7/27/2019 DC_DC_1_PE_AU_06_11_08

1/42

Voltage regulator (VR) provides:

nearly constant dc output voltage that is essentially independent of the

input voltage

output load current

temperature

Chapter 5 (P#166)

DC-DC Converters

Input DC Constant output ?

DC VoltageConstant OR

Variable (Dynamic)

Load

Constant

OR

Variable Voltage

7/27/2019 DC_DC_1_PE_AU_06_11_08

2/42

DC Voltage

Regulator

Series Shunt Step-

Down

Linear Switching

Step-up Up & Down

(Negative)

Chopper

s

+ Positive - Negative

Dual

BuckBoost

Buck-boost

Regulated

Power Supply

-Low Efficiency (Disadvantage)

-Low Noise (Advantage)

-High Efficiency (Advantage)

-High Noise (Disadvantage)

In Power Electronics

We discuss only

Switching Power

supplies

In Power Electronics

Why not Linear PS?

7/27/2019 DC_DC_1_PE_AU_06_11_08

3/42

SMPS

7/27/2019 DC_DC_1_PE_AU_06_11_08

4/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

5/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

6/42

5.2(Page 166) PRINCIPLE OF STEP-DOWN OPERATION

VDC RloadVload

Iload+

VDC

t

VDC

t

Vload

VAV

Figure 5.1

offon

DConAV

tt

V.tV

T

V.tV

DCon

AV

T

ON Off

DCon

AV VT

tV K = Duty Cycle

)......(

k

R

RkV

V

I

V)cetansisReInput(R load

loadDC

DC

load

DCi 45

BJT

MOSFET

IGBT

Ri

DCAV VkV

7/27/2019 DC_DC_1_PE_AU_06_11_08

7/42

Figure 5-3(page 171) DC Converter with RL - Load

VDC

t

t

Vload

Figure 3-40

RL

Vload

Iload

VDC+

SCR

v1(t)D1

Free-wheelingdiode

Makes current

flow during tofftON=? T

ON Off

7/27/2019 DC_DC_1_PE_AU_06_11_08

8/42

1. Constant-frequency Operation (PWM) Page 168 (Self)

2. Variable-frequency Operation (FM) Page 168 (Self)

Which of the above method preferable ? Why?

7/27/2019 DC_DC_1_PE_AU_06_11_08

9/42

The duty cycle k can be generated by comparing a dc reference

signal vcrwith a saw-tooth carrier signal vr

Vg

0

Vr

0

V

Vcr

vrVcr

T

kT T

t

t

+5V

0

0

VcrVcr

T

kT T

t

t

)9.5....(tT

Vvr

r kTVv

crcr kt

T

VVr

cr M

VVk

r

cr

M = Modulation index

Vr= Maximum valueof saw tooth wave

vr= Instantaneous Value of

saw tooth wavettanConsV

V

VV cr

r

ino

7/27/2019 DC_DC_1_PE_AU_06_11_08

10/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

11/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

12/42

http://upload.wikimedia.org/wikipedia/commons/1/1f/PWM_3L.gif

7/27/2019 DC_DC_1_PE_AU_06_11_08

13/42

Section 5.3 to 5.7 (Page 171 to 185)

Will Refer When Required

NOW START

Section 5.8(Page 186)

Switching Mode Power Supply

7/27/2019 DC_DC_1_PE_AU_06_11_08

14/42

DC to DC: CHOPPER [Switching Mode Power Supply] (SMPS)

7/27/2019 DC_DC_1_PE_AU_06_11_08

15/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

16/42

V

7/27/2019 DC_DC_1_PE_AU_06_11_08

17/42

PRINCIPLE OF STEP-DOWN OPERATION

VDC RloadVload

Iload+

VDC

t

VDC

t

Vload

VAV

BJTMOSFETIGBT

Ri

VDC

t

VDC

t

t

Vload

t

Vload

Figure 3-40

RL

Vload

Iload

VDC

+

SCR

v1(t)D1

Figure 3-40

RL

Vload

Iload

VDC

+

SCR

v1(t)D1

T

ON Off

T

ON Off

7/27/2019 DC_DC_1_PE_AU_06_11_08

18/42

5.8 (Page#186) Switching-Mode Regulators

7/27/2019 DC_DC_1_PE_AU_06_11_08

19/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

20/42

BUCK

BOOST

BUCK-BOOST

This circuit is

different form

the circuit givenin the book (Fig

5.18). Find the

difference, why

this difference?

7/27/2019 DC_DC_1_PE_AU_06_11_08

21/42

5.8.1 (Page#186) Buck Regulator (Step Down)

Figure 5.15 (Similar); Elements of switching-mode regulators - Buck (Step Down)

l i

7/27/2019 DC_DC_1_PE_AU_06_11_08

22/42

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

7/27/2019 DC_DC_1_PE_AU_06_11_08

23/42

Variable

PWM

Linear Mode

Vin

Vout

Switching Mode

Vin

Vout

7/27/2019 DC_DC_1_PE_AU_06_11_08

24/42

Variable

PWM

Square Voltage

(Ripples)

Variable

PWM

Variable

PWM

LPF(C)

Required to

remove DC

ripples

LPF (LC)

Required to

remove DC

ripplesL Remove?

C Remove?

5.8 (page#186) SMPS - Step Down (Buck) Regulator

Do we

Need PWM

in Linear

Power

Regulators

WHY?

Do we

Need LPFin Linear

Power

Regulators

WHY?

Fig 5 16 SMPS Step

7/27/2019 DC_DC_1_PE_AU_06_11_08

25/42

LPFVariable

PWMFree Wheeling:

Diode required to

pass energy trapped

in L&C during tOFF

Do we need Free

Wheeling Diode in

Linear Power

Regulators WHY?

CONTROL CIRCUIT

Fig 5.16 SMPS - Step

Down (Buck) Regulator

Vin

Vout ON ON

T=1/f

OFF

Continuous I and Discontinuous I in DC DC Converters

7/27/2019 DC_DC_1_PE_AU_06_11_08

26/42

Continuous IL and Discontinuous IL in DC-DC Converters

Continuous Inductance Current (IL)

ON

OFF

ON

OFF

7/27/2019 DC_DC_1_PE_AU_06_11_08

27/42

FIGURE 5 16 Buck regulator with continuous i

7/27/2019 DC_DC_1_PE_AU_06_11_08

28/42

Control

vc

eL

iL,ILis,Is

ic,Icio,Ia

IGBT

io,Iaic,Ic

is=iL

ON

Mode 1

io,Iaic,Ic

IL

OFF

Mode 2

FIGURE 5.16 Buck regulator with continuous iL

Mode 1-When transistor

is switched ON at t=0.

The current flows

through L, C & load

Mode 2-When transistor is

switched OFF at t=t1. Thecurrent flows through

Diode, C & loadt

ONMode

OFF

ModeON

t=0 t=t1

osL VVv oL Vv 0

i I

i I

7/27/2019 DC_DC_1_PE_AU_06_11_08

29/42

),.....(I

I

V

V

T

tkCycleDuty

)o(a

s

s

oon 49485

).....()VV(V

VLIT

fttperiodSwitching

asa

s

s

5051

21

).....(LfV

)VV(VIcurrentripplepeaktoPeak

s

asa 515

).....(C

TIVvoltageripplepeaktoPeak

c535

8

io,Iaic,Ic

is=iL

ON

Mode 1

io,Iaic,Ic

is=iL

ON

Mode 1

Control

vc

eL

iL,ILis,Is

ic,Icio,Ia

Control

vc

eL

iL,ILis,Is

ic,Icio,Ia

io,Iaic,Ic

IL

OFF

Mode 2

io,Iaic,Ic

IL

OFF

Mode 2Equation required for Designing

Buck Converter

http://www hills2 u net com/electron/smps htm#buck

7/27/2019 DC_DC_1_PE_AU_06_11_08

30/42

http://www.hills2.u-net.com/electron/smps.htm#buck

7/27/2019 DC_DC_1_PE_AU_06_11_08

31/42

V

t

VL=eL

Vs

LVV

dtdi osL

?dtdiL

LV

LV oo

0

kT T

ONON OFF

kT T

dt

diLe LL

VLCurrent in the inductor

will not change thedirection

Voltage in the inductor

will change the

direction (polarity)

osL VVv

oV

IL

IL

I1

I2

(1-k)T

7/27/2019 DC_DC_1_PE_AU_06_11_08

32/42

I

t

VL=eL

Vs

IC

kT T

ONON OFF

kT T

VC

Current in the Capacitor

changes its direction

I1-I

a

I2-Ia

(1-k)T

Voltage in the capacitor

will not change

direction (polarity)

Figure 5 16

7/27/2019 DC_DC_1_PE_AU_06_11_08

33/42

Figure 5.16

io,Iaic

,Ic

is=iL

ON

Mode 1

io,Iaic

,Ic

is=iL

ON

Mode 1

io,Iaic,Ic

IL

OFF

Mode 2

io,Iaic,Ic

IL

OFF

Mode 2

Please Read S it hi R l t

7/27/2019 DC_DC_1_PE_AU_06_11_08

34/42

Its percent voltage ripple is 100%.

The output voltage with such a high ripple content may be satisfactory forelectric 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 inductorcannot 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

7/27/2019 DC_DC_1_PE_AU_06_11_08

35/42

Please Read Switching Regulator Basic Design Procedure

7/27/2019 DC_DC_1_PE_AU_06_11_08

36/42

oCalculate D to obtain required output voltage.

o Select a particular switching frequency (f) and device

preferably f>20KHz for negligible acoustic noise

higher fs 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

Please Read Switching Regulator Basic Design Procedure

7/27/2019 DC_DC_1_PE_AU_06_11_08

37/42

oCalculate Lmin. Choose L>>10 LminoCalculate C for ripple factor requirement.

Capacitor ratings:

must withstand peak output voltage

must carry required RMS current. Note RMS current fortriangular w/f is Ip/3, where Ip is the peak capacitor

current given by iL/2.

oWire size consideration:

Normally rated in RMS. But iL is known as peak. RMS

value for iL is given as:

Please Read- Switching Regulator Basic Design Procedure

SMPS - Step UP (Boost) Regulator

7/27/2019 DC_DC_1_PE_AU_06_11_08

38/42

Variable

PWM

inout VD

V

1

1

p ( ) g

SMPSVoltage Inverter

7/27/2019 DC_DC_1_PE_AU_06_11_08

39/42

PWM

g

Configuration

Buck-Boost

little modified call Cuk

in_out VD1

D=V

7/27/2019 DC_DC_1_PE_AU_06_11_08

40/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

41/42

7/27/2019 DC_DC_1_PE_AU_06_11_08

42/42