billings 2.17 & 2.20 Ripple regulator application (re. 2.17)

http://focus.ti.com/lit/ug/slvu007/slvu007.pdf

Case study of using a DC-to-DC transformer (re. 2.17) http://www.reed-

electronics.com/ednmag/archives/1995/031695/06df4.htm Choosing the right power converter article (re. 2.20)

http://www.analogzone.com/col_0106.htm

History of swtiched-mode power supplies (re. 2.20) http://www.steve-w.dircon.co.uk/fleadh/mphil/history.htm

Lots of Cuk information (re. 2.20) http://www.energychallenge.org/2001Reports/UTEP.pdf

Cuk converter applet (re. 2.20)

http://www.ece.umr.edu/links/power_electronics/CukConverter.html

LM 2611 datasheet (Cuk converter IC) (re. 2.20) http://www.national.com/pf/LM/LM2611.html

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Step-Down

Switching


Chopper s


Up & Down(Negative)

Cuk

Step-Down

Switching


Choppers


Up & Down(Negative)

Cuk

Documents

DC_DC_2_PE_AU_20_11_08