Điện tử công suất (DC DC converter chapter 3 )

DC-DC Converter Chopper DEFINITION: Converting the unregulated DC input to a controlled DC output with a desired voltage level.. Fully off cut-off– When switch is open, no current flo

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 fromfeedback circuit)

– Switched-mode power supply (SMPS), DC

motor control, battery chargers

• Power loss is high at

high current due to:

L ce

o

T L

o

I V

P

R I

V = −

Fully off (cut-off)

– When switch is open,

no current flow in it

– When switch is

closed no voltage

drop across it.

• Since P=V.I, no losses

occurs in the switch.

– Power is 100%

transferred from

source to load.

(for ideal switch):

(OFF) open

(ON) closed

Buck (step-down) converter

CIRCUIT OF BUCK CONVERTER

CIRCUIT WHEN SWITCH IS CLOSED

CIRCUIT WHEN SWITCH IS OPENED

Switch is turned on (closed)

dt

di L v

L L

L L

Switch turned off (opened)

through the diode.

• The inductor voltage

can be derived as:

i t

i dt

di

L

V dt

di

dt

di L V

v

DT L

V V

i

L

V

V DT

i t

i

dt

di

i i

L

V

V dt

di

dt

di L V

V

v

o opened

L

o L

L L

o L

L o

L

o d

closed

L

o d

L L

L

L L

o d

L

L o

d

L

) 1

(

) 1

(

opened, switch

constant.T

positive a

is of Derivative

: (on) closed is

switch When the

Steady-state operation

s

o s

o d

opened L

T

D L

V DT

L

V

V

i i

∆

0 )

1 (

0

: i.e zero, is

period one

over

of

change the

is That cycle.

next the

of begining

at the same

the is cycle switching

of

end

at the that

requires operation

state -

min max

min

max

L

: ripple current

Inductor

2

) 1

(

1 2

: current Minimum

2

) 1

( 1

) 1

( 2

1 2

: current Maximum

R

in current Average

current inductor

Average

I I

i

Lf

D R

V

i I

I

Lf

D R

V

T

D L

V R

V i

I I

R

V I

I

L

o

L L

o

o o

L L

o R

=

− +

=

∆ +

=

=

=

=

Average, Maximum and

Minimum Inductor Current

Continuous Current Mode (CCM)

min min

min min

be b chosen is

Normally

operation.

of mode continous

ensure

current to inductor

minimum the

is This

2

) 1

(

0 2

) 1

( 1

, 0

operation, continuous

For

2

) 1

(

1 2

analysis, previous

From

L L

R f

D L

L

Lf

D R

V

I Lf

D R

V

i I

I

o

o

L L

Output voltage ripple

size.

capacitor Increasing

3)

size inductor Increasing

2)

frequency switching

Increasing

1)

: by reduced be

can Ripple

:

Note

8

) 1

(

factor, ripple

the

So,

8

) 1

( 8

peak) to

(Peak voltage

-Ripple

8

2 2

2

1

: formula area

triangle

Use

: as witten be

can charge

The

: current Capacitor

V

r

LCf

D C

i T

V

i

T

i T

Q

C

Q V

V C

V o +

Basic design procedures

RL

Po = ?

Io = ?

• Calculate D to obtain required output voltage.

• 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

• Calculate Lmin Choose L>>10 Lmin

• 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 Ipis the peak capacitor current given

by ∆iL/2

• ECAPs can be used

• Wire size consideration:

– Normally rated in RMS But iLis known as peak RMS value

for iLis given as:

22

2

∆ +

= L L

RMS

I

• 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.

Boost (step-up) converter

V d

C

RLS

V d

C

RLS

CIRCUIT OF BOOST CONVERTER

CIRCUIT WHEN SWITCH IS CLOSED

CIRCUIT WHEN SWITCH IS OPENED

Boost analysis:switch closed

( )

L

DT V

i

L

V dt

di

DT

i t

i dt

di

L

V dt

L

d L

L L

L

d L

Switch opened

L

DT V

V i

L

V

V dt

di

T D i t

i dt

di

L

V

V dt

v

o

d opened

L

o d

L

L

L L

o d

L

L

o d

L

) 1

(

) 1

+ v L

-i L

+

v o -

T D V

V L

DT V

i i

d o

o d

d

opened L

closed L

=

∆ +

∆

1

0

) 1

– When switch is opened, the output stage

receives energy from the input as well as from the inductor Hence output is large.

– Output voltage is maintained constant by

virtue of large C.

Average, Maximum, Minimum

Inductor Current

L

DT

V R

D

V i

I I

L

DT

V R

D

V i

I I

R D

V I

R D

V R

D

V I

V

R

V I

V

d d

L L

d d

L L

d L

d

d L

d

o d

d

2 )

1 ( 2

: current inductor

Minimum

2 )

1 ( 2

: current inductor

Maximum

) 1

(

: current inductor

Average

) 1

(

) 1

(

power Output

power Input

2 min

2 max

2

2 2 2

DT V

V

V C

DT R

V

Q

f

R D D

TR D

o

o o

d d

0 2

• The boost converter has the following parameters: Vd=20V,

D=0.6, R=12.5ohm, L=65uH, C=200uF, fs=40KHz Determine (a) output voltage, (b) average, maximum and minimum

inductor current, (c) output voltage ripple.

• Design a boost converter to provide an output voltage of 36V from a 24V source The load is 50W The voltage ripple factor must be less than 0.5% Specify the duty cycle ratio, switching frequency, inductor and capacitor size, and power device.

CIRCUIT OF BUCK-BOOST CONVERTER

CIRCUIT WHEN SWITCH IS CLOSED

CIRCUIT WHEN SWITCH IS OPENED

S

V i

L

V T

D

i t

i

L

V dt

di

dt

di L V

v

L

DT V

i

L

V DT

i t

i

L

V dt

di

dt

di L Vd

v

o opened

L

o L

L

o L

L o

L

d closed

L

d L

L

d L

L L

) 1

( )

(

) 1

(

opened

Switch

) (

closed Switch

Output voltage

• NOTE: Output of a buck-boost converter either be higher or lower than input.

– If D>0.5, output is higher than input

– If D<0.5, output is lower input

• Output voltage is always negative.

• Note that output is never directly connected to load

• Energy is stored in inductor when switch is closed and transferred to load when switch is opened.

=

∆ +

∆

D

D V

L

T D

V L

DT V

s

o d

opened iL

closed iL

1 V

: tage Output vol

0

) 1

(

0

: operation state

Steady

o

) (

) (

Average inductor current

2 2

2

2

) 1

(

, for ng

Substituti

: as current inductor

average

to related is

current source

supplied

power

equal must

load

by the

absorbed

power

converter,

in the loss

power no

Assuming

D R

D

V D

V

P RD

V

V I

V

D I

V R

o d

o L

o

L d o

L

s

s d

L and C values

RCf

D V

DT

V

V

V C

DT R

V

f

R D

L

L

DT

V D

R

D V i

I I

L

DT

V D

R

D V i

I I

o

o o

o

o o

d

d d

L L

d d

L L

=

Q

ripple, tage

Output vol

2

) 1

(

0 2

) 1

1 ( 2

2 )

1 ( 2

current, inductor

min and

Max

2 min

2 d

2 min

2 max

D V

V

D V

V

o o d o

2

) 1

( 8 1 Buck

L

RCf

D V

V

D V

V

o o d o

2

) 1

(

1 1 Boost

RCf

D V

V

D

D V

V

o o d o

2

) 1

(

1

Boost Buck

2min = −

+

Vo

−

Vo (desired)

Vo (actual)

+-

Switch control signal

SawtoothWaveform

Vcontrol 1

Switchcontrolsignal

ton 2

T

Vcontrol 2

ton 1

Isolated DC-DC Converter

• Isolated DC-DC requires isolation transformer

• Two types: Linear and Switched-mode

• Advantages of switched mode over linear power supply

• However above certain ratings,

SMPS is the only feasible choice

Linear and SMPS block diagram

Basic Block diagram of linear power supply

Basic Block diagram of SMPS

Base/gate Drive

Error Amp.

High Frequency rectifier and filter

Base/

gate drive

PWM Controller errorAmp

Vo

Vref

DCRegulated

DC-DC CONVERSITION AND

ISOLATIONDC

Unregulated

High frequency transformer

: Models

;

ip relationsh output

input

-Basic

voltage varying

- me up/down ti

step

2)

isolation electrical

output -

Input

1)

: function

Flyback converter circuit

Model with magnetising

inductance

Operation: switch closed

N 2 +

0

Therefore,

off turned diode

i.e.

, 0

er, transform the

of side load

On the

1 2 1

2 1

2 1

N

N V

V

v

N

N V

N

N v

v

L

DT

V i

L

V DT

iL dt

iL dt

diL

dt

di L

V

v

d o

D

d

m

d closed

L

m

d m

m m

Lm m

d

m

2 1 2 1 2

1 1

2

1 2

1 2

1

2 2

1 1

: switch the

across Voltage

) 1

( 1

But

N

N V

V v

N

N L

T D V

i

N

N L

V T

D

i dt

i dt

di

N

N V

dt

di L v

N

N V

N

N v

v

V v

N

N V

v

o d

SW

m

o open

m L

m

o m

L m

L m

L

o m

L m

o o o

1 2

2 1

1

0 1

0

operation, state

steady For

-N

N D

D V

V

N

N L

T D

V L

DT V

i i

d o

m

o m

d

opened

L closed

2 1

2 2

2 0

2 0

2 0 0

) 1

(

) 1

(

: as

n can writte

for Solving

: as

to related

is

N

N R D

V

N

N R D

D V I

I

DR V

V I

R

V D I

V

I

D

I T

DT

I

I

I I

L

d L

m

Max, Min inductor current

( )

RCf

D V

V

r

N

N f

R D

V L

f L

D

V L

DT

V N

N R D

N R D

D

V

i I

I

L

DT

V N

N R D

D

V

i I

I

d m

m

d m

d d

L

m

d d

L L

L

m

d d

L L

L

m

m m

m

m m

=

0

0

2 2

1

2 min

2 1

2 2

min

2 1

2 2

min

,

2 1

2 2

max

,

boost, similar to

is

n calculatio

Ripple

2

) 1

(

2 2

)

1

(

0 ,

CCM,

For

2 )

1

(

2

2 )

1

(

2

Max peak-peak inductor current ripple:

25% of the average inductor current.

Maximum peak-peak output voltage: 0.1V

Switching frequency: 75kHz

Based on the abovementioned specifications, determine a) Transformer turns ratio

c) Maximum and minimum inductor current.

d) Value of capacitor C.

Full-bridge converter

SW2 SW4

−

v p SW1

Full bridge: basic operation

• Switch “pair”: [S1 & S2];[S3 & S4].

• Each switch pair turn on at a time as shown The other pair is off.

• “AC voltage” is developed across the primary

Then transferred to secondary via high frequency transformers.

• On secondary side, diode pair is “high frequency full wave rectification”.

• The choke (L) and (C ) acts like the “buck

converter” circuit.

• Output Voltage

D N

N V

V

p

s s